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AERATION AND AIR-CONTENT 
 
 THE ROLE OF OXYGEN IN ROOT ACTIVITY 
 
 BY 
 
 FREDERIC E. CLEMENTS 
 
 Published by the Carnegie Institution of Washington 
 Washington, 1921 
 
CARNEGIE INSTITUTION OF WASHINGTON 
 Publication No. 315 
 
 PRESS OF ANDREW B. QRAHAM CO. 
 WABHINaTON, D. C. 
 
AERATION AND AIR-CONTENT 
 
 THE ROLE OF OXYGEN IN ROOT ACTIVITY 
 
 By Frederic E. Clements 
 
 A r^ rrr r: % 
 
H'^- 
 
 Ss.** 
 
 '^^Q&ii^^ 
 
 CONTENTS. 
 
 Page 
 
 Introduction 7 
 
 1. Respiration and oxygen 8 
 
 Normal respiration of roots 8 
 
 Early researches 8 
 
 Later researches 12 
 
 Recent researches 16 
 
 Summary 26 
 
 Aerotropism 28 
 
 Summary 32 
 
 Air of soil and plants 33 
 
 Air-content of the soil 33 • 
 
 Earlier researches 33 
 
 Later researches 38 
 
 Summary 42 
 
 Air-content of water 45 
 
 Summary 49 
 
 Influence of algse and water plants 
 
 on oxygen-content 50 
 
 Summary 52 
 
 Air-content of plants 53 
 
 Summary 56 
 
 Anaerobic respiration 56 
 
 Respiration 57 
 
 Early researches 57 
 
 Later researches 57 
 
 Recent researches 62 
 
 Summary 65 
 
 Photosynthesis 67 
 
 Summary 68 
 
 Transpiration 69 
 
 Summary 71 
 
 Germination 71 
 
 Early researches 71 
 
 Later researches 73 
 
 Page 
 
 1. Respiration and Oxygen — Cont'd. 
 
 Germination — Continued. 
 
 Recent researches 75 
 
 Summary 78 
 
 Growth 79 
 
 Early researches 79 
 
 Later researches 80 
 
 Recent researches 83 
 
 Summary 87 
 
 Protoplasmic streaming and mitosis 88 
 
 Summary 91 
 
 Irritability 91 
 
 Summary 94 
 
 Fungi 95 
 
 Summary 97 
 
 Aeration as an ecological factor. . . 98 
 
 Summary 107 
 
 2. Bog xerophytes and acid soils Ill 
 
 The nature of bog xerophytes Ill 
 
 Earlier views Ill 
 
 Later views 117 
 
 Causes and interpretations of bog 
 
 xerophytes 123 
 
 Acidity 130 
 
 Summary 140 
 
 Conclusions 141 
 
 3. Toxic exudates and soil toxins 144 
 
 Early views 144 
 
 The Woburn researches 146 
 
 Researches of the Bureau of Soils 151 
 
 Other researches 154 
 
 Conclusions 158 
 
 Bibliography 163 
 
AERATION AND AIR-CONTENT. 
 
 INTRODUCTION. 
 
 Recent studies of the so-called bog xerophytes have shown the 
 air-content of the soil to be an ecological factor of primary and often 
 of unique importance (Clements, 1916, 1920; Dosdall, 1919; Berg- 
 mann, 1920). In organizing a comprehensive investigation of bog 
 and swamp plants and of related problems, it has become desirable 
 to analyze much of the literature dealing with the respiration of 
 roots and with anaerobiosis. An attempt has been made to present 
 a complete digest of the results in so far as they have to do with 
 transpiration, growth, or movement, or serve to throw light upon the 
 mooted questions of bog toxins, acid soils, or toxic exudates. In 
 addition, a complete account is given of the development of views 
 upon bog xerophytes and soil toxins, primarily to afford a clear 
 grasp of existing opinion and to reveal the points to which further 
 study should be directed. 
 
 In spite of the enormous amount of work done upon the respira- 
 tion of roots and the significance of soil-air, physiologists and eco- 
 logists generally have ignored this subject or have given it httle 
 consideration. A cursory survey of a score of text-books shows that 
 the air-content of the soil is not even mentioned by the majority, 
 while it is given slight attention by a few, and accorded recognition 
 as a factor of primary importance by three or four only. This was 
 probably a natural outcome of the laboratory development of physi- 
 ology. At least, it appears certain that the basic importance of the 
 air-content of the soil could not be appreciated fully until ecology 
 had begun the instrumental measurement of the habitat. Even the 
 latter has found it difficult to distinguish between sequelae and causes 
 in this field (Plant Succession, p. 90), and general recognition of 
 air-content as a primary factor in many habitats, and a controlling 
 one in wet soils and water, is yet to come. 
 
 In the following treatise, the development of our knowledge of 
 the respiration of roots and other underground parts is first taken 
 up in historical sequence, together with an account of studies deal- 
 ing with the nature and composition of soil-air. This is succeeded 
 by a digest of results in the field of anaerobic respiration, with 
 especial reference to its relation to germination, growth, and move- 
 ment. A section is devoted to bog xerophytes and swamp plants, 
 with which are considered bog toxins and acid soils, while soil toxins 
 and toxic exudates are discussed in a final section. 
 
 lull 
 
I. RESPIRATION AND OXYGEN. 
 NORMAL RESPIRATION OF ROOTS. 
 
 The necessity of oxygen for the proper functioning of roots has 
 not only been repeatedly demonstrated by direct experiment, but 
 further evidence of it has also been gained from the behavior of other 
 organs. This would be expected from the fact that the basic re- 
 sponses of protoplasm are more or less identical for all green plants. 
 As a consequence, studies of the oxygen requirements of leaves and 
 shoots have hkewise much significance for the behavior of roots. 
 In the case of underground shoots, rootstocks, tubers, bulbs, and 
 corms, the process of respiration is not only identical with that of 
 roots, but the relation to the air-content is also the same. This is 
 true to a large extent of all soil organisms and especially of the host 
 of aerobic fungi. Soil algse, on the contrary, free more oxygen than 
 they use and serve to maintain the air-balance of the soil. 
 
 In organizing the evidence derived from the results of many inves- 
 tigators, it has been desirable to preserve the historical sequence as 
 far as possible, but at the same time to give a coherent discussion 
 of the various phases of the subject. In consequence, our knowledge 
 of the oxygen requirements of roots is divided into three sections, 
 namely, normal respiration, anaerobic respiration, and field studies 
 of aeration. Each of these furnishes its own particular body of 
 evidence and all must be taken into account for a complete view. 
 For convenience, the first two have been divided into three general 
 periods determined by the years 1870 and 1900. One of these 
 marks the first studies of intramolecular respiration by Pfeffer and 
 his students, and the other the beginning of quantitative ecology, 
 as well as a more intensive attack upon the problems of respiration 
 by Palladin, Stoklasa, Nabokich, and their associates. 
 
 Early researches. — The importance of oxygen for germination and 
 growth was first demonstrated toward the close of the seventeenth 
 century by several investigators. The first of these were Mayow 
 (1668), who found that oxygen was indispensable to plants, and 
 Huygens and Papin (1674), who showed that plants die under the 
 air-pump in the absence of air. Malpighi (1687) was the first to 
 determine that air was required for germination, while Ray (1690) 
 discovered that lettuce seeds would not germinate in a vacuum, but 
 did so readily upon the return of air. Homberg (1699) found that 
 the seeds of Portulaca oleracea, Lactuca sativa, and Lepidium sativum 
 germinated slowly or not at all in rarefied air. 
 
 Hales (1727) first discovered that CO2 was secreted by roots. 
 Corti (1774 : 210) placed plants of Chara in a vacuum and left them 
 
RESPIRATION AND OXYGEN. 9 
 
 for 48 hours. The movement of the protoplasm ceased, but it 
 began again in 8 to 12 hours after the plants were returned to atmos- 
 pheric air. Scheele (1777) was the first to prove that oxygen was 
 used during germination and carbon dioxid released, as in animals. 
 Ingenhousz (1779) was the pioneer in recognizing that plants give 
 cfif carbon dioxid at night or in the dark, in contrast to the evolu- 
 tion of oxygen during the day. He stated that roots, flowers, and 
 fruits, as well as green parts, behaved in this manner. In 1796, he 
 emphasized the fact that roots and other colorless parts always 
 excrete CO2, while green parts in the light give off oxygen. He 
 likewise denied that plants absorb their necessary carbon from the 
 soil by means of the roots. 
 
 Senebier (1791) stated that the access of air is indispensable to 
 plant life and that it was Hkewise necessary to the germination of 
 seeds in the soil. He also pointed out that the roots of plants perish 
 in stagnant water. Huber and Senebier (1801) found that the 
 amount of air during germination diminished in proportion to the 
 oxygen in it, and that this was accompanied by the formation of 
 carbon dioxid. They hkewise noted that seeds would not germinate 
 in air whose oxygen had been exhausted by bees and they confirmed 
 the need of oxygen for plant-growth by a comprehensive series of 
 studies in various gases. Rollo (1798) beheved that oxygen dis- 
 appeared and was replaced by carbon dioxid in the course of germi- 
 nation, and he determined that barley grains gave off CO2 for several 
 days in the absence of oxygen. 
 
 Saussure (1804) regarded water and oxygen as the two factors 
 essential to germination and stated that a small amount of oxygen 
 is present even in the case of the seeds of water-plants and others 
 that germinate under water. He proved this by showing that peas, 
 lentils, and seeds of Alisma and Polygonum were unable to germinate 
 in an amount of boiled water 7 to 8 times their weight, but did so 
 readily when the amount was 200 times greater. He further showed 
 that oxygen disappears and is replaced by carbon dioxid when the 
 seeds are in direct contact with it, but, when oxygen is scanty or 
 lacking, decomposition results, with the evolution of hydrogen as 
 well as carbon dioxid. The amount of oxygen necessary to effect 
 germination differed with the species, beans, kidney-beans, and lettuce 
 requiring more than peas, and these more than wheat, barley, or 
 purslane. 
 
 Roots of carrot consumed their own volume of oxygen, as did 
 turnip, while a potato used 0.4 of its volume and a hly-bulb approxi- 
 mately the same. These differences were reflected in those of the 
 leaves. Leaves of deciduous trees and shrubs consumed from 2.2 
 to 8 times their volume of oxygen in 24 hours, beech and apricot 
 being the most active and lilac the least. The average consumption 
 for the 15 species was approximately 6 times the volume. For 
 
10 AERATION AND AIR-CONTENT. 
 
 evergreens, both broad-leaved and needle-leaved, the range was from 
 0.8 to 4 times and the average 2 times the volume. For terrestrial 
 herbs, the range was from 0.5 for Liliuin to 5 for Triticum, and the 
 average 2.4 times the volume. Aquatic and marsh herbs varied 
 from 0.7 for Alisma plantago to 2.3 for Lythrum and Carex, the aver- 
 age being 1.6 times the volume, while the range in fleshy plants was 
 from 0.6 for Saxifraga cotyledon to 1.7 for Sedum and Mesembryan- 
 themum. The average was 1.1 times the volume. In most cases 
 the respiration was 50 to 100 per cent greater in May or June than 
 in September. 
 
 To obtain further proof of the need for oxygen, Saussure treated 
 the roots of the chestnut with nitrogen, hydrogen, and carbonic 
 acid, using other plants in atmospheric air as checks. The plants 
 whose roots were in contact with CO2 died first at the end of 7 or 
 8 days, while those with the roots in nitrogen and hydrogen died at 
 the end of 13 or 14 days. The chestnuts with roots growing in 
 ordinary air were still vigorous at the end of 3 weeks, when the experi- 
 ment was finished. The conclusion that the presence of oxygen is 
 necessary for growth was further supported by the observation that 
 plants with roots submerged in stagnant water suffered more quickly 
 than those in running water, and also by the fact that roots which 
 grew^ into manure or into water conduits became very greatly divided 
 in the endeavor to increase their contact with the very small amount 
 of oxygen found in such places. 
 
 Grischow (1819 : 143) determined that roots take up oxygen and 
 give off carbon dioxid. He likewise proved that fungi absorb oxy- 
 gen and evolve carbon dioxid, and hence that their respiration is 
 essentially the same as that of green plants. 
 
 Marcet (1829) thought to have demonstrated that fungi also evolve 
 hydrogen in respiration, but later (1834) concluded that this was due 
 to fermentation processes set up by bacteria and that fungi respired 
 in the usual manner. 
 
 Meyen (1838) declared that respiration is general for all plant 
 parts, and he also distinguished clearly between respiration and 
 photosynthesis in the economy of the plant. 
 
 Dutrochet (1840) stated that, since all plant parts absorb oxygen 
 and excrete carbon dioxid, they should also produce heat, and he 
 was able to demonstrate this experimentally. He also emphasized 
 the need of distinguishing between respiration and the decomposition 
 of CO2 in the hght, and determined that neither periodic nor tropistic 
 movement was possible in the absence of oxygen. 
 
 Becquerel (1833), Wiegmann and Polstorff (1842), and Oudemans 
 and Rauwenhoff (1858) early demonstrated that the excretion of the 
 root was acid, as shown by its reaction to litmus. 
 
 Garreau (1851), in the study of respiration in the different organs 
 of various plants, determined the amount of CO2 exhaled by the 
 
RESPIRATION AND OXYGEN. 11 
 
 fleshy roots of carrot and the fibrous roots of Senecio during 24 hours. 
 The respiration of the latter was more than 6 times greater than that 
 of the former. The amount of CO2 expired by plants was found to 
 be the greater the richer they were in protein and the more extended 
 surface they presented relative to their mass. When green parts 
 of plants were submerged they respired as aerial plants, but with the 
 difference that the amount of CO 2 was reduced because the medium 
 was poorer in oxygen. 
 
 While Liebig (1858) was apparently the first to note the etching of 
 hmestone and ascribe it to the action of roots, Sachs (1860, 1865) first 
 demonstrated experimentally that this was the case. He grew the 
 roots of corn on pohshed marble plates and found that they etched 
 the surface. He concluded that this was due to the excretion of 
 CO2 from the roots, since roots grown in distilled water quickly 
 charged this with CO2. He also thought it possible that the corrosion 
 might be due to the acid cell-sap which saturated the cell-wall. His 
 early work was confirmed by experiments in 1860 and 1864, showing 
 the corrosion of the surface of dolomite, magnesite, etc. He as- 
 sumed that this might be produced by CO2, but also thought that it 
 might perhaps be due to the acid sap of the roots themselves. He 
 detailed at some length the studies of 1864 with several species of 
 plants on various surfaces. According to Mayen (1838, 1 :1 1), Molden- 
 hawer thought that the root-hairs secreted a sap which served to 
 dissolve any other material. 
 
 Knop (1861, 1864) confirmed the results of Sachs in regard to root 
 excretion. He found that a considerable amount of CO2 accumu- 
 lated during the growth of grasses in neutral solutions. In further 
 researches, proof was obtained that CO 2 was excreted by the roots 
 of growing plants and this was regarded as a device for increasing 
 the absorption of solutes. He concluded that all organs absorb 
 oxygen in producing CO2, and hence that the tissues of land plants 
 were filled throughout with air containing CO 2. It was regarded 
 as probable that the CO 2 excreted by the root serves universally 
 for the solution of mineral substances in the soil, and as possible 
 also that other more permanent acids aid in this work. 
 
 Nobbe (1865) showed that potato tubers respire during storage in 
 the winter and that the amount of starch was correspondingly 
 reduced. Fleury (1865) confirmed the results of Huber and Senebier 
 and of Saussure in showing that dry seeds begin to evolve carbon 
 dioxid soon after the absorption of water. 
 
 Coren winder (1867 : 63) grew roots of Cuphea attached to the 
 aerial parts in air with a known quantity of CO 2 in order to deter- 
 mine whether roots absorb the latter. He found, on the contrary, 
 that a considerable amount of CO2 was exhaled in the case of a 
 number of roots. Similar experiments were made with cabbage and 
 with Eupatorium with the same results. The author found, more- 
 
12 AERATION AND AIR-CONTENT. 
 
 over, that certain marsh plants died quickly when their roots were 
 kept in contact with water charged with CO2. 
 
 Later researches. — As a consequence of the work of Pfeffer and his 
 students, active investigation was directed more to the problems of 
 intramolecular respiration, and direct results relating to normal 
 respiration were largely a by-product. Extensive studies were made, 
 however, of the respiratory curve and quotient during germination 
 and growth at the beginning of this period by Wiesner (1871), 
 Sachsse (1872), Heintz (1873), Deherain and Landrin (1874), De- 
 herain and Moissan (1874), Wolkoff and Meyer (1874), Meyer 
 (1875), Borodin (1875), Detmer (1875), Rischawi (1876), and Saike- 
 wicz (1877). Sachsse and Detmer in particular, together with 
 Muntz (1876), Deherain and Vesque (1876, 1877), and Bonnier and 
 Mangin (1884), gave complete confirmation to the earlier results, 
 which had showed that carbon dioxid was the only gas produced by 
 respiration in all green plants and in most fungi. At the end of 
 this period Pfeffer (1878) declared that it was impossible to ascribe 
 any of the energy used by higher plants to intramolecular respira- 
 tion, and that the share of oxygen in metabolism was so important 
 that normal functioning was impossible in its absence. He regarded 
 the old maxim, ''No life without respiration," as still effective, since 
 normal respiration is indispensable if the organism is to remain 
 capable of life. 
 
 In his study of the respiration of water plants (1875 : 694), 
 Bohm reached the conclusion that the amount of oxygen used in 
 the respiration of water plants in atmospheric air is much smaller 
 than under similar conditions in the case of land plants. Likewise, 
 carbon dioxid is formed by water plants in an atmosphere com- 
 pletely without oxygen or otherwise indifferent, in consequence of 
 intramolecular respiration, but more feebly than under similar con- 
 ditions with land plants. Freyberg and Mayer (1879 : 463) studied 
 the respiration of swamp plants with special reference to the com- 
 parative behavior of swamp plants and land plants. They proceeded 
 upon the assumption that the presence of air-passages was supple- 
 mented by a difference in the intensity of respiration in the roots 
 of swamp plants. Their chief results are expressed in table 1. 
 
 The respiration maximum for the roots of mature swamp plants 
 was 38 c.c. 0, and for seedling roots it was 56 c.c. O, while the corre- 
 sponding maxima for mesophytes were 68 and 83 c.c. The respira- 
 tion was found to increase with the nitrogen-content, and this was 
 thought to explain why swamp and water plants have a uniformly 
 low respiration. The final conclusion was that the roots of swamp 
 plants require less oxygen than those of mesophytes, whether this 
 be determined with respect to volume, mass, or dry weight. 
 
KESPIRATION AND OXYGEN. 
 
 13 
 
 Table 1. — Respiration of roots and leai\ 
 
 Species. 
 
 Root-length 
 (equal volume). 
 
 Oxygen used in 
 
 24 hours per 1 gm. 
 
 dry weight. 
 
 Per cent of 
 
 nitrogen in dry 
 
 weight. 
 
 Roots. 
 Seedling 
 
 jnm. 
 15.6 
 35.0 
 14.6 
 27.6 
 
 29.0 
 13.2 
 39.0 
 41.0 
 37.0 
 
 c.c. 
 67.9 
 82.8 
 44.4 
 55.1 
 
 62.5 
 37.2 
 46.1 
 19.1 
 27.5 
 
 27.2 
 29.4 
 22.4 
 24.7 
 24.8 
 15.0 
 12.8 
 11.8 
 11.2 
 29.6 
 18.9 
 19.2 
 
 3.2 
 
 Do 
 
 
 1.6 
 
 Do 
 
 Mature. 
 Lamium album 
 
 3.4 
 1.5 
 2.7 
 1.7 
 
 Mentha aquatica 
 
 Ranunculus bulbosus 
 
 Caltha palustris .... 
 
 Do 
 
 Leaves. 
 
 4.2 
 
 Do 
 
 
 
 
 3.0 
 
 Do 
 
 
 Do 
 
 
 
 
 
 2.6 
 
 Do 
 
 
 Glyceria fluitans 
 
 
 1.9 
 
 Do 
 
 
 Ranunculus bulbosus ... 
 
 
 4.6 
 2.9 
 
 Ranunculus fluitans 
 
 
 Do 
 
 
 
 
 
 Cauvet (1880 : 113) demonstrated that roots constantly excrete 
 CO2 and that this excretion is weaker at night than during the day. 
 He also determined that the root does not absorb CO2 from the soil, 
 and that the CO 2 excreted has for its immediate effect the solution 
 of the solid matter of the soil for the use of the root. 
 
 Schwarz (1883 : 135) at first assumed that the production of root- 
 hairs was suppressed in many plants because of lack of oxygen. 
 The addition of abundant oxygen to the water-cultures failed to 
 cause the production of hairs and he concluded that the absence of 
 hairs could not be ascribed to its lack. It seems probable, however, 
 that the amount of oxygen used was too great for growth. Van 
 Tieghem and Bonnier (1882) found that 7.976 gm. of peas, sealed 
 in air, yielded 3.82 per cent of carbon dioxid and reduced the oxygen 
 to 14.44 per cent in the course of 2 years. 
 
 Bonnier and Mangin (1884 : 215, 220) pointed out that, in pro- 
 longing the sojourn of plants in containers, the respiration was no 
 longer normal. At the end of a certain time, when nearly all the 
 oxygen was consumed, fermentation proper entered and carbon 
 dioxide was released in great quantity without oxygen being ab- 
 sorbed. They were also (1885) the first to show that respiration 
 increases with increased humidity of the air. 
 
14 AERATION AND AIR-CONTENT. 
 
 Miiller-Thurgau (1885 : 857) found that potato tubers respired 
 more vigorously just after being harvested than they did several 
 days later. But tubers that were separated from sound stocks re- 
 spired about doubly. Ultimately, however, the respiration sank to 
 about the same level and then remained constant for a long time 
 during the resting condition of the potato. 
 
 In an investigation of the relation of cell-turgor in growing organ- 
 isms to respiration, Palladin (1886 : 328) reached the following conclu- 
 sions : Since the absorption of oxygen is necessary for the production 
 of substances which cause turgor, it is to be expected that the latter 
 will be reduced by lack of oxygen, and, in fact, plants that have 
 lived some time without oxygen appeared to be withered. In 
 growing organs the accumulation of the organic acids that produce 
 the turgor of the cells appears as a result of respiration. In the ab- 
 sence of oxygen, growth ceases on account of the interruption of the 
 formation of these substances. 
 
 Molisch (1887 : 84) studied the secretions of roots and concluded 
 that they influence also organic materials, and in a higher degree 
 than mineral and rock constituents of the soil. He found that the 
 root-secretions have both reducing and oxidizing action. They 
 oxidize different organic substances, such as guaiaconic, pyrogaUic, 
 and gallic acids, and, most important of all, humus substances. As a 
 consequence, they favor in a high degree the decomposition of the 
 organic material of the soil of fields and of forests. The root- 
 secretion changes cane sugar to reducing sugar, and exerts a weak 
 diastatic effect. 
 
 Kny (1889 : 163) found that a 12-day exclusion of oxygen pre- 
 vented the formation of cell-walls, but was not sufficient to kill the 
 cells completely. He concluded that the free oxygen of the air is 
 not only necessary for the beginning of the formation of cell-divisions 
 in the wound-periderm, but also for cutinization of the membrane. 
 The cell-divisions of the latter seem to be favored a little by the action 
 of a very small amount of hydrogen peroxide. 
 
 Loven (1891) studied the respiration of a number of marine algae, 
 Ascophyllum, Laminaria, Ulva, Enteromorpha, Ceramium, etc., and 
 found that they are able to absorb every trace of oxygen present in 
 the water. In water entirely free from oxygen, algse can produce con- 
 siderable quantities of CO 2. He gave the oxygen-content per Hter 
 and also that of CO2 before and after the respiration experiments. 
 
 Aubert (1892 : 280) showed that the internal air of fleshy plants 
 differed considerably from the atmospheric air in the relative pro- 
 portions of the component gases. The variations in the ^ ratio 
 in the fleshy plants bore a certain relation to the amount of water 
 they contained. They were greater when succulence was most pro- 
 nounced. Fleshy plants subjected to the same temperature in the 
 dark absorbed a nearly constant volume of oxygen, but evolved 
 
EESPIRATION AND OXYGEN. 
 
 15 
 
 during the day a greater amount of CO 2 than at night. This 
 difference between the day and night volumes of CO2 was more 
 important when the plants were most fleshy. Plants exhibited a more 
 active gas-exchange in proportion as their fleshiness is less 
 pronounced, and hence ordinary plants showed a greater respiration 
 intensity than fleshy ones. Among fleshy plants, Crassulacece and 
 Mesembryanthemacece, which possess a thin cuticle, exhibited a more 
 active exchange than most of the Cactacece. The fleshy euphorbias 
 and trees with evergreen leaves were nearly intermediate in this re- 
 spect. The extent of surface-contact with the atmospheric air was 
 regarded as of the first importance in explaining differences in in- 
 tensity and respiration, and the amount of water the plant contains 
 as next most important. 
 
 Aeroboe (1893) studied the respiration of roots of Vicia faba in 
 relation to light and reached the conclusion that this exerted an 
 indirect effect. He found that when plant parts were placed in the 
 dark the production of CO 2 steadily decreased. 
 
 Bohm (1893), in a study of the respiration of potatoes, found that 
 wounding, relatively low and high temperatures, partial exclusion 
 of oxygen, continued exposure to pure oxygen, and infection by 
 Phytophthora infestans, all produced energetic respiration. In a 
 medium poor in oxygen, thin cyhnders of sound or stimulated tubers 
 used only a small amount of the gas. 
 
 Mangin (1896 : 747) in experiments with flax, radish, peas, car- 
 rots, etc., found that the accumulation of CO2 and a reduction in 
 oxygen effects a diminution of respiratory activity in the seeds and 
 tubers. This occurred in air that contained 1 to 3 per cent of CO2 
 in one case and 2 to 5 per cent in the other. 
 
 Jost (1893 : 100) observed that wounded potatoes, especially those 
 cut into many pieces, showed earlier growth of the buds. Ziegenbein 
 (1893 : 594) determined the respiration rate of potato tubers, seed- 
 lings, and shoots at different temperatures to be as follows, expressed 
 in miUigrams of CO2 produced by 100 gm. in an hour. 
 
 Czapek (1896 : 321) studied the 
 excretions of roots of Phaseolus, 
 Pisum, Helianthus, Cucurbita, Zea, 
 Linum, Picea, etc., in water and in 
 air saturated with moisture. He 
 found that the roots of all of these 
 secrete various substances, partly 
 organic, partly inorganic. The lat- 
 ter are potassium, calcium, mag- 
 nesium, and hydrochloric, sulphuric, 
 and phosphoric acids. Of these, 
 only potassium and phosphoric acid 
 are present in any considerable 
 
 Table 2. 
 
 Temp. 
 
 Potato 
 tubers. 
 
 Seedlings, 
 Vicia 
 faba. 
 
 Shoots, 
 
 Abies 
 
 excelsa. 
 
 10 
 20 
 30 
 35 
 40 
 45 
 50 
 55 
 60 
 
 1.17 
 2.22 
 4.62 
 7.85 
 10.24 
 12.22 
 11.14 
 10.30 
 2.71 
 
 
 
 
 
 55.20 
 78.72 
 65.10 
 57.80 
 80.80 
 
 185.00 
 206.40 
 198.40 
 168.90 
 33.30 
 
 
 
 
 
16 AERATION AND AIR-CONTENT. 
 
 quantities, and they occur in the form of primary potassium phos- 
 phate. Acetic acid and lactic acid are not found in the root excre- 
 tions, but formic acid in the form of potassium salts is not at all rare. 
 This diffuses out of the living cells of the root-tips, and is, therefore, 
 not a product of decomposition. Oxalic acid, as calcium oxalate, was 
 found but once, in the excretions of Hyacinthus orientalis. The cor- 
 rosion phenomena produced by roots are due in the largest degree to 
 the excretion of CO2. The reddening of litmus paper and the corrosion 
 of rock surfaces is due to two different substances, carbon dioxid and 
 monopotassium phosphate. No acid other than CO2 is regularly ex- 
 creted by the roots of higher plants. The excretion of diastatic or 
 inverting ferments by the roots of higher plants is not physiologically 
 impossible, but a critical repetition of the experiments of Molisch, 
 who assumed the regular occurrence of these ferments in the root- 
 secretions, gave only negative results. 
 
 Richards (1896 : 551) determined that a greatly increased respira- 
 tion results after injury to plant-tissue, varying in intensity and dura- 
 tion with the character of the tissue and the extent of the wound. 
 This increased activity usually reaches a maximum within two days 
 and then falls gradually to the normal as the wound heals over. 
 
 Palladin (1897:827) concluded, since completely immersed etiolated 
 leaves do not become green, that oxygen, to an amount greater than that 
 freed in assimilation, is necessary for the production of chlorophyll. 
 
 Wacker (1898 : 70) has studied the effect of soil and water upon 
 water and land plants. He found that Vicia faba, Lupinus albus, 
 Helianthus annuus, and Cucurhita pepo undergo a retardation in the 
 growth in length of their main roots when they are cultivated in 
 water. On the other hand, such water plants as Lernna viinor and 
 trisulca, Azolla filiculoides, and Hydrocharis viorsus-rancB, in a nor- 
 mally moist garden soil, show almost no root-growth. In both cases 
 this difference in growth is not a consequence of the different amount 
 of oxygen in the two media, or of the greater density of the soil so- 
 lution. The almost complete cessation of root-growth in Lemna 
 minor in soil seems to indicate that these plants do not have the 
 ability to draw water from the soil particles in sufficient amount. 
 Roots are only formed when the soil is saturated with water, so that 
 the plant may come in intimate contact with the water in the soil. 
 In mud, the roots of land plants, Vicia faba and Lupinus albus, died 
 off, either due to the absence of free oxygen, to the presence of vari- 
 ous decomposition products, or to both of these factors together. 
 While swamp plants are able to secure a sufficient amount of oxygen 
 for the roots from the aerial parts, land plants are unable to do this, 
 owing to the absence of aeration passages. 
 
 Recent researches. — Maze (1900 : 350) found that a short immer- 
 sion of seeds in water restricted their further germination and that 
 
RESPIRATION AND OXYGEN. 17 
 
 after 8 to 12 days they completely lost their power of growth and 
 died for the most part, 
 
 Arker (1901 : 431) investigated the influence of the surrounding 
 medium upon roots and showed that the rate of growth in roots of 
 Lupinus albus was increased when a stream of atmospheric air was 
 draw^n through the soil and when the soil-air was diluted to a certain 
 degree. The growth of roots of Lupinus and Helianthus in water 
 was faster when atmospheric air was bubbled through the solution. 
 The growth in mud was promoted by frequently renewing the water 
 and thus increasing the access of air. The retardation of root- 
 growth in water was less marked, since the oxygen is absorbed more 
 readily from water. The drawing of air through water does not 
 increase the oxygen-content, but keeps it nearer saturation. 
 
 Strohmer (1903 : 933), in a comprehensive study of the respiration 
 of the sugar-beet, showed that some roots under normal conditions 
 excrete no other carbon-containing gas than CO 2. When wounded, 
 the sugar-beet showed an evident increase in the respiration inten- 
 sity. Ethyl alcohol was found to be a product of intramolecular 
 respiration. 
 
 Newcombe (1902) observed that roots were distorted when grown 
 in closed tubes with water at 23° C. or above, and ascribed this 
 response to the possible absence of oxygen or the accumulation of 
 root secretions. 
 
 Vochting (1902 : 87) studied the influence of aeration upon the 
 germination of potato tubers, chiefly by means of cylinders so ar- 
 ranged that the upper half would have well-aerated soil and the 
 lower half soil in which oxygen was more or less absent. In other 
 experiments, three layers of different air-content were arranged in 
 each cyUnder. In the case of cylinders of two layers, the upper pro- 
 duced many shoots and none or relatively few tubers, while in the 
 lower the results were just opposite. In the cyUnders with more 
 than two layers, the access of oxygen to the lowermost was so re- 
 duced that these decayed before further results could be obtained. 
 In the two upper layers the results were as indicated above. In 
 testing Stich's discovery that 3 to 4 per cent of oxygen did not mea- 
 surably decrease the amount of CO2 produced, tubers were placed in 
 an atmosphere of 4 per cent. After a few days sprouts appeared 
 and the amount of oxygen was then reduced to 3 per cent. Under 
 these conditions new sprouts still appeared and the oxygen was then 
 reduced to 2 per cent, where no further growth occurred. The shoots 
 apparently remained fresh and the experiment was continued for 
 4 weeks, when it was found that all tubers had decayed more or less. 
 He also found that the roots of potato tubers ceased to produce root- 
 hairs when the amount of oxygen was reduced to 3 per cent. In 
 experimenting with willow twigs, he showed that there was enough 
 oxygen in the water to maintain life, but not for the production 
 
18 
 
 AERATION AND AIR-CONTENT. 
 
 of new organs, and that an aerial supply was needed for the growth 
 of shoots and roots. 
 
 Smirnoff (1903 : 26) has shown that wounding causes an increase 
 in the intensity of normal respiration in the bulbs of Allium, but 
 not of intramolecular respiration. This increase may sometimes 
 amount to more than 50 per cent. 
 
 Kossowitch (1904), in studies with hemp, has reached the con- 
 clusion that the respiration of roots is significant, and that it can 
 not be left out of consideration in the biological processes of the soil. 
 
 Putter (1904) found that Beggiatoa and Euglena, as well as Para- 
 mcecium and Spirostomum, were obligate aerobes that were harmed 
 by very low oxygen pressures, while complete withdrawal of oxygen 
 killed them with great rapidity. 
 
 Duval (1904 : 76) has shown by studies of the seeds of beans, cab- 
 bage, carrot, lettuce, and onion that the respiration of seeds is in- 
 tense if moisture be present and is accompanied by a rapid loss in 
 vitality. Table 3 gives the results when seeds were allowed to ab- 
 sorb from 4 to 10 per cent of water. 
 
 Table 3. 
 
 Kind. 
 
 Weight. 
 
 CO2 per 
 year. 
 
 Germination. 
 
 Exper. 
 
 Control. 
 
 
 grams. 
 25 
 10 
 10 
 10 
 10 
 
 c.c. 
 
 2.5 
 24.0 
 27.0 
 19.5 
 26.5 
 
 
 
 
 
 
 100 
 
 89 
 84 
 94 
 97 
 
 Cabbage 
 
 Carrot 
 
 Lettuce 
 
 
 
 When onion seeds were kept for a year and 13 days in sealed bot- 
 tles of air or illuminating gas, the vitality was not impaired except 
 in the case of those with additional moisture, which were all dead. 
 In this bottle the oxygen had completely disappeared and the amount 
 of CO2 was 13.35 per cent, while in the two other bottles containing 
 air, the oxygen was 8 and 12 per cent and the carbon dioxid was 
 18.8 and 6^8 per cent. 
 
 Snow (1905 : 33) found that the absence of oxygen stopped the 
 production of root-hairs and retarded growth, while root-hairs also 
 developed better in tap-water than in distilled water. In the case 
 of corn, the reduction of the oxygen-pressure to zero completely 
 suppressed the development of hairs, even when the CO2 was re- 
 moved. The roots of wheat quickly died in the absence of oxygen, 
 but in the case of several roots that lived for a day, no root-hairs 
 were formed. The roots of willow twigs developed root-hairs in 
 about half the normal oxygen-content, but this may have been due 
 
RESPIRATION AND OXYGEN. 19 
 
 to the presence of chlorophyll or to included air. Bergmann 
 (1920 : 17) and Cannon and Free (1920 : 62) have likewise shown 
 that the formation of root-hairs is dependent upon oxygen. Raci- 
 borski (1905 : 338) found that the ability of the root-surface to 
 oxidize easily oxidized substances about it is controlled by oxygen. 
 
 Stoklasa and Ernest (1905 : 723) pointed out that roots act strong- 
 ly upon rock through the excretion of CO 2, as is often seen in nature 
 on smooth stone-walls corroded by roots. This effect has often 
 been ascribed to organic acids produced in the roots, but it is now 
 clear that this role is played only by the CO2, and it is particularly 
 noticeable in young root-systems. This makes it possible to under- 
 stand how the plant prepares its necessary mineral nutrients in so- 
 lution through its root-system. This results from the excretion of 
 CO 2 by its own root-system and through the micro-organisms of the 
 soil, the number of which is materially increased in cultivated soils. 
 
 Brizi (1906 : 89) found that the roots of rice are not of the aquatic 
 type, and demonstrated by means of water-cultures that oxygen is 
 absolutely essential for their growth. He concluded that the algse 
 of the rice-fields are doubly useful, in that they utilize a large amount 
 of CO2 and set free a corresponding quantity of oxygen available for 
 the roots. 
 
 Day (1906 : 37) has grown wheat, barley, oats, and peas in jars, 
 one set of which was aerated daily during the growing season by 
 drawing enough air through to completely change the air in the jars. 
 In germination there was no marked difference between the two sets, 
 except in the case of the peas. The aerated jars germinated 81 per 
 cent and the unaerated ones 61 per cent. Moreover, the growth of 
 the pea plants was nearly twice as great in the aerated as in the un- 
 aerated jars for the first period. There was but little difference 
 between the grains grown in the aerated and unaerated conditions. 
 In a second study of aeration (1907 : 36), alfalfa and soy beans were 
 grown in addition to wheat, barley, oats, and peas. There were four 
 jars of each species and air was forced through two of them once a 
 day. The results for wheat and barley were negative, while oats 
 and peas yielded slightly better when aerated. This lack of response 
 was thought to be due to the fact that frequent rain produced good 
 natural aeration in all jars. One striking fact was that the pea 
 plants grown from seed from the aerated jar of the previous year 
 gave double the yield given by the plants from seed from unaerated 
 jars. Alfalfa and soy beans did not mature, but the beneficial 
 effect of increased aeration was very noticeable. 
 
 Hall (1906 : 57) concluded that from several points of view it is 
 not necessary to assume the existence of an excretion from the roots 
 of a plant of a permanent acid, organic or inorganic, to attack the 
 solid mineral particles of the soil and to bring them into solution for 
 the nutrition of the plant. The growing portions of a plant-root are 
 
20 AERATION AND AIR-CONTENT. 
 
 always giving ofif CO2, and CO2, especially in the concentrated solu- 
 tion which must be momentarily formed in the cell-wall of the root- 
 hairs, has an appreciable solvent effect upon the majority of the 
 minerals composing the soil. This CO2 alone is capable of giving 
 rise to such solutions as are required for the nutrition of the plant. 
 As the direct evidence is also adverse to the idea of an excretion of 
 acid, the principle of not seeking remote causes would lead us to 
 attribute to CO 2 only, the long-recognized solvent power of the 
 plant upon the soil. 
 
 Kunze (1906) has shown that corrosion studies and culture experi- 
 ments in powdered stone prove that the higher plants can not obtain 
 necessary nutrients from unweathered stone. Moreover, plants 
 which are marked by vigorous excretion of acid, such as white cab- 
 bage and vetch, show a stronger development, due to the energetic 
 decomposition of the soil than those with more or less of this quality, 
 e. g., mustard and sainfoin. Rapidly growing plants of relatively 
 short vegetation period show marked excretion, e. g., Cucurbita, 
 Helianthus, Phaseolus, Zea, etc. Among the grasses, Secale and 
 Avena show strong excretion, Hordeum and Triticum less. More- 
 over, while the ability of the grasses to dissolve soil nutrients through 
 root excretions is small or lacking, this is compensated by the greatly 
 branched root-systems and their energetic transpiration. The 
 marked excretion found among the Boraginacece is explained by the 
 vigorous development of the plant-body, relative short vegetative 
 period, and the dry habitat. Since only a small amount of acid or 
 none at all is produced by a great number of plants, and since in 
 many cases CO 2 plays only a subordinate part in breaking down the 
 soil particles, it is assumed that many higher plants must possess 
 another means of producing nutrient solutions in the soil, as by the 
 aid of soil fungi. 
 
 Stoklasa and Ernest (1908 : 64), in a study of the chemical nature 
 of the root secretion, conclude that carbon dioxid is the sole gas 
 secreted in normal respiration. The injurious action which the 
 lack of oxygen in the soil produces upon the root is evident, and 
 appears especially in soils crusted over or supersaturated. The 
 unsatisfactory aeration of the soil always produces certain patho- 
 logic phenomena of the plant, which can only be connected with the 
 improper oxidation of the products of the decomposition of the 
 carbohydrates and proteins. 
 
 Griiss (1907 : 69) studied the chemical changes involved in 
 wounded potatoes and found the oxidizing and diastatic enzymes 
 increased. Wismewski (1912 : 1045) found that the rest-period of 
 buds of Hydrocharis morsus-rance was shortened by wounding. 
 Muller-Thurgau and Schneider-Orelli (1910:309; 1912:386) ob- 
 served increased respiration in potato tubers and bulbs of Conval- 
 laria after the warm-bath, as did Iraklionow (1912 :515) in potato 
 
RESPIRATION AND OXYGEN. 21 
 
 tubers after treatment with warm water. Hoffman and Sokolowski 
 (1910) showed that varieties of potatoes differed among themselves 
 in the intensity of respiration and the evolution of carbon dioxid, 
 and that the effect of water and nitrogen-content was not always 
 the same as in grains, since potatoes with high water and protein 
 content may respire less actively than those with low water and 
 protein. 
 
 Appleman (1914) found that potato tubers could be sprouted 
 at any time during the rest period by removing the skin and supply- 
 ing proper conditions for growth, including the maximum partial 
 oxygen-pressure of the atmosphere. Subdued light stimulates 
 growth in buds on new tubers with slightly suberized skins, probably 
 owing to the greater oxygenation of the tissues by photosynthesis, 
 as this disappears when the skin is removed. The rest-period in 
 new potatoes is shortened by treatment with hydrogen peroxide, 
 which is decomposed by catalase in the tissue, liberating free oxy- 
 gen. It is concluded that the elimination or abbreviation of the 
 period of rest is correlated with increased absorption of oxygen. 
 
 In a study of the relation of catalase and oxidases to respiration 
 in the potato tuber (1916 : 223) Appleman determined the rate of 
 respiration and the activity of catalase and the oxidases under a 
 variety of conditions. He confirmed his earlier result that exposure 
 to ethyl bromide gas greatly increased respiration, and found a simi- 
 lar increase in the catalase activity, but none in that of the oxidases. 
 The effect of cold storage was to nearly treble the respiration, and to 
 increase catalase activity nearly 50 per cent, while slightly reducing 
 oxidase activity. Greening increased respiration and catalase activ- 
 ity a little and reduced slightly the oxidase action. The former were 
 also greater in the seed end than the stem end, while the latter was 
 practically the same. A difference of about 40 per cent in the respi- 
 ration of two varieties was closely reflected in the catalase, but not 
 in the oxidase activity. The author's conclusion is that there is no 
 correlation between oxidase activity and the rate of respiration, but 
 a very striking one between this and the catalase activity of the 
 potato juice. He also finds (1918 : 209) that the catalase activity 
 of the juice of sweet corn is a fair index of the respiration of the 
 tissues, and concludes that catalase activity is invariably correlated 
 with the oxidation processes of respiration. 
 
 Crocker and Harrington (1918 : 171) have shown that there is a 
 similar correlation between catalase activity and respiration in the 
 seeds of Johnson grass, but not in those of Amarantus. As a rule, 
 the catalase activity of seeds seems to parallel physiological behavior 
 much more generally than does oxidase activity. 
 
 Bartholomew (1913, 1915) applied the name blackheart to a dis- 
 ease of the potato tuber in which much of the tissue is dead and black. 
 He showed that it was due to tissue change caused by overheating 
 
22 AERATION AND AIR-CONTENT. 
 
 in an atmosphere with insufficient oxygen to meet the demands of 
 rapid respiration. Experiments proved that the only conditions 
 necessary for the production of blackheart were excessive tempera- 
 tures and insufficient oxygen. 
 
 Stewart and Mix (1917 : 277) found that no sprouts start and any 
 already present blacken and die when jars full of potatoes are sealed 
 at 70° F. Moisture appears on the surface of tubers after 10 days 
 and the condition of blackheart develops in the interior. The 
 amount of air for the proper maintenance of tubers was determined by 
 two series of experiments. When the number of volumes of air 
 ranged from 4.6 to 9.5 per volume of potatoes, sprouts barely 
 started and were dead at the end of 40 days. From 10.5 to 14.7 
 volumes they were very feeble, and were likewise dead at the end. 
 With 15.7 to 18.5 volumes the growth was feeble and the sprouts 
 dead, while at 19.7 to 33.2 volumes the sprouts were normal. In 
 the one series examined for it, blackheart was present from 4.6 to 
 22.2 volumes. 
 
 Coville (1910) has emphasized the importance of good aeration 
 for the swamp blueberry and other heaths. The ideal condition of 
 the peat about the roots is one of constant water-content during the 
 growing-season, but with such drainage that thorough aeration is 
 secured. The high degree of aeration obtained is thought to explain 
 the success attending the use of coarse kalmia peat, since pure peat 
 was not used by the earlier heath-growers. Without understanding 
 the importance of air, they secured aeration by mixing pieces of 
 pots or sandstone with the soil. 
 
 Shull (1911 : 476) decided that the seeds of Xanthium can not 
 grow without comparatively large amounts of oxygen, while the 
 naked embryos of the two seeds require very different amounts of 
 oxygen for germination. The delay in germination is largely due 
 to the nature of the coats which exclude the oxygen. In a later 
 study (1914 : 64) he was able to show that the seed-coats of seeds 
 of Xanthium glabratum used 20 per cent as much oxygen as the seeds 
 themselves. The respiration of the lower seeds was greater than 
 that of the upper in a ratio of 1.35 : 1. The lower seeds with coats 
 off used from 2 to 5 times as much oxygen as those with the coats on. 
 
 Babcock (1912 : 111) found that, while intramolecular respira- 
 tion might occur in seeds containing more than 10 per cent of water, 
 germination never took place except when direct respiration was pos- 
 sible. Free oxygen was essential, though it was not always neces- 
 sarily present in the gaseous state. Some seeds in water utilize 
 dissolved oxygen, but only those of water plants can grow properly 
 under these conditions. A number of seeds of cultivated plants de- 
 composed hydrogen peroxid rapidly, and then germinated as readily 
 as in the air. 
 
RESPIRATION AND OXYGEN. 23 
 
 Chambers (1912 : 203) summarizes his results upon the relation 
 of algae to gases as follows: 
 
 "There is an intimate and mutual relation between the algae and submerged 
 aquatics in a body of water and the gases dissolved in that water. They 
 fluctuate together. Air, or its constituents, oxygen and CO2, are as essential 
 to water-plants as water is to land-plants, and equally difficult to secure. 
 Warm and stagnant water is poorer in these essentials than colder water 
 gentl}^ agitated by wind or currents. Currents are especially beneficial to 
 attached plants by renewing or removing these gases. Some species demand 
 more aeration than others. Some species are more tolerant of stagnant 
 water than others. Filamentous forms with large cells and thin outer walls 
 are best adapted to stagnant waters. Such forms predominate in warm, 
 tropical fresh waters, which are poorly aerated. Stagnant water, on account 
 of the large amount of CO2 and the small amount of oxygen, favors the forma- 
 tion of colonies and filaments rather than of free individual cells. Colonies 
 and filamentous forms may be produced artificially with some plants by in- 
 creasing the amount of CO2 or diminishing the amount of oxygen in the 
 culture solutions. Narrow, much-branched filaments are adapted to and 
 produced by poorly aerated waters. Aeration or abundance of oxygen 
 apparently favors the formation of chlorophyll ; and algae are brighter green 
 when well aerated. The periodicity of spore formation is not readily influ- 
 enced by aeration or gas content of the water. It seems to be more a matter 
 of heredity." 
 
 Hunter (1912 : 183) has grown plants in five different types of 
 soil as follows: (1) soil with small lumps, loose; (2) soil fine, loose; 
 (3) soil fine, firm below with loose surface; (4) soil fine, firm; (5) 
 soil fine, hard. Seeds of Helianthus, Pisum sativum, Triticum, and 
 Lepidium sativum were sown in the pots with a more or less equal 
 water-content. The plants made very different growths in the sev- 
 eral soils. The roots were longest in pot No. 1 and shortest in No. 5. 
 The plants in the latter were small and the roots were unable to 
 penetrate the soil much below the surface. The plants in the loose 
 fine soil had the largest leaves and the best developed root-system of 
 the series. The differences in growth were attributed to variations 
 in the amount and movement of the soil-air. This was supported 
 by determining the resistance offered to the movement of air through 
 the various soils. The loosest soil was taken as unity and the rela- 
 tive restistance of the other four was as 2, 17, 42, and 310. Further 
 proof was obtained by planting badly developed seedlings with weak 
 stems and curled leaves in soil through which 15 liters of air were 
 drawn each day for 3 weeks. In 2 or 3 days the seedlings sub- 
 jected to the air-currents became more robust and the stems 
 stronger, while the leaves grew rapidly and became much larger 
 than those of the control plants. By comparative water-cultures, 
 in which one series was not aerated at all, while another had a con- 
 tinuous current of air bubbling through, Hall, Brenchley, and Under- 
 wood (1914 : 278) have shown that barley and lupine in the aerated 
 
24 
 
 AERATION AND AIR-CONTENT. 
 
 series gained more than 50 per cent in dry weight over the plants in 
 the non-aerated one. 
 
 Harrison and Aiyer (1914 : 93) have determined the beneficial 
 effect of drainage on the growth of rice in pots. The results are 
 shown in table 4. 
 
 Table 4. 
 
 Experiment. 
 
 Weight of grain. 
 
 Weight of straw and chaff. 
 
 I. 
 
 II. 
 
 I. 
 
 II. 
 
 
 grams. 
 11.15 
 13.47 
 14.45 
 11.92 
 12.08 
 
 grams. 
 12.15 
 13.80 
 17.65 
 14.25 
 13.60 
 
 grams. 
 24.70 
 19.75 
 26.40 
 20.57 
 24.37 
 
 grams. 
 21.45 
 21.80 
 27.45 
 26.10 
 26.00 
 
 
 
 Drained on second day 
 
 Drained every day 
 
 
 When the drainage was made complete, so that air could enter the 
 soil, there was Ukewise a definite increase in the production, which 
 reached a maximum between 1 and 2 days' aeration. This was less 
 marked than in simple drainage, however, owing to the greater 
 amount of oxygen in the soil-water as a result of the activity of algae. 
 
 Hole and Singh (1914), in studying the relation of sal (Shorea 
 rohusta) to air-content, found that germination was best in the well- 
 aerated sandy soil and that it was 8 to 32 per cent less in loam and 
 leaf -mold. As to the seedlings, 12 per cent died in watered loam and 
 26 per cent in watered leaf-mold to none in watered sand. In porous 
 pots, germination was best in sand, 8 per cent less in loam, and 28 
 per cent less in leaf-mold. In the case of the last two, germination 
 was 50 per cent less in painted than in porous pots. The plants in 
 the painted sand pots remained healthy throughout the rains, while 
 24 per cent died in the loam and 40 per cent in the leaf-mold. When 
 the pore-space of a soil is reduced by mechanical pressure, 25 per 
 cent more plants may die in it during the rains than in the normal 
 soil, while the survivors have an abnormal root-system confined to 
 the upper, better-aerated layer. When seedlings are grown in 
 glazed pots, some of which are corked, they become unhealthy in 
 the open in about 10 days and eventually die, while in uncorked pots 
 they remain healthy. Similar consequences ensue when the drain- 
 age holes are not corked, but the surface of the soil is covered with a 
 thick layer of sal leaves. 
 
 Cannon (1915 : G4) aerated roots of Opuntia at 32° C. for alternate 
 periods of 2 hours and found that the average rate of growth was 1.59 
 mm. for the aerated period and 1.25 mm. for the unaerated one. 
 The increase due to aeration thus amounted to more than 25 per 
 cent. 
 
RESPIRATION AND OXYGEN. 25 
 
 Free and Livingston (1915 : 60) have shown that cutting off the 
 supply of oxygen from the roots of Coleus stops absorption, and this 
 results in cessation of growth and wilting. When wilting did not go 
 too far, the plants could be revived by renewing the access of oxygen 
 to the soil. Cannon, and Cannon and Free, have carried out ex- 
 tensive investigations of root behavior under anaerobic conditions, 
 and their results are discussed in a later section. 
 
 Butler (1919) has demonstrated that the sprouting of potatoes 
 was retarded by reducing the oxygen-supply or by lowering the tem- 
 perature to 3.7° C, and that the former was more effective in this. 
 Respiration and consequent loss of weight were much influenced by 
 the humidity of the air. They were also rapid in wounded potatoes 
 during the first week of storage at 8° to 10°C. and then decreased. 
 
 Rose (1915 : 435) determined that Martynia and Laduca gave 
 germinations of 90 and 44 per cent respectively when treated with 
 80 per cent oxygen, but none when untreated. A 0.15 per cent solu- 
 tion of hydrogen peroxid increased the germination of Taraxacum 
 from 56 to 72 per cent and that of Datura wrightii from 20 to 100 
 per cent. 
 
 Pember (1917 : 25) stated that aerating daily solutions in which 
 barley plants were growing did not noticeably change the growth 
 of the plants. This is explained, apparently, by the fact that the 
 solutions were changed every 2 weeks and water added at frequent 
 intervals to make up for water-loss. 
 
 Fred and Haas (1919 : 631) have shown that the presence of soil 
 bacteria increases the etching power of the roots of Canada field peas. 
 This is ascribed to the normal CO 2 excretion from the living cells of 
 the root, together with carbonic and other acids evolved from the 
 dead or dying root-cells broken down by bacteria. 
 
 Bergman (1920 : 13) has made a comprehensive study of the be- 
 havior of plants when their roots are submerged. When seedlings 
 of beans, balsam, and geranium in pots were submerged in bog-water 
 and tap-water until the top of the soil was covered, balsam began 
 to wilt in 2 days, was badly wilted in 3 days, and beyond recovery 
 in 4 days. The beans and geraniums began to wilt in 4 or 5 days and 
 at the end of 5 or 6 days the leaves turned yellow and dropped. 
 Under similar conditions Cyperus and Ranunculus grew vigorously. 
 In a later series, air or oxygen was supplied as soon as the plants 
 began to wilt, with the result that they regained their turgor, if 
 not too badly wilted, and grew normally as long as aeration was con- 
 tinued. In 8 to 10 days all the plants developed new roots at or 
 near the surface of the water, after which aeration was no longer 
 needed. Balsam plants aerated by bubbling air continuously 
 through the water in which they were submerged and by placing 
 Philotria and Spirogyra in it, wilted not at all or but slowly in con- 
 
26 AERATION AND AIR-CONTENT. 
 
 trast to plants without aeration, and continued to live throughout 
 the 3 weeks of the experiment, while the unaerated ones died. Root- 
 hairs developed on some roots of the aerated plants. Coleus showed 
 wilting in 1 or 2 days when its roots were submerged, and lost nearly 
 all its leaves, while geranium did not show ill effects for more than a 
 week. In the case of Vicia faba, plants in garden soil, peat, and 
 Sphagnum with submerged roots began to wilt in 4 to 5 days. 
 
 The determination of the root-pressure of plants of Coleus and 
 Fuchsia in soil and submerged pots showed it to be 2 to 3 times 
 greater in the former. If the plants with submerged roots were 
 aerated by means of bubbling air or by placing Philotria or Spiro- 
 gyra in the water, the root-pressure was nearly as great and as well 
 maintained as in the soil. The effect of submergence on the trans- 
 piration of geranium was to greatly increase it at the outset, but it 
 quickly fell off in 2 days to a point below that in moist soil. In 2 
 days more the leaves began to turn yellow, and at the end of 8 days 
 the transpiration had fallen to less than 30 per cent of the normal and 
 in 11 days to less than 15 per cent. Seedlings of Quercus rnacrocarpa 
 in moist and submerged soil showed similar results. Seedlings in 
 moist soil gave a transpiration rate of 7 to 13 gm. from the second to 
 the twenty-fourth day, while those in the submerged soil lost but 2 
 to 4 gm. per day. 
 
 Schley (1920 : 79) has found that the respiration of a root geotrop- 
 ically stimulated is greater than that of one unstimulated. The 
 respiration-rate decreases as the time of stimulation increases. 
 
 Knudson (1920 : 379) concluded from experiments with the roots 
 of Pisum arvense and Zea in solutions containing sucrose that the in- 
 crease in reducing sugars in the latter is due to the excretion of these 
 from the roots and not to the excretion of invertase. 
 
 Bergman (1921 : 50) has recently studied the relation between 
 the oxygen-content and the injury of the cranberry vine due to 
 flooding. Injury is most apt to occur during cloudy weather, when 
 the oxygen is lowest. When submerged cranberry vines are shaded, 
 injury results as a consequence of the reduction in the oxygen-content 
 of the water. No essential difference was seen when the shaded 
 vines were in pond-water or bog-water. The flowers and growing 
 tips were most affected, owing to their higher rate of respiration and 
 consequent greater demand for oxygen. 
 
 Summary. — The essential features of the respiration of plants were 
 established in the short period from Ingenhousz (1779) to Saussure 
 (1804). It was shown not only that all organs of the plant possessed 
 this function in common, but also that roots respired in exactly the 
 same manner as stems and leaves, in spite of the difference in medium. 
 This is supported by practically all the evidence drawn from germi- 
 nation, since the early stages of this have to do with the radicle. In 
 
RESPIRATION AND OXYGEN. 27 
 
 consequence, there is a complete chain of evidence from Mayow 
 (1668) to the present time as to the necessity of oxygen for root 
 activity. The direct proof of this has been obtained by showing the 
 use of oxygen by roots, and this has been confirmed again and again 
 by their behavior in the absence of oxygen. It is a fact significant 
 of the influence of the nature philosophers that the researches of 
 Ingenhousz, Senebier, Rollo, Huber, and Saussure were followed 
 by an almost barren period of 50 years. In spite of the experiments 
 of Grischow, Boussingault, and a few others the labors of the earlier 
 investigators were largely lost sight of, and their results were in a 
 large degree to be gained anew. 
 
 The direct evidence of the necessity of oxygen for the proper 
 functioning of roots has been furnished bj^ Ingenhousz, Senebier, 
 Saussure, Grischow, Garreau, Aeroboe, Vochting, Kossowitch, Snow, 
 Raciborski, Brizi, Day, Hunter, Hall, Brenchley and Underwood, 
 Harrison and Aiyer, Hole and Singh, Howard and Howard, Cannon, 
 Livingston and Free, Bergmann, and Schley. In addition to a vast 
 amount of evidence derived from studies of germination and anaero- 
 bic respiration, the respiratory behavior of tubers, bulbs, and other 
 underground parts affords further confirmation of that of roots. 
 Such studies have been made chiefly upon the potato since the pio- 
 neer work of Nobbe (1865), and the investigators concerned have 
 been Kny, Bohm, Miiller-Thurgau, Mangin, Ziegenbein, Jost, Rich- 
 ards, Strohmer, Vochting, Smirnoff, Griiss, Miiller and Schneider- 
 Orelli, Iraklionow, Appleman, Bartholomew, Hasselbring and Haw- 
 kins, Stewart and Mix, and Butler. 
 
 The excretion of carbon dioxid by roots was first noted by Hales 
 (1727) and was confirmed by Ingenhousz, Saussure, Garreau, and 
 Corenwinder, as well as by many recent observers. Its acid nature 
 was first demonstrated by Wiegmann and Polstorf (1842) and by 
 Oudemans and Rauwenhoff (1858). The significance of the excreted 
 CO2 in the economy of the soil was first suggested by Moldenhawer 
 (1812), but was proved by Sachs's studies of the etching power 
 exerted by roots upon rock (1860-1864), and confirmed by Knop 
 (1861, 1864) and Cauvet (1880). Recently Fred and Haas (1919) 
 have returned to this point to show that the presence of bacteria 
 greatly increases the etching power of roots. Sachs had thought 
 it possible that other organic acids might have a part in corrosion 
 phenomena, but Stoklasa and Ernest (1905, 1908) have shown that 
 carbonic acid alone is concerned in this under conditions that permit 
 normal respiration. 
 
 Molisch (1887) concluded from his experiments that root excretions 
 have both an oxidizing and reducing action, as well as a weak dia- 
 static one. Czapek (1896) was unable to confirm the presence of a 
 ferment, but he found that in addition to carbon dioxid, roots 
 secreted considerable quantities of monopotassium phosphate, which 
 
28 AERATION AND AIR-CONTENT. 
 
 had a small share in the corrosion of rock surfaces. Raciborski 
 (1905) has confirmed the existence of oxidizing power in the roots 
 of a large number of plants, as have other investigators. Knudson 
 (1920) has recently come to the conclusion that roots do not secrete 
 invertase. 
 
 It was early shown by Saussure (1804) that the shoots of the vari- 
 ous species exhibit different respiration intensities, and that this 
 was true of roots as well. He found that fleshy plants required on 
 an average but 1.1 times their volume of O, while aquatic and marsh 
 plants needed but 1.6 times their volume, in contrast to an average 
 of 6 times for deciduous trees and shrubs. Differences in the respira- 
 tory power of roots were also found by Garreau (1851), but the 
 most comprehensive study was that of Freyberg and Meyer (1879), 
 who showed that the roots of swamp plants require less oxygen than 
 those of land plants, a fact already noted for their shoots by Bohm 
 (1875). The respiration maximum for the roots of mature swamp 
 plants was 78 per cent greater and for the roots of seedlings 50 per 
 cent greater than in land plants. Hoffmann and Sokolowski, and 
 Appleman have determined that varieties of potatoes possess dif- 
 ferent rates of respiration and exhibit corresponding differences in 
 their relation to oxygen. The ability of roots to endure the absence 
 of oxygen has been shown to be most variable, as indicated in the 
 section on anaerobic respiration, and similar results have been ob- 
 tained from field studies, which are discussed in the corresponding 
 section. 
 
 AEROTROPISM. 
 
 Mohsch (1884 : 111) made an exhaustive study of the growth 
 responses of corn and pea roots to various gases and reached the 
 following conclusions: If a growing root is exposed to a certain gas 
 upon one side, so that the latter is present in unlike quantities for a 
 considerable time on the two opposed sides of the root, the root 
 departs from its normal direction of growth in a definite manner. 
 This phenomenon is termed aerotropism. Such an effect by gases 
 upon growing roots has been shown for oxygen, CO2, ether, ammonia, 
 etc. The roots are sensitive in different degrees to different gases. 
 The effect of oxygen is weak, of CO 2 stronger, and of chlorine very 
 strong. If a gas is too strong, the root bends towards the source of 
 the gas (positive aerotropism); with a more moderate amount of 
 gas it bends away (negative aerotropism). With reference to oxy- 
 gen, the facts are somewhat more complex. The positive bending 
 arises from the fact that the concave side is injured and its growth 
 in length is less than on the opposite side. Decapitated roots react to 
 CO2, chlorine, and illuminating gas just as uninjured ones, though 
 in a weaker manner. 
 
RESPIRATION AND OXYGEN. 29 
 
 Aerotropism is regarded as a paratonic nutation, in which the 
 external factors influence the growing region directly, and not indi- 
 rectly through the root-tip, as in hydrotropism. If young seedhngs 
 of corn are fastened so that their root-tips touch a water-surface, 
 they show irregular bending in the water, or they turn and grow 
 along the surface. The irregular nutations are due to abnormal 
 influences, among them the lack of oxygen. This is confirmed by 
 the fact that corn roots show exactly the same bendings in air lacking 
 in oxygen or mixed with illuminating gas. The horizontal growth of 
 the roots upon the water is an aerotropic movement determined by 
 the high oxygen-content of the uppermost water-layer. 
 
 Goebel (1886 : 249) pointed out that the two tropical genera, 
 Sonneratia and Avicennia, growing in swamps but not related sys- 
 tematically to each other, exhibit roots which grow upright. These 
 roots were regarded as air-roots, which permitted the roots creeping 
 in oxygen-free mud to come in contact with the air, and therefore 
 as organs of respiration. He endeavored to test this conception 
 experimentally, and found that upright roots developed when Rumex, 
 Nymphaea, etc., were planted too deeply, and that similar results 
 occurred when Saccharum was kept in a very wet pot. These obser- 
 vations confirmed the view that such roots are the result of growth 
 processes operating under a lack of oxygen. 
 
 Jost (1887 : 601) studied the roots of a number of palms and 
 Pandanacece, as well as those of Saccharum, Cyperus, and Luff a, and 
 reached the conclusion that aerotropic roots serving as organs of 
 respiration have a much wider distribution than has been supposed. 
 This is shown, moreover, in greenhouses, where the roots of Cyperus, 
 Papyrus, Richardia, and Musa appear on the upper surface of pots. 
 It was assumed that the lack of oxygen is the stimulus which pro- 
 duces aerotropic roots. This is shown also by cases where roots 
 regularly grow at the top of the pot and where they form a layer 
 on the inner surface of the latter. Fraxinus and Alnus produce a 
 great number of adventitious roots which run horizontally above 
 the oxygen-free swamp soil, and such air-roots may permit trees to 
 grow in the soil of moors. Aerotropism was considered to be widely 
 distributed and probably to possess great biological significance. 
 
 Schenck (1889 : 526) gave the name aerenchym to a tissue devel- 
 oped from the phellogen of the shoots and older roots, chiefly of low 
 shrubs growing in swamps or wet soil. It consists of thin-walled, 
 non-suberized cells, with large communicating air-spaces much larger 
 than the cylindric cells. It occurs in a large number of genera, 
 Jussicea, Epilohium, Ly thrum, Cuphea, Hypericum, Cleome, Sesbania, 
 etc. Most of these are shrubs, but several, Jussicea repens, J. natans, 
 Epilohium hirsutum, Ly thrum salicaria, etc., are herbs. In a num- 
 ber also, Jussicea peruviana, J. pilosa, J. repens, J. natans, are found 
 
30 AERATION AND AIR-CONTENT. 
 
 aerotropic rootlets with the same great air-spaces. Perseke (1877 : 27) 
 has shown that a similar development may occur in terrestrial 
 plants, such as Phaseolus muUiflorus. When grown in water, the 
 latter develops great air-spaces in the cortical parenchyma, and in 
 roots 3 months old appear tears, due to the expansion of the air 
 in this tissue. The aerenchym cells are in but slight contact, as 
 in diaphragms, but the spaces are much larger. The aerenchym 
 breaks the epidermis and cortex and its cells are then in direct 
 contact with the medium, but the air is so firmly embedded in 
 the tissue that water can not enter through the rifts. 
 
 The air of the aerotropic roots of Jussicea grandiflora was found 
 by Martins (1866) to contain 87.5 N and 12.5 oxygen, and that of 
 J. repens to have 86.3 N and 13.7 oxygen. Immendorff determined 
 for Schenck that the aerenchym of Lythrum salicaria contained 30 
 per cent of oxygen, and also a small quantity of CO2. In hydro- 
 phytic shrubs without aerenchym, access of oxygen is taken care of 
 by means of numerous lenticels, which produce in many cases a 
 mass of stopper-cells closely resembling true aerenchym. In Salix 
 dminalis the number of lenticels is much greater on submerged 
 than on aerial shoots. The tissue contains air-spaces, is white in- 
 stead of brown as in the air, and makes a plate often 2 mm. high. 
 Similar lenticel plates are found in Eupatorium cannabinum, Bidens 
 tripartitus, Malachra gaudichaudiana, Scoparia dulcis, Aeschynomene 
 sensitiva and hispida, and Solarium sp. When Artemisia vulgaris 
 grows in w^ater, the few cortical layers of the roots develop into a 
 tissue 2 to 3 mm. thick, and very like aerenchym, differing only in 
 not arising from phellogen. Scott and Wager (1888) concluded that 
 the floating tissue of the roots of Sesbania also facilitates the access 
 of oxygen. 
 
 Wilson (1889) observed that the number and size of the "knees" 
 of Taxodium distichum were determined by the height of the water 
 and the duration of flooding. Young roots often turn directly 
 upward until they reach the surface, when they again bend beneath 
 the water. On old trees the roots often grow together and the knees 
 arise at the point of union. In dry soil cypress trees show no trace 
 of such development and there seems no question that they are to 
 be regarded as aerating organs. Similar structures are found about 
 the base of trunks of Nyssa aquatica in swamps, the roots bending 
 sharply upward to a distance of 6 to 8 inches above the surface of 
 the water and then bending downward into it again. 
 
 Ewart (1894 : 238) found that, when peas were suspended above 
 water free from oxygen with the radicles touching, the radicles that 
 pointed downward soon bent laterally or curved sharply upwards, 
 while those pointed upward grew in that direction for a longer period 
 than usual before responding to geotropism. When they reached 
 the water they bent upward or grew along over the surface. Over 
 
RESPIRATION AND OXYGEN. 31 
 
 well-aerated water, peas grew downward, regardless of their original 
 position. When the aeration was stopped, growth ceased or went 
 on but slowly. If the stagnant oxygenless water was aerated, the 
 radicles grew downward into it, in spite of the carbon dioxid in it, 
 thus indicating that the previous curvature was a response to oxygen. 
 In consequence, the term oxytropic was proposed for this type of 
 irritability. Similar experiments with hemp and wheat gave the 
 same ifesults, though hemp showed less marked response. 
 
 Later studies (1896 : 191) with seedlings of hemp and peas in 
 tubes filled witht boiled sterilized water and closed with a plug of 
 cotton confirmed the existence of oxytropic curvature. The primary 
 roots of sunflower were strongly oxytropic, the secondary ones but 
 slightly so, while the radicles of Cucurbita showed distinct oxytro- 
 pism. Wieler (1898) has studied the pneumatodes of Phoenix and 
 Chamcerops and finds that in some respects they correspond to the 
 aerenchym of Schenck, but he was unable to find any proof that they 
 were definitely related to aeration. 
 
 Pfeffer (1900 : 182) stated that oxytropic reactions are feeble and 
 that it is yet to be shown that they play a prominent part in the 
 orientation of roots in water and soil. He also regarded it as uncer- 
 tain whether the upward growth of roots in mud or water-logged 
 soil is due to oxytropism or to an alteration of geotropic irritability 
 produced by the deficiency of oxygen. The absence of oxygen pro- 
 duces disturbances of growth which often result in irregular curva- 
 tures. When curvature is toward the region with more oxygen, 
 growth is more rapid. It appears, however, as though the avoidance 
 of regions poor in oxygen is in part aided by the suppression or rever- 
 sal of the geotropic irritability, for on repeating Ewart's experiments 
 on a klinostat he was unable to obtain constant and definite curva- 
 tures away from the deoxygenated region. 
 
 Bennett (1904 : 241) repeated Molisch's studies of the behavior 
 of roots to different gases, employing his methods, and came to the 
 conclusion that the curvatures noted were due to hydrotropism. In 
 further experiments, roots of Zea mays, Pisum sativum, Raphanus 
 sativus, Cucurbita pepo, and Lupinus albus were subjected to one- 
 sided access of oxygen, hydrogen, and carbon dioxid to determine 
 the presence of aerotropism. When the roots were grown in water 
 between submerged chambers containing air on one side and CO2 or 
 H on the other, no constant or regular curvatures occurred, as was 
 also true when they were placed in a similar position in a damp 
 chamber. Similar results were obtained when the roots were grown 
 in a thin vertical layer of earth separating air and CO2 or air and 
 hydrogen, or in earth with air on one side and carbon dioxid or hydro- 
 gen on the other, or in gelatin under similar disposition of the air 
 and gases. The conclusion is reached that, in so far as the repre- 
 sentative land plants used were concerned, definite curvatures are 
 
32 AERATION AND AIR-CONTENT. 
 
 not produced in roots by the one-sided access of such gases as oxy- 
 gen, hydrogen, or carbon dioxid, and their roots are therefore not 
 aerotropic. The evidence is regarded as being decidedly against a 
 beUef in the aerotropism of roots, 
 
 Sammet (1905 : 621), at the suggestion of Pfeffer, has investigated 
 the degree to which roots in water, roots in earth, and roots, shoots, 
 and fungal hyphse in saturated air respond to the unequal distribu- 
 tion of various substances. In saturated air the roots responded in 
 different ways. To oxygen they reacted only by positive curvature, 
 while to carbon dioxid, ether, etc., higher concentrations gave posi- 
 tive and lower, negative curvatures. A stream of air produced no 
 reaction in completely saturated air, but in air somewhat less than 
 saturated, hydrotropic curvature occurred. The roots in soil showed 
 combinations of chemotropism and hydrotropism, but when oxygen 
 was applied to the surface of dry soil, the roots curved upward to 
 the surface, showing that aerotropism was stronger than the influence 
 of hydrotropism. Intact and decapitated roots showed the same 
 behavior to chemotropic stimuli, but hydrotropic response was 
 suppressed in the latter. 
 
 Bergmann (1920 : 16) found that, a week or so after the sub- 
 mergence of roots in water, new laterals appeared on the base of the 
 stem at the surface of the water. An examination showed that the 
 submerged roots had died. In the case of plants in soil wet from 
 below, the upper roots made the greatest growth; the lower ones 
 remained alive in Impatiens, but died in Pelargonium. Cannon and 
 Free (1920:62) have found that the roots of sunflower behave simi- 
 larly when placed in an atmosphere of nitrogen. 
 
 Summary. — In spite of the conclusions of Bennett and the con- 
 firmation given by the experiment of Schreiner and Reed (1903), 
 the evidence in support of the aerotropic curvature and growth of 
 roots is practically conclusive. While Bennett has succeeded in 
 throwing doubt upon Molisch's results, her own conclusions are too 
 sweeping, owing to the failure to reckon with the oxygen already in 
 the roots of the media used. Most of the experiments were carried 
 on for too brief a period, 10 to 30 hours as a rule, and no account was 
 taken of the inhibiting effect of the amount of the gases used. Mol- 
 isch, Goebel, Ewart, and Sammet have furnished the experimental 
 evidence of aerotropism or oxytropism, while Goebel, Jost, Schenck, 
 and Wilson have observed aerotropic effects in the plants of swamps 
 especially. 
 
 In connection with the comprehensive study of bog and swamp 
 vegetation, preliminary experiments have been made upon the 
 behavior of roots in saturated soil. In three successive series of 
 pot experiments, seeds of sunflower and bean were planted in various 
 positions in glazed pots filled with sandy loam. In two of these an 
 
RESPIRATION AND OXYGEN. 33 
 
 unglazed area 2 inches square was left on the opposite side; in others 
 the unglazed area was a zone; while in others all water was added 
 from the bottom. After 2 days the pots with unglazed areas for 
 the access of air were sealed by pouring the usual wax over the surface 
 of the soil. After a period varying from 3 to 5 days in the different 
 series, the soil-mass was carefully removed from the pots in such a 
 way as to preserve the relation to the unglazed area. In all cases 
 the bean seeds were found to have decayed without germinating and 
 sunflower alone was used in the remaining series. Even the sun- 
 flower failed to germinate in the soils constantly saturated from 
 below. The two pots with the unglazed square gave positive curva- 
 tures toward this area in all three series, though this was less true 
 of those with the unglazed zone, owing to the more general diffusion 
 of the air. In the former, all the seedlings showed right-angled 
 curvatures toward the direction of air access, without a single nega- 
 tive or opposite response. 
 
 AIR OF SOIL AND PLANTS. 
 AIR-CONTENT OF THE SOIL. 
 
 Earlier researches. — Boussingault and Lewy (1853 : 5) were the first 
 to determine the composition of the air contained in the soil. They 
 analyzed the air-content of various soils, namely, soil recently man- 
 ured, soil of a field of carrots, soil of a vineyard, forest soil, sandy 
 soil, humus soil, bed of asparagus, beet field, field of alfalfa, etc. 
 The highest amount of carbon dioxid, 3.83 per cent, was found in 
 the soil containing abundant humus, and conversely, the smallest 
 amount of oxygen, 16.43 per cent. Soil recently manured yielded 
 2.17 to 2.25 per cent of carbon dioxid, but after frequent rains this 
 had increased to 9.74 per cent and the oxygen had decreased to 
 10.35 per cent. Sand contained the smallest amount of CO 2, 0.11 
 to 0.19 per cent, and the largest amount of oxygen, over 20 per cent. 
 They found the average amount of CO2 in soils not fertilized for a 
 year to be 22 times the amount in normal air, while in manured 
 soils the amount was 245 times greater.. They concluded that the 
 greater part of the carbon dioxid is derived from the oxidation of 
 organic matter in the soil, and that a small but constant part of the 
 oxygen combines with the hydrogen derived from fermentation. 
 
 Petenkoffer (1871 : 395) determined the amount of carbon dioxid 
 in the soil-air at Munich from September 1870 to November 1871. 
 Samples were taken at 0.66, 01.5, 2.5, 3, and 4 meters. He found 
 that the air of the upper soil-layers contained less carbon dioxid 
 from August to June, and more throughout June and July. At 1.5 
 meters the amount rose from 0.26 per cent in January to about 0.8 per 
 cent in July, and reached its maximum at about 1 per cent in August 
 to fall to 0.5 per cent in November. At 4 meters the amount rose 
 
84 
 
 AERATION AND AIR-CONTENT. 
 
 from 0.35 per cent in January to 0.6 per cent in July, reached its 
 maximum at over 1.5 per cent in August and fell to 0.6 per cent in 
 November. Another series of determinations (1873 : 250) of the 
 CO2 content of soil-air at Munich at depths of 1.5 and 4 meters was 
 made from November 2, 1871, to November 29, 1873. It was found 
 as before that the CO2 was greater at the greater depth, and in mid- 
 summer sometimes twice or nearly thrice as much. The mean for 
 July was 0.26 per cent at 4 meters and for January, 0.05 per cent. 
 The author also gave the results of Fleck's observations at 2 meters, 
 4 meters, and 6 meters in Dresden. The soil of the botanic garden 
 in Dresden contained several times as much CO 2 as the Munich 
 soil studied. 
 
 Risler (1872; cf. Mangin, 1896) studied the variation in CO2 in soil 
 at depths of 0.25 meter and 1 meter in relation to both tempera- 
 ture and weather. He found the maximum amount, 2 per cent, pre- 
 sent late in June, as well as a greater amount always at the 
 lower depths. 
 
 Fleck (1872; cf. Pettenkofifer, 1873) found the intensity of the de- 
 composition process in the soil and the amount of CO2 determined 
 by mechanical pressure, size of particles, and the water-content. 
 Garden soil without a plant cover showed more CO2 in the lower 
 layer, while that covered with plants gave the largest amount in the 
 upper layer, as also did sandy soil covered with forest. The total 
 amounts in the latter were usually less than one-tenth those of the 
 garden soil. 
 
 Table 5. 
 
 Soil-air of garden soil. 
 
 Soil-air of a sandhill covered with forest. 
 
 2 meters. 
 
 4 meters. 
 
 6 meters. 
 
 2 meters. 
 
 4 meters. 
 
 6 meters. 
 
 CO2 
 
 
 
 CO2 
 
 
 
 CO2 
 
 
 
 CO2 
 
 CO2 
 
 CO2 
 
 p.ct. 
 1.68 
 2.42 
 2.89 
 4.82 
 
 p.ct. 
 18.9 
 18.1 
 16.3 
 16.2 
 
 p. ct. 
 2.75 
 3.59 
 4.00 
 5.56 
 4.98 
 4.60 
 4.32 
 
 p.ct. 
 17.3 
 17.0 
 15.7 
 16.8 
 16.2 
 15.6 
 16.7 
 
 p.ct. 
 3.38 
 3.63 
 4.52 
 6.33 
 6.36 
 6.11 
 7.96 
 
 p.ct. 
 16.7 
 17.0 
 14.9 
 14.8 
 14.8 
 14.9 
 13.6 
 
 p.ct. 
 0.47 
 0.56 
 0.75 
 0.79 
 0.51 
 0.36 
 0.22 
 
 p.ct. 
 0.42 
 0.51 
 0.64 
 0.66 
 0.52 
 0.35 
 0.28 
 
 p.ct. 
 0.34 
 0.21 
 0.52 
 0.58 
 0.47 
 0.34 
 0.29 
 
 2.91 
 2.21 
 
 18.6 
 19.7 
 
 Schloessing (1873 : 203) studied the intensity of oxidation in a 
 calcareous soil containing different amounts of oxygen, namely, 
 1.5, 6, 11, 16, and 21 per cent. The oxidation was considerably less 
 in the first, identical in the next three and somewhat greater in the 
 last. He later (1889 : 673) determined the composition of the soil- 
 air at 15 to 65 cm. of depth and at various places and times during 
 the growing-season. The amount of CO2 varied widely from 0.5 
 per cent to 11 per cent, and the oxygen from 10 per cent to 21 per 
 
RESPIRATION AND OXYGEN. 
 
 35 
 
 cent.l There was a general tendency for the carbon dioxid to in- 
 crease with the depth, but this was not absolute. Habitats at the 
 base of a slope tended to have more CO2 than those higher up. 
 
 Smolensk! (1877 : 383) concluded that the degree of contamina- 
 tion of the soil is a predominant factor in the amount of CO 2. For 
 example, while he found from 0.1 per cent to 2 per cent in ordinary- 
 soil, the amount rose to 10.2 per cent in contaminated soil. Renk 
 (1878) confirmed the results obtained at Munich by Pettenkoffer. 
 ■i.'Moller (1878 : 121) summarized his results from the study of soil- 
 air as follows : The air in purely mineral soil or in absolutely dry soil 
 contains no more carbon dioxid than the atmosphere. Soils with 
 organic constituents possess a constant source of carbon dioxid. 
 The formation of carbon dioxid shows but slight variations when 
 the external conditions are the same. Soil may become so dry that 
 the production of carbon dioxid ceases. On the other hand, a very 
 small amount of water suffices to bring about the production of the 
 same amount of carbon dioxid as when it is saturated. When an 
 air-dry soil is abundantly watered, there results a considerable but 
 temporary increase in the carbon-dioxid content. In a later paper 
 (1879 : 329), a comparative study was made of the carbon dioxid in 
 fallow and cultivated fields and in several kinds of soil. The culti- 
 vated field regularly showed 5 to 6 times as much carbon dioxid as 
 the fallow field, while calcareous soil contained as a rule considerably 
 more carbon dioxid than clay, and both these soils several times as 
 much as sand. 
 
 Audoynaud and Chauzit (1879) obtained somewhat different re- 
 sults from those of Boussingault and Lewy in regard to the compo- 
 sition of soil-air, as shown in table 6. 
 
 Table 6. 
 
 
 1876 
 
 Time. 
 
 1879 
 
 I 
 
 II 
 
 III 
 
 CO2 
 and 
 
 N 
 
 C02... 
 
 
 
 N.... 
 
 p.ct. 
 4.5 
 8.9 
 
 86.6 
 
 p.ct. 
 
 1.65 
 11.59 
 
 86.79 
 
 V.ct. 
 
 3.4 
 
 10.3 
 
 86.3 
 
 1. August 
 
 4. August 
 
 5. August 
 
 6. August 
 
 7. August 
 
 p.ct. 
 14.5 
 12.9 
 15.5 
 17.1 
 17.7 
 
 p.ct. 
 85.5 
 87.1 
 84.5 
 82.9 
 82.3 
 
 The soil-air was much richer in nitrogen and poorer in oxygen 
 than in the studies of Boussingault and Lewy. The authors assumed 
 that the decrease of the oxygen was due not alone to its use in respi- 
 ration or oxidation in the soil, but also to the different osmotic rela- 
 tion of nitrogen and oxygen in the passage through the soil. 
 
 Fodor (1881) found that the amount of CO 2 in soil-air was very 
 variable, the maximum being 14.3 per cent. He concluded that 
 
36 
 
 AERATION AND AIR-CONTENT. 
 
 there was no relation between the amount of CO2 and of organic 
 matter found in the soil, and that the former depends primarily 
 upon permeability. The soil-air contained more carbon dioxid and 
 less oxygen at 4 meters than at 1 meter. 
 
 
 Table 7. 
 
 
 
 1 meter. 
 
 4 meters. 
 
 3 meters. 
 
 C02 . . . . 
 
 
 
 p.ct. 
 0.89 to 1.03 
 18.79 to 21.33 
 
 p.ct. 
 2.6 to 5.4 
 
 p.ct. 
 
 17.29 to 18.53 
 
 
 Salger (1882) confirmed the previous results as to the increase of 
 CO 2, and found the amount greater at great depths, such as 20 meters. 
 Further, the amount increased with the contamination of the soil, 
 while regular ventilation diminished it rapidly in the surface layers, 
 but more slowly than at greater depths. Contrary to the usual 
 results, Bentzen (1882 : 446) stated that the atmosphere of the 
 upper layers was richer in CO 2 than that of the lower. 
 
 Ebermeyer (1878 : 158) determined the amount of carbon dioxid 
 in soil-air with particular reference to the difference in this respect 
 between forest and field soils. His results for the four summer 
 months. May to August, are given in Table 8. 
 
 Table 8. 
 
 p. ct. 
 Bare, unfertilized, unworked soil at 1 
 
 meter 2.3 
 
 Same soil under an Acacia 1.4 
 
 Free air 2 meters above the soil . 04 
 
 Forest air 2 meters above the soil ... O.OS 
 
 p. ct. 
 
 Forest soil in the humus layer 0. 14 
 
 Forest soil at 0.5 meters 0.45 
 
 Forest soil at 1 meter 0.5 
 
 Soil of cultivated field at 0.5 meter. . 2.6 
 Soil of cultivated field at 1 meter. ... 2.5 
 
 He concluded that forest air contains twice as much carbon dioxid 
 in summer as free air and that forest soil is much poorer in CO2 than 
 unforested soil, while a cultivated field contains 5 to 6 times as much. 
 With an increase of temperature, the C02-content of the cultivated 
 field increases more rapidly than that of the forest soil. The move- 
 ment of CO 2 in the soil seems to be very slow, as shown by the differ- 
 ence in amount in contiguous places in the soil. 
 
 He also (1890 : 15) found that the air in mineral soil is always 
 richer in CO2 and poorer in oyxgen than ordinary air. The soil-air 
 in the upper layers was 4 to 5 times richer, and at 70 cm. deep, 10 
 to 20 times richer in CO2 than the atmosphere. Among the soils, 
 quartz sand was poorest in carbon dioxid, while calcareous sand and 
 clay were at least once again as rich. Dry moor soil was far richer 
 in CO2 than the pure mineral soils. This was true in the uppermost 
 layers, but especially at 70 cm. deep, where it was 22 times greater 
 than in sand and 10 times greater than in hme and clay soils. In 
 
RESPIRATION AND OXYGEN. • 37 
 
 forested soil, the soil-air was much poorer in CO2 than that in neigh- 
 boring manured or humus fields. The C02-content of the soil-air 
 stands in a definite relation to the chemical activity of the soil, and 
 constitutes a definite measure of the latter. Under similar condi- 
 tions the C02-content of the soil-air in deep beech forests during the 
 growing-season is only about half that of a pine forest of equal age. 
 The C02-content of the soil-air is always less in closed forests than 
 in cultivated fields or in open bare soil. Soil-air is, as a rule, poorer 
 in oxygen as it is richer in CO2. Humus and calcareous soils in 
 bare situations are more active, richer in CO2, and poorer in oxygen 
 than all other bare soils. They even exceed dry moor-soil in this 
 respect. A living cover reduces decomposition and the production 
 of carbon dioxid. 
 
 Wollny (1880 : 1373) studied the effect of plant cover and shade 
 upon the amount of carbon dioxid in the soil-air and reached the 
 following conclusions : Soil covered with living plants contained less 
 carbon dioxid than fallow soil or mulched soil during the warmer 
 half of the year, the respective amounts being 0.19, 0.88, and 0.67 
 per cent. During the colder half, the soil beneath the grass cover 
 contained more carbon dioxid in the ratio of about 2 to 1, the amount 
 being only about one-third as much as in the summer. The amount 
 of carbon dioxid increased more rapidly in fallow or mulched soil 
 with rising temperatures than in that with a plant cover. With the 
 increase of rainfall the amount of carbon dioxid increased several 
 times in cultivated soil and fell in bare soil or soil mulched with straw. 
 The soil under a cover of living plants was poorer in carbon dioxid in 
 proportion to the density of the plants, owing to the reduced water- 
 content and the lower temperature resulting from the shade. He 
 also found that the CO2 content under similar conditions rises and 
 falls in general with the amount of organic material in the soil. 
 The atmospheric air is materially concerned in the formation of CO2 
 in the soil. This is not completely prevented by the removal of 
 the air through one of the gases not concerned in the decomposition 
 of organic material. Carbon dioxid is also formed in the soil through 
 the agency of lower organisms. 
 
 Wollny (1886 : 165) investigated the influence of the physical 
 properties of the soil on the amount of CO2 in it. He found the 
 latter to be greatest at a certain slope (20°), while it decreased both 
 with less slope (10°), or a steeper one (30°). As to exposure, on an 
 average the south slopes were the richest and the north the poorest 
 in CO 2, while the latter was intermediate on east and west slopes. 
 With reference to the color of the soil, the dark-colored soils were 
 poorer in CO2 than the lighter ones. This was true, however, only 
 under dry conditions, for, when the water-content of the two soils 
 was the same, the darker contained more CO2. The C02-content 
 of the soil under like amounts of organic material was the greater 
 
38 AERATION AND AIR-CONTENT. 
 
 the finer the particles. Carbon dioxid increased with the depth of 
 the soil layer. A soil shaded by living plants contained during the 
 warm season a smaller amount of CO2 than a fallow soil, and this 
 again less than one covered with a layer of dead plant material. 
 In the last case the amount of CO2 increased with the thickness of 
 the layer. It was also found (1889 : 385) that the soil in bright, 
 warm, and dry weather is the richer and in cloudy, cool, and moist 
 weather the poorer in CO 2 under conditions otherwise similar, the 
 darker the color of the surface. 
 
 Wollny (1890) summarized his results upon the influence of organic 
 matter as follows: The amount of CO 2 in the soil-air is proportional 
 to the amount of organic material present only when this is small. 
 The production of CO 2 with a high content of the soil in organic 
 material increases in smaller degree than the amount of organic 
 material, or remains the same, because at a higher C02-content the 
 activity of the organisms of decomposition is Hmited, and with the 
 increase of the organic material above a certain limit the properties 
 of the soil most important for decomposition are changed in a fashion 
 unfavorable for the intensity of the process. The amount of CO 2 
 present in the soil gives neither a measure of the intensity of or- 
 ganic processes nor of the amount of humus material present. In a 
 later study (1896 : 151) it was shown that soil covered with plants 
 possesses a higher content of CO2 than bare soil under conditions 
 otherwise similar. The reverse is true when the bare land is ma- 
 nured. The soil-air in soil covered with grass or with birches is 
 richer in CO2 than one covered with pine. The soil under the pines 
 without a covering of straw contains larger amounts of CO 2 than one 
 with a straw cover. 
 
 Mangin (1895 : 1065) analyzed the soil-air for the purpose of de- 
 termining the cause of retardation in the leafing of AUanthus and 
 Ulmus at Paris. In the case of AUanthus with open buds, he found 
 the C02-content of the soil to range from 0.35 to 3.43, with the aver- 
 age at about 1.5 per cent, while in AUanthus, with buds still closed, 
 it varied from 3.89 to 24.84, the average being about 10 per cent. 
 The oxygen in the first case ranged from 17.86 to 20.3 per cent, with 
 an average of about 19 per cent, and in the second from 31.6 to 15.92 
 per cent, the average about 10 per cent. In the case of elms with 
 normal leafing, the carbon dioxid varied from 0.67 to 2.12 per cent, 
 the average being about 1 per cent, while those with leafing retarded 
 15 to 20 days showed a range of 1.71 to 5.81 per cent, with the aver- 
 age about 3 per cent. Similarly, the oxygen ranged from 17.86 to 
 20.05 per cent, and from 6.26 to 17.72 per cent, the average being 
 about 14 per cent. 
 
 Later researches. — Letts and Blake (1900 : 213) have made a com- 
 prehensive summary of the results of Pettenkoffer, Fodor, Wollny, 
 and others with respect to soil-air. This results from atmospheric 
 
RESPIRATION AND OXYGEN. 39 
 
 air diffused into the soil and changed by loss of oxygen and consider- 
 able increase of CO2. The sum of these two, however, is not very 
 different from the original proportion of oxygen. More CO 2 is 
 found at lower than at higher soil-levels. The amount increases 
 during spring and summer, diminishes after August, and remains 
 more or less stationary during the winter months, the variations 
 being due to temperature changes. The amount of CO 2 in the soil- 
 air, depends upon its porosity and its organic matter, the relation 
 being inverse in the first and direct in the second. Rainfall has a 
 marked influence on the CO 2 of the soil-air. 
 
 Stoklasa and Ernest (1905 : 723) have shown that the C02-content 
 of the soil-air is derived either from the respiration processes of 
 micro-organisms, especially bacteria, molds, and algae, or the respira- 
 tion of the root-systems of plants. With greatly limited access of 
 air, decomposition begins, as a consequence of which the mineraUza- 
 tion of the organic materials takes place much more slowly. Proof 
 that the source of the CO 2 is not in the chemical processes, but in 
 the activity of the micro-organisms, is afforded by the fact that no 
 development of CO2 is noticed when the soil is sterilized or the 
 organisms destroyed by antiseptics. 
 
 Lau (1906 : 33) reached the following conclusions as a conse- 
 quence of his study of the air in the soil: Soil-air is richest in CO 2 
 in summer, next in autumn, then in spring, and least in winter. 
 The maximum occurs in the months of July and August, the mini- 
 mum in February. The C02-content of soil-air increases with the 
 depth. Soil-air is poorest in CO2 in sand, intermediate in clay, and 
 richest in moor-soil. It is poorer in oxygen as it is richer in CO2. 
 At the same depth the soil-air is richer in CO2 at the root-level in a 
 cultivated field than in a bare one. It is richer in CO 2 in the root- 
 layer than beneath it. The amount of carbon dioxid increases with 
 the greater development of plants and increasing soil-temperature. 
 Soil-air is richer in CO2 in soil containing potatoes or lupines, which 
 respire intensively, than in soil with oats or barley, which respire 
 weakly. Soil-air is poorer in CO2 about 2 a. m. than it is at 2. p. m. 
 It is richer in sandy soils manured with organic material than in 
 those unfertilized. The soil-air in unfertilized lupine beds is but 
 little poorer in CO2 than in fertilized ones. 
 
 Vageler (1907 : 19) has made a comprehensive study of the soil- 
 air of various moor communities. A large number of analyses of 
 the soil-air of moors shows that the greatest amount of CO 2 and the 
 least oxygen occurs in the Molinietum, namely, 2.68 per cent and 16.68 
 per cent, while the Arundinetum, for example, has 0.13 per cent of 
 CO2 and 20.23 per cent oxygen. The carbon dioxid in the moor-soil 
 is an ecological factor in so far as its amount furnishes a measure of 
 the decomposition processes in the soil, modified through the higher 
 or lower air-content of the rhizosphere and the nature of the soil 
 
40 
 
 AERATION AND AIR-CONTENT. 
 
 material. A high content of CO2 in the soil-air is identical with a high 
 degree of activity of the soil, and this, with the ability to bear a 
 flora of high requirement. A poisonous effect from soil-air with a 
 few per cent of CO 2 was nowhere found in the moor. It is regarded 
 as probable that it does not occur until the amount reaches such as 
 those which Mangin found in Paris. 
 
 Jodidi and Wells (1911 : 146) measured the amount of carbon 
 dioxid and oxygen in the soil of 22 field plots during the months of 
 April, May, June, July, and August. The averages for each month 
 are shown in table 9. 
 
 Table 9. 
 
 Time. 
 
 CO2 
 
 
 
 April 
 
 V.ct. 
 0.191 
 0.182 
 0.299 
 0.256 
 0.344 
 
 p.ct. 
 20.93 
 20.60 
 20.37 
 20.39 
 20.30 
 
 May 
 
 June . . . 
 
 July 
 
 August 
 
 The oxygen-content of the various plots varied little more than 
 1 per cent throughout the period, but the carbon dioxid showed a 
 maximum range from 0.04 per cent to 0.82 per cent. This bore 
 little relation to the previous treatment of the plots, as plots A and C, 
 with the same treatment for the preceding 4 years, gave values of 0.10 
 and 0.82 per cent respectively during the month of June. 
 
 Harrison and Aiyer (1913) have determined the composition of 
 the gases from rice fields in India under various treatments and 
 shown that manure greatly increases the production of CO2. 
 
 Table 10. 
 
 Gas derived from — 
 
 Year. 
 
 CH4 
 
 N 
 
 CO2 
 
 
 
 H 
 
 Cropped manured 
 plot 
 
 fl909 
 \ 1910 
 [1911 
 
 1911 
 
 1911 
 
 p.ct. 
 21.0 to 74.0 
 17.0 to 71.0 
 16.6 to 66.1 
 
 26.4 to 57.2 
 
 0.0 to 53.8 
 
 p.ct. 
 11.0 to 73.0 
 11.0 to 78.2 
 
 15.8 to 78.8 
 
 39.9 to 66.4 
 41.9 to 96.8 
 
 p.ct. 
 4.5 to 14.6 
 2.0to21.0 
 1.4 to 6.6 
 
 1.4to 5.1 
 
 1.3 to 5.9 
 
 p.ct. 
 . to 2.8 
 O.Oto 1.9 
 0.0 to 1.9 
 
 . to 5.7 
 
 O.Oto 1.4 
 
 p.ct. 
 
 O.Oto 1.2 
 9.0 to 11.3 
 
 O.Oto 6.3 
 
 O.Oto 3.1 
 
 Uncropped unma- 
 nured plots .... 
 
 Cropped unma- 
 nured plot .... 
 
 Leather (1915 : 108) has made a comprehensive study of the gases 
 in the soils of fields with different treatments. In unmanured fallow 
 land in the spring of 1907, the amount of oxygen varied from 17.2 
 per cent at 3 to 6 inches to 15.5 per cent at 6 feet, the maximum being 
 18.6 per cent at 9 to 12 inches and the minimum 13.4 at 5 feet. The 
 carbon dioxid ranged from a minimum of 2.9 per cent at 3 to 6 inches 
 
RESPIRATION AND OXYGEN. 41 
 
 to 8.2 per cent at 6 feet, the maximum being 8.5 per cent at 1 and 5 
 feet approximately. In manured land in October, the oxygen was 
 
 11.4 and 13.2 per cent, and the carbon dioxid 1.98 and 5 per cent at 
 9 to 12 and 29 to 30 inches respectively. In December, the oxygen 
 was 14.7, 14.7, 13.6, and 17.8, and the carbon dioxid 6.8, 4, 10, and 
 4.3 per cent at 9 to 12, 19 to 20, 29 to 30, and 40 to 43 inches re- 
 spectively. Fallow land after green-manuring gave a range of 10 to 
 
 12.5 per cent of oxygen at various depths, and of 3.5 to 10.1 per cent 
 of carbon dioxid with one crop and a maximum of 18.4 per cent with 
 another. The gas from swamp rice-land yielded the following aver- 
 ages: nitrogen, 85.57 per cent; oxygen, 0.54 per cent; carbon dioxid, 
 4.42 per cent; hydrogen, 5.75 per cent; methane, 2.81 per cent; argon, 
 0.893 per cent. The amount of oxygen in soil at various depths up 
 to 15 inches about the roots of Crotolaria juncea ranged from 2.23 to 
 9 per cent, and the CO2 from 4.84 to 16.99 per cent; about the roots of 
 Indigofera arrecta, the respective ranges were 3.33 to 5.24 per cent 
 and 12.62 to 21.14 per cent, and about those of Zea mays, 6.28 to 
 13.82 per cent and 3.34 to 12.30 per cent. The amount of oxygen 
 in fallow land decreased about 1 per cent after nitrification, and the 
 carbon dioxid increased slightly, while nitrification in vessels usually 
 reduced the oxygen from 20.93 per cent to less than 1 per cent, and 
 resulted in the production of 12 to 24 per cent of carbon dioxid. The 
 outstanding results are the relatively low content of oxygen and high 
 carbon-dioxid content about the roots of plants, and the great 
 amounts of oxygen used and carbon dioxid evolved in nitrification. 
 
 Russell and Appleyard (1915 : 1) have studied the free air of 
 Rothamsted soils and found that to a depth of 6 inches it differs 
 from atmospheric air in containing about 10 times as much carbon 
 dioxid, namely, 0.25, in contrast to 0.03 per cent, and practically the 
 same amount of oxygen, 20.6, as against 20.96 per cent. During 
 rapid nitrification there is a perceptible falling-off of oxygen and a 
 still greater one in water-logged soils. The air dissolved in the water 
 and the colloids of the soils consists chiefly of carbon dioxid and nitro- 
 gen, and contains practically no oxygen. Variations in the compo- 
 sition of the free air arise mainly from fluctuations in the chemical 
 processes of the soil, the curves following those of the amount of 
 nitrate and the bacterial counts. Weather conditions appear to 
 have little effect upon the soil-air, and no evidence was obtained 
 that growing crops greatly increase the carbon dioxid in it. The 
 dissolved oxygen brought in by rainfall is a factor of considerable 
 importance in renewing the soil-air and facilitating chemical change. 
 
 Hole and Singh (1916) find that when rain-water with an initial 
 content of 1 mg. of carbon dioxid and 7 mg. of oxygen was kept in 
 contact with the loam of a sal forest, the oxygen fell to 1 mg. and the 
 CO2 rose to 60 to 70 mg. in 2 days, and then to 230 mg. in 28 days. 
 
42 
 
 AERATION AND AIR-CONTENT. 
 
 As no plants were growing in the soil, this effect was due to the living 
 soil organisms. 
 
 Howard and Howard (1920) give in table 11 the results obtained 
 by Mukherjee in determining the amount of CO 2 in three plots with 
 different treatment. 
 
 Table 11. 
 
 Month. 
 
 1. Grassed down. 
 
 2. Grassed down 
 
 but partially 
 
 aerated by 
 
 trenches. 
 
 3. Surface 
 cultivated. 
 
 January 
 
 V.ct. 
 
 0.444 
 
 0.472 
 
 0.427 
 
 0.454 
 
 0.271 
 
 0.341 
 
 1.540 
 
 1.590 
 
 1.908 
 
 1.297 
 
 0.853 
 
 p.ct. 
 
 0.312 
 
 0.320 
 
 0.223 
 
 0.262 
 
 0.257 
 
 0.274 
 
 1.090 
 
 0.836 
 
 0.931 
 
 0.602 
 
 0.456 
 
 p.ct. 
 
 0.269 
 
 0.253 
 
 0.197 
 
 0.203 
 
 0.133 
 
 0.249 
 
 0.304 
 
 0.401 
 
 0.450 
 
 0.365 
 
 0.261 
 
 February 
 
 March . ... 
 
 April 
 
 May 
 
 
 July 
 
 August 
 
 September 
 
 October 
 
 
 Summary. — The amount of carbon dioxid regularly increases with 
 the depth of the soil, and the amount of oxygen decreases corre- 
 spondingly. Since the carbon dioxid is derived from respiration and 
 decomposition in the soil, it is most abundant in soils and at depths 
 where its escape into the air is most difficult. On the other hand, the 
 supply of soil-oxygen is obtained from the air, and the amount de- 
 creases with the distance from the source and the increasing tribute 
 taken by the roots. The complementary nature of the two gases in 
 the soil is a direct result of the respiration ratio. Pettenkoffer (1871) 
 was the first to determine that the amount of carbon dioxid was 
 greater with the depth, and that in midsummer it was 2 or 3 times 
 as much at 4 meters as at 1.5 meters. Similar results were obtained 
 by Fleck (1872), Risler (1872), Salger (1882), Schloessing (1889), 
 Ebermeyer (1890), Lau (1906), and Leather (1915), the results of 
 the last two being especially detailed and complete. Fleck showed 
 that the reverse relation existed in sand, and this probably furnishes 
 the explanation of Bentzen's view that carbon dioxid was more 
 abundant in the upper layers. 
 
 Seasonal changes, especially temperature and rainfall, have a 
 marked influence upon the composition of the soil-air, though part 
 of this is often to be ascribed to the effect of vegetation. Petten- 
 koffer found that the carbon dioxid was least abundant in January 
 and most abundant in August, the ratio being about 1 : 4. Risler 
 obtained the maximum amount late in June, while in Lau's studies 
 it fell in July and August and the minimum in February. The lat- 
 
RESPIRATION AND OXYGEN. 43 
 
 ter also showed that soil-air is richest in CO 2 in the summer, less so 
 in autumn, still less in spring, and least in winter. For the growing- 
 season, Jodidi and Wells (1911) found the smallest amount of carbon 
 dioxid in May and the largest in August. In India, the minimum 
 occurred in May and the maximum in September, the ratio being 
 1 :7 (Howard and Howard, 1920). 
 
 As would be expected, sand contains the least carbon dioxid and 
 the most oxygen. This was early shown by Boussingault and Lewy 
 (1853), who found 0.11 to 0.19 per cent of CO2 and over 20 per cent 
 of oxygen in sand, while Fleck showed that the soil-air of sand con- 
 tained about one-fourth as much carbon dioxid at 2 meters as gar- 
 den soil, and about one-tenth as much at 6 meters. Moller (1878) 
 obtained considerably more CO2 from calcareous soil than from clay, 
 and several times as much from both as from sand, while Ebermeyer's 
 results showed quartz sand poorest in CO2, calcareous sand and clay 
 twice as rich, and dry moor-soil far richer. Lau likewise determined 
 that sand was poorest, clay intermediate, and moor-soil richest in 
 carbon dioxid. 
 
 The general effect of organic matter and manure is to greatly 
 increase the amount of carbon dioxid and to decrease the oxygen. 
 The most CO 2 and the least oxygen were found by Boussingault and 
 Lewy in soils containing abundant humus, and in those recently 
 manured. The latter yielded 10 times as much CO 2 as soils not 
 fertilized for a year. Smolenski (1877) obtained from 5 to 100 
 times as much carbon dioxid in contaminated as in ordinary soil, 
 and Salger confirmed his general results. Wollny (1880) determined 
 that the amount of carbon dioxid in the soil rose and fell in general 
 with that of organic material, but the amounts were proportional 
 only when the organic matter was not too abundant. Lau found 
 that organic material increased the production of CO2, while Jodidi 
 and Wells found little relation to previous differences of treatment. 
 Harrison and Aiyer (1913) obtained as much as 14 to 21 per cent of 
 carbon dioxid from manured plots, while Leather observed a maxi- 
 mum of 18 per cent in fallow land after green-manuring. 
 
 Cultivation and plant-growth generally augment the carbon dioxid 
 of the soil-air and diminish the oxygen in proportion. Moller 
 showed that the soil of cultivated fields regularly contained 5 to 6 
 times as much carbon dioxid as that of fallow fields, while Fleck 
 observed that in a soil covered with plants, carbon dioxid was most 
 abundant in the upper layers. On the other hand, Ebermeyer 
 concluded that a living cover reduced decomposition and the conse- 
 quent production of CO2. He found forest-soil much poorer in car- 
 bon dioxid than unforested, and 5 to 6 times poorer than cultivated 
 soil. The soil-air of a deep beech forest contained but half as much 
 carbon dioxid as that of a pine forest. Wollny's results as to the 
 effect of a living cover appear somewhat contradictory, but his final 
 
44 AERATION AND AIR-CONTENT. 
 
 conclusion (1896) was that soil covered with plants possesses a higher 
 content of CO2 than bare soil under conditions otherwise similar. 
 The composition of the soil-air varies with the plant cover concerned, 
 as would be expected. Lau stated that the soil-air contains more 
 carbon dioxid at the root-level in a cultivated field than in a bare one, 
 and that there is more in the root-layer than beneath it. The car- 
 bon dioxid increases with the greater development of plants, and 
 also varies with the kind of plant. The effect of plants upon the 
 composition of the soil-air is most graphically shown by the results 
 of Leather, who found minimum amounts of 2 to 6 per cent of oxy- 
 gen and maximum amounts of 12 to 21 per cent of CO 2 about the 
 roots of crop plants in India. The results obtained by Howard and 
 Howard indicate that the amount of carbon dioxid is usually 2 to 
 4 times as great in grass plots as in those with the surface cultivated. 
 
 While investigators have differed much as to the importance of the 
 various factors in modifying the composition of the soil-air, it seems 
 evident that this depends in the first place upon the intensity of 
 respiration and oxidation in the soil and secondly upon the possibility 
 of diffusion through it. The soil having the largest amount of 
 living organisms in it will consume the most oxygen and produce the 
 most carbon dioxid. Since many of the most active organisms are 
 microscopic, cursory observation is insufficient to determine the 
 actual importance of a plant cover or the presence of organic matter. 
 In addition to the flowering plants, ferns, mosses, and larger fungi 
 which may constitute the visible cover are a host of microscopic 
 fungi, molds, bacteria, and algae, together with soil animals from 
 amoebae to rodents. All of these have a larger or smaller part in 
 increasing the CO 2 and decreasing the oxygen of the soil, but the 
 algse may reverse this process under conditions permitting photosyn- 
 thesis. Any factor that promotes respiration or oxidation, such as 
 higher temperature or water-content, will increase the carbon dioxid 
 at the expense of the oxygen, and those that retard respiration will 
 have the opposite effect. The amount of organic material in the 
 form of humus, manure, or green manure is naturally of great im- 
 portance as a source of energy for molds, bacteria, protozoa, etc. 
 The porosity of the soil is directly important in determining the rate 
 at which oxygen can enter and carbon dioxid escape and indirectly 
 in that it affects the water-content, temperature, organic matter, and 
 number of organisms. 
 
 Since water contains but 6 to 7 c.c. of oxygen per hter when 
 saturated during the growing season, the water-content and air- 
 content of the soil stand in inverse relation to each other, in so far as 
 respiration is concerned at least. The greater the water-content, 
 the smaller the air-content and the amount of available oxygen, and 
 the reverse. Dry soils contain relatively large amounts of air, and 
 wet soils httle air, regardless of their fineness. Under the same rain- 
 
RESPIRATION AND OXYGEN. 45 
 
 fall, porous soils have a larger air-content than fine or compact ones, 
 and this is true even when they approach saturation, owing to the 
 readier movement of water in them. Water-logging is practically 
 confined to heavy soils, and swamps are typical of alluvium. Satu- 
 rated soils contain air only in solution, and the amount available is 
 sufficient only for water-plants, which possess some device for aera- 
 tion in the case of helophytes and plotophytes. As many investi- 
 gators have shown, land-plants, with the rarest exceptions, fail to 
 find enough oxygen in saturated or water-logged soils, and die as a 
 consequence. 
 
 As shown in a following section, the amount of carbon dioxid 
 necessary to produce injury ranges from 2 to 20 per cent, depending 
 upon the plant. Such percentages are far from infrequent, especially 
 in manured and water-logged soils. Boussingault and Lewy found 
 9.74 per cent in manured soil wet by frequent rains, while Fleck 
 obtained 7.96 per cent in garden soil, and Smolenski 10.2 per cent 
 in soil that had been contaminated. Mangin determined that 
 amounts of carbon dioxid ranging from 3.89 to 24.84 per cent were 
 sufficient to retard the leafing of Ailanthus, the higher percentages 
 being fatal to the trees. The elm was much more sensitive, retarda- 
 tion occurring from 1.71 to 5.81 per cent. Harrison and Aiyer found 
 maxima ranging from 6.6 to 21 per cent in the gas of cropped and 
 manured rice plots. Leather obtained maxima of 10.1 and 18.4 per 
 cent in fallow fields after green manuring, and of 12.30, 16.99, and 
 21.14 per cent about the roots of plants. These results make it 
 practically certain that injury from CO2 is much more frequent than 
 is commonly supposed, especially in field and garden soils that have 
 been manured, and indicate that it must be taken into account in 
 all cases of toxic action. 
 
 AIR-CONTENT OF WATER. 
 
 Morren and Morren (1841 : 9) determined the composition of the 
 gas of a vivarium 20 feet in each direction, following the changes 
 throughout the growing-season from March to September. They 
 found that the CO2 varied from 1.27 per cent to 23.04 per cent, and 
 often very rapidly, these two extremes occurring on the 9th and the 
 19th of August. This maximum amount of CO2 led to the death of 
 small animals and finally to that of the fishes. The amount of oxy- 
 gen varied from 18.01 per cent to 60.43 per cent, the close relation 
 to the CO2 being shown by the fact that the maximum occurred 
 on August 9 and the minimum on August 19, the dates for the 
 minimum and maximum of CO2. 
 
 Whipple and Parker (1902 : 104) have given a comprehensive 
 series of tables of oxygen and carbon dioxid in dealing with the effect 
 of gases on microscopic organisms. At 20° C, 1 Hter of water can 
 absorb 28 c.c. of oxygen, 14 c.c. of nitrogen, and 901 c.c. of carbon 
 
46 
 
 AERATION AND AIR-CONTENT. 
 
 dioxid. The amount of oxygen in distilled water when saturated 
 varies from 9.7 c.c. per liter at 0° C. to 7.77 c.c. at 10° C, 6.28 c.c. 
 at 20° C, and 5.43 c.c. at 30° C. Hence the amount of oxygen in 
 water varies from about 9 c.c. in midwinter to 6 c.c. in midsummer. 
 Ponds and reservoirs in Massachusetts often showed 100 per cent 
 of saturation at the surface and at 10 and even 20 feet, ranging usually 
 to zero at 40 to 50 feet. River-water in Ohio ranged for the most 
 part from about 80 to 100 per cent, and under conditions of super- 
 saturation up to 169 per cent. Tap- water varied from 54 to 100 
 per cent, averaging about 85 per cent in surface-water suppHes and 
 from 9 to 100 per cent, averaging 61 per cent, in driven-well waters. 
 The amount of free carbon dioxid in rain-water varied from 1.8 to 
 29 parts per million, with an average of about 7 parts. Streams 
 showed 2 to 19 parts, with an average of 7 parts, and shallow ponds 
 in summer, 3 parts. A reservoir gave 2 to 3 parts at 1 foot, 4 to 11 
 at 24 to 26 feet, and 16 at 46 feet, while a lake yielded 2 parts at 1 
 foot, 2.25 at 10 feet, 9 at 25 feet, 1*1 at 40 feet, and 17 at 50 feet. 
 Volk (1906 : 13) found the amounts of oxygen in the Elbe River 
 shown in table 12 during the autumn of 1904 and 1905. 
 
 Table 12. 
 
 Date. 
 
 Upper 
 Elbe. 
 
 Lower Elbe. 
 
 North side. 
 
 Middle. 
 
 South side. 
 
 Average. 
 
 1904 
 Sept. 9 
 
 c.c. 
 7.03 
 7.51 
 7.48 
 7.56 
 8.53 
 8.51 
 
 c.c. 
 5.01 
 5.52 
 6.05 
 6.34 
 
 7^68 
 
 c.c. 
 5.75 
 5.85 
 6.34 
 6.62 
 7.60 
 7.83 
 
 c.c. 
 5.75 
 5.82 
 6.41 
 6.52 
 
 7^76 
 
 c.c. 
 5.50 
 5.73 
 6.26 
 6.49 
 7.60 
 7.76 
 
 Sept. 13 
 
 Sept. 20 
 
 Sept. 27 . . 
 
 Sept. 30 
 
 Oct. 11 
 
 Averages 
 
 1905 
 Sept. 5 
 
 7.77 
 
 
 
 
 6.66 
 
 
 
 
 7.26 
 
 7.68 
 7.55 
 
 7.78 
 8.55 
 8.78 
 
 5.90 
 5.60 
 5.98 
 6.07 
 6.43 
 6.52 
 
 6.20 
 5.94 
 6.16 
 6.65 
 6.62 
 6.84 
 
 6.20 
 5.96 
 5.93 
 6.67 
 6.54 
 6.64 
 
 6.10 
 5.83 
 6.02 
 6.46 
 6.53 
 6.66 
 
 Sept. 12. . . . 
 
 Sept. 19 . 
 
 Sept. 26 
 
 Oct. 3 
 
 Oct. 10 
 
 Averages 
 
 7.93 
 
 
 
 
 6.27 
 
 
 
 
 Hesselmann (1910 : 117) has made an exhaustive study of the 
 oxygen-content of various bog and water habitats in Sweden. The 
 oxygen-content of the water in slowly growing pine forest was 2.49 
 to 6.08 c.c. per hter, and in spruce moors and swamps, 0.25 to 3.73 
 c.c, with higher amounts only exceptional. In streams and lakes 
 the amount varied from 5.17 to 7.61 c.c, and in springs and brooks 
 
RESPIRATION AND OXYGEN. 47 
 
 from 3.36 to 6.84 c.c. In the soil-water of rapidly growing pine 
 forest the range was from 0.88 to 3.30 c.c, the average being approxi- 
 mately 2 c.c. in contrast to 8.2 c.c. at saturation. In pine forest 
 with poor growth, the range was from to 0.88 and the average 0.13 
 c.c, half of the situations showing no oxygen whatever. 
 
 Birge and Juday (1911) have thoroughly investigated the gases 
 in Wisconsin lakes. They have found that the amount of oxygen 
 decreases during the winter in Lake Mendota, the decrease being 
 sHght from 1 to 15 meters, but often falhng to zero in the bot- 
 tom water before the ice breaks up in the spring. The layer 
 beneath the ice may reach 130 per cent of saturation on clear days 
 when the algae receive enough sunhght for photosynthesis. The per 
 cent of saturation due to the evolution of oxygen by algae was great- 
 est at 4 to 5 meters, where a maximum of 364 per cent was reached. 
 The spring overturn occurs about the time of the disappearance of 
 the ice and results in the equal distribution of the gases throughout 
 the depth of the lake, the oxygen being 8 cc per liter. The process 
 of decay reduces the amount of oxygen in the lower water, so that it 
 has entirely disappeared at 18 to 22 meters before the middle of 
 July. During August it may be entirely absent below 10 meters, 
 and is present in small quantities until the fall overturn in October. 
 During the four months, June to September, the amount in the 
 surface 5 feet varied chiefly between 5 and 8 c.c. per Hter, with fre- 
 quent periods of supersaturation for the surface. The amount be- 
 fore the autumnal overturn in October was about 5 c.c. to a depth of 
 15 meters, while after the overturn about the same amount occurred 
 to 22 meters. From this time the oxygen in the upper water gradu- 
 ally decreased, due to the falHng-off of photosynthesis and the rapid- 
 ity of decay. The amount tends to rise again in November and to 
 approach saturation in December. 
 
 The demands of the algae of Lake Mendota for carbon dioxid are 
 greater than the supply of this gas in the free state. The excess 
 demand is met by the half-bound carbon dioxid, and this results in 
 changing the water from acid to alkahne. The free carbon dioxid is 
 uniform from top to bottom as a result of the autumnal circulation. 
 The amount of carbon dioxid, and hence the acidity, increases dur- 
 ing the winter, reaching a maximum of 5 to 9 c.c. near the bottom 
 in March. The spring overturn brings the entire body of water near 
 to the neutral point, after which it becomes increasingly alkaline 
 until the latter part of May, when the water below 18 meters devel- 
 oped free carbon dioxid and became acid. From June to October, 
 the water was alkahne above 8 feet and acid below 15 feet, the 10 
 and 12 foot depths being acid in midsummer. The overturn in 
 October again made the water uniformly alkaline in reaction. 
 
 Chambers (1912) found the oxygen-content of a lagoon to vary 
 from 3.2 c.c. per hter, June 23, to the saturation-point (7.8 cc.) on 
 
48 
 
 AERATION AND AIR-CONTENT. 
 
 July 20, when the water-bloom appeared. He found 4.6 c.c. of oxy- 
 gen and 7.7 c.c. of CO2 in tap-water per liter. The lowest amount 
 of oxygen was 2.32 c.c, when the water was muddy after heavy 
 rains, and the highest amount of CO2 was 10 c.c, when the lagoon was 
 covered with ice. Richards (1917 : 331) concluded that rain-water 
 is very nearly saturated with oxygen when its temperature as col- 
 lected is below 15° C, but is less than saturated to as much as 25 per 
 cent when the temperature is above this. The amount of oxygen in 
 summer rain has been shown by Russell and Richards (1919 : 328) 
 to be 95 per cent of saturation and in winter rain 99 per cent. The 
 number of pounds per acre is 26 for the winter from November to 
 February inclusive, and 20.8 for the summer. May to August. It is 
 also estimated that 66.4 pounds of dissolved oxygen per acre is 
 brought down by rain in a year. 
 
 Bergmann (1920 : 23) has shown that the oxygen-content of lake 
 and bog water decreases with the amount of vegetation, while that 
 of carbon dioxid correspondingly increases, as seen in table 13. 
 
 Table 13. 
 
 Source. 
 
 0, 
 
 i 
 
 02 
 
 Gas- 
 content 
 per liter. 
 
 Source. 
 
 m 
 
 Gas- 
 content 
 per liter. 
 
 CO,. 
 
 Oxy- 
 gen. 
 
 CO2. 
 
 Oxy- 
 gen. 
 
 Hubert Lake (spring water). . 
 Do 
 
 ... 
 
 2 
 3 
 4 
 5 
 6 
 
 c.c. 
 1.0 
 1.4 
 1.2 
 1.2 
 1.2 
 1.0 
 
 c.c. 
 7.6 
 7.2 
 7.4 
 7.6 
 7.6 
 7.8 
 
 Mud Lake: Carex-Calamagros- 
 
 tis; Sphagnum abundant. . . 
 
 Do 
 
 No. 
 
 1 
 2 
 .3 
 1 
 
 1 
 
 1 
 
 2 
 
 c.c. 
 
 5.8 
 0.0 
 6.0 
 
 8.4 
 8.4 
 
 9,8 
 10.2 
 
 c.c. 
 
 3.2 
 3.0 
 3.0 
 2.8 
 4.6 
 
 3.9 
 
 3.8 
 
 Do 
 
 Do 
 
 Do 
 
 Mud Lake, Larix-Picea 
 
 Henderson's Bog: Carex 
 
 Henderson's Bog: Andromeda, 
 below Sphagnum 
 
 Do 
 
 Do 
 
 Do 
 
 
 
 Pearsall (1920 : 171) has determined the changes in the dissolved 
 gases of Enghsh lakes during the growing-season in relation to the 
 plant communities. The avergages were expressed in per cent as in 
 table 14. 
 
 Table 14. 
 
 
 Apr. 
 
 May. 
 
 June. 
 
 July. 
 
 Aug. 
 
 Sept. 
 
 Dec. and Jan. 
 
 
 0.63 
 
 0.62 
 .058 
 
 0.56 
 .049 
 
 0.31 
 .049 
 
 0.41 
 .066 
 
 0.46 
 .079 
 
 0.70 
 .070 
 
 C02 
 
 
 
 Samples taken above different communities showed no important 
 difference in the carbon-dioxid content (table 15). 
 
RESPIRATION AND OXYGEN. 
 
 Table 15. 
 
 49 
 
 Soil type. 
 
 Plants. 
 
 Windermere. 
 
 Coniston. 
 
 Esthwaite. 
 
 June. 
 
 Aug. 
 
 Sept. 
 
 5-15 p. ct. humus. 
 15-30 p. ct. humus. 
 
 Potamogeton spp . . 
 IsoetesorNitella. . . 
 
 0.050 
 .048 
 
 0.066 
 .067 
 
 0.076 
 .072 
 
 . 055 
 .0.54 
 
 When the CO2 ranged from 0.055 to 0.067 per cent in the open 
 lake, it varied from 0.066 to 0.084 per cent in a closed bay contain- 
 ing Caslalia, Potamogeton, and Carex. 
 
 Bergmann (1921 : 50) has estabhshed a relation between the vari- 
 ation in carbon dioxid and oxygen-content of water and the condi- 
 tion of the sky. In one pond with clear water, Uttle vegetation, 
 and a white sandy bottom, the oxygen averaged 6.2 c.c. and the 
 carbon dioxid 0.46 c.c. on clear days, and 5.5 c.c. and 0.17 c.c. on 
 cloudy days. In one filled with vegetation and with a muck bot- 
 tom, the oxygen averaged 4.7 c.c. and the carbon dioxid, 0.7 c.c. on 
 clear days. In bog ditches the general average was about 4 c.c. of 
 oxygen on clear days and 5.1 of carbon dioxid on clear days, and 2.7 
 c.c. and 4.5 c.c. respectively on cloudy ones. Where algse and other 
 vegetation were present, as in the last two cases, there was a much 
 wider variation in the gas-content of the water between day and 
 night, or between clear and cloudy days. 
 
 Summary. — Since the amount of oxygen in water is usually deter- 
 mined in cubic centimeters per liter or in percentages of saturation, 
 and the carbon dioxid often in parts per milhon, it is difficult to 
 estabhsh comparisons with soil-air. This is directly possible only 
 from the researches of Morren and Morren, who found the carbon 
 dioxid in the gas of a vivarium to vary from 1.27 to 24 per cent and 
 the oxygen from 18 to 60 per cent. The maximum amount of CO2 
 caused the death of animals and must have had a similar effect upon 
 the plants. Whipple and Parker stated that the amount of oxygen 
 varied from about 9 c.c. in midwinter to about 6 c.c. in midsummer, 
 which approximates saturation at both times. The water of ponds, 
 reservoirs and rivers is usually somewhere near saturation, and often 
 exceeds it. Tap-water is more rarely saturated, the averages being 
 61 and 85 per cent. Volk found the water of the Elbe to be generally 
 above saturation in September, the amount being greater in the 
 Upper than in the Lower Elbe. Birge and Juday noted 8 c.c. of 
 oxygen in the water of Lake Mendota at the spring overturn, and 
 from 5 to 8 c.c. in the surface 5 feet during June to September, the 
 surface itself often showing supersaturation. Chambers found 3.2 
 c.c. of oxygen in the water of a lagoon in June, and 7.8 c.c. in July, 
 while Bergman observed an average of 3.4 c.c. in bog water and of 
 7.4 c.c. in a lake fed by springs. Whipple and Parker obtained an 
 
50 AERATION AND AIR-CONTENT. 
 
 average of 7 parts of COo per million in rain-water and streams, and 
 of 3 parts in shallow ponds, while a lake ranged from 2 parts at 1 foot 
 to 17 at 50 feet. Birge and Juday found 5 to 9 c.c. per Hter near the 
 bottom in the winter, the water above 10 feet being practically 
 without acid during the summer. Bergmann showed that bog- 
 water contained several times as much carbon dioxid as lake-water, 
 the respective averages being 7.8 c.c. and 1.15 c.c. 
 
 INFLUENCE OF ALG^ AND WATER PLANTS ON OXYGEN-CONTENT. 
 
 Brizi (1906 : 89) was the first to point out the significance of 
 algae in the aeration of higher plants. He demonstrated that the 
 alg2e from the rice-fields, when placed in cultures of rice containing 
 CO2 but no oxygen, produced sufficient oxygen to aerate the roots 
 and insure healthy growth. He concluded from this that the algse 
 of the rice-fields greatly increased the amount of oxygen in the water 
 and were the principal factor in the aeration of the roots. They 
 were regarded as of further advantage in consuming the carbon 
 dioxid that might otherwise accumulate in injurious quantities. 
 Chambers (1912 : 203) found that the photosynthesis of rapidly 
 growing algae and aquatic plants in a body of water may diminish 
 or deplete the supply of carbon dioxid and increase the oxygen-content 
 beyond saturation. In the absence of free CO2, the plants may use 
 the half-bound carbon dioxid of the dissolved carbonates, chiefly 
 those of calcium and magnesium. Photosynthesis may be so active 
 as to use up the half-bound carbon dioxid and make the water alka- 
 line, but more carbonates may then be formed as a result of respira- 
 tion and absorption from the air. Waters rich in calcium carbon- 
 ates are also rich in vegetation, but bog-waters containing humic 
 acids, and hence poor in carbonates of lime, are poor in vegetation. 
 
 Harrison and Aiyer (1913 : 94) were led to the conclusion that the 
 surface film of algae on rice soils is the chief agency in the aeration 
 of the roots, as indicated by the evolution of oxygen from such 
 soils. In a special study of this film (1914), they determined that 
 green algae were generally, and diatoms invariably, present, and 
 showed that it decomposed CO2 in the sunlight, with evolution of 
 oxygen. The film probably also contains bacteria capable of oxi- 
 dizing methane and hydrogen to carbon dioxid and water, as it 
 carries on these processes actively. The organic film on the surface 
 of the swampy soil of rice-fields utilizes the gases in such a way as to 
 increase the evolution of oxygen and the consequent aeration. The 
 bacterial activity results in the production of more carbon dioxid, 
 and this is used by the algae with increased production of oxygen. 
 The film permits the maximum oxygen concentration of the water 
 entering the soil. Green manuring increases the soil-gases, supplies 
 more material to the film, and thus augments the evolution of oxygen 
 and consequent aeration. 
 
RESPIRATION AND OXYGEN. 
 
 51 
 
 Bergmann (1920 : 22) has found that Pkilotria decreasas the car- 
 bon dioxid and increases the oxygen during the day, as shown by 
 tables 16 and 17. 
 
 Table 16. — Gas-content per liter of water with and without Philotria. 
 
 No. 
 
 Kind of water. 
 
 COj. Oxygen. 
 
 Standing tap-water 
 
 Do 
 
 Tap-water, with Philotria, forenoon. 
 
 Do 
 
 Tap-water, with Philotria, afternoon 
 
 Do 
 
 c.c. 
 2.8 
 2.6 
 2.8 
 2.6 
 0.6 
 0.8 
 
 c.c. 
 5.4 
 5.6 
 5.4 
 5.6 
 8.2 
 7.8 
 
 Table 17. — Gas-content per liter of water with Philotria. 
 
 No. 
 
 Time. 
 
 C0». 
 
 Oxygen. 
 
 1 
 2 
 3 
 
 10 a m 
 
 c.c. 
 1.1 
 0.8 
 0.5 
 
 c.c. 
 6.2 
 7.0 
 
 7.4 
 
 10 h 30 naam . 
 
 11am 
 
 
 Esmarch (1910, 1914) first called attention to the algal flora of 
 soils, and pointed out that algae occur in the lower layers as well as 
 in the surface-soil. He concluded that cultivated soils are richer 
 in blue-green algae than uncultivated ones. A later study dealt with 
 the depth and distribution of blue-green algae in the soils of Schleswig- 
 Holstein. He found that blue-green' algae in cultivated soils are not 
 confined to the surface layers, but that many occur to depths of 10 
 to 25 cm. and some as deep as 40 to 50 cm., ranging throughout the 
 entire depth. This is due to cultivation, percolation of water, and 
 the burrowing of worms and other small animals. It was also ob- 
 served that blue-green algae inclosed in the soil keep green for a time, 
 but gradually become yellowish, and after a further interval the 
 filaments become distorted and disintegrate. This period is 3 to 6 
 weeks long, or, in one case, as much as 10 weeks long. The author 
 thought that in certain soils the presence of blue-green algae is an 
 important factor in the fixation of nitrogen by bacteria. 
 
 Robbins (1912) found 21 species of algae in Colorado irrigated soils, 
 all belonging to the blue-green slimes, except Pleurococcus viridis and 
 Navicula sp. The algae occurred in all kinds of soil, from sandy 
 loam to heavy adobe, and were probably derived for the most part 
 from the irrigation-water. The most important forms were Phor- 
 midium tenue, Nostoc sp., Anabcena sp., Nodularia harveyana, and 
 Stigonema sp. The organic matter furnished by the algae was 
 
52 AERATION AND AIR-CONTENT. 
 
 regarded as an important source of energy for the nitrogen-fixing 
 organisms, especially Azotobacter, and this serves to explain the 
 accumulation of unusually large amounts of nitrates in certain Colo- 
 rado soils. This has been confirmed by Hutchinson (1918), who has 
 shown that the activity of Azotobacter is appreciably increased by 
 the addition of plant residues to the soil. 
 
 Moore and Karrer (1919 : 281) have studied the subterranean 
 algal flora of several soils from Missouri, Massachusetts, and Cali- 
 fornia, and have concluded that this flora is independent of the 
 nature of the soil and locality. The variety of algae was not large, 
 though comparable with that on the surface. Protoderma viride was 
 the most common species by far, while the blue-green algse supplied 
 the majority of the other forms. The former occurred to the great- 
 est depth, 1 meter, and in every soil, indicating its special ability to 
 live underground. It is concluded that the algal flora rises from 
 the surface forms, but that its persistence at considerable depths 
 indicates that the algse actually grow in the soil, since it is assumed 
 that surface waters are unable to carry them so deeply in such 
 compact soils as clay. 
 
 Bristol (1920 : 35) has found 64 species of algse in soils from 
 widely separated locahties. Of these, 20 are diatoms, 24 blue-green 
 algse, and 20 yellow-green algse. The most common species are 
 Hantzchia mnphioxys, Trochiscia aspera, Chlorococcum humicola, 
 Bumillera exilis, and Ulothrix subtilis variabilis, with much moss 
 protonema. These were found to be able to withstand complete 
 desiccation for 4 to 26 weeks. The author suggests that the presence 
 of algae in the soil must affect the soil gases. 
 
 Summary. — It has been shown by a number of workers that algae 
 may bring about the supersaturation of lakes and streams as a con- 
 sequence of photosynthesis. This frequently amounts to 200 to 300 
 per cent of saturation and has been found to reach 550 per cent. At 
 the same time, algae and other water-plants prevent the accumula- 
 tion of large amounts of CO2 as a result of the same process. Brizi, 
 and Harrison and Aiyer, have shown the importance of the algal film 
 of rice-fields for the aeration of the roots, and Bergmann has demon- 
 strated the value of Philotria and Spirogyra in furnishing a supply 
 of oxygen for submerged roots. Esmarch, Robbins, Moore and 
 Karrer, and Bristol have studied the algal flora of soils, and the 
 first two have concluded that the algae are an important source of 
 organic material for nitrogen-fixing bacteria. Bristol suggests that 
 the presence of algae may affect the soil-gases. It seems certain 
 that this must be the case in soils sufficiently moist to permit their 
 growth, and that they then increase the aeration and prevent the 
 harmful action of carbon dioxid in the same manner as in rice- fields. 
 
RESPIRATION AND OXYGEN. 
 
 53 
 
 AIR-CONTENT OF PLANTS. 
 
 Aim^ (1841 : 537) analyzed the bubbles on the surface of marine 
 algse, such as Ulva, as well as those in the interior, with the results 
 shown in table 18. 
 
 Table 18. 
 
 Bubbles. 
 
 Before sunrise. 
 
 Before sunset. 
 
 Internal: 
 
 
 
 p.ct. 
 17 
 83 
 
 21 
 
 79 
 
 p.ct. 
 36 
 64 
 
 55 
 45 
 
 N 
 
 External: 
 
 
 
 N 
 
 
 Unger (1854) determined that the floating leaves of Pistia texensis 
 contained the maximum amount of air, 71.3 per cent by volume, while 
 the fleshy leaves of Begonia hydrocoiylifolia showed the least, 3.5 
 I per cent. 
 
 Faivre and Dupr^ (1866) investigated the composition of the in- 
 ternal air in the organs of the mulberry under various conditions, 
 with the results shown in table 19. They found that the carbon 
 dioxid diminished and the oxygen increased as activity was reduced, 
 and that the amount of oxygen was less and of CO2 greater in roots 
 than in branches during the growing-season. 
 
 Table 19. 
 
 Date. 
 
 Organ. 
 
 0. 
 
 CO2. 
 
 1865 
 Mar. 
 May 15 
 June 15 
 July 2 
 July 7 
 Aug. 17 
 Oct. 15 
 Oct. 16 
 Nov. 17 
 Nov. 24 
 
 1866 
 Jan. 31 
 
 Leafless branches 
 
 p.ct. 
 21.0 
 13.33 
 
 2.5 
 10.21 
 
 1.9 
 10.7 
 13.96 
 
 7.5 
 13.1 
 16.6 
 
 20.9 
 
 p.ct. 
 
 3.33 
 15.7 
 6.3 
 14.6 
 9.0 
 3.19 
 3.76 
 3.8 
 1.6 
 
 0.01 
 
 Leafy branches 
 
 Do 
 
 
 
 
 Branches, activity reduced . 
 
 
 Roots, after leaf-fall 
 
 
 Heintz (1873 : 358) analyzed the composition of air inside the 
 sugar-beet with the results given in table 20. 
 
 Table 20. 
 
 
 1 
 
 2 
 
 3 
 
 4 
 
 5 
 
 CO, 
 
 
 
 N 
 
 30.52 
 
 0.14 
 
 69.34 
 
 35.10 
 
 0.56 
 
 64.34 
 
 11.49 
 1.53 
 
 86.98 
 
 41.02 
 
 2.10 
 
 56.88 
 
 78.90 
 
 0.06 
 
 21.04 
 
54 
 
 AERATION AND AIR-CONTENT. 
 
 Barthelemy (1874 : 175) gave a summary of the analyses of in- 
 ternal air in connection with his own determinations (table 21). 
 
 Table 21. 
 
 COs. 
 
 Aim6 (marine algce) 
 
 Saussure (branch of apple) 
 
 Boussingault (oleander) 
 
 Martins and Moitessier: 
 
 Jussisea 
 
 Aldrovandia 
 
 Pontederia crassipes 
 
 Dutrochet (Nuphar luteum): 
 
 Rootstock 
 
 Root 
 
 Leaves 
 
 Lechartier (Nuphar luteum, petiole) 
 Barthelemy (Nelumbo) 
 
 Do 
 
 Do 
 
 Do 
 
 p. ct. 
 17 
 9 
 6.64 
 
 15.8 
 15.5 
 14.1 
 
 p. ct. 
 
 5 
 5.34 
 
 p. ct. 
 83 
 86 
 
 84.92 
 
 84.5 
 
 85.9 
 
 84 
 82 
 82 
 88 
 87 
 76 
 83 
 78 
 
 Wille (1889) found the air of Fucus bladders in water to contain 
 35 to 37 per cent of oxygen, 20.7 to 20.8 per cent in those dried in 
 air 10 hours, and but 2.7 per cent in those lying in the dark for 12 
 hours. Carbon dioxid was completely absent in all cases. 
 
 Devaux (1889 : 115), studying the gas-exchange of aquatic plants, 
 made the analyses of the gas in and about the plant and of the air 
 shown in table 22. 
 
 Table 22. 
 
 CO. 
 
 Air from interior of stem of Elodea 
 Air at surface of leaves and stems . 
 
 Air collected by wax funnel 
 
 Air contained in the water 
 
 Composition of surrounding air. . . . 
 
 p. ct. 
 2.14 
 0.69 
 0.30 
 21.10 
 0.04 
 
 p. ci. 
 18.86 
 23.08 
 23.59 
 31.04 
 18.66 
 
 p. ct. 
 79.40 
 76.23 
 76.11 
 66.87 
 81.30 
 
 The author concluded that the air dissolved naturally in water 
 possesses essentially the same pressure as in the atmosphere. As 
 to the internal air of the plants, if the water is normally aerated, the 
 air of the air-passages or spaces is nearly pure. The air arrives at 
 each cell with nearly the same pressure as that which it possesses 
 in the surrounding water and in the air-passages. There is some air 
 simply dissolved in the substance of the cell itself, and this possesses 
 the same pressure as at the exterior. 
 
 In measuring the oxygen-content of a tubercle (1890 : 257), 
 Devaux concluded that the oxygen never wholly disappears in the 
 
RESPIRATION AND OXYGEN. 
 
 55 
 
 midst of massive tissue. In conditions especially unfavorable to 
 gas exchange, the amount of oxygen falls very low, to about 0.25 
 per cent. In all the cases observed, however, its total disappearance 
 never occurred. The diffusion of free air is vastly more rapid than 
 that of dissolved air, a fact which permits very perfect aeration of 
 porous tissues, but very imperfect in culture fluids. The central 
 cavity of a fruit of Cucurhita maxima contained air with a compo- 
 sition of 2.52 per cent CO2, 18.29 per cent oxygen, and 79.19 per cent 
 N. When air was forced into the fruit under water, numerous larger 
 and smaller bubbles appeared over the surface, coming from what are 
 essentially lenticels. In the fruits of cucurbits that lack lenticels 
 the air enters through the stomata. 
 
 Aubert (1892 : 203) found the composition given in table 23 for 
 the internal air of several species of succulent plants. 
 
 Table 23. 
 
 
 CO2. 
 
 0. 
 
 N. 
 
 Crassula arborescens (sunny) 
 
 p.ci. 
 1.50 
 0.22 
 0.85 
 0.41 
 0.67 
 0.47 
 
 V.ct. 
 26.45 
 22.78 
 18.35 
 25.33 
 24.02 
 25.63 
 
 p.ct. 
 72.05 
 77.00 
 80.80 
 74.26 
 75.31 
 73.90 
 
 Crassula arborescens (cloudy) 
 
 Sedum dendroideum (sunny) 
 
 Phyllocactus grandiflorus (sunny) 
 
 Opuntia tomentosa (sunny) 
 
 Opuntia dejecta (sunny) 
 
 
 Magness (1920 : 308) has analyzed the gas in the intercellular 
 spaces of apples, potatoes, and carrots at different storage temper- 
 atures, with the results included in table 24. 
 
 Table 24. 
 
 Products stored. 
 
 Tempera- 
 ture 
 of storage. 
 
 No. of 
 determi- 
 nations. 
 
 CO2. 
 
 0. 
 
 Apples 
 
 2 
 6 
 11 
 20 
 30 
 11 
 22 
 11 
 22 
 
 5 
 30 
 27 
 31 
 29 
 8 
 8 
 
 2 
 
 V.ct. 
 6.7 
 8.4 
 12.2 
 17.2 
 21.4 
 19.6 
 34.4 
 12.2 
 28.6 
 
 V.ct. 
 14.2 
 12.9 
 10.7 
 
 5.5 
 
 3.2 
 10.9 
 
 5.7 
 13.1 
 
 5.2 
 
 Do 
 
 Do. 
 
 Do. 
 
 Do. 
 
 Potatoes 
 
 Do 
 
 
 Do 
 
 
 The removal of the peel from the ends of apples resulted in a 
 marked reduction in the amount of CO2 and a similar increase of 
 oxygen, due to the greater ease of escape and entrance. The range 
 of variation in the amounts of these two gases at 20 C. was 12.5 
 
56 AERATION AND AIR-CONTENT. 
 
 to 25.7 per cent for carbon dioxid and 1 to 9.5 per cent for oxy- 
 gen. The chief factors determining the amounts are the rate of 
 respiration, the permeabiUty of the peel or skin, and the difference 
 in the pressure of the two gases within and without the tissue. 
 
 Summary. — The composition of the internal air of plants depends 
 primarily upon the presence or absence of photosynthesis. Stems, 
 leaves, and other chlorophyllous parts contain air with much oxygen 
 and little carbon dioxid, while roots, tubers, ripe fruits, etc., show 
 much carbon dioxid and relatively small amounts of oxygen. Aim^ 
 found 17 per cent of oxygen in the internal gas-bubbles of Ulva and 
 21 per cent in the external before sunrise, and these increased during 
 the day to 36 and 55 per cent respectively. No carbon dioxid was 
 present. According to Wille, the bladders of Fucus contained 35 
 to 37 per cent of oxygen in the water, 20 per cent when exposed to 
 the air 10 hours, and but 2.7 per cent after 12 hours in darkness. 
 Saussure obtained 9 per cent of oxygen and 5 per cent of CO2 from 
 an apple twig, Boussingault, 6.64 per cent of oxygen and 5.34 per 
 cent of carbon dioxid from oleander, and Barthelemy, 10 to 22 per 
 cent of oxygen and to 3 per cent of CO 2 from Nelumbo. Dutrochet 
 found 18 per cent of oxygen in the leaves of Nuphar, Lechartier, 12 
 per cent in the petiole, and Martins and Moitessier, 14 to 15.8 per 
 cent in other water-plants. The air of the interior of the stem of 
 Elodea was shown by Devaux to contain 8.86 per cent of oxygen and 
 2.14 per cent of CO2; the air at the surface of leaves and stems, 23 
 and 0.69 per cent respectively; that of the water, 31 and 21 per cent; 
 and the surrounding air, 18.66 and 0.04 per cent. The air in the cen- 
 ter of a squash was composed of 18.29 per cent oxygen, 2.52 per cent 
 carbon dioxid, and 79.19 per cent nitrogen (of. Pf offer, 1900, 1 : 205). 
 
 According to Aubert, the air of succulent plants yielded 18.35 to 
 26.45 per cent of oxygen and 0.47 to 1.50 per cent of carbon dioxid 
 in the sun. Crassula arborescens contained 7 times as much CO2 in 
 the sun as under a cloudy sky. Dutrochet found but 8 per cent of 
 oxygen in the root of Nuphar, while Magness has recently shown 
 that apples, potatoes, and carrots stored at 11° C. and above, regu- 
 larly contain much more CO 2 than oxygen. At 20° to 22° apples 
 averaged 17.2 per cent of carbon dioxid and 5.5 per cent of oxygen; 
 potatoes, 34.4 and 5.7 per cent; and carrots, 28.6 and 5.2 per cent. 
 
 ANAEROBIC RESPIRATION. 
 
 The significance of reduced air-content and of the respiratory prod- 
 ucts arising from it in saturated soils is so great as to warrant a 
 comprehensive account of anaerobic respiration. This lacks com- 
 pleteness chiefly with reference to the theories concerning the 
 nature of intramolecular respiration and fermentation, and to the 
 nature and role of the enzymes concerned. In organizing the large 
 
RESPIRATION AND OXYGEN. 57 
 
 mass of results, the primary divisions are based upon the functions 
 and organs studied. These are respiration itself, photosynthesis, 
 transpiration, germination, growth, protoplasmic movement, and 
 irritability. The fungi are discussed separately, in consequence of 
 their more or less peculiar behavior. The effects of the various 
 agents employed in producing anaerobic conditions are summarized 
 in the discussion at the end of each section and the significance of 
 anaerobic respiration in explaining the problems discussed is dealt 
 with in the succeeding chapters. 
 
 RESPIRATION. 
 Early researches. — As already indicated, Huygens and Papin (1674) 
 were the first to show that plants could not grow in vacua, while 
 shortly afterward Ray (1686) and Homberg (1699) found that under 
 similar conditions seeds failed to germinate or did so only with diffi- 
 culty. Ingenhousz (1779) observed that plants died in air unfavor- 
 able to animal life as well as in pure carbon dioxid. Humboldt 
 (1794) and Rollo (1798) thought that seeds germinated more readily 
 in oxygen than in air, and Lefebure (1801) observed that germination 
 was greatly retarded by low oxygen-content. The classic experi- 
 ments were those of Huber and Senebier (1801) and Saussure (1804) 
 upon germination and growth, which are considered in detail under 
 these sections. Rollo (1798 : 37) was the first to observe the phe- 
 nomenon of anaerobic respiration in noting that barley grains gave 
 off a considerable quantity of carbon dioxid for several days when 
 oxygen was absent. Dumont (1819) and Dobereiner (1822) found 
 that fruits, especially apples and pears, developed a demonstrable 
 amount of alcohol after they had been subjected to an atmosphere of 
 CO2 for a certain time. Berard (1821) demonstrated that fruit kept 
 in an atmosphere of carbon dioxid exhibited the phenomena of fer- 
 mentation. When oxygen was withheld, green fruits were unable to 
 ripen, but the ripening process was resumed when they were again 
 placed in air, providing the anaerobic conditions were not too pro- 
 longed. Cahours (1864 : 635) Hkewise found that oranges respired 
 in the absence of air, evolving carbon dioxid in an atmosphere of 
 nitrogen. 
 
 Later researches. — The proper understanding of the nature of anae- 
 robic respiration was first made possible by Pasteur's researches in 
 fermentation (1861, 1872, 1876). He stated that the yeast of beer 
 behaved altogether like an ordinary plant and that it was probable 
 that certain lower plants could live without air in the presence of 
 sugar, producing under these conditions a fermentation similar to 
 that of yeast. He later experimented with wine grapes kept in an 
 atmosphere of CO2 and came finally to the view that the respiration 
 was analogous to that of yeast. He concluded that the formation 
 of alcohol was due to the fact that the cells of the fruit continue to 
 
58 AERATION AND AIR-CONTENT. 
 
 live under the new conditions in a manner similar to that of ferments. 
 He found that when fruit is placed in an atmosphere of CO 2, respira- 
 tion proceeds in consequence of the decomposition of sugar. The 
 cells are in the condition of ferments which live without free oxygen, 
 as in the case of cells of Mycoderma vini when submerged. In fact, 
 as soon as the fruit is placed in CO2, both carbon dioxid and alcohol 
 are produced in small amounts, while the fruits remain firm and 
 entirely sound in appearance. 
 
 Lechartier and Bellamy (1869; 1872; 1874) determined that C02and 
 alcohol were formed when fruits were kept in a closed receptacle 
 without oxygen, though it was impossible to find an alcohoHc fer- 
 ment in them. Further study of the respiration of fruits showed that 
 cherries, gooseberries, figs, and lemons, as well as beets and potatoes, 
 and the leaves of cherry, gooseberry, etc., behaved as did pears and 
 apples in the earlier experiments. They concluded that life does 
 not stop in the cells of a fruit or leaf as soon as detached from the 
 plant. Activity continues under the exclusion of the air through the 
 consumption of sugar and with the production of alcohol and CO2. 
 The moment at which the production of CO2 ceases is at the final 
 death of the cell. As a consequence, fruits, seeds, and leaves can 
 remain inert indefinitely if micro-organisms do not develop in them. 
 
 Deherain and Moissan (1874 : 356) found that leaves placed in 
 an atmosphere deprived of oxygen continued to evolve CO 2 for 
 several hours, the evolution apparently ceasing only when all the 
 cells were dead. The resistance to death in the absence of oxygen 
 varied greatly from one species to another. They also showed that 
 CO2 produced a slowing down of respiration, while Borodin (1875) 
 obtained similar results in contrast to the behavior in an atmosphere 
 of hydrogen. Deherain and Vesque (1876 : 327) studied the ab- 
 sorption of oxygen and the emission of carbon dioxid by roots, in 
 extension of the work of Saussure. In addition to confirming the 
 normal behavior of roots in air, they placed them in pure oxygen, 
 and also in air deprived of oxygen. The former gave no bad effects 
 in the case of Veronica. Plants of the latter lived for 7 days in an 
 atmosphere of nitrogen, and those of ivy for 15 days, but finally the 
 leaves fell and the plants died. Plants of ivy placed in an atmosphere 
 of pure carbon dioxid died speedily, the leaves drying up and the 
 vegetative point falling away. They also showed (1877 : 959) that 
 a feeble excretion of carbon dioxid occurs when oxygen is absorbed 
 by the root. Plants did not seem to suffer by the substitution of 
 oxygen for the air in the soil about the roots, but the emission of 
 CO2 was somewhat greater. When small amounts of CO2 were 
 added to the soil-air or to oxygen, the plant did not appear to suffer, 
 but it died if kept in pure CO 2. It also died when the roots were 
 kept in an atmosphere of nitrogen, but the action was much less 
 rapid. 
 
RESPIRATION AND OXYGEN, 59 
 
 De Luca (1878 : 301) stated that fruits, flowers, and leaves in an 
 atmosphere of carbon dioxid or hydrogen, in a vacuum, or in re- 
 stricted air, gave off CO 2, nitrogen, and, in some cases, hydrogen, 
 with the formation of alcohol and acetic acid, as a consequence of 
 the action of a ferment. 
 
 Godlewski (1882 : 521) found that a reduced oxygen-pressure 
 exercised a definite effect upon respiration. The absorption of 
 oxygen was greatest in the case of the plants in air, and, as the 
 oxygen-content of the air was reduced, absorption decreased. The 
 reduction of the respiration intensity was due solely to reduced 
 oxygen-pressure, as the plants regained their earher intensity when 
 the apparatus was opened, and the reduced absorption of oxygen was 
 not observed in the case of plants in pure oxygen. The intensity 
 of respiration was found to be different with the seeds of different 
 species. Under similar conditions, flaxseed respired more than those 
 of hemp and radish, and these again more than alfalfa. The respira- 
 tion energy was much less in the case of germinating starchy seeds 
 than with oily ones. Moreover, the starchy seeds differed among 
 themselves strongly in this respect; 1 gram of germinating wheat 
 grains absorbed in the same time and under similar conditions con- 
 siderably more oxygen than 1 gram of peas. Swollen peas in the 
 first stage of germination in pure oxygen absorbed considerably more 
 of this gas in the same time than in the air. Moreover, the produc- 
 tion of CO 2 in such seeds was hastened by the action of pure oxygen, 
 though in less degree. When the swelling of seeds took place with 
 the air excluded, as under water, intramolecular respiration began. 
 This did not cease immediately when the seeds were placed again 
 in the air, but was gradually replaced by normal respiration. As 
 soon as oily seeds showed rootlets, the volume of CO 2 evolved lagged 
 more and more behind that of the oxygen absorbed. In the ger- 
 mination of starchy seeds, the volume of CO2 evolved was nearly the 
 same as that of the oxygen absorbed in all stages. The changes of 
 the pressure of the oxygen affected the respiration energy of different 
 plant parts in a very different manner. When the respiration dealt 
 with fats, the intensity depended more upon the oxygen-pressure 
 than where it took place at the expense of carbohydrates. 
 
 In a study of intramolecular respiration, Wortmann (1882 : 520) 
 concluded that while oxygen takes a part in the formation of new 
 chemical compounds through intramolecular activity, its presence 
 has a critical effect upon the continuance of molecular change. Even 
 when plants possess the abiUty to exist for a time when deprived of 
 oxygen, this must be regarded not as a normal but as a pathological 
 condition, since both photosynthesis and growth are inhibited. 
 
 Johannsen (1885 : 716) confirmed Bert's general results that the 
 physiological results of compressed air were to be ascribed to the 
 increased partial pressure of oxygen. Higher oxygen-pressures up 
 
60 AERATION AND AIR-CONTENT. 
 
 to 5 atmospheres at first increased the evolution of CO2 for the differ- 
 ent seedUngs investigated and in varying degree for the different 
 species. With longer action, the CO 2 decreased gradually until 
 death ensued, and the more rapidly, the greater the pressure. After 
 two to four hours' exposure to oxygen of a pressure of 2 to 5 atmos- 
 pheres, the plants showed a considerably greater evolution of CO2 
 than before the exposure when they were again returned to the 
 original conditions. 
 
 Pfeffer (1885 : 645) found that the amount of CO2 evolved when 
 seedhngs were placed in hydrogen was regularly less than in the air. 
 Respiration in the latter was often several times greater, with the 
 exception of Viciafaba, where the amounts were about equal. The 
 respiration of sun-roses in ordinary air, in mixtures of equal parts of 
 air and hydrogen, and of 1 part of air and 4 parts of hydrogen was 
 essentially the same. Intramolecular respiration, as shown by a 
 considerable decrease of the CO2 produced, occurred in a mixture 
 of 19 parts of hydrogen and 1 of air. 
 
 Diakonow (1886 : 411) demonstrated that the excretion of carbon 
 dioxid by cotyledons of the bean was 50 per cent greater in the ab- 
 sence of oxygen than in its presence. He reached the conclusion 
 that fermentation begins immediately upon the withdrawal of oxy- 
 gen and disappears again just as soon as the cells resume their full 
 capacity for respiration. As a consequence, there can be no respira- 
 tion, and hence no life, without the presence of oxygen or the action 
 of fermentation as the one means of meeting the energy needs of 
 the cell. 
 
 Palladin (1886 : 44) determined that rootlets of Vicia faba con- 
 sumed 4.6 per cent of dry material in 20 hours of normal respiration, 
 and 11 per cent during the same period in air free from oxygen. 
 Since they used more than twice as much dry substance in the latter 
 in spite of the gradual decrease of activity, anaerobic respiration 
 must be regarded as true fermentation. 
 
 Clausen (1890 : 893) investigated the behavior of dead protoplasm 
 with respect to the evolution of CO2 and also the decomposition of 
 albumen in the living protoplasm when oxygen is absent. With 
 reference to the former, he found that living plants evolved 27 to 28 
 mg. of CO2 in an hour, while the same plants when killed gave from 
 1.5 to 2.1 mg., which was little more than within the limits of error. 
 He concluded in agreement with Detmer, Johannsen, and Pfeffer, 
 and contrary to Reinke's opinion, that dead plants do not evolve 
 CO2. The albumen of the protoplasm of plant-cells when this is 
 in contact with oxygen, as is well known, breaks down into acid 
 amides and amino-acids, and his experiments showed that a similar 
 breaking-down occurred in the absence of oxygen. 
 
 Stich (1891 : 1) concluded that respiration is independent of the 
 oxygen-content of the environment within fairly wide limits. With 
 
RESPIRATION AND OXYGEN. 61 
 
 oxygen-contents of 20.8, 8, 6, 4, 2, and per cent, the point at which 
 a decrease of the CO2 production appeared was different for different 
 plants. On an average, however, a striking reduction of the carbon 
 dioxid produced did not take place between 4 and 2 per cent. At 
 2 per cent, flowers of Anemone japonica, fruits of Prunus domestica, 
 and seedlings of Helianthus, Triticum, and Vicia respired normal 
 amounts of CO2, while flowers of Stenactis annua, Cacalia verbasci- 
 folia, fruits of Hippophaea, and seedhngs of Brassica napus and 
 Cucurbita melanosperma showed a marked decrease of CO 2. With a 
 number of fruits the amount of CO2 was not influenced by the oxy- 
 gen-content, since these produced as much CO 2 in hydrogen as in 
 the air. The respiration quotient was not influenced by a content 
 of 8 per cent and an exposure period of 3 to 24 hours, but at 3 to 
 4 per cent of oxygen the quotient was changed, with a resulting 
 increase of COa- 
 
 Ziegenbein (1893 : 564) found that seedlings of Lupinus exhibit a 
 breaking down of the albumen in the absence of oxygen, as well as 
 in its presence. Diakonow (1894) repeated his earlier experiments 
 upon the dependence of molds upon the presence of oxygen, using 
 Penicillium glaucum, Aspergillus niger, and Mucor stolonifer. With 
 the first two, activity was found to be wholly dependent upon oxygen, 
 the excretion of carbon dioxid ceasing instantly in pure hydrogen. 
 Penicillium, moreover, died quickly in the absence of oxygen, even 
 in solutions containing sugar or glycerin. Mucor, on the contrary, 
 showed no injury when grown in a sugar solution, and continued to 
 produce CO 2 in a stream of pure hydrogen. 
 
 Palladin (1894), in applying Diakonow's discovery to flowering 
 plants, found that, while etiolated leaves of Vicia faba and Lupinus 
 luteus, free from carbohydrates, evolved but small amounts of CO 2 
 and quickly died in the absence of oxygen, those that had absorbed 
 sugar produced carbon dioxid much more rapidly and retained their 
 activity longer. 
 
 Godlewski and Polzeniusz (1897) showed that peas in oxygen-free 
 media were able to use 40 per cent of their original weight in respira- 
 tion. In spite of the accumulation of alcohol in the culture, intra- 
 molecular respiration continued for 6 weeks, the intensity being 
 maintained for 3 weeks at the level of that of normal respiration. 
 They also showed that the seeds retained their power of germination 
 for 2 weeks under anaerobic conditions. 
 
 Buchner (1897 : 117) first showed that yeast contains a ferment, 
 zymase, capable of transforming glucose into alcohol and CO2. 
 Devaux (1899) found that the deep-seated tissues of woody stems 
 above a certain diameter are lacking in free oxygen, and hence 
 undergo fermentation with the formation of carbon dioxid and 
 alcohol. This condition exists at ordinary temperatures, but is 
 augmented by raising the temperature. Direct analyses of the gas of 
 
62 AERATION AND AIR-CONTENT. 
 
 such tissues showed oxygen to be present in only one-thirty-fifth of 
 the volume. 
 
 Recent researches. — Gerber (1903 : 269) found that increased oxy- 
 gen increased the respiratory quotient and hastened the ripening of 
 unripe bananas, but decreased the respiratory quotient of ripe 
 bananas. 
 
 Smirnoff (1903 : 32) showed that wounding produced no increase 
 in intramolecular respiration. The latter at first decreased in hydro- 
 gen, but nearly or quite regained its original intensity in about 40 
 hours, beginning to fall off again on the fifth day. If, however, the 
 wounded bulbs were placed in air instead of hydrogen between 
 experiments, there was an increase in the energy of intramolecular 
 respiration nearly proportional to that of the normal respiration. 
 
 Stoklasa, Jelinek, and Vitek (1903 : 493) reached the conclusion 
 that the anaerobic respiration of the sugar-beet is essentially identical 
 with alcoholic fermentation by yeast. In both cases, CO2 and alco- 
 hol are the chief products, while by-products appear in insignificant 
 quantity. The same quantitative relations between CO 2 and alco- 
 hol occur as in alcoholic fermentation by yeast. They regard it as 
 fully demonstrated that the anaerobic respiration of sugar-beet 
 under the complete exclusion of microbes is an alcoholic fermenta- 
 tion, and that its products, alcohol and CO2, are true excretions. 
 The presence of enzymes similar to those of invertase and zymase 
 shows that the anaerobic respiration of the sugar-beet is extraordi- 
 narily like that of the yeast-cell. 
 
 Godlewski (1904) observed that lupine seeds in pure water with- 
 out oxygen developed only a very weak intramolecular respiration 
 for 6 to 8 weeks. This could be greatly strengthened by adding 
 the proper sugar. In the latter case the intramolecular respiration 
 depended upon alcoholic fermentation. In solutions of both fruit- 
 sugar and cane-sugar the seeds may germinate partially without 
 access of oxygen and the radicles reach a length of 3 to 6 mm., when 
 they slowly die. He concluded that the decomposition of albumen 
 can occur in the absence of oxygen, but that the synthetic formation 
 of asparagin as the beginning of albumen regeneration was impos- 
 sible with the higher plants. 
 
 Gola (1905) studied the seeds of Trapa natans and the rhizomes 
 of Nuphar luteum and Nymphcea alba during the period of rest as 
 well as of germination and found alcohol present in the storage 
 tissues. He thought this to be due to the unfavorable conditions 
 for aeration in swamps, which result in anaerobic respiration. 
 
 Palladin and Kostytschew (1906) made further studies of anaero- 
 bic respiration in relation to alcoholic fermentation, with the follow- 
 ing results : A considerable amount of alcohol is formed in the anaero- 
 bic respiration of living seeds and seedlings of Lupinus, and the 
 
RESPIRATION AND OXYGEN. 63 
 
 anaerobic respiration of these is, therefore, really identical with 
 alcoholic fermentation. The anaerobic respiration of frozen seeds 
 and seedhngs produced no alcohol, and that of a frozen stem-tip of 
 Vicia faba no amount worth noting, and hence bears no relation to 
 alcoholic fermentation. A considerable amount of alcohol was pro- 
 duced in the anaerobic respiration of hving and frozen seeds of peas 
 and castor beans and of wheat germs. The anaerobic respiration of 
 these objects is, therefore, chiefly alcohoHc fermentation, since the 
 zymase was not destroyed. With living peas an accumulation of 
 alcohol was observed only in the absence of oxygen. Frozen peas, 
 on the contrary, accumulated considerable amounts of alcohol with 
 complete access of oxygen. This is explained by the fact that the 
 oxidation processes in plant-cells are weakened in consequence of 
 killing. With both normal and anaerobic respiration of living and 
 frozen plants, acetone and various acids were formed under certain 
 conditions. 
 
 Stoklasa, Ernest and Chocensky (1906 : 302) concluded that in 
 most cases anaerobic respiration is an enzymatic process identical 
 with alcoholic fermentation. This is true of the anaerobic respira- 
 tion of the frozen organs of seed-plants, as well as of the leaf and root 
 of the sugar-beet, and the tubers of the potato. Under anaerobic 
 conditions they isolated noticeable quantities of lactic acid from 
 sugar-beets. 
 
 Palladin and Kostytschew (1907 : 51) studied the anaerobic res- 
 piration of etiolated plant parts and concluded that seed-plants can 
 produce alcohol only in the presence of carbohydrates. In the 
 absence of these, anaerobic respiration leads to the production of 
 CO2 without the formation of alcohol. 
 
 Kostj^tschew (1908 : 537) assumed that the intermediate products 
 of alcohoHc fermentation are oxidized under the access of oxygen 
 and that alcohol in consequence is to be regarded as a by-product of 
 respiration which is not produced under normal conditions. He 
 further concluded (1913 : 129) that in the majority of cases the anae- 
 robic respiration of seed-plants is not identical with zymase fermen- 
 tation, since in most cases other reactions occur along with the latter. 
 
 Stoklasa and Ernest (1908) subjected roots of Hordeum vulgare 
 and Zea mays to an atmosphere consisting of 94 per cent nitrogen 
 and 6 per cent oxygen, and roots of buckwheat to 94 per cent hydro- 
 gen and 6 per cent oxygen, and found that they secreted acetic and 
 formic acids. They concluded that these acids are formed only 
 when there is a lack of oxygen for the normal oxidation processes of 
 the root. With the proper access of oxygen, acetic and formic acids 
 are further changed in the living cell, and are finally broken up into 
 carbon dioxid and hydrogen, the latter probably being largely oxi- 
 dized to water. The roots of buckwheat, rye, oats, and corn all 
 showed formic acid in the absence of oxygen, while rye yielded acetic 
 
64 AERATION AND AIR-CONTENT. 
 
 acid as well. They cited the work of Aso (1906) which showed that 
 acetic and formic acids, even in greatly diluted solutions, exerted an 
 unfavorable influence upon lower as well as higher plants. More- 
 over, the salts of both of these acids in similar concentrations were 
 poisonous to plant organs. Aso explained this poisonous effect of 
 the salts of formic and acetic acids as probably due to hydrolytic 
 dissociation in the hving cell of these salts into acids and bases, 
 by which the base was absorbed by the proteids, while the acid was 
 released and acted unfavorably upon the living protoplasm. Acid 
 aldehyde and acetone were always found alongside of acetic and 
 formic acid, all of which have been shown by Aso to be poisonous. 
 
 Babcock (1912 : 150) confirmed the results of earlier investigators 
 upon the behavior of apples and pears under anaerobic conditions, 
 and noted that there was practically no difference in the final results 
 when hydrogen, nitrogen, carbon dioxid, or the residual gas of respi- 
 ration was employed, or the fruits immersed in cotton-seed oil. 
 Succulent tissues of all kinds behaved in similar fashion in the ab- 
 sence of oxygen, but in young tissues the production of acids was 
 found to be more rapid and the life of the cells short. 
 
 Hill (1913) reached the following conclusions respecting the anae- 
 robic respiration of fruits: The anaerobic production of CO2 by 
 ripe cherries, blackberries, and grapes is as rapid as the aerobic 
 production for a considerable length of time. Ripe fruits that spoil 
 quickly, such as cherries, have a higher respiratory rate than those 
 that do not spoil so quickly, such as grapes. This is due possibly 
 to a higher enzyme-content. Fruit tissues that respire as actively 
 anaerobically as aerobically seem to be those that have finished 
 their growth and are ripe. Growing tissues, such as green peaches 
 and germinating wheat, respire more than twice as rapidly aerobi- 
 cally as anaerobically. The activity of the protoplasm would 
 seem to be connected with the more direct use of oxygen in the 
 production of CO2. If growing tissues, such as green peaches, are 
 placed in an oxygen-free gas for a few days and then brought back 
 into air, the rate of production of CO2 does not entirely return to 
 the normal. This would indicate a permanent injury to the proto- 
 plasm or to some of the enzymes, due to insufficient oxygen. 
 Ripe apples lose their color, texture, and flavor, and take on the 
 quahties of half-baked apples, by being kept for a sufficient length 
 of time in oxygen-free gases (N, H). This emphasizes the need of 
 good aeration for apples. The softening of peaches seems to be de- 
 creased greatly by CO2 and to a considerable extent by hydrogen 
 and nitrogen. Peaches become brownish and acquire a very bad 
 flavor when oxygen is withheld from them. ''Ice-scald" seems to 
 be an injury due to insufficient oxygen and to an accumulation of 
 CO2 within the paper wrappers in which peaches are so often 
 shipped. With good ventilation in conjunction with good refrigera- 
 
RESPIRATION AND OXYGEN. 65 
 
 tion, such injury may be greatly reduced. This appUes to fruits 
 in storage as well as to those in transit. 
 
 Noyes (1914 : 792) gradually saturated the soil in which corn and 
 tomato plants were growing with washed CO2. The lower parts of 
 the plants were first affected and in a week the leaves drooped, 
 turned brownish, and withered. The plants were practically browned 
 at the end of 2 weeks' treatment, the tomato showing the most 
 marked effect. After the treatment, oxygen was given access to 
 the plants. The tomato plant soon died, but the corn plant revived 
 and was growing normally at the end of a week. 
 
 Hasselbring (1918 : 284) found that sweet potatoes are killed under 
 an oxygen-pressure of 5 atmospheres, and that starch hydrolysis is 
 greatly depressed or prevented in the killed tissues. The hydrolysis of 
 starch and the formation of cane-sugar take place in the absence of 
 oxygen as in the air or in an atmosphere of oxygen, and the presence 
 of oxygen is thus not always necessary to the formation of cane-sugar. 
 The material consumed and the output of carbon dioxid is greater in 
 the sweet potato during anaerobic than during normal respiration. 
 
 Summary. — The general effect of the reduction or absence of oxy- 
 gen upon respiration is to decrease its intensity, as shown by Godlew- 
 ski, Wilson, Johannsen, Palladin, Stich, Hill, and others. On the 
 contrary, Diakonow found the intensity to be greater in the cotyle- 
 dons of the bean, but this is probably due to their nature as special- 
 ized storage-organs. Even in Penicillium and Aspergillus he found 
 that the production of carbon dioxid ceased immediately upon the 
 withdrawal of oxygen. The reduction in oxygen-content required 
 to affect respiration differs more or less with the species, and this 
 serves in some measure to explain the discordant results. Saussure 
 determined that the oxygen could be reduced one-half without 
 weakening respiration, while Wilson found that the latter was but 
 slightly affected in a mixture containing 20 per cent of air, though 
 distinctly decreased in one with 5 per cent. Stich stated that in 
 general a striking reduction in the amount of carbon dioxid did not 
 occur between 4 and 2 per cent of oxygen. 
 
 Saussure and Grischow concluded that the rate of respiration was 
 somewhat increased in pure oxygen, but Bert observed a decrease in 
 the amount of CO 2 evolved after several days' exposure. While 
 Bohm and Rischawi found that plants were more or less indifferent 
 to high oxygen-content, Godlewski obtained a strikingly greater pro- 
 duction of carbon dioxid for the first day, after which it fell to a mini- 
 mum. Johannsen likewise noted that the respiration intensity under 
 an air-pressure of 10 to 15 atmospheres, corresponding to a pure- 
 oxygen pressure of 2 to 5 atmospheres, was more or less increased for 
 a few hours, after which it gradually decreased to the death point, 
 and the more rapidly under the greater pressure. 
 
66 AERATION AND AIR-CONTENT. 
 
 Pfeffer showed that the evolution of carbon dioxid in the air was 
 several times greater than in hydrogen, and Pasteur, Borodin, 
 Deh^rain and Moissan, Vesque, and others that it is greatly reduced 
 in an atmosphere of CO 2 itself, continued exposure resulting in the 
 death of the plant. Pfeffer (1900) has stated that most land-plants 
 die eventually in an atmosphere containing from 4 to 20 per cent of 
 carbon dioxid. 
 
 As would be expected, respiration under anaerobic conditions dif- 
 fers with the species, much as does normal respiration. This fact 
 may be readily gained from practically all experiments dealing with 
 two or more species, and it has been studied especially by several 
 investigators. Deh^rain and Moissan stated that in the absence of 
 oxygen the resistance of leaves to death varied greatly from species 
 to species and they confirmed this for roots as well. 
 
 Godlewski found that the energy of respiration was much smaller in 
 starchy than in oily seeds during germination under reduced oxygen- 
 pressure, and starchy seeds also differed much among themselves. 
 The increased evolution of carbon dioxid under oxygen-pressure up 
 to 5 atmospheres varied with the species in the investigations of 
 Johannsen, while Pfefifer observed a wide range in the amount evolved 
 by seedlings in hydrogen from Vicia faba, which gave nearly equal 
 amounts in air and in hydrogen, to those in which the normal respi- 
 ration was several times greater. At 2 per cent of oxygen, Stich 
 showed that certain flowers, fruits, and seedlings respired normal 
 amounts of CO2, while others showed a marked decrease. Hill found 
 that ripe fruits which spoil quickly, such as cherries, respired more 
 intensely under anaerobic conditions than those that keep better, 
 such as grapes. 
 
 Finally, Stoklasa and Ernest have determined that the roots of 
 rye secrete both formic and acetic acid in the absence of oxygen, but 
 those of buckwheat, oats, and corn acetic acid alone. 
 
 Carbon dioxid and alcohol are the regular products of anaerobic 
 respiration, and the latter is consequently regarded by most investi- 
 gators as essentially identical with alcoholic fermentation when 
 carbohydrates are present. Under certain conditions it approaches 
 other types of fermentation, and acetic, formic, and lactic acids have 
 frequently been noted as excretions from the roots and other parts 
 of flowering plants. Among the fermentation products arising from 
 anaerobic respiration are amyl, butyl, and ethyl alcohols, acetic, 
 butyric, citric, formic, lactic, oxalic, propionic, and valerianic acids, 
 acid aldehyde, and acetone, while the decomposition products are 
 ammonia, fatty amido-acids, leucin, skatol, tyrosin, sulphureted hydro- 
 gen, mercaptan, etc. Hydrogen is a frequent product, and methane, 
 carbon monoxid, and nitrogen rarer ones (Pfeffer, 1900 : 533). 
 
RESPIRATION AND OXYGEN. 67 
 
 PHOTOSYNTHESIS. 
 
 The first studies of the effect of pure carbon dioxid upon photo- 
 synthesis were made by Grischow (1819), who found that this proc- 
 ess was greatly reduced but not wholly prevented in the sunlight 
 and with the gas at the ordinary air-pressure. Boussingault 
 (1868 : 269) confirmed Grischow's results, and concluded that the 
 reduction was due to the excessive partial pressure of the carbon 
 dioxid, rather than to the absence of oxygen, as the latter was con- 
 stantly produced in the sunlight. When the carbon dioxid was 
 mixed with other gases, or its pressure reduced, its depressing effect 
 was less marked. He also found (1865 : 605) that oleander leaves 
 lost the ability to free oxygen from a mixture of CO 2 and ordinary 
 air after 48 hours' retention in carbon dioxid, nitrogen, or marsh- 
 gas in the dark at a temperature of 22° to 23° C. In the case of a 
 single leaf, which had been kept for 48 hours in hydrogen in the dark, 
 2.6 c. c. of carbon dioxid were broken down after 5 hours of insola- 
 tion. In the case of many plants, while photosynthesis was slight 
 in pure carbon dioxid, it was marked in mixtures containing 30 and 
 even 40 per cent. 
 
 Pfeffer (1871) observed that both 8 and 16 per cent of carbon 
 dioxid in the air were without noticeable effect upon the evolution 
 of oxygen from the leaves of Prunus laurocerasus. 
 
 Bohm (1873 : 230) found that leaves of laurel plants, after re- 
 maining several hours in an atmosphere without oxygen, perished 
 or completely lost the capacity to break down carbon dioxid. How- 
 ever, they continued to live, obtaining the necessary energy for their 
 functions through intramolecular respiration. He also studied the 
 influence of CO 2 upon the greening and growth of seedlings, and found 
 that this gas exerted a strikingly injurious effect. In a mixture of 
 2 per cent its unfavorable effect upon chlorophyll formation was 
 noticeable. In air whose oxygen-content corresponded to that of 
 ordinary air, but contained 50 per cent of CO2, not only did no growth 
 occur, but the plants themselves died after a short time. In view 
 of the fact that plants visibly sickened in an atmosphere which con- 
 tained only a few per cent of CO2, it was concluded that the present 
 plant world would perish, at least in part, in an atmosphere not much 
 richer in CO 2 than the existing one. From this it results either that 
 the compostion of the atmosphere on earth has been the same from 
 the beginning, which seems a necessary consequence of its being 
 limitless, or, what was regarded as less probable, that there were 
 plants in earlier periods which could endure a greater amount of 
 CO2 without damage. 
 
 Godlewski (1873 : 243) endeavored to determine the optimum 
 amount of CO 2 for photosynthesis, and found it to lie between 5 and 
 10 per cent for Typha, Glyceria and Nerium, while greater amounts 
 
68 AERATION AND AIR-CONTENT. 
 
 were more or less injurious. His experiments lasted but a short 
 time and gave no light upon the effect of long-continued exposure. 
 
 Schiitzenberger and Quinquand (1873) found that the maximum 
 photosynthesis in Elodea was at 5 to 10 per cent of CO 2 dissolved in 
 water; it was reduced by 20 to 30 per cent, and ceased in saturated 
 CO2. 
 
 Warburg (1886 : 122) found that the differences in photosynthesis 
 for Nerium were not very significant at 5 to 25 per cent, while in 
 Bryo'phyllum, photosynthesis was slight at 12 per cent and minimum 
 at 20 per cent. Hoplophyturn grande showed a marked reduction at 
 10 to 15 per cent. 
 
 Pfeffer (1887) likewise observed that photosynthesis was suppressed 
 in the absence of oxygen. Correns (1892) confirmed the conclusion 
 of Wiesner that oxygen is necessary for the greening of etiolated 
 plants. This occurred at 4 per cent for Helianthus, 5 per cent for 
 Sinapis, and 8 per cent for Lepidium. 
 
 Brown and Escombe (1902 : 397) determined that a leaf responds 
 to increased carbon dioxid in the air around it within certain limits. 
 With 2 to 4 times as much CO2 in the air, the activity of photosyn- 
 thesis was increased, but the gain in dry weight was less than in 
 ordinary air. The effect upon the plant was shown by shorter, 
 thicker internodes, a more bushy habit due to the development of 
 axillary buds, smaller leaves, and the almost complete suppression 
 of flowers. They concluded that the sudden increase of CO2 in the 
 atmosphere to 2 to 3 times the present amount would destroy nearly 
 all flowering plants. 
 
 Crocker and Davis (1914) determined that no chlorophyll de- 
 veloped in seedhngs of Alisma during a month's exposure to an air- 
 pressure below 5 mm. 
 
 Summary. — All results are in accord as to the harmful effect of 
 carbon dioxid upon photosynthesis and the production of chlorophyll. 
 The latter function appears the more susceptible, as Bohm observed 
 that injury was produced at 2 per cent, but an extensive study of 
 this relation in a wide range of plants is much to be desired. Bohm 
 also determined that several hours' exposure to an atmosphere de- 
 void of oxygen was sufficient to destroy the power of photosynthesis, 
 and Boussingault obtained similar results from the use of carbon 
 dioxid, nitrogen, or marsh-gas for 48 hours. Grischow found that 
 pure carbon dioxid greatly reduced but did not wholly prevent 
 photosj^nthesis in the sunlight, and Boussingault stated that, while 
 it was slight in the pure gas, it was marked at 30 and even 40 per 
 cent. These percentages are much higher than those obtained by 
 other investigators, and it seems probable that they were due to the 
 dilution of the gas by the oxygen freed in the sunlight. 
 
 The varying response of different species is shown by the fact 
 that Pfeffer discerned no effect from 8 and 16 per cent of carbon 
 
RESPIRATION AND OXYGEN. 69 
 
 dioxid in the case of Prunus, and Warburg but little in Nerium at 
 5 to 25 per cent, while Schiitzenberger and Quinquand found 5 to 10 
 per cent the optimum for Elodea, and Warburg observed that photo- 
 synthesis was markedly reduced in Hoplophytum at 10 to 15 per 
 cent, and was but sHght in Bryophyllum at 12 per cent. Although 
 Brown and Escombe showed that photosynthetic activity was 
 increased with 2 to 4 times the normal amount of CO 2 in the air, the 
 gain in dry weight was less, the stems and leaves were reduced, and 
 the flowers were almost completely suppressed. 
 
 The significance of the injurious effect of carbon dioxid on photo- 
 synthesis upon the question of the greater amount of this gas in the 
 atmosphere during earlier geological periods was first pointed out by 
 Bohm, who regarded it as probable that the composition of the 
 atmosphere has remained the same since the beginning of plant life 
 on the land. This view receives further support from the conclusion 
 of Brown and Escombe that a sudden increase in the carbon dioxid 
 of the atmosphere to 2 to 4 times the present amount would destroy 
 most flowering plants. 
 
 TRANSPIRATION. 
 
 Saussure (1804) was the first to observe that pea plants wilted in 
 an atmosphere of carbon dioxid, as well as in mixtures containing 
 three-fourths and two-thirds of it when this gas was led into the 
 nutrient solution in which they were growing, and Wolff (1870 : 134) 
 confirmed this in the case of barley and beans. Barthelemy (1873) 
 also found that transpiration was reduced by the action of dry 
 carbon dioxid. In a study of the effect of very dilute acids upon 
 transpiration, Burgerstein (1876 : 202) determined the influence of 
 carbon dioxid. In three preliminary experiments with corn seed- 
 lings in which the amount of CO2 was not known, the transpiration 
 generally was much greater for the plants in distilled water. In the 
 series proper, the solutions contained respectively 0.08 and 0.04 per 
 cent of carbon dioxid. The plants in the stronger solution trans- 
 pired slightly more than those in distilled water, and those in the 
 weaker shghtly less in the case of corn. All the other plants studied, 
 peas, beans, pumpkin, broad beans, Celiis, Fagus, Tilia, Cratoegus, 
 and Salisburia, regularly showed considerably greater water-loss in 
 the CO2 solution. 
 
 In a study of the absorption of free nitrogen by legumes, Kosso- 
 witsch (1892 : 702) found that an atmosphere of 80 per cent CO2 
 and 20 per cent oxygen worked injury to the root-system. When 
 the roots of peas had been in such an atmosphere only 2 days, the 
 plants began to wither and they grew no further. As soon as the 
 CO 2 was removed from the inclosed soil and the latter aerated, the 
 peas regained their normal turgescence. He also found that when 
 the soil was penetrated by a stream of mixed oxygen and hydrogen, 
 the plants were not injured. 
 
70 AERATION AND AIR-CONTENT. 
 
 Kosaroff (1897 : 604) in a study of the effect of temperature and 
 gases upon absorption, found that a lack of oxygen and the accumu- 
 lation of CO2 exerted an unfavorable influence upon the functions of 
 the plant. He was especially concerned to determine how far the 
 absence of oxygen was injurious and what part the presence of unde- 
 sirable gases played. He found that pure CO2 diminished both 
 absorption and transpiration. Plants whose roots were placed for 
 some time in an atmosphere rich in CO 2 soon lost their turgor, be- 
 came limp, and commonly died after further action. The injurious 
 effect of CO2 is its particular property, but it also emphasizes the 
 withdrawal of oxygen, and its influence upon absorption is, there- 
 fore, of a double nature. On the contrary, the depressing effect of 
 hydrogen operates only through the withdrawal of oxygen, and is 
 much weaker than that of CO2. Both gases also influence absorp- 
 tion unfavorably when the root is cut off. Pure CO2 affects the 
 transpiration of plants whose roots have been killed by scalding. 
 The author concluded that not only is the activity of the root 
 influenced by this gas, but it also enters the roots and probably 
 exerts an influence on the width of the stomatal opening, thus 
 further reducing the transpiration. In the case of seedlings, leafy 
 and leafless shoots and twigs (1900 : 138), carbon dioxid strongly 
 depressed absorption and hence transpiration, regardless of the part 
 used. It worked injury wherever it came into contact with living 
 cells. This was both a direct consequence of the action of CO2 
 and an indirect result of the exclusion of oxygen. The wilting of 
 plants with continued access of CO 2 was ascribed to the depression 
 of the transpiration stream. 
 
 Livingston and Free (1917 : 183) found that absorption by the 
 roots of Coleus hlumei and Heliotropiuin peruvianum ceased within 
 24 hours after replacing the soil-air with nitrogen. Within 1 to 6 days 
 this was followed by loss of turgor in shoot and leaves, and finally 
 by wilting and death. In Nerium oleander, the disturbance of water- 
 relations in the shoot was indicated by the yellowing and loss of 
 leaves. Coleus recovered slowly with renewed access of oxygen, 
 while Heliotropium failed to do so after wilting became extensive. 
 The roots of the injured plants were found to be dead and partially 
 disintegrated. New roots were formed from the base of the stem in 
 Coleus on the readmission of oxygen. The injury due to the lack 
 of oxygen was found to be greatest with the plants possessing the 
 larger root-systems. The roots of Salix were found to function 
 normally in the absence of oxygen. 
 
 Bergmann (1920 : 14) observed that geranium plants wilted in a 
 few days after the roots were subjected to carbon dioxid, the wilting 
 evidently beginning before all the oxygen was replaced. After 
 wilting, the leaves turned yellow and fell off to the end of the experi- 
 ment. Impatiens halsamina, under similar treatement, was slightly 
 
RESPIRATION AND OXYGEN. 71 
 
 wilted on the second day and badly on the third. It was then given 
 access to the air, but failed to recover. 
 
 Summary. — The depressing action of high percentages of carbon 
 dioxid upon absorption and transpiration has been shown in all the 
 investigations concerned. A similar result has been obtained by 
 Livingston and Free by the use of nitrogen. In practically all cases 
 the pure gas was employed, and hence it is uncertain at what points 
 injury begins. It appears probable that with many species a dis- 
 turbance of the water-relations occurs at 5 or 10 per cent, or even 
 lower, and that in water-logged soils and in bogs carbon dioxid may 
 operate by reducing absorption as well as respiration and photo- 
 synthesis. Kossowitsch was the first to show that CO 2 exerted a 
 specific effect, regardless of the presence of oxygen, and this was 
 confirmed by Kosaroff, who emphasized the fact that the injury 
 wrought was due to the poisonous property of carbon dioxid as well 
 as to the withdrawal of oxygen. The results of Burgerstein with 
 exceedingly dilute solutions do not contravene the rule, but serve 
 to show that carbonic acid in minute quantities behaves like many 
 other acids that stimulate absorption and transpiration. 
 
 GERMINATION. 
 
 Early researches. — The earliest studies of germination in a vacuum 
 were made by Boyle (1660), Ray (1686), Romberg (1699), Boer- 
 haave (1724), and Musschenbroek (1729). Their results were all in 
 agreement to the effect that seeds germinated poorly or not at all in 
 vacua, though they did readily upon renewed access of air. Hum- 
 boldt (1794) and Rollo (1798) concluded that seeds germinated more 
 readily in oxygen than in ordinary air, while Huber and Senebier 
 (1801) found that germination was poorer in oxygen obtained chemi- 
 cally. Rollo also determined that seeds would not grow in hydrogen 
 or nitrogen. Ingenhousz (1786) germinated cress seeds in oxygen, 
 but was unable to do so in hydrogen. 
 
 Lefebure (1801 : 94) investigated the germination of radish seeds 
 in a number of gases. When placed in nitrogen, they failed to ger- 
 minate during a sojourn of 12 days, while in oxygen nearly all had 
 germinated by the end of 3 days. Repeated experiments gave the 
 same results in oxygen. No germination occurred in carbon dioxid 
 or in hydrogen in a period of 12 days. When oxygen was mixed with 
 nitrogen, carbon dioxid, hydrogen, or a combination of nitrogen and 
 CO2 in the proportion of 1 to 8 or 1 to 16 parts, the seeds germinated 
 as readily as in ordinary air. When the amount of oxygen was re- 
 duced to one thirty-second, however, some failed to grow and the 
 others did so more slowly than in the air. 
 
 Huber and Senebier (1801) were the pioneers in an exhaustive 
 study of the relation of air and other gases to germination. Inter- 
 
72 AERATION AND AIR-CONTENT. 
 
 estingly, Huber performed all the experiments, while Senebier fur- 
 nished the suggestions and wrote the text. It was seen that seeds 
 would not germinate in air whose oxygen had been exhausted by 
 bees. Peas, beans, lentils, and wheat germinated under water, while 
 many other seeds would not. When the water was deep, the radicle 
 appeared at first, but fermentation ensued and the plants died. 
 Peas began to decompose in 24 hours in water that had been boiled. 
 It is perhaps significant that peas, beans, and spinach alone germi- 
 nated readily under water, and that only a few seeds of other species 
 succeeded in pushing forth the radicle. Carbon dioxid alone was 
 given off at first, but hydrogen appeared with the beginning of 
 fermentation. 
 
 Seeds of lettuce placed in oxygen derived from green plant parts 
 were seen to germinate more rapidly than in ordinary air, as well as 
 to develop their seedlings more rapidly, but the latter were sometimes 
 injured by the gas. In oxygen obtained from manganese the seeds 
 germinated less rapidly. The favorable effect of oxygen drawn from 
 green parts was confirmed by the seeds of wheat, beans, kidney 
 beans, and spinach. Seeds germinated better in a mixture of 3 
 parts of nitrogen or hydrogen and 1 of oxygen than in 3 of the latter 
 with 1 of either of the other two. In 1 to 4 parts of oxygen and 
 hydrogen germination was very good, in 1 to 5 it was slow and the 
 seedlings perished sooner, while in 1 to 7 but five seeds had germi- 
 nated at the end of 3 days and the radicles died immediately. While 
 Lefebure found that the seeds of turnip germinated in nitrogen con- 
 taining one thirty-second part of oxygen, Huber was able to germi- 
 nate lettuce only when the oxygen reached a sixth. Carbon dioxid, 
 when mixed with varying amounts of oxygen, prevented germination 
 in all cases, while some germination took place with all but the 
 minimum amount of oxygen in hydrogen. 
 
 Lettuce seeds refused to germinate in pure nitrogen, regardless of 
 its manner of derivation, but most of them germinated readily 
 when placed again in the air. Peas, however, germinated readily 
 enough in pure nitrogen. With varying mixtures of nitrogen and 
 oxygen, germination did not occur until the one containing 4 times 
 the initial amount of oxygen. The greater number of seeds did not 
 germinate in pure hydrogen, though they grew readily enough in 
 mixtures of it with ordinary air or oxygen. With the latter, growth 
 failed only with the mixture containing the smallest amount of oxy- 
 gen. While peas germinated in hydrogen, lettuce required 7 days 
 instead of 22 hours, and wheat, barley, and oats did not grow at all. 
 All seeds exposed to pure carbon dioxid refused to germinate, and 
 many of them were unable to do so when brought into ordinary 
 air afterward. 
 
 Saussure (1804) found that no germination took place in pure 
 hydrogen, but green plants persisted in it practically as well as in 
 
RESPIRATION AND OXYGEN. 73 
 
 nitrogen. In the best vacuum obtainable some seeds, such as peas, 
 germinated to the point of the appearance of the radicle, but no 
 further. Seeds were unable to germinate in pure carbon dioxid, and 
 even a small amount retarded germination, both in sun and shade. 
 
 John (1819 : 282) found that swollen peas showed no germination 
 after 4 weeks in carbon dioxid in the light, but that they began to 
 germinate in 3 days in a mixture of one-third carbon dioxid and two- 
 thirds ordinary air. No further growth occurred in the course of 
 2 weeks, even with access of air, as decomposition had set in. He 
 concluded that CO2 killed the embryo as well as the young seedling, 
 and that germination was possible only when the amount of air 
 exceeded that of carbon dioxid. 
 
 Dobereiner (1822 : 212) germinated barley in one-half normal 
 atmospheric pressure and under a pressure of 2 atmospheres. The 
 germination proceeded at an equal rate under both bell-glasses, 
 but the seedlings grew faster in the compressed air than in the 
 rarefied. 
 
 Later researches. — Bohm (1873) observed that wet seeds in pure 
 oxygen at the ordinary pressure did not go beyond the first stage 
 of germination, but that they thrived just as well as in atmospheric 
 air when the oxygen was mixed with four-fifths of its volume of 
 hydrogen or reduced to a pressure of 150 mm. The development of 
 beans was at a minimum in the case of seeds in pure oxygen. It was 
 not the lack of nitrogen but too high a pressure of oxygen that pro- 
 duced this, since the germination of the bean proceeded normally 
 in pure oxygen when the pressure was reduced to one-fifth of the 
 normal. The same amount of oxygen was absorbed when the ger- 
 mination took place in pure oxygen or in the air. He obtained the 
 same results with peas, lentils, and corn, while seeds of sunflower, 
 cress, and flax were much less influenced by high oxygen-pressure. 
 The development of the seedlings was somewhat weaker in pure 
 oxygen than in ordinary air. The results also indicated that oily 
 seeds can germinate under a much higher oxygen-pressure than 
 starchy ones. He later found (1874 : 180) that germination was 
 delayed by 5 per cent CO2, while in germinating cress the formation 
 of chlorophyll was delayed by 2 per cent CO 2, prevented by 20 per 
 cent, and entirely suppressed by 2 days' stay in the gas. 
 
 Deh^rain and Landrin (1874 : 382) determined that seeds germi- 
 nate in pure oxygen, but less rapidly than in atmospheric air. When- 
 ever oxidation in the seed had begun, it continued even in an atmos- 
 phere deprived of oxygen, and the volume of CO 2 produced was 
 greater than the original volume of oxygen. Hydrogen appeared 
 ordinarily only in an atmosphere in which the oxygen had com- 
 pletely disappeared, and carbon dioxid was shown to be more 
 injurious to germination than nitrogen or hydrogen. 
 
74 AERATION AND AIR-CONTENT. 
 
 Bert (1876 : 1493) observed that an increase of the air-pressure 
 to 4 or 5 atmospheres, or of the oxygen-content to 60 per cent, was 
 either without any influence upon germination or merely hastened 
 it. But when the oxygen rose to 80 or 90 per cent, or the air was 
 compressed beyond 5 atmospheres, the harmful effect of the increased 
 oxygen-pressure was quickly evident. Germination was delayed and 
 the growth of the plants was weaker than under normal conditions, 
 the greater the pressure the weaker being the plant. The germina- 
 tion of starchy seeds suffered much more from the increased pres- 
 sure than that of oily seeds, and the latter could stand a much 
 higher pressure without damage. As for respiration itself, a much 
 smaller volume of oxygen was absorbed at a pressure of 11 atmos- 
 pheres than under normal conditions. Under reduced air-pressure, 
 germination took place more slowly the lower the pressure. It 
 ceased between 4 and 10 cm. without the seeds dying. 
 
 GigHoh (1879 : 477; 1895 : 544) found that seeds of wheat, 
 Cynara, Vicia, and Phaseolus kept their power to germinate longer 
 in CO2 than in ordinary air when they were dry, but that they were 
 killed very quickly in it when wet. Wet seeds kept in oxygen and 
 in carbon monoxide failed to germinate. Seeds of alfalfa that had 
 been kept in hydrogen for 16 years gave a germination of 56 per 
 cent, while those in carbon monoxid gave 84 per cent. 
 
 Bernard (1883 : 200) noted that seeds of cress were not able to 
 germinate in a mixture of air with one-sixth volume of CO2. When 
 exposed afterward to air, germination took place in normal fashion. 
 
 Van Tieghem and Bonnier (1882 : 25) found that peas left in 
 open air for 2 years gave 90 per cent of germination, while those in 
 restricted air gave but 45 per cent, and in carbon dioxid, none ger- 
 minated. For beans, the respective percentages were 98, 2, and 0. 
 Castor-beans gave similar results, while wheat and flax showed little 
 difference in open and in closed air, but all agreed in failing to ger- 
 minate in CO2. 
 
 Linossier (1889 : 820) concluded that amounts of CO2 up to 19 
 per cent delayed germination, and above this the number of germi- 
 nating seeds decreased with increasing content, but that germina- 
 tion was entirely suppressed only at higher percentages. Lettuce 
 seeds still germinated at 36 per cent, though cress seeds would not. 
 Carbon dioxid was considered to be a poison to which seeds were 
 variously susceptible. 
 
 Lukas (1886 : 298) found that a varying air-pressure from 22 to 
 72 mm. was sufficient for the germination of seeds of Avena saliva, 
 Triticum vulgare, Panicum miliaceum, and Cucurhita pepo, but not 
 sufficient for the further development of the young seedling, or for 
 the germination of Brassica rapa, Lactuca saliva, etc. Atmospheric 
 air under a pressure of 70 to 168 mm. was sufficient for the growth 
 
RESPIRATION AND OXYGEN. 75 
 
 of the young plants of practically all of these, although in some it 
 resulted in a slower rate of growth than in normal air. 
 
 Deherain (1892 : 8) proved that lack of oxygen prevented germina- 
 tion in water by placing seeds in a tube traversed by a current of 
 aerated water. The seeds at the entrance germinated perfectly, 
 while the ones at the other end did not grow and finally decayed, 
 owing to the consumption of the oxygen by the former. 
 
 Ewart (1894 : 215) demonstrated that prolonged immersion in 
 unchanged water greatly reduced germination. While the per- 
 centage varied from 44 for barley to 95 for beans after 5 days, germi- 
 nation failed after 10 days in the case of beans, 14 days for peas, and 
 3 weeks for wheat, barley, and flax. This was confirmed in later 
 experiments (1896 : 185), which also showed that the same seeds 
 retained their viability several weeks longer in sterile water free 
 from oxygen, 
 
 Lopriore (1895) found that pollen-grains form protuberances in 
 pure CO2, but these soon burst; others are unable to germinate and 
 still others burst in it. Pollen-tubes formed in the air and then 
 placed in pure CO 2 burst for the most part. A content of 1 to 10 per 
 cent carbon dioxid markedly promoted the growth of the pollen-tube, 
 but not its turgor-pressure. The latter increased steadily when 
 tubes were placed in air after 20 minutes in CO 2. 
 
 Jodin (1897 : 442) observed that peas submerged in mercury for 
 more than 4 years showed 80 per cent of normal germination, but 
 when submerged nearly 6 years longer, gave but 22 per cent of nor- 
 mal germination and 22 per cent of abnormal, while 56 failed to germi- 
 nate. Seeds of pea and cress gave no indication of germinating in 
 58 per cent CO2. When returned to ordinary air, all the cress seeds 
 germinated within 2 days, but none of the peas, showing that the 
 latter were killed by the gas. It was found that 7 per cent CO2 
 did not hinder or greatly modify the germination of peas, while at 
 13 per cent the radicle appeared only with difficulty and its growth 
 stopped at 2 to 3 mm. At 50 per cent the power of germination was 
 quickly lost, although respiration continued for some time. Peas 
 in stagnant or sterilized water showed only the first stage of germi- 
 nation, while they germinated normally in running water. 
 
 Recent researches.— Maze (1900 : 350) observed that the seeds of 
 most land-plants will not germinate under water, and that immersion 
 soon leads to the loss of the power to germinate. Germination of 
 immersed seeds was produced by thorough aeration of the water or 
 by means of hydrogen peroxid. 
 
 Schaible (1900 : 93) studied the germination of seeds of Phaseolus, 
 Lepidium, Satureia, Vicia and Hortensia under reduced air-pressure 
 of 3 different grades and confirmed the results of Bert to the effect 
 that it was slower and less complete than in the air. 
 
76 AERATION AND AIR-CONTENT. 
 
 Kolkwitz (1901 : 285) has shown that the respiration of barley with 
 a moisture content of 10 to 11 per cent is very weak, averaging less 
 than 1 mg. CO2 per kilogram per day. It rises rapidly with increas- 
 ing moisture and reaches 2,000 mg. per day for each kilogram at 33 
 per cent, while if the temperature and oxygen-content of the air are 
 raised, it reaches astounding amounts. When seeds are cut into 
 halves, the part containing the embryo respires three times as much 
 as the one with endosperm alone. If the grains are ground in a coffee- 
 mill, respiration increases 50 per cent, due either to wounding or to 
 easier access of oxygen. Respiration does not stop if the grains are 
 ground to meal. 
 
 Duval (1904 : 79) kept the seeds of cabbage and onion in a vacuum 
 for 182 days and found that they germinated 75 and 73 per cent 
 respectively, in comparison with 81 and 74 per cent for controls. 
 During the period no evolution of gases occurred. 
 
 Takahashi (1905 : 439) found that peas could not germinate in 
 the absence of air, although intramolecular respiration was carried 
 on for a number of weeks. Rice grains were able to germinate in 
 plain water and in the absence of air. This was explained by the 
 fact that rice grows naturally in places where germination must occur 
 in the presence of very little oxygen. In the experiment, however, 
 growth seemed to stop when the young plumule reached the length 
 of 3 cm. 
 
 Crocker (1906 : 273) has shown that delayed germination in the 
 cocklebur is due to the exclusion of oxygen by the seed-coats. While 
 the coats of both upper and lower seeds reduce the amount of oxygen, 
 reduction is greatest for the upper. With the coats removed, lower 
 seeds absorb 1.6 to 1.7 times as much oxygen as with them intact, 
 while the upper take up 2 to 2.4 times as much. Seeds of Xanihium 
 canadense soaked for 12 to 18 hours and then kept at 21° to 23° C. for 
 6 days showed complete germination in pure oxygen and none in air. 
 He further demonstrated (1907 : 378) that delayed germination of 
 the seeds of water-plants is due to the exclusion of water by the seed- 
 coats. Oxygen does not seem to be concerned, as increased pressures 
 do not effect germination with the coats intact, and Httle oxygen is 
 needed, moreover. Seeds with intact coats showed no germination 
 after 10 days, while those with the coats broken gave the following 
 percentages at the end of 1 and 10 days respectively: Alisma 
 plantago, 86, 98; Eichhornia, 96, 98; Polygonum amphibium, 81, 85; 
 Potamogeton natans, 42, 51; P. pectinaius, 47, 53; Sagittaria variabilis, 
 92, 92; Typha latifolia, 85, 89. 
 
 Shull (1911 : 475) has found a marked difference in the demand for 
 oxygen by the embryos of the upper and lower seeds of Xanihium. 
 In seeds without coats, the minimal oxygen pressure at 21° C. is 
 about 9.5 mm. for the lower seeds and 12 mm. for the upper. A rise 
 of 10° in temperature lowers these minima to 3 mm. and 7 mm. 
 
RESPIRATION AND OXYGEN. 77 
 
 respectively. The seeds of cocklebur can not germinate without 
 relatively large amounts of oxygen, being opposite in this respect 
 to seeds of Alisma and rice. They do not support the view that 
 the seeds of higher plants can grow in the entire absence of oxygen. 
 
 Babcock (1912 : 99) determined that corn containing 6 to 8 per 
 cent of water kept for a year in CO 2 germinated more slowly than that 
 kept in the air, while corn with 30 per cent of water kept in carbon 
 dioxid for 8 to 12 months entirely failed to germinate. Kernels of 
 corn immersed for one or more days in water boiled free from air did 
 not germinate until oxygen was supphed. 
 
 Becker (1912 : 21) demonstrated that seeds of Dimorphotheca 
 germinated more readily in oxygen than in air, especially those from 
 the ray-flowers. Exposure to oxygen for 30 hours was found to pro- 
 mote germination in the air. When the seed-coats were removed, 
 germination was favored by 10 hours' exposure to oxygen, but de- 
 layed by exposure for 13 hours. The normal seeds of Calendula 
 germinated much more readily in oxygen, while those of Atriplex 
 were injured by increased oxygen-pressure. 
 
 At wood (1914 : 386) concluded that the delay in the germination 
 of the seeds of Avenafatua was due to the oxygen supply as a limiting 
 factor. This is indicated by the results obtained by breaking or 
 searing the seed-coats or the removal of the embryo, by the amount 
 of germination occurring in different concentrations of oxygen, and 
 by measuring the rate of oxygen absorption in seared and intact 
 seeds. The increased permeability of the seed-coat to oxygen was 
 regarded as a factor in the process of after-ripening. 
 
 Pack (1921 : 41) has shown that the catalase activity of seeds 
 stored 45 days at 25° C. rises steadily with the increase of oxygen 
 from 30 to 100 per cent and that the activity at 80 per cent equals 
 that in air. 
 
 Kidd (1914) has found that CO2 in relatively small quantities in 
 the atmosphere inhibits germination, the actual percentage varying 
 with temperature and oxygen supply. Inhibition was produced by 
 2 to 4 per cent at 3° C, while 25 to 30 per cent was required at 20° C. 
 With 5 per cent oxygen, 9 to 12 per cent CO 2 brought about inhibi- 
 tion, while 20 to 25 per cent was required with 20 per cent oxygen. 
 In all seeds tested, except Brassica alba, germination followed nor- 
 mally when CO2 was removed. 
 
 Kidd and West (1917 : 457) have shown that the effect of CO2 
 in inhibiting germination in Brassica alba is followed by a secondary 
 effect of prolonged dormancy after the gas is removed. If the con- 
 ditions during the primary period of CO 2 inhibition are injurious, due 
 to lack of oxygen or excess of carbon dioxid, secondary dormancy 
 does not occur. A high percentage of the latter can only be produced 
 by a mixture of 20 to 30 per cent CO2 and not less than 15 per cent 
 oxygen. Secondary dormancy in the seeds of Brassica alba is not 
 
78 AERATION AND AIR-CONTENT. 
 
 due to increased mechanical pressure of the seed-coats or to their de- 
 creased permeabiHty to gases, but to a stable condition of the embryo 
 slowly established during the period of inhibition by carbon dioxid. 
 This is contrary to the results of Crocker on Xanthium (1916), ShuU 
 on Xanthium (1911), and Rose on Datura and Martynia (1915), all 
 of whom ascribed the effect to the low permeability of the testa to 
 oxygen, and a similar conclusion is indicated by At wood for Avena 
 fatua (1914). 
 
 Summary. — Since germination is essentially a matter of respiration 
 and growth, it shows much the same response to anaerobic condi- 
 tions. The earliest investigators found that it was poor or alto- 
 gether absent in a vacuum, and similar results were obtained when 
 the oxygen was removed by other means. Rollo and Lefebure ob- 
 served that seeds would not germinate in pure nitrogen or hydrogen, 
 while Huber and Senebier stated that peas would do so, but lettuce 
 would not. They were unable to germinate most seeds in hydrogen, 
 in agreement with the results of Ingenhousz and Saussure, who ob- 
 tained no germination in it. The experiments of Huber and Senebier, 
 Saussure, Deherain, Ewart, and Maze have shown that seeds germi- 
 nate but poorly or not at all under water, especially when this is deep 
 and quiet. Rice germinates more readily under water than the seeds 
 of mesophytes, but the growth of the seedling seems to be inhibited 
 sooner or later. Reduction in the amount of oxygen or its pressure 
 beyond a certain point regularly delays or decreases germination. 
 Lefebure found that it took place normally in one-eighth or one- 
 sixteenth oxygen, but failed for the most part in one thirty-second. 
 Huber, however, was able to germinate lettuce seed only when the 
 oxygen reached a sixth. Dobereiner germinated barley at half the 
 normal air-pressure, and Bert, Lukas, and Schaible showed that con- 
 siderable reductions in pressure retarded germination more or less in 
 proportion. 
 
 Contrary to the opinion of Humboldt and Rollo that seeds germi- 
 nate more readily in pure oxygen, most investigators have found that 
 the latter delays germination. Bohm noted that wet seeds in pure 
 oxygen stopped their development in the first stage, and Deherain 
 and Landrin observed that germination was less rapid than in at- 
 mospheric air, while Huber and Senebier had long before shown that 
 it was better in a mixture of 3 parts of hydrogen or nitrogen and 1 of 
 oxygen than when the amounts were reversed. In Bert's experi- 
 ments, an air-pressure of 4 to 5 atmospheres or an oxygen content of 
 60 per cent affected germination little or not at all, but above this 
 it was more or less delayed. 
 
 Small amounts of carbon dioxid usually delay germination and 
 higher percentages inhibit it. Saussure stated that a small amount 
 retarded germination, and Bohm found that 5 per cent was sufficient 
 
RESPIRATION AND OXYGEN. 79 
 
 to produce delay. Bernard obtained no germination with cress in 
 16 per cent CO 2, while Linossier concluded that it was merely delayed 
 at percentages up to 19, and completely suppressed only at much 
 higher ones. Jodin observed that 7 per cent affected the germina- 
 tion of peas but little, but that it occurred only incompletely at 13 
 per cent. The explanation of this wide variation apparently has 
 been furnished in part by Kidd, who finds that germination is in- 
 hibited by 2 to 4 per cent of carbon dioxid at 3° C, while 25 to 30 
 per cent was required to bring it about at 20° C. Even more sig- 
 nificant differences in response are occasioned by the use of different 
 species. Huber and Senebier early showed that peas, beans, lentils, 
 and wheat would germinate under water, though the seeds of most 
 species would not, and Bohm found similar differences in germination 
 in pure oxygen. Bert observed that the germination of starchy 
 seeds suffered more from high oxygen-pressure than that of oily 
 ones, and Jodin noted that peas were more susceptible to CO2 than 
 cress seeds. 
 
 GROWTH. 
 
 Early researches. — While Scheele (1777) thought that plants could 
 not develop well in pure oxygen, and Ingenhousz (1786) found that 
 seedlings thrived better in it, Saussure (1804) was the first to make a 
 study of growth under anaerobic conditions. In pure oxygen, pea 
 plants in the shade were found to gain only half the weight that they 
 did in ordinary air, while in the light the gain was practically the 
 same in both cases. Plants without green parts were unable to 
 grow in pure nitrogen, nor were seeds able to germinate in it, but 
 decomposed quickly, as shown by seeds of pea, cress, and Polygonum 
 amphibium. Buds of Populus nigra and Salix alba, which were 
 ready to open, were unable to develop further in nitrogen, even in the 
 sunshine, but decomposed at the end of 15 days. Buds of roses, lilies, 
 and violets about to open were unable to develop in pure nitrogen. 
 Opuntia in pure nitrogen died in the shade in 6 days; in sunshine, it 
 survived for 3 months, but with great difficulty. Some pea plants 
 were able to resist the effect of nitrogen for 4 to 5 days. Such marsh- 
 plants as Lyihrum salicaria, Epilobium hirsutum, molle, and mon- 
 tanum, and Polygonum persicaria grew as well in nitrogen in weak 
 light as they did in ordinary air. Plants of pea, bean, and kidney- 
 bean died in 3 days in a vacuum, either in sun or shade, while fleshy 
 plants, such as Opuntia, lived for a month in the sun. The marsh- 
 plants mentioned above grew as readily in vacua as in ordinary air, 
 but this was considered to be due to the large amount of oxygen 
 contained in their tissues. It was concluded that plants can survive 
 or grow in vacua only by the aid of the oxygen evolved from their 
 green parts. Both seeds and roots died in a vacuum, woody plants 
 could not open their leaf-buds in it, and flower-buds showed the same 
 effect. With an eighth of CO2 the average growth was 371 mg. and 
 
80 AERATION AND AIR-CONTENT. 
 
 with a twelfth 583 mg. In the shade, the smallest addition of carbon 
 dioxid proved injurious. The plants died in 6 days in a fourth, and 
 in 10 days in a twelfth. A superabundance of CO2 was more in- 
 jurious to a plant in nitrogen than in ordinary air, for, while a twelfth 
 of carbon dioxid in ordinary air was not injurious to marsh-plants, 
 they died in a few days with such an amount in nitrogen. 
 
 John (1819) showed that plants which were brought into an at- 
 mosphere of 33 to 50 per cent of CO 2 or more developed poorly. 
 Davy (1821 : 205) also observed that plants showed but poor growth 
 in air that contained one-third to one-half of its volume of CO2. 
 Birner and Lucanus (1866 : 160) endeavored to determine the effect 
 of CO 2 in a nutrient solution on growth. Carbon dioxid was led into 
 the solution every 2 or 3 days, but an unfavorable effect w^as observed 
 in neither of the two plants. On the contrary, they developed ex- 
 cellently and were much stronger than those not acted upon by CO2. 
 The conclusion was reached that the action of free CO2 is favorable 
 to the production of dry weight, though it was thought that this 
 was due to the actual use of the gas by the leaves, either by absorp- 
 tion through the roots or by diffusion through the air. 
 
 Later researches. — Bert (1873, 1878) made a comprehensive study 
 of growth in reduced air-pressure and in reduced oxygen-content, as 
 well as under increased air-pressure and increased oxygen-content. 
 He was able to show that air-pressure as such had no influence upon 
 growth, but that the latter was influenced only by the partial pres- 
 sure of the oxygen. Studies with the germination of corn, barley, 
 and cress showed that the process took longer in a pressure of 50 to 
 25 cm. than with a normal air-pressure, and that germination finally 
 ceased at a pressure of 8 to 7 cm. In the case of Mimosa pudica it 
 was found that the leaves fell and the plant quickly perished under 
 a pressure of 25 cm. AlgaB were found to cease growth at a similar 
 pressure. 
 
 Bohm (1873 : 141) found that the development of seedlings in 
 pure oxygen was usually restricted to the first stage in germination, 
 and concluded that the injury was not due to the lack of nitrogen, 
 but to the density of the oxygen itself. In general, the growth of 
 plants from a supply of reserve material was as a rule reduced to a 
 minimum in pure oxygen of ordinary density. However, growth took 
 place with the same intensity as in atmospheric air when the oxygen 
 was so rarefied by the air-pump or the admixture of hydrogen that 
 it possessed a pressure equal to or even smaller than the partial 
 pressure of atmospheric oxygen. 
 
 Rischawi (1876) repeated the experiments of Bohm, but he did not 
 find the development of bean, pea, and corn seedlings so near the 
 minimum as Bohm, although it was weaker than in ordinary air. 
 The most important result was that the rootlets of the plants in 
 
BESPIRATION AND OXYGEN. 81 
 
 oxygen were 4 to 5 times shorter than in the case of those in the air. 
 He found further that the respiration intensity remained the same, 
 whether the seedhngs were in oxygen or in air. 
 
 Wortmann (1879) carried out an extensive series of experiments 
 with seeds, seedhngs, and shoots in order to determine the nature 
 and significance of intramolecular respiration. His results indicated 
 that the latter was unable to furnish energy for growth processes, 
 though he concluded that life could persist for several days after the 
 withdrawal of oxygen. He was unable to find the shghtest growth 
 in the absence of oxygen. 
 
 Detmer (1881) repeated and extended the studies of Wortmann, 
 employing N2O, H, and CO2 with seeds and seedlings of Pisum and 
 Triticum. He found neither germination of seeds nor growth of 
 seedhngs to occur, while both heliotropism and geotropism were 
 suppressed, as well as the formation of chlorophyll. The plants 
 suffered markedly and quickly perished. 
 
 Wilson (1882 : 93) repeated the experiments of Wortmann with 
 seedlings of Vicia faba and confirmed the observation that, for a 
 short period, CO2 was excreted just the same, whether oxygen was 
 available or not. When the experiment continued in the absence of 
 oxygen there was a gradual decrease in the production of CO 2, be- 
 cause the plant became injured. With other seedlings, as with 
 flowers and other plant parts, just as soon as the access of oxygen was 
 cut off a direct decrease took place in the evolution of CO 2. This 
 decrease was usually one-half to three-fourths of the normal respira- 
 tion. In an atmosphere consisting of one-fifth air and four-fifths 
 hydrogen, the seedhngs of Helianthus annuus showed no noticeable 
 decrease in CO 2, but a mixture of one-twentieth air and nineteen- 
 twentieths hydrogen resulted in a marked decrease. 
 
 Wieler (1883 : 223) determined that plants require atmospheric 
 oxygen for growth, with the exception of certain fungi of fermentation 
 and decomposition. Growth ceased at once in the absence of oxygen, 
 but the amount necessary for growth was very small, ranging from a 
 fraction of a cubic centimeter for Helianthus annuus and Vicia faba 
 to 1 to 7 c.c. for Brassica and Ricinus. With decreasing pressure, 
 growth was at first increased, then reached an optimum, decreased 
 with further rarefication, and finally ceased. The growth optimum 
 was determined for Helianthus annuus and Vicia faba. For the 
 former it was about 3 per cent and for the latter 5 to 6 per cent of 
 oxygen in relation to the contents of the bell-glass. A slowing down 
 of growth in comparison with growth in normal air occurred at 0.14 
 to 0.5 per cent for Helianthus, 1.45 per cent for Vicia and Lupinus 
 luteus, and 5 to 6 per cent for Cucurbita pepo. In pure oxygen, 
 Helianthus and Vicia grew more rapidly than under normal pressure. 
 In an oxygen-content corresponding to an air-pressure of 2 to 2.5 
 atmospheres, growth seemed to be slowed down in comparison with 
 
82 AERATION AND AIR-CONTENT. 
 
 that ill normal air. In the case of Helianthus, no growth occurred 
 under entire exclusion of oxygen, and it ceased as soon as the oxygen 
 was excluded. Growth began again when the plants were placed in 
 atmospheric air, if they had not been without oxygen for too long a 
 time. Continued exposure to an oxygen-free medium worked serious 
 injury in most cases to the plant. Helianthus suffered exposure for 
 24 hours without damage, and grew vigorously when brought again 
 into atmospheric air. Vicia faba, on the contrary, suffered so after 
 22 hours that the plants blackened when placed in atmospheric air, 
 and Cucurhita died on the second day after being returned to it. 
 
 Moller (1884) repeated Detmer's experiments with N2O, using 
 seedlings of Vicia faba, and found that no growth occurred, though 
 the seedlings suffered no damage in the 48-hour exposure. 
 
 Palladin (1886) exposed seedhngs of Vicia faba for 20 hours to 
 hydrogen and was entirely unable to find any growth, in contrast to 
 the behavior in air. 
 
 Jentys (1888 : 452) stated that, with a single exception, in no case 
 was a complete stoppage of growth observed under greater oxygen- 
 pressure, but in all there was more or less marked reduction. Under 
 approximately equal periods of exposure, the reduction of the growth 
 of the stem parts was the greater the higher the pressure. He pointed 
 out that these results were in opposition to those of Bert, but ex- 
 plained this by the fact that Bert's experiments lasted 4 days in con- 
 trast to 4 hours. Raising the partial pressure of oxygen to one 
 atmosphere was decidedly favorable to the growth of seedlings of 
 Raphanus, Sinapis, and Brassica, but without any influence upon 
 the growth of Vicia, Helianthus, and Pisum. Sinapis and Raphanus 
 were found to grow better in pure oxygen, but more poorly in com- 
 pressed air, while indifferent gases, such as hydrogen and nitrogen, 
 worked unfavorably under increased pressure, as did also the re- 
 duction of the air-pressure below the normal. Peas and other 
 plants grew in air richer in oxygen just as well as in ordinary air. 
 
 Montemartini (1892) observed that seedlings of spinach grew best 
 in 4 per cent CO 2, less well in 7 per cent, and poorly in 22 per cent. 
 Those of Tropaeolum grew vigorously in 4 per cent, poorly in 7 per 
 cent, and perished in 22 per cent. The roots of peas grew shghtly 
 better in 4 per cent CO2 than in normal culture, while growth was 
 considerably retarded at 7 per cent and reduced more than half at 
 22 per cent. The higher amounts of carbon dioxid called forth 
 striking modifications in leaf-structure. The thickness of the pali- 
 sade increased in relation to the sponge parenchyma; its intercellular 
 spaces were smaller and the cells narrower. 
 
 Jentys (1892 : 306) cultivated beans, wheat, rye, and lupines in 
 glass jars under the influence of varying amounts of CO2, namely, 
 4, 5, 8, and 12 per cent. The action of such mixtures was less in- 
 jurious than would be assumed from Bohm's results. In periods of 
 
RESPIRATION AND OXYGEN. 83 
 
 31 to 64 days, beans showed not only a quantitative decrease in the 
 wet and dry weight of shoots and roots, amounting in the case of the 
 latter sometimes to nearly one-third, but they were also shorter and 
 more bushy, with fewer rootlets. The roots of lupines and rye in a 
 mixture of 5 per cent CO2 were both more shallow and less developed, 
 while the roots of wheat were practically indifferent to the gas. 
 
 Jaccard (1893 : 289) found that rarefied air brought about an 
 acceleration of growth, accompanied by modifications in the general 
 aspect of the plant, while air compressed between 3 and 4 atmos- 
 pheres frequently produced an acceleration of growth which was 
 weaker and less general than in the case of rarefied air. Its injurious 
 effect did not occur until a pressure of 8 atmospheres was reached. 
 When oxygen was added up to 90 per cent, there was in general no 
 injurious effect upon growth, and if such air was rarefied it brought 
 about an increased rate of growth. When a mixture containing 70 
 per cent of oxygen was compressed until the oxygen tension equaled 
 that of 10 atmospheres of air, growth was greatly hindered. 
 
 Chudiakow (1894 : 333) investigated the effect of temperature 
 upon the length of Hfe in the absence of oxygen, using young seed- 
 lings of Vicia faba, Triticum vulgare, Pisum sativum, and Helianthus 
 annuus. The results showed that the increased respiration due to 
 higher temperature did not make the plants more resistant to the 
 consequences of the withdrawal of oxygen. On the contrary, in 
 spite of the greater rate of respiration or even on account of it, the 
 plants died much more rapidly than at low temperatures. Experi- 
 ments with swollen seeds of Brassica napus, Triticum vulgare, Vicia 
 faba, and Zea mays gave the same results. 
 
 Recent researches. — In the case of roots of Vicia saliva and Pisum 
 sativum, Chapin (1902 : 375) observed that the slo wing-down of 
 growth was noticeable at 5 per cent CO 2 and growth stopped com- 
 pletely at 25 to 30 per cent. An injurious after-effect was not no- 
 ticed when 20 per cent CO2 acted for 120 hours. At 40 per cent no 
 significant damage was noticed after 24 to 28 hours, but if the plants 
 remained 72 to 96 hours in 20 to 40 per cent CO2, the after-effect 
 became evident. The formation of lateral roots was completely 
 suppressed in 20 to 25 per cent CO2. Seedlings of Pisum sativum 
 were unable to develop at 20 per cent and were killed at 25 per cent 
 in 96 hours. The corresponding values for Vicia saliva were 35 per 
 cent and 80 per cent in 40 hours. The hypocotyl of Sinapis and 
 Trifolium began to stop growth at 15 per cent and completely ceased 
 to grow at 25 per cent. The time necessary to kill plants in CO2 
 decreased as the content increased. With roots and shoots a small 
 amount of growth was still evident after 24 hours with all the higher 
 percentages of CO2, and the effect of the gas therefore was not 
 instantaneous. There was a great difference in the resistance to 
 
84 AERATION AND AIR-CONTENT. 
 
 carbon dioxid between flowering plants and fungi. The first were 
 killed by long immersion in 20 to 30 per cent CO2, while the fungi 
 could not be entirely killed in any intensity. With the former the 
 growth stopped at 20 to 30 per cent, with the fungi at 40 to 80 per 
 cent. The optimum amount of CO2 for the growth of the higher 
 plants studied was about 0.5 per cent. Carbon dioxid in small 
 amounts acted as a stimulus, but in greater amounts as a poison. 
 
 Nabokich (1903 : 272) combated the views of Wieler and of God- 
 lewski and Polzeniusz as to the suppression of growth in the absence 
 of oxygen. He found that hypocotyl of sunflower, 48 to 49 mm. 
 long, made an average growth of 6.3 mm. during 35 hours in pure 
 hydrogen, while another set gave an average of 4.8 mm. during a 
 36-hour culture. Seedhngs were also grown in a series of containers 
 of about 100 c.c. volume, with difl'erent amounts of oxygen for 20 
 hours with the results shown in table 25. 
 
 In a later paper (1909 : 51), the same author table 25 
 
 has given an extensive and unsympathetic ac- 
 count of the work of other investigators upon 
 growth in the absence of oxygen. The authors 
 considered were Borodin, Pfeffer, Wortmann, 
 Detmer, Moller, Wieler, Palladin, Pringsheim, 
 Clark, Correns, Chudiakow, Iwanowsky, Jodin, 
 Kuhne, Celakowsky, Ritter, Maze, Godlewski 
 and Polzeniusz, Polowzow, and Dude. In addi- 
 tion to extending his earlier studies of growth, 
 he found that shoots of Pisum exposed to oxygen- 
 free media for 53^ hours showed all the stages of 
 mitosis, just as in the control shoots, but that in 50-hour exposure, 
 all nuclei were in the resting-stage except those that had dissolved, 
 probably as a result of the death of the protoplasm. He concluded 
 that anaerobic growth in certain relations is completely identical 
 with normal aerobic growth. This is chiefly shown by the fact that 
 the development of plants in oxygen-free media takes place in accord- 
 ance with the usual rules, that the plants retain the abiUty to respond 
 to external stimuli, such as gravity, in the ordinary manner, and 
 that the growth of tissue results from the formation of cells by nor- 
 mal mitosis. On the contrary, other phenomena are in disagreement 
 with normal aerobic growth. These are the universal death of cells 
 in oxygen-free media, the peculiar course of the curve of growth 
 in different periods, and the specific dependence of growth upon the 
 temperature and upon the sugar solutions. 
 
 Mitscherlich (1910 : 158) has studied the effect of increasing the 
 C02-content of the soil upon the production of oats, and finds that 
 it has no effect in increasing the harvest. He thinks that the 
 amount of CO2 in the soil derived from the roots and from the com- 
 position of humus is so great that the solubility of nutrients can not 
 
 0. 
 
 Average 
 growth. 
 
 c.c. 
 
 mm. 
 
 none 
 
 0.9 
 
 0.02 
 
 1.3 
 
 0.04 
 
 1.0 
 
 0.06 
 
 0.8 
 
 14. 
 
 6.2 
 
 Unlimited 
 
 9.0 
 
RESPIRATION AND OXYGEN. 
 
 85 
 
 be increased by greater amounts. While he found that the use of 
 CO2 has no effect upon the harvest, it seemed that in some cases the 
 development of roots was small. 
 
 Lehmann (1911 : 90) showed that many plants, e. g., Vicia faba, 
 Brassica napus, Lupinus albus, Pisum sativum, and Cucurbita, stop 
 growth very quickly when oxygen is withdrawn and are unable to 
 grow even in sugar solution. He found no growth in epicotyls at 
 0.5 mm. pressure, though in some cases, e. g., Helianthus annuus, 
 slight growth occurred in 0.5 to 1 per cent sugar solution at 20° C, 
 and in distilled water at 25° C. 
 
 Crocker and Davis (1914 : 312) have shown that seedhngs of 
 Alisma plantago grew 30 to 32 mm. in 21 days in vacua, or 11 to 12 
 times the length of the embryo, while those in the air grew 50 to 
 57 mm. or 18 to 22 times the length of the embryo. No leaf branches 
 were formed in the seedlings grown in vacua, and chlorophyll did 
 not develop. With the air-pressure below 5 mm. no chlorophyll 
 developed within a month. In the dark, an air-pressure of more 
 than 15 mm. was needed to develop leaf branches. The formation 
 of the primary root seemed to be dependent upon oxygen, as seed- 
 lings in drop cultures and on the surface of water produced primary 
 roots, while few showed them beneath a layer of water, and none 
 in an air-pressure much below 5 mm. 
 
 Cannon (1916 : 75) determined that the exposure of roots of Pro- 
 sopis and Opuntia to pure carbon dioxid for 15 minutes did not 
 affect growth, but exposure during periods of over 30 minutes 
 inhibited it. Growth began again soonest after the shortest exposures, 
 and sooner at high than at low soil temperatures. Roots of Prosopis 
 recovered their normal rate of growth more readily than those of 
 Opuntia. In later studies upon the effect of low percentages of oxygen 
 (1917:82; 1918 : 82), he found that root-growth stopped sooner in 
 Opuntia versicolor than in Prosopis julifiora, as shown by table 26. 
 
 Table 26. 
 
 Oxygen in 
 soil-air*. 
 
 Root-growth in days. 
 
 Opuntia. 
 
 Prosopis. 
 
 p.cl. 
 0. (Nitrogen) 
 2.67 
 
 
 to 1 
 to 2 
 
 1 
 4 to 5 
 
 4 to 6 
 
 1 to 5 
 
 2 to 5 
 2 to 5 
 
 9 to 11 
 
 4 . 50 
 
 7.00 
 
 10.00 
 
 
 Root-growth regularly ceased in nitrogen, though it continued in 
 some cases for a day or two (1919 : 71). The effect upon the shoot 
 was less marked, in Eriogonum growth continuing at a rate of 8 to 
 
86 AERATION AND AIR-CONTENT. 
 
 15 mm. per day for the 8 days the roots were subjected to nitrogen. 
 Shoots of potato, on the contrary, stopped growing very soon after 
 the access of nitrogen. Growth was decreased in the case of morning- 
 glory, tomato, and corn. Carbon dioxid in mixtures containing 25, 
 50, and 75 per cent exerted a depressing effect upon root-growth in 
 Krameria canescens, and growth ceased after a time, to be renewed 
 upon the re-entrance of air. In Covillea the effect was very harm- 
 ful, growth being quickly retarded and soon ceasing. It was renewed 
 but slowly in the air. In Mesembryanthemum growth was decreased, 
 but it ceased only after relatively long exposures, and was renewed 
 with relative rapidity. The difference in sensitivity to carbon dioxid 
 was shown by the fact that the root-growth of Covillea ceased in 
 1.5 hours in 25 per cent CO2, that of Krameria in 2.5, and of Mesem- 
 hnjanthemum in over 4 hours. 
 
 Experiments with such low amounts of oxygen as 2 and 1 per cent 
 and with nitrogen (1920 : 60) again demonstrated the varying sensi- 
 bility of different species. With 2 per cent oxygen, root-growth 
 stopped promptly in the case of onion, and was much retarded in 
 most of the other species. However, in the plants of wet soils, 
 *Juncus, Potentilla, and Salix, and in the succulent Mesembryanthe- 
 mum, as well as in alfalfa, growth was decreased but slightly. In 1 
 per cent oxygen, growth ceased quickly, though in the species of 
 wet soils and in Mesembryanthemum growth continued for several 
 days. In commercial nitrogen root-growth continued for over 5 
 days in Mesembryanthemum, over 7 days in Juncus and Potentilla, 
 and for 13 days in rice, while it was inhibited in the other species. 
 Cannon and Free (1919 : 86) have shown that the root-growth of 
 the sweet-pea stopped at once in static atmospheres of nitrogen or 
 helium, but continued for 3 days in a stream of helium at a rate 
 little below normal growth in air. Injury to potato roots took place 
 soonef in nitrogen than in helium. In later experiments (1920 : 61), 
 this difference in the action of nitrogen and helium was confirmed, 
 though it does not occur when conditions are entirely anaerobic or 
 when an ample amount of oxygen is present. With nitrogen, 1.5 per 
 cent of oxygen is required for the root-growth of the garden pea, but 
 with helium 0.5 per cent suffices. Similar differences have been found 
 in the germination of peas, the greening of rice seedlings, leaf move- 
 ments in acacia and oxalis, stigma movements of Diplacus glutinosus, 
 etc. It is suggested that this difference is due to the fact that 
 oxygen diffuses more rapidly through helium than through nitrogen. 
 Kidd (1919 : 248) has shown that 5 to 10 per cent of oxygen is the 
 optimal amount for the sprouting of potato tubers. Above this it 
 is harmful, while 80 per cent inhibits it in 4 to 5 weeks. Its injuri- 
 ous effect is increased in the presence of carbon dioxid. The latter 
 inhibits sprouting at 20 per cent, and higher concentrations cause 
 marked injury and death. 
 
BESPIRATION AND OXYGEN. 87 
 
 Summary. — The general rule that growth is decreased or prevented 
 by the absence of oxygen appears to suffer but a single exception. 
 Jaccard stated that rarefied air brought about an acceleration of 
 growth accompanied by modifications in aspect, but it seems prob- 
 able that this was due to elongation and did not involve an actual 
 increase in dry weight. Saussure found that most plants and plant 
 organs died in a vacuum in the absence of sunlight, and explained 
 the exceptional behavior of marsh-plants as due to the large amount 
 of oxygen in their tissues. Crocker and Davis have shown that 
 seedHngs of Alisma grow considerably in vacua, but that this is Httle 
 more than 50 per cent of their aerobic growth. Saussure, Jentys, 
 Cannon, and Cannon and Free have found pure nitrogen to stop 
 growth quickly or to greatly retard it, except in marsh-plants, and 
 helium has much the same effect. Detmer and Palladin obtained 
 no growth with seedlings in hydrogen, and Wilson noted that a 
 mixture of nineteen-twentieths hydrogen and one-twentieth air pro- 
 duced a marked decrease, while Jentys Hkewise found this gas 
 unfavorable to growth. Bert, Wieler, Jentys, Lehmann, and Crocker 
 and Davis have all demonstrated that reduced air-pressure retards 
 or inhibits growth, and Nabokich and Cannon have obtained the 
 same results with amounts of oxygen of 2 per cent and below. 
 
 While pure oxygen or air-pressures of several atmospheres have 
 generally been shown to retard growth, Bert, Jentys, and Jaccard 
 have all observed exceptions to the rule. It is certain that both 
 the species and the time of exposure account for very wide variations 
 in response. Carbon dioxid naturally produces the same effect upon 
 growth that it does upon germination, and all investigators are in 
 accord in finding it very injurious. As a rule, the amount required 
 to cause injury in mesophytes ranges from 5 to 10 per cent, but in 
 hydrophytes and some xerophytes especially, it may be as high as 
 20 per cent, and, in exceptional cases, even higher. 
 
 The question of the possibility of the growth of phanerogams in 
 the continued absence of oxygen has been discussed chiefly by Wieler, 
 Nabokich, Godlewski and Polzeniusz, and Lehmann. While Nabo- 
 kich has maintained that growth can regularly occur in the absence 
 of oxygen, he is forced to admit that it differs from normal aerobic 
 growth in its dependence upon sugar solutions and upon temperature, 
 and in the universal death of the cells in oxygen-free media. His 
 own experiments are hardly free from criticism, as he has taken no 
 account of the air contained in the seedlings at the outset, which 
 might well be enough to meet the exceedingly low requirements of 
 the sunflower, as shown by Wieler. Moreover, it would seem difli- 
 cult to prove that the small amounts of growth observed were not 
 due to imbibition or turgor forces, as suggested long ago by Saussure. 
 Finally, the most significant result was the death of 63 per cent of 
 the seedlings at the end of a period of 34 to 36 hours without oxygen. 
 
«» AERATION AND AIR-CONTENT. 
 
 Consequently, the question of a short period of anaerobic growth 
 under special conditions has little more than academic value. Even 
 if shown to exist beyond question, it would be without significance 
 for the functioning of organs or the growth of plants. 
 
 PROTOPLASMIC STREAMING AND MITOSIS. 
 
 Corti (1774 : 210) placed Char a in a vacuum under an air-pump 
 and left the plants for 48 hours. The movement of protoplasm in 
 the cells ceased, but began again in 8 to 12 hours after the plants 
 were returned to atmospheric air. On the contrary, Dutrochet 
 (1838 : 31) found that movement continued for 22 days under simi- 
 lar conditions, but Hofmeister (1867 : 49) was able to refute his 
 conclusions by repeating Corti's experiments. Kabsch (1862 : 341) 
 observed that protoplasmic movement in stamen-hairs of Tradescan- 
 tia ceased after 45 minutes in CO2, but in air it returned after 15 to 
 20 minutes. After 24 hours in CO2 the protoplasm became coagu- 
 lated and was no longer capable of response. The assumption was 
 made that the cessation of movement was due to the lack of oxygen 
 rather than to the direct effect of carbon dioxid. Klihne (1864 : 
 88, 104) also showed that there was no protoplasmic movement in 
 the absence of air or in an atmosphere of hydrogen or CO2. This 
 held also for Myxomycetes, which showed no movement and no 
 development in the absence of oxygen. 
 
 Frank (1872) kept shoots of Elodea under oil for several days, and 
 found that movement ceased, to begin again with access of oxygen. 
 Moller (1884) found that nitrogen stopped the movement of proto- 
 plasm in Elodea in 1 to 2 hours when the leaves were placed in the 
 dark, and that the protoplasm finally contracted under longer expo- 
 sure. He concluded that all gases, with the exception of oxygen 
 and air, had an injurious effect, while some of them were actually 
 poisonous. 
 
 In a study of protoplasmic movements in the absence of oxygen, 
 Pfeffer (1887) reached the conclusion that not only movement but 
 photosynthesis also was suppressed in the absence of free oxygen. 
 He found some movement for a time after the withdrawal of oxygen 
 and explained this as a result of minimal amounts which could not 
 be absolutely excluded. 
 
 Clark (1888 : 273) observed that the streaming of the plasmodia 
 of slime-molds ceased in the absence of oxygen in a few minutes, 
 and readily began again upon the access of air. The complete with- 
 drawal of oxygen also stopped the movement in Tria7iia and Urtica, 
 but it was renewed under a pressure of 1.2 mm. and 2.8 mm. respec- 
 tively. In the case of flagellate forms, such as Euglena, the with- 
 drawal of oxygen produced a resting-stage in the zoospores in a few 
 minutes. With Stylonichia, a pressure of 2.5 mm. caused the indi- 
 vidual to come to rest and to flatten out, while an increase of the 
 
RESPIRATION AND OXYGEN. 89 
 
 pressure to 6 mm. caused the latter process to cease, and the animal 
 began to move again. 
 
 Hauptfleisch (1892 : 219) pointed out that protoplasmic stream- 
 ing naturally ceased in the absence of oxygen, since this reduces the 
 activity of aerobic plants, and movement is hkewise renewed with 
 the access of oxygen. He stated that streaming in the root-hairs of 
 Trianea was gradually brought to a stop when hydrogen was led 
 into the moist chamber. 
 
 Demoor (1894 : 163) studied the effects of different gases, of vacua, 
 and cold upon living cells under the microscope. The plant mate- 
 rial consisted of stamen-hairs of Tradescantia, plasmodia of Chon- 
 drioderma difforme, and chloroplasts of leaves of Funaria. Carbon 
 dioxid brought streaming in the stamen-hairs to a stop in 3 to 6 
 minutes, but the mitotic process continued. However, no cell-wall 
 was formed, unless conditions permitted the renewal of motion. 
 Carbon dioxid stopped the activity of the chloroplasts of Funaria 
 and killed the leucocytes of the blood in 2 to 3 hours. In hydrogen 
 the protoplasm assumed a granular condition, and after 15 to 40 
 minutes movement was no longer visible. Oxygen increased the 
 activity of the protoplasm and its movement. The author's view 
 is that both carbon dioxid and hydrogen hinder respiration by with- 
 drawing oxygen and stopping the protoplasmic movement. The 
 former is the more injurious, because of its poisonous effect. He 
 assumed that the activity of the nucleus is independent of that of 
 the protoplasm, inasmuch as it can continue after the conditions 
 are no longer favorable to the life of the protoplasm. 
 
 Lopriore (1895 : 547) found that CO2 stopped the movement of 
 protoplasm for a time, but did not have a permanently injurious 
 effect, and concluded that its influence is specific and not due alone 
 to the absence of oxygen. When CO2 was mixed with 10 or 20 per 
 cent of oxygen, the streaming was not long suppressed. It accom- 
 modated itself gradually to a high content of CO2 and finally was 
 not inhibited in nearly pure gas. Pure oxygen sometimes exerted a 
 stimulating effect upon slow streaming, but not upon movement of 
 normal rapidity. Pure hydrogen at first hastened movement, but 
 later slowed it down markedly without entirely stopping it. 
 
 Celakowski (1892) found that movement of the protoplasm con- 
 tinued in cells of Tradescantia engulfed by the Plasmodium of a slime- 
 mold, indicating that the cells were supplied with oxygen. He also 
 showed that movement could take place in the absence of oxygen 
 in a number of one-celled and several-celled organisms. In con- 
 trast to his earlier results, Kiihne (1898) observed that protoplasmic 
 movement sometimes persisted for 1 to 3 weeks in the absence of 
 oxygen. Ritter (1899) confirmed the observations of Kiihne and 
 Celakowsky upon anaerobic movement in the Characece, and recog- 
 nized that sugar promoted movement and growth in bacteria facul- 
 
90 AERATION AND AIR-CONTENT. 
 
 tatively anaerobic. He was unable to find either growth or tro- 
 pistic movement under the total exclusion of oxygen. 
 
 Samassa (1900 : 320) confirmed Lopriore's observation that pure 
 oxygen did not increase the rate of protoplasmic movement in the 
 stamen-hairs of Tradescantia. The lack of oxygen stopped the 
 movement quickly, and in nitrous oxid it ceased in 15 to 20 minutes. 
 In agreement with Lopriore, he found an adaptation to carbon dioxid, 
 when successive mixtures were employed with more CO2 and less 
 oxygen, but in all cases pure carbon dioxid caused the motion to 
 cease. The action of CO2 was considered to be that of an acid, since 
 the normal resting nucleus assumed the same foamy appearance 
 noted in dilute solutions of sulphuric, acetic, and formic acids. The 
 conclusion was reached that the cessation of motion due to the with- 
 drawal of oxygen also stops mitosis, contrary to the observations of 
 Demoor. 
 
 Pfeffer (1900 : 546) called in question Kiihne's assumption that 
 the streaming of the protoplasm of Nitella for 50 days in darkness 
 was due to traces of oxygen in the protoplasm, and thought the 
 proper explanation to be that Nitella is a partial anaerobe. He 
 regarded the whole matter as demanding more study, since Ewart 
 has shown that a minute trace of oxygen is sufficient to maintain 
 movement in Chara almost indefinitely. 
 
 SabUne (1903 : 488) found a considerable number of mitotic fig- 
 ures in root-tips of Vicia faba deprived of oxygen for a period of 2 
 hours. The figures were more or less modified, chiefly by the absence 
 of the formation of a cell-plate and the faintness of the spindle. 
 
 O. Nabokich (1904 : 62) subjected young seedlings of Helianthus 
 annuus, Pisum sativum, and Phaseolus vulgaris to an oxygenless 
 atmosphere for 5 to 51 hours. Her chief conclusion was that a 
 normal anaerobic mitosis occurs with some higher plants. In the 
 young leaves and growing-points of sunflower all mitotic stages were 
 found in the cultures for 5, 18, 23, 30, and 43 hours, while with 50 
 hours no figures were seen. The buds of pea and bean behaved 
 similarly, though mitotic figures disappeared earlier. Pea roots 
 were much more sensitive, the figures disappearing after 20 hours, 
 while in root-tips of the bean practically none were found after 5 
 hours. In nearly all cases the cell- wall was formed normally in Heli- 
 anthus, but it was suppressed after 5 hours in Phaseolus. The author 
 concluded that cell-division is independent of the presence of oxygen, 
 and that the injury observed was due to secondary influences. 
 
 Andrews (1912 : 473) found that hydrogen stopped protoplasmic 
 movement in Mucor mucedo and M. stolonifer in 20 minutes in moist 
 air, while Schroter found 5 minutes sufficient. In dry air it required 
 54 minutes for hydrogen to stop the movement. In this case, 5 
 minutes were required for streaming to begin again instead of 1 
 minute after moist air. 
 
RESPIRATION AND OXYGEN. 91 
 
 Summary. — The absence of oxygen stops the movement of proto- 
 plasm in all plant-cells studied, with the possible exception of Nitella. 
 This is equally true whether the air be removed by exhaustion, re- 
 placed by hydrogen, or its access prevented in various ways. Since 
 Ewart has shown that exceedingly minute quantities of oxygen suffice 
 for streaming in Chara, it is probable that this is likewise the case 
 with Nitella and that the movement in the latter is not really anae- 
 robic. Carbon dioxid and nitrous oxid inhibited protoplasmic move- 
 ment in every instance, but both Lopriore and Samassa found a 
 gradual accommodation to the former, so that higher and higher 
 percentages were required to check streaming. As to the effect of 
 an increased oxygen-content, Demoor thought that movement was 
 promoted by it, but Lopriore found this to be true only of slow 
 streaming, and Samassa obtained no increase at all. 
 
 With respect to the movements involved in mitosis, Demoor 
 observed that mitotic processes continued in the stamen-hairs of 
 Tradescantia after carbon dioxid had caused streaming to cease. No 
 cell-plate was formed, however, and division remained incomplete 
 as long as movement was impossible. He concluded that the activ- 
 ity of the nucleus is independent of that of the protoplasm, but it 
 seems that this can be but partially true at most, since mitosis is 
 imperfect in the absence of oxygen, and streaming comprises but a 
 part of the protoplasmic activity. 
 
 Sabline obtained similar results with root-tips of Vicia grown 
 without oxygen, the cell-plate being absent and the spindle faint, 
 while Samassa found that the withdrawal of oxygen inhibited mitosis 
 as well as movement. This discrepancy may be explained by the 
 results of O. Nabokich, who found that different plants and different 
 parts of the same plant varied much in sensibility to the lack of 
 oxygen. Mitotic figures disappeared in the sunflower only with 50 
 hours' exposure, but earlier in the buds of pea and bean, while in 
 root-tips of the pea they were absent after 20 hours and in those of 
 the bean after 5 hours. In most cases the cell-plate appeared in the 
 sunflower, but it was suppressed after 5 hours exposure in the bean. 
 The conclusion that nuclear division is independent of oxygen and that 
 the injuries observed were due to secondary factors does not seem to 
 be warranted by the facts. It seems not improbable that mitosis 
 is still possible in the presence of minute quantities of oxygen, as 
 Ewart has shown for streaming in Chara, and that such amounts 
 may have persisted in the tissues for the relatively short periods of 
 exposure. This explanation is supported by Pfeffer's observation 
 that movement continued for some time after the exclusion of oxygen 
 
 IRRITABILITY. 
 
 The first observations upon the relation of oxygen to irritability 
 were those of Dutrochet (1838), who found that the leaflets of 
 Mimosa pudica folded together with the beginning of air exhaustion, 
 
92 AERATION AND AIR-CONTENT. 
 
 and with further pumping partially unfolded. After 2 hours in the 
 receiver the plants responded again to mechanical stimulus, but 
 with 12 hours in vacuum their irritability had disappeared. The 
 leaves of Mimosa pudica failed to show sleep movements, and the 
 heads of Leontodon taraxacum and Sonchus oleraceus were unable 
 to close or to open in rarefied air, while "sleeping" leaves of Robinia 
 pseudacacia did not open in water free from air. 
 
 Payer (1842 : 1194) found that seedlings of Lepidium sativum 
 exhibited distinct phototropic curvatures in hydrogen or nitrogen, 
 as well as under water, but these were reduced in degree. 
 
 Kabsch (1862 : 341) investigated the effect of carbon dioxid and 
 other gases upon irritabihty. In pure CO2 the sensibility of fila- 
 ments of species of Berberis to mechanical stimuli ceased almost 
 instantly. It returned in the air after a few minutes, when immer- 
 sion in the CO 2 had lasted not more than 5 to 10 minutes, but only 
 after several hours when immersion had lasted for 3 to 4 hours. 
 Even after 6 to 12 hours receptivity had not completely disappeared. 
 A mixture of 30 to 40 per cent of carbon dioxid with air was without 
 effect, but irritabihty disappeared under greater amounts. The 
 inflorescence of Berberis developed buds and flowers normally after 
 2 to 3 days in pure CO2, but a longer stay produced ill effects. Oxalis 
 acetosella and 0. corniculata in carbon dioxid failed to show the nor- 
 mal sleep movements of the leaflets. After being brought into the 
 air, the movements were not resumed until 3 days had elapsed. 
 Movements of the leaflets occurred at a pressure of 15 mm., but at 
 2 to 3 mm. they were no longer irritable. The leaves of Oxalis and 
 the heads of Bellis suspended their periodic movements in vacua, but 
 resumed them upon renewed access to air. 
 
 Wiesner (1878 : 58) subjected seedlings of Lepidium sativum, Pha- 
 seolus multiflorus, Sinapis alba, and Vicia faba to an atmosphere 
 free from oxygen. No curvature occurred after exposure to the 
 fight for several hours, but when air was permitted to enter it became 
 visible in an hour. His conclusion was that heliotropism is impos- 
 sible in the absence of oxygen. 
 
 Wortmann (1879:509) placed germinating seeds of Phaseolus multi- 
 florus, P. vulgaris, and Vicia faba in an oxygen-free atmosphere, 
 which completely inhibited geotropic curvature in the horizontal 
 roots, as well as in the stems of Pceonia peregrina. He stated that 
 irritability is entirely lost when oxygen is lacking, but that it returns 
 with renewed access, even after deprivation for weeks. In a special 
 study of geotropism (1884 : 705), seedhngs of Helianthus, Lepidium, 
 and Phaseolus were placed in a partial vacuum. The slight curva- 
 ture already begun ceased after a short time, and could not be 
 again induced by the admission of air. Shoots exposed to hydrogen 
 in a horizontal position exhibited no geotropic after-effect when 
 placed in the air, in spite of further growth in length. Detmer 
 
RESPIRATION AND OXYGEN. 93 
 
 (1881) confirmed the results of Wortmann, finding that heliotro- 
 pism and geotropism in Pisum and Triticum were suppressed by 
 nitrogen, hydrogen, and CO2. 
 
 Kraus (1884 : 199) exposed horizontal flower-shoots of Anthriscus, 
 Ranunculus, Taraxacum, etc., to streaming carbon dioxid and hydro- 
 gen. After 6 hours no geotropic effect was visible, but upon expo- 
 sure to the air curvature took place in 5 hours. He regarded this as 
 proof that the plants merely pass into a state of rigor. 
 
 Correns (1892 : 87) made an exhaustive study of the relation of 
 irritability to the presence of oxygen, and at the same time considered 
 the results of Kabsch and Dutrochet. He observed that, at suffi- 
 ciently low pressures, rigidity occurred, the petiole rising and the 
 leaflets closing toward each other. According to Kabsch, reduced 
 pressure effected a movement of the stamens in Berberis and Maho- 
 nia, results verified by Correns. The latter showed also that both 
 hydrogen and nitrous oxid gave similar results, while the sudden 
 increase of the air-pressure did not act as a stimulus. After a period 
 in hydrogen irritability ceased. In pure oxygen the filaments of 
 Berberis did not react, but they remained irritable after 24 hours' 
 immersion. Both Kabsch and Correns found that the filaments did 
 not react to CO2, 50 per cent CO2 destroying irritabihty in 10 minutes. 
 The stigmatic lobes of Mimulus closed under reduced pressure, as 
 they did also in hydrogen. In pure oxygen the stigmas retained 
 their irritability as long as 48 hours. The leaves of Securigera, 
 Tetragonolobus, Oxalis, Lupinus, Trigonella, Medicago, Trifolium, 
 and Amicia lost their irritability under reduced pressure, both night 
 closing and morning opening being suppressed. Oxalis was unable 
 to withstand the lowered pressure and was completely dead after 
 12 hours. For the appearance of sleep movements in Oxalis, 3.5 
 per cent of the oxygen originally present was necessary; for Securi- 
 gera, 5 per cent closed the leaves quickly, while it acted more slowly 
 for Tetragonolobus, Sleep movements ceased in pure hydrogen, as 
 they did in CO2, nitrogen, and nitrous oxid. Correns confirmed the 
 discovery of Wortmann that geotropic movements were suspended 
 in vacua, and that of Wiesner, which gave similar results for heHo- 
 tropic movements. He also corroborated the results of Wieler as to 
 the cessation of growth in the absence of oxygen. He noted that 
 the growth of shoots ceased before that of seedlings, and that shoots 
 of Helianthus died at 1 per cent of the original amount of oxygen, 
 and Vicia at 2 per cent. 
 
 Reduction of the pressure did not produce reaction in tendrils, 
 though they responded shghtly to contact in oxygen-free air. For 
 distinct response the lowest limit was 3 to 4 per cent for Passiflora, 
 Sicyos, and Bryonia, and 2 to 3 per cent for Cyclanthera. In a mix- 
 ture of 12 per cent air and 80 per cent CO 2, irritability was lost in 
 a short time. Reduced pressure did not produce reaction in the 
 
94 AERATION AND AIR-CONTENT. 
 
 tentacles of Drosera, but the power to perceive stimuli and to re- 
 spond persisted in the presence of the minutest quantities of oxygen. 
 
 Czapek (1895 : 274) showed that geotropic induction is possible 
 in lupine seedlings placed in a vacuum and then in hydrogen, al- 
 though no growth took place. 
 
 Paal (1912 : 1) noted the time required for the geotropic curvature 
 of bean roots at different air-pressures, and found that the reaction- 
 time was lengthened by reduced pressure. Further experiment 
 showed that the presentation-time was also increased by decreasing 
 pressure, gradually at first and then very rapidly. 
 
 Kenkel (1913) studied the effect of reduced air-pressure upon irri- 
 tability, and found that the geotropic reaction still occurred at an 
 oxygen-content at which heliotropic curvature w^as no longer possible. 
 
 Van Ameijden (1917 : 211) has carried out a comprehensive inves- 
 tigation as to the irritability of Avena saliva in an oxygen-free 
 atmosphere obtained by means of nitrogen. He finds that when 
 seedlings, long enough removed from the effect of oxygen, receive geo- 
 tropic or heliotropic stimuli in the absence of oxygen and are placed 
 at once in the air, they are unable to react. Reaction does occur, 
 however, after a similar fore-period, if the seedlings are both stimu- 
 lated and left in the air. In the absence of oxygen the perception of a 
 stimulus can not take place if there has been a sufficiently long fore- 
 period. After the perception of a stimulus the seedlings do not 
 react if left in an oxygen-free atmosphere, showing that oxygen is 
 necessary for the reaction. Seedlings retain the power of perception 
 for a long time in a low oxygen-content, but this is weakened by a 
 prolonged stay. Complete or partial withdrawal of oxygen produced 
 no difference in the reaction of seedlings to geotropic or heliotropic 
 stimuli. 
 
 Summary. — Practically all tropistic responses are suppressed in the 
 absence of oxygen, though reaction to contact may persist in the 
 presence of minute quantities. The relation of geotropism to oxy- 
 gen has been studied by Wortmann, Detmer, Kraus, Correns, 
 Czapek, Ritter, Paal, Kenkel, and Van Ameijden, and all are in 
 agreement that geotropic curvature is impossible without oxygen, 
 whether this is secured by means of a vacuum or by the use of hydro- 
 gen, carbon dioxid, or nitrous oxid. Wiesner, Detmer, Correns, 
 Kenkel, and Van Ameijden have shown that heliotropic response is 
 similarly inhibited. The nyctotropic movements of leaves and the 
 anthotropic ones of flower-heads have been found by Dutrochet and 
 Kabsch to cease in a vacuum, as well as under greatly reduced pres- 
 sure or in carbon dioxid. Correns obtained the same result with the 
 leaves of several genera under reduced pressure, or in pure hydrogen, 
 nitrogen, carbon dioxid, or nitrous oxid, and both Kabsch and Cor- 
 rens observed that the irritability of stamen filaments was suppressed 
 by carbon dioxid. 
 
RESPIRATION AND OXYGEN. 95 
 
 FUNGI. 
 
 Humboldt (1793) was the first to state that agarics gave off hydro- 
 gen during respiration, and DeCandolle (1832) later determined that 
 Sphceria and Peziza exhaled hydrogen when exposed under water 
 to sunlight. 
 
 Marcet (1829, 1834) found that fungi excreted a small quantity 
 of CO 2, but no hydrogen, in bell-glasses containing air. When 
 placed under water, however, they rapidly disengaged both nitro- 
 gen and hydrogen in sunlight as well as in darkness. In his later 
 experiments the respiration of fungi was studied in the air, in oxygen, 
 and in nitrogen. In every case a certain amount of CO2 was ob- 
 tained, abundantly in the case of oxygen and less so in that of nitro- 
 gen. In these experiments no generation of hydrogen occurred, and, 
 contrary to his preceding opinion, he admitted that the hydrogen 
 was produced by the beginning of decomposition. 
 
 Pasteur (1861, 1876) was the first to show that yeast and certain 
 bacteria could grow in the absence of oxygen if a supply of energy 
 was available, but that anaerobic growth was not indefinitely pos- 
 sible, in the case of yeast at least. 
 
 Brefeld (1874, 1876) found that yeast was still capable of develop- 
 ment in an atmosphere of CO2 which contained one six-thousandth 
 part of oxygen. Yeast was shown to possess the ability to grow 
 without free oxygen in the midst of sugars, which were fermented to 
 CO2 and alcohol. The limit of growth occurred at 12 per cent and 
 that of fermentation at 14 per cent of alcohol. A similar ability was 
 exhibited by Mucor racemosus, but the fermentation proceeded more 
 slowly under similar conditions, and the respective limits of growth 
 and fermentation were 4.5 and 5.5 per cent of alcohol. In the case 
 of Mucor stolonifer the fungus produced fermentation without grow- 
 ing and became inactive at 1.5 per cent of alcohol. He concluded 
 that in the case of all plants, from the simplest to the highest, ab- 
 normal decompositions occur in the absence of oxygen, which, in 
 certain respects, such as the constant formation of CO2 and alcohol, 
 are in harmony with the alcoholic fermentation of yeast. 
 
 Selmi (1874) observed the evolution of hydrogen from molds and 
 from Agaricus ccesarea and supposed that this occurred also in the 
 presence of free oxygen. Gugini (1876 : 1 1 1) pointed out that this was 
 due to the use of sulphur or arsenic in the experiment, while Missaghi 
 (1875) found no hydrogen when moulds grew in atmospheric air. 
 
 Miintz (1876 : 67) determined that Agaricus campestris produced 
 no hydrogen in atmospheric air constantly renewed. When, how- 
 ever, this mushroom was placed in an atmosphere without oxygen, 
 it produced a small quantity of hydrogen, as well as large amounts of 
 CO2. He stated that all fungi in an atmosphere without oxygen 
 transform the sugars which they contain into alcohol and CO. 
 
96 AERATION AND AIR-CONTENT. 
 
 When the sugar is mannite this transformation is accompanied by 
 the evolution of hydrogen, and this is notably the case whenever 
 bacterial decomposition sets in. 
 
 Sadebeck (1881) observed that the spores of Ascomyces tosquinetii, 
 when placed in a sugar solution in the complete absence of oxygen, 
 exhibited an extraordinarily energetic germination, even better and 
 more rapid than in the air. 
 
 Foth (1889 : 279) observed that carbonic acid, like other acids, 
 exerted a strong limiting action on the budding of yeast, and that its 
 fermentative activity was influenced by even small amounts, while 
 different races of yeast were resistant to CO2 in varying degrees. 
 
 Frankel (1889 : 332) found carbon dioxid to limit the growth but 
 not to kill rose yeast and black yeast, while the true yeasts thrived 
 in it. He divided the bacteria into several classes, namely, those 
 which grow as well in CO2 as in air, those which can develop in CO2 
 but whose growth is greatly reduced by it, and those which can not 
 develop at ordinary temperatures in pure CO2, but can in incubation 
 temperatures. The remaining bacteria, which are in general sa- 
 prophytes, do not develop in CO 2, but are not killed by it, and they 
 grow again when placed in air. Finally, there are bacteria which 
 are killed in CO2, among which are the most important pathogenic 
 forms. 
 
 Frankland (1889 : 13) found the widest range of response to un- 
 favorable conditions among one-celled organisms, the majority of 
 individuals dying quickly, while a few remained unharmed. 
 
 D'Arsonval (1891 : 667) showed that carbon dioxid under a pres- 
 sure of 45 atmospheres was a sterihzing agent of great significance, 
 replacing the autoclave. While the resistance of the bacteria was 
 very variable, all were killed after long treatment at a temperature 
 of 40°C. 
 
 Freudenreich (1892 : 7) found that the anthrax bacillus and one 
 other found in milk were resistant to a C02-pressure of 80 to 90 
 atmospheres and an oxygen-pressure of 60 atmospheres, combined 
 with a rise of temperature to about 65° C. 
 
 Van Slyke and Bosworth (1908) have found that carbon dioxid 
 under pressure exerts a marked retarding action upon the develop- 
 ment of lactic-acid bacteria, to the extent that fresh milk so treated 
 exhibits practically no increase in acidity after 9 months at 40° to 
 60° F. 
 
 Bosio (1893 : 61) stated that carbon dioxid under pressure sup- 
 pressed the development of Mycoderma aceti and M. vini in beer. 
 
 Lopriore (1895 : 621) observed that spores of Mucor mucedo could 
 not germinate in pure CO2 when they remained in the gas 3 months, 
 but that they would germinate and grow normally when again 
 brought into air. He found, moreover, that spores germinated at 
 90 per cent CO 2 and that the number of germinated spores increased 
 
RESPIRATION AND OXYGEN. 97 
 
 with decreasing CO2. Pure CO2 stopped the growth of hyphae 
 in 24 hours. Mixtures which contained 10 to 30 per cent could 
 neither suppress the growth of the hyphse nor the production of the 
 sporangia, but growth was much slower. A higher content of CO 2 
 checked growth and suppressed sporangia, but the latter developed 
 when air was substituted. Bursting of the hypha3 occurred when 
 cultures were exposed for several days to high C02-content. The 
 higher the latter, the greater the number of vacuoles in the proto- 
 plasm. The propagation of yeast was inhibited in pure CO 2, but 
 was resumed upon the replacement with air. Mycoderma cerevisice 
 was much more susceptible, and it lost its power of propagation after 
 a 12-hour exposure. 
 
 Ortloff (1900 : 763) found that the increase of yeast-cells was re- 
 duced by a stream of CO 2, but that the amount of the fermented 
 sugar during 28-hour exposure was greater than in normal cultures. 
 
 Chapin (1902 : 375) found that the spores of Mucor, Aspergillus, 
 and Penicillium did not germinate in a high content of CO2, but after 
 being exposed to pure CO2 for 4 months, they were able to germinate 
 when again brought into air. The amount of CO2 necessary to hin- 
 der germination was 60 per cent for Mucor and 90 per cent for Peni- 
 cillium and Aspergillus. Cessation of the growth of the hyphse took 
 place at 30 to 40 per cent for Mucor and at 80 per cent with the other 
 two. The production of spores was hindered at 20 per cent in Mucor, 
 40 per cent in Aspergillus, and 50 per cent in Penicillium. 
 
 Kostytschew (1907 : 178) showed that the normal and anaerobic 
 respiration of Penicillium glaucum and Aspergillus niger, when nour- 
 ished with mannite, took place without the formation of hydrogen, 
 and apparently without anything in common with alcoholic fermen- 
 tation. He likewise found that in the case of Agaricus campestris 
 the normal and anaerobic respiration of fungi containing mannite 
 took place without the formation of hydrogen, and regarded it as 
 clear that the hydrogen found by Miintz was due to the activity 
 of bacteria. In a later publication (1907^ : 188) he stated that no 
 trace of ethyl alcohol was found during the anaerobic respiration of 
 Agaricus campestris, and hence the process is distinct from zymase 
 fermentation. 
 
 Summary. — Practically all the yeasts and molds and the great ma- 
 jority of bacteria are more or less aerobic in nature. They may live 
 and even carry on certain functions for a time under anaerobic con- 
 ditions, but growth for an indefinite period is impossible. In the soil 
 especially it seems probable that many of them live and function un- 
 der conditions alternately aerobic and anaerobic. A considerable 
 number of bacteria are obhgate aerobes, as well as some yeasts, and 
 are able to withstand the absence of oxygen for but a short time. 
 The obhgate anaerobes are practically all bacteria, some of which 
 
98 AERATION AND AIR-CONTENT. 
 
 are able to grow aerobically under certain conditions. In fact, 
 Beijerinck (1897) has called in question the existence of permanent 
 anaerobes. 
 
 The bacteria, yeasts, and molds that produce fermentation are 
 the commonest of temporary anaerobes, since their ability to use 
 sugar enables them to dispense with free oxygen for a time (Smith, 
 1895). They differ greatly in their ability to hve without oxygen, 
 Ascomyces growing better than in the air, and some of the yeasts 
 reproducing repeatedly in its absence, while the bread-mold soon 
 ceases growth and becomes inactive. Fermentation may continue 
 until stopped by the accumulation of alcohol, while growth ceases 
 more quickly and reproduction still earlier. 
 
 Practically all fungi are affected by carbon dioxid, but to a much 
 smaller degree than chlorophyllous plants. While a few bacteria 
 and yeasts may grow more or less normally in pure carbon dioxid, 
 the growth of the great majority is retarded or inhibited, and many 
 are quickly killed by it. The common molds, Mucor, Aspergillus, and 
 Penicillium, were unable to germinate in pure carbon dioxid, though 
 the spores were not killed by an exposure of 3 months. In Mucor spore 
 formation was stopped by 20 per cent CO 2, growth stopped at 30 to 
 40 per cent, and germination at 60 per cent, while for Aspergillus and 
 Penicillium the respective percentages were 50, 80, and 90. 
 
 While the importance of the bacteria in the soil has been univer- 
 sally recognized, much less attention has been given to the molds and 
 other fungi found in the soil. Waksman (1916) has recently sum- 
 marized the investigations in this field and has given an account of 
 his own studies (1917). He has isolated over 200 species of fungi 
 from 25 different soils in North America and the Hawaiian Islands, 
 representing the Mucor acece, Sphceriacece, Mucedinacece, Demaiiacece, 
 Tuber culariacecB, and Saccharomycetacece. It appears certain that 
 molds must play an important part in many soils, and especially 
 in those with deficient aeration. While too httle is known of their 
 products under anaerobic conditions, there is no question that they 
 produce organic acids and other substances that must be taken into 
 account in connection with acidity and toxicity. 
 
 AERATION AS AN ECOLOGICAL FACTOR. 
 
 Sorauer (1873, 1886, 1895, 1909) was perhaps the first to thor- 
 oughly appreciate the importance of aeration in practice, doubtless 
 because he has been the chief exponent of the non-parasitic diseases 
 of plants. In the successive editions of his "Handbuch der Planz- 
 enkrankheiten" he has dealt with the effects of deficient aeration in 
 detail. These are discussed under various captions, namely, lack 
 of oxygen (1909 : 312), puddling of the soil (190), flooding and for- 
 swamping (195), souring of seeds and potted plants (201), excessive 
 
RESPIRATION AND OXYGEN. 99 
 
 watering (206), and deep planting of trees and seeds. The relation 
 of aeration to growth and the treatment of soils to promote it are 
 considered at considerable length in his ''Popular Treatise on the 
 Physiology of Plants" (1895 : 61), under the headings, "How can the 
 soil best meet the requirements of the roots for air?" "How can we 
 improve our fields so as to obtain the best possible crops?" "How is 
 the nutrition of pot-plants effected?" and "How do ordinary roots 
 obtain their necessary supply of air?" 
 
 Sorauer stated that plants without access of oxygen gradually die. 
 When the living cells can absorb no more oxygen their functions un- 
 dergo a change of direction; later they pass into a state of rigidity, 
 in which movement of the protoplasm ceases, sensibihty to stimuli 
 is lost, and growth stops. However, the plants do not die immedi- 
 ately; they exhale carbon dioxid for a long time and can resume their 
 functions upon renewed access of oxygen, even after apparent death. 
 Sour soil is immediately recognized by its peculiar odor and a wholly 
 different process of decomposition of the organic matter occurs in it. 
 There probably arise acid combinations in the little-known series of 
 humus compounds, in addition to the free acids formed. If iron is 
 present, the harmless ferric salts may be reduced to the ferrous ones, 
 since perceptible lack of oxygen must result from the filling-up of the 
 soil spaces with water. Water filled with carbon dioxid derived from 
 the root secretion as well as from the decomposition of organic mat- 
 ter alone suffices to kill plants after protracted action. 
 
 Vonhausen (1877 : 724) placed a clay drain-tube in the middle of 
 a third of the length of a seed-bed sown with seeds of Platanus. 
 The ends of the tube led out to the surface of the soil. At first the 
 non-aerated portion showed no difference, but from the beginning of 
 August the growth in the aerated portion was much greater both in 
 height and in luxuriance. It was suggested that a similar method 
 of aeration could be used in all nursery and seed-beds, and that it 
 might be employed in vegetable gardening as well. 
 
 Bohm (1881) found an example of disease due to faulty aeration in 
 the case of dying Ailanthus that had been planted too deeply in the 
 Ringstrasse in Vienna. These trees had been diminishing in growth 
 for a number of years, as the annual rings formed soon after planting 
 were often more than 3 mm. wide, while those of the years preceding 
 the death of the tree were but 0.5 mm. At the death of the trees the 
 earth of the root-ball was so injurious that seeds of different plants 
 quickly decayed when placed in it. The seeds readily developed, 
 however, after the earth was repeatedly soaked with water and ex- 
 posed in a thin layer to the action of the summer sun. 
 
 Wollny (1889^: 379) stated that in general the most important rule 
 for agricultural practice was to increase the access of atmospheric 
 air to the soil in the most complete manner possible. Soil in the 
 powdery condition contains less air than a crumbly one, and the dif- 
 
100 AERATION AND AIR-CONTENT. 
 
 ference increases with increasing water-content. The soil of fields 
 in the latter condition is significantly more permeable to air than one 
 in the powdery condition. 
 
 Hartig (1894 : 275) pointed out the importance of oxygen for the 
 roots of trees, and stated that the latter die from asphyxia if excluded 
 from a constant supply. Physiological root-rot of pines and spruces 
 is due to lack of soil-air, owing to the density of the soil or to its 
 water-logged condition, and a similar root-rot occurs in plants 
 grown in glazed pots. The remedy for both of these lies in better 
 aeration of the soil. In beech woods the failure of natural regenera- 
 tion is often due to poor aeration resulting from the thick layer of 
 humus. 
 
 Warming (1895 : 96; 1909 : 43) regards soil-air as of the most 
 fundamental significance to plants, since roots and underground 
 shoots, like all other living parts, require oxygen for respiration. 
 Plants adapted to ordinary soils are suffocated in very wet soils, and 
 this results in alcoholic fermentation, followed by death and putre- 
 faction. Soils poor in oxygen exhibit a different type of decompo- 
 sition, and they become ''sour" in consequence of the formation of 
 great quantities of humous acids. The production of acid humus in 
 the forest leads to the exclusion of air and the death of the trees. 
 Ramann (1895, 1911) has likewise insisted that aeration is one of 
 the most important of soil processes for the plant, since it has to do 
 both with the access of oxygen and the removal of excessive carbon 
 dioxid. 
 
 Mangin (1896 : 67) made a comprehensive study of the relation 
 of the amount of CO2 and oxygen in the soil-air about the roots of 
 trees to the well-being of the latter. He found that soils which are 
 packed are less aerated than other soils, and that the grills placed at 
 the base of trees in porous soils were sufficient to assure good aeration. 
 In compact soils or in consequence of watering, which makes the soil 
 of the basin but slightly permeable, the renewal of the air by grills 
 did not suffice to prevent the accumulation of CO2 under the bitumen 
 in quantities sometimes considerable. The watering of the basins 
 presented serious difficulty in consequence of the compacting, which 
 diminished the permeability of the soil to both air and water. While 
 the aeration of the soil was very good in many places in the prome- 
 nades, there were other places where the amount of CO2 reached 8, 16, 
 and even 24 parts per 100, while the oxygen was reduced as low as 
 15 or 10, or even 6 parts. Since CO2 in the soil noticeably decreased 
 growth, all of the trees found in the badly aerated soil slowly perished 
 because of it, as well as owing to the lack of oxygen. 
 
 Deh^rain (1896 : 468) reached the following conclusions with ref- 
 erence to the effect of tillage on aeration: Untilled soil is very well 
 aerated. It inclosed as much air in the prairies and woods as when 
 covered by spontaneous vegetation. Although the quantity of air 
 
RESPIRATION AND OXYGEN. 101 
 
 contained in tilled soils is greater than in untilled, the differences are 
 insufficient to explain the value of tillage. Fallow mellow soil un- 
 
 dergoes movements which increase the total amount of air-space. 
 Rolling the soil diminishes its aeration. 
 
 Brizi (1906 : 89) regared the disease called "brusone" as being non- 
 parasitic, at least in part, and probably due to irregular or incom- 
 plete respiration in water or soil low in oxygen. 
 
 Ehrenberg (1906 : 193) noted the case of turnips that had re- 
 ceived so much liquid manure that the plants stood in water up to 
 the crowns. At first they showed no injury, while the water was 
 receding and being absorbed by the soil. Soon, however, a marked 
 wilting developed rapidly among the leaves and finally extended to 
 the petioles. For several days the leaves remained hanging and then 
 gradually began to recover, although certain portions, especially at 
 the tip of old leaves, died off. Later, similar observations were made 
 on sugar-beets and carrots, while corn, grass, and other plants showed 
 no injury from the flooding. The injury to fleshy roots was ascribed 
 to the lack of oxygen, since these require a large amount at the time 
 of their most active development. -^ 
 
 Clements (1907 : 19) regarded air-content as a factor of impor- 
 tance in all soils, but particularly in acid ones, owing to the constant 
 use of oxygen by the roots. The air-content varies inversely as the 
 water-content, and hence water-plants show characteristic modifica- 
 tions called forth in response to a low air-content. Plants which 
 grow in saturated soils or in water apparently do not compete for the 
 latter, though it is probable that a new factor, air-content, enters the 
 problem. 
 
 Hesselmann (1910 : 91) has emphasized the fact that the swamping 
 of pine forests in Sweden is not a question of water, but of oxygen. 
 The water of the moors and swamp forests is almost completely free 
 from oxygen, and the pine forests suffer greatly in consequence. On 
 the other hand, the pines thrive on the banks of spring brooks, where 
 the water contains oxygen as a result of its rapid motion. In north*^ 
 ernmost Sweden the swamping of the soil is gradually increasing 
 through a marked rise in the level of the ground-water. The latter 
 has been deprived of its oxygen through contact with the peat and 
 thus brings about a significant depression of the functioning of the 
 tree roots. Wherever the soil is better aerated or the motion of the 
 water more rapid, so that it absorbs oxygen, the water works no in- 
 jury, even though it had originally been rendered oxygen-free in 
 passing through the moor. 
 
 Hole (1911) pointed out that the Saccharum-Shorea community 
 in India grew in moist but well-aerated soil, while the Erianthus- 
 Terminalia community occurred in soil moist to wet, and hence less 
 aerated and apt to become somewhat water-logged during rains. 
 Shorea was stated to be decidedly sensitive in regard to aeration 
 
102 AERATION AND AIR-CONTENT. 
 
 and was likely to establish itself only on well-drained soil not subject 
 to water-logging. 
 
 Balls (1912 ; 38) found that the roots of the cotton plant in Egypt 
 were locally asphyxiated in water-logged soil, and in a few weeks even 
 the stout woody roots were not merely dead, but also decomposed. 
 When the depth of available soil was decreased by raising the 
 water-table and thus asphyxiating or killing the lower part of the 
 root -system, greater absorption was demanded of the surface roots. 
 Excavations of a root-system which had been partially submerged in 
 subsoil water showed all the original tap-roots and branches dead 
 from 160 cm. to the maximum, 220 cm. This level coincided with 
 the maxim.um height of the water-table, which had been maintained 
 for 10 days at the end of September. Alongside of the brown and 
 partly decomposed roots were new white roots in abundance, which 
 ended at various depths up to 210 cm. These new roots were all 
 found to arise from laterals which had not been reached by the 
 water-table. With the fall of the latter, these healthy laterals had 
 developed hundreds of rootlets that grew downward as the water 
 receded. 
 
 Harrison and Aiyer (1913 : 106) have reached the conclusion that 
 the surface film of algae is the chief agent in the aeration of the 
 roots of the rice crop. The oxygen evolved by the algal film is 
 dissolved by the irrigation water to produce a highly aerated solution. 
 In undrained soils this can not enter the soil, with the result that the 
 roots are congested near the surface, thus Hmiting the area of root 
 action. In drained soils the aerated water is carried downward, and 
 the roots consequently penetrate to greater depths. The mass of 
 soil subject to absorption is increased and the crop is correspondingly 
 benefited. Too rapid drainage hinders the formation of the algal 
 film and lessens the consequent aeration. The optimum rate of 
 drainage for all swamp paddy soils is a comparatively slow one. 
 This is due to the fact that aeration by atmospheric oxygen is less 
 effective in promoting root aeration than that by the aerated water 
 draining through them. The use of green manures in drained paddy 
 soils promotes the activity of the algal film and thereby increases 
 the aeration of the roots. 
 
 The studies of Hole and Singh (1914 : 10) upon aeration in forest 
 soils in India show that lack of oxygen is a factor of great impor- 
 tance and wide extent. The general summary of their results is as 
 follows (101); 
 
 "1. The present experiments have confirmed the results previously ob- 
 tained regarding the very injurious effect of bad aeration on the growth of 
 sal seedlings in the local forest soil. 
 
 "2. When water is long held in contact with this soil, which is the case 
 under conditions of bad aeration, it becomes heavily charged with carbon 
 dioxid and impoverished as regards its supply of oxygen. 
 
RESPIRATION AND OXYGEN. 103 
 
 "3. The bad growth of sal seedlings in this soil is correlated with an accumu- 
 lation of carbon dioxid in the soil-solution and a low oxygen-content, and this 
 possibly explains the evil effects of bad aeration. Further work, however, 
 is required to prove this and also to decide the relative importance of carbon 
 dioxid and oxygen, respectively. 
 
 "4. Liming this soil, immediately before sowing, has an injurious effect 
 upon sal seedhngs, and during the rains, soil which has been thus limed 
 appears to contain more carbon dioxid and less oxygen than the unlimed soil. 
 It seems possible that this may be due to accelerated bacterial activity. 
 
 "5. As carbon dioxid is rapidly dissipated and a deficiency of oxygen made 
 good under the ordinary conditions of water cultures, it is not easy to prove 
 the effect of varjdng quantities of these gases on plants grown in cultures. 
 For the same reason artificial aeration of such cultures may not show any 
 beneficial result. 
 
 "6 As sal seedhngs can be successfully grown in water cultures, the inju- 
 rious effect of bad aeration is not due to water as such This probably 
 explains the fact that sal can grow on the banks of the rivers or even of stag- 
 nant lakes, in which the water is kept well aerated by exposure to the air or 
 by the presence of green aquatic plants." 
 
 Bembeck (1914 : 26) has pointed out the importance of fresh 
 soil-air for the growth of tree roots and emphasized the relation of 
 the amount of air in the soil to the porosity. Graves (1915 : 213) 
 has studied a disease of coniferous seedlings growing in clay seed- 
 beds. The disease caused most havoc during the wet months, while 
 many cases of recovery occurred in the drier months. In porous 
 soil, in the same nursery, the disease has never been known to occur, 
 and he concludes that it is due to a lack of oxygen in a soil saturated 
 with water. 
 
 Howard and Howard (1915 : 19) have concluded that the so-called 
 disease of Java indigo in India is due to long-continued saturation 
 of the soil, which leads to the death of the young absorbing roots, 
 consequent leaf-fall, and more or less complete wilting. In dealing 
 with soil ventilation (1915^ : 35; 1915^ : 11), they have discussed the 
 relation between aeration and manuring, green manuring, fallow- 
 ing, packing of the surface soil, earth mulches, rice cultivation, grass 
 effects, simulated diseases, peach yellows, surface crusts, and the 
 saving of irrigation-water. Of especial interest are the so-called 
 natural aerators, plants such as Cajanus indicus, Trifolium resupi- 
 natum, ''busunduk,'' and alfalfa, which serve to break up the soil 
 by reason of deep-seated tap-roots or large laterals. They empha- 
 size the fact that crops differ greatly in the air requirement of their 
 roots. Gram is cited as an example of a crop that requires a great 
 deal of air and but a moderate amount of water. Hence great care 
 must be taken to secure and maintain the proper relation between 
 air and water in the soil. 
 
 Howard (1916) has discussed the improvement in the aeration of 
 field soils under surface drainage, and has given a resume of the 
 whole subject of aeration in another bulletin (1916^). In this it is 
 
104 AERATION AND AIR-CONTENT. 
 
 pointed out that water-logging during September reduced the pro- 
 duction of wheat at Pusa somewhat more than 100 per cent. Defi- 
 cient aeration handicaps the deeper-seated roots, and also exerts 
 an unfavorable effect upon the development of the root itself. The 
 saving of irrigation water with the consequent improvement of soil- 
 aeration is further considered in the report for 1916-17. 
 
 Clements (1916 : 90) has emphasized the importance of oxygen in 
 wet habitats in which plant remains accumulate so abundantly as 
 to make the access of air difficult. The decomposition is slow and 
 partial, and the water or soil becomes more or less acid. Lack of 
 oxygen seems a very necessary condition, and the possible effect 
 of the acid upon plant growth is compHcated by that of deficient 
 aeration. Both, apparently, act together in diminishing the absorp- 
 tive power of roots, probably in consequence of decreased respira- 
 tion. So far as succession is concerned, the production of acid in 
 swamps marks a series of stages which dominate for a time, owing 
 to a favorable response to poor aeration. A recent study of the 
 transpiration and growth of plants in aerated bog-water indicates 
 that the acid is a concomitant only and not a cause. The acid is 
 evidently a by-product of decomposition in the absence of oxygen, 
 and deficient aeration is to be regarded as the effective factor. As a 
 consequence, the measurement of the primary reaction in acid habi- 
 tats must be directed toward the effect upon the oxygen-content, 
 i. e., upon aeration. 
 
 Coventry (1917) stated that for practical purposes it may be 
 assumed that the failure of natural regeneration in the deodar is 
 due to the accumulation of humus and other organic substances, 
 which have interfered with the proper aeration and drainage of the 
 soil. In support of this is the well-known fact that deodar shows a 
 distinct Hking for ridges and spurs and similar well-drained places. 
 It is usually found on light, well-aerated soils and does not grow 
 in the heavier clay soils. Some of the best natural reproduction 
 takes place on grassy slopes, which had formerly been subjected to 
 fires, and are consequently well-aerated and drained, owing to the 
 absence of humus. 
 
 Hesselmann (1917) has shown that in the pine heaths of Sweden 
 operations which increase the soil-air and the organic matter avail- 
 able for energy promote nitrification, and consequently tree-growth. 
 This may be accomplished by the mixture of decaying leaves or 
 wood with the mineral soil, and especially by logging, which works 
 the surface layer into the soil. A marked increase of nitrification 
 has been secured by mixing sod with the soil or by stirring the 
 latter with the hoe. 
 
 Howard (1918 : 187) emphasizes the neglect of aeration as a fac- 
 tor in growth, and reviews briefly the work of Hall on the effect 
 of increased aeration on the root development of barley, and that 
 
RESPIRATION AND OXYGEN. 105 
 
 of Hunter on the relation of soil-texture to aeration, as well as his 
 own work on the effect of potsherds and sand upon aeration and 
 consequent growth. A somewhat fuller account is given of the 
 studies of Russell and Appleyard on the composition of the soil-air, 
 and the relation of irrigation and water-saving to proper aeration 
 and growth is discussed. The importance of air-content as a limit- 
 ing factor is indicated, and the relation of quality in barley, tobacco, 
 and cotton to aeration suggested. 
 
 Hole (1918 : 202) reviews his study of the relation of aeration to 
 the growth of sal seedlings, and gives the experimental evidence to 
 show that soil organisms greatly decrease the oxygen and increase 
 the carbon dioxid in soils which bear no green plants. He deals with 
 the poisonous effect of carbon dioxid in various quantities, and 
 concludes that injury in badly aerated soils is due to an excess of 
 carbon dioxid as well as to the deficiency of oxygen, while admitting 
 that further investigation of this point is needed. He states that 
 soil aeration depends chiefly upon the amount of water and organic 
 matter in the soil, the number and kind of soil organisms, and the 
 rate at which air or water with oxygen in solution penetrates the 
 soil. Finally, he suggests, as Howard has earlier, that the injurious 
 effect of grass on fruit trees may be due to poor aeration. 
 
 Howard and Howard (1918 : 36) have described a new method of 
 pit cultures for the study of air and water relations under essentially 
 natural conditions, and have employed it for determining the effect 
 of mixing the Pusa soil with potsherds and sand on the growth of 
 Java indigo. The average length in soil only was 36.7 cm., in equal 
 parts of soil and sand, 51.6 cm., in soil with one-tenth of potsherds, 
 48.3 cm., and in soil with three-tenths, 50.9 cm., the respective per- 
 centages of increase being 40, 31, and 38. Potsherds at the rate of 
 1 inch per acre increased the yield of oats 18 per cent, wheat 20 per 
 cent, and tobacco 10 per cent. In the case of alfalfa, one-third 
 potsherds gave an increase of 24 per cent, and one-half windblown 
 sand an increase of 42 per cent (19 18^). 
 
 Sen (1918) has shown that the addition of 30 per cent of potsherds 
 to soil greatly increases nitrification. The dissolved oxygen is much 
 greater with 10 to 30 per cent of potsherds than with none, and the 
 oxidation of organic matter is correspondingly hastened. A fall of 
 rain leaches out some of the nitrates and is apt to give rise to denitri- 
 fication, but the oxygen of the rain-water increases the amount 
 in the soil-air and hence tends to stimulate nitrification. The down- 
 ward movement of rain, and especially of the water-table, causes 
 greater aeration of the soil, and results in more active nitrification. 
 
 Howard and Howard (1919) have given a further account of the 
 effect of water-logging on the development of roots and of the influ- 
 ence of drainage upon crop production, and have summarized the 
 various experiments upon the saving of irrigation water (1919^). 
 
106 AERATION AND AIR-CONTENT. 
 
 In a final paper on indigo wilt (1920), they state that the conclusion 
 is irresistible that the trouble results from the destruction of roots 
 and nodules under conditions in which regeneration is impossible. 
 If floods cause the ground-water to rise, or if heavy rainfall water- 
 logs the surface soil for long periods, the defective aeration makes 
 root regeneration very difficult, and wilt ensues. In confirmation, 
 it has been shown that other deep-rooted species exhibit wilt, while 
 varieties of these with shallow roots do not. Moreover, wilt is 
 common in years of heavy rainfall, and rare or of shght importance in 
 dry years, while it is seldom found in porous soils. 
 
 Clements (1920 : 85) recognizes that plants may serve as indicators 
 of good or bad aeration and has discussed the subject as follows: 
 The efi"ects of wet and acid soils upon plant behavior have long con- 
 stituted a puzzling problem. The leading role in such habitats as 
 marshes and bogs has been assigned to various factors, such as 
 acids, bog-toxins, toxic exudates, the absence of lime, and the lack 
 of oxygen. Probably all of these are more or less concerned in the 
 problem, with the exception of the supposed exudates, but the view 
 held here is that lack of oxygen is the cause, and the other conditions 
 consequences or concomitants (Clements, 1916 : 90). The presence 
 of acids and bog-toxins is regarded as the direct result of the activity 
 of the roots and bog-flora under deficient aeration (C/. Stoklasa and 
 Ernest, 1908 : 55; Livingston, 1918 : 95). 
 
 The absence of lime is apparently a concomitant of acid produc- 
 tion, since the addition of lime to an acid soil either neutralizes the 
 acid or affects the colloidal relations in such fashion as to make the 
 soil agriculturally productive. It is significant, however, that lime 
 is not the only substance that has this effect, since it is also produced 
 by other materials which improve aeration. An acid soil is regarded 
 as unfavorable to plant-growth primarily because of the deficit in 
 oxygen, and consequently also because of the poor development of 
 the micro-organisms that reconvert organic nitrogen into available 
 form. The current assumption that bog-water contains acids or 
 toxins which are in themselves unfavorable to absorption seems dis- 
 proved by the experiments of Bergman. 
 y ,^, Plants may indicate good or bad aeration. The former are natu- 
 rally of little importance as aeration indicators, since their impress 
 is due to some other factor or factor-complex. Aeration indicators 
 proper are correlated with a deficiency of soil-oxygen, and are natu- 
 rally confined to wet soils and water, owing to the inverse relation 
 existing between the amount of water and of oxygen. They may be 
 conveniently arranged in four groups, based upon the kind of 
 response to deficient aeration. In the first two the species have 
 developed adaptations which enable them to live so successfully in 
 swamps and bogs that the habit is now obUgate for the majority of 
 them. The species of swamps regularly possess a special aerating 
 
RESPIRATION AND OXYGEN. 107 
 
 system of air-passages and diaphragms, often supplemented by super- 
 ficial roots and a marked movement of the transpiration stream. 
 Such indicators are found typically in Equisetum, Juncus, Heleocharis, 
 Scirpus, Alisma, Sagittaria, Sparganium, etc. Air-passages also 
 occur in some bog-plants, but they are little or not at all developed 
 in the shrubby species, such as Vaccinium, Ledum, Andromeda, 
 Kalmia, Empetrum, etc. In most of these the aeration devices are 
 subordinate to those designed to conserve the water-supply during 
 drought, especially in winter (Gates, 1914). Coville (1911, 1913) has 
 emphasized the importance of good aeration for the successful culture 
 of the blueberry, pointing out that this is secured in nature by the 
 superficial roots as well as by their position in hummocks. It is 
 probable also that mycorrhiza plays an important role, partly in 
 increasing the available nitrogen, and partly also perhaps in directly 
 compensating for the deficit in oxygen. 
 
 The other two groups of aeration indicators consist of plants which 
 grow normally in well-aerated soil. Hence they lack special adap- 
 tations for aeration and consequently serve to indicate a lack of 
 oxygen by their growth or distribution. Those which are some- 
 what tolerant of water-logged and poorly aerated soils respond to 
 reduced oxygen-content by decreased growth and reproduction. 
 Intolerant species drop out, and their reduced number or absence 
 serves as an indicator of conditions. 
 
 Summary. — The results of field studies of aeration are in complete 
 agreement with those obtained from physiological investigations as 
 to the basic importance of oxygen for root activity and the injury 
 wrought by the accumulation of carbon dioxid. The detailed sig- 
 nificance of the lack of oxygen and the abundance of carbon dioxid 
 as ecological factors is discussed in connection with bog xerophytes 
 and soil toxins. Here it will suffice to point out that field research 
 has approached the problem of an oxygen deficit from four different 
 angles, and that the results and conclusions are all in essential accord. 
 The agricultural approach has been made by Sorauer, Deh^rain, 
 Wollny, Brizi, Ehrenberg, Balls, Harrison and Aiyer, Howard and 
 Howard, Main, and Allan, and that of forestry by Vonhausen, Bohm, 
 Mangin, Hesselmann, Bernbeck, Hole, Hole and Singh, and Coventry. 
 Pathological considerations have entered into many of the studies, 
 but they have received especial attention at the hands of Sorauer, 
 Hartig, Mangin, Howard, and Graves, while the ecological outlook 
 has been represented by Warming and Clements. Moreover, a 
 large number of the papers in the next two sections have a more or 
 less direct bearing upon the ecological and practical significance of 
 aeration, but are discussed later, owing to their relevance to the 
 special problems concerned. 
 
108 
 
 AERATION AND AIR-CONTENT. 
 
 Table 27. 
 
 Inches of 
 
 Bushels of 
 
 Bushels of 
 
 water 
 
 wheat for 
 
 grain to 
 
 applied. 
 
 each inch. 
 
 the acre. 
 
 5.0 
 
 7.56 
 
 37.81 
 
 7.5 
 
 6.39 
 
 41.54 
 
 10.0 
 
 4.35 
 
 43.53 
 
 15.0 
 
 3.05 
 
 45.71 
 
 25.0 
 
 1.86 
 
 46.46 
 
 35.0 
 
 1.39 
 
 48.55 
 
 50.0 
 
 0.99 
 
 49.38 
 
 The inverse relation of water and soil-air is chiefly discussed in 
 the following sections, but the significance of irrigation-water in this 
 connection has been largely determined by Howard and Howard in 
 their investigations of aeration as a primary factor in agriculture. 
 
 Widtsoe (1914 : 249) has probably been the first to show that there 
 is a steady decrease in the yield of wheat per inch of water as the 
 irrigation of a field is increased, and that excessive irrigation may 
 produce an actual decrease in the total yield, though he did not rec- 
 ognize that this was chiefly due to faulty aeration. The rapid de- 
 crease in production for each inch of water used is shown in table 27. 
 
 Two irrigations amounting to 
 7.5 inches are regarded as sufficient 
 for a crop of wheat on deep soil, 
 and 4 to 5 irrigations, totahng 18 
 inches, on shallow gravelly soil. 
 On many soils a single irrigation 
 of 5 inches is better. 
 
 Howard and Howard (1915, 
 1919) have pubHshed two bulletins 
 on the saving of irrigation-water 
 in wheat-growing, which deal with 
 the principles underlying water- 
 saving, and with experiments at 
 Quetta and in India. The six principles are the following: 
 
 (1) The irrigation- water available should be spread over the 
 largest possible area. 
 
 (2) Irrigation-water must be applied in such a manner as to inter- 
 fere as little as possible with the natural aeration of the soil. 
 
 (3) Heavy waterings reduce the proportion of grain to total crop. 
 
 (4) The growth-period of wheat is increased by heavy watering. 
 
 (5) When the water-supply is limited, the root development of 
 the wheat crop must be deep. 
 
 (6) The soil-moisture must be preserved as far as possible by a 
 surface mulch of dry soil. 
 
 While all of these relate to aeration as well as water economy, the 
 first three have to do directly with a proper supply of soil-air. Other 
 things being equal, the soil-air is increased as the irrigation-water 
 is diminished, and with respect to the plant alone, the best irrigation 
 method involves the most effective compromise between too much 
 water and too little air (table 28). 
 
 The economic waste involved in using irrigation-water beyond 
 the optimum is threefold. The most serious waste occurs when an 
 actual reduction of the yield per acre takes place, but scarcely less 
 important is the waste resulting from a rapidly diminishing return 
 per acre-inch of water used. In average seasons such waste amounts 
 at least to much of the cost of the water, and may amount to the 
 
RESPIRATION AND OXYGEN. 
 
 109 
 
 value of the crop that could be produced with the superfluous water, 
 while in seasons of drought it may often reach the total value of the 
 average crop. In the third place, excessive irrigation works injury 
 to the fertility of the soil, largely as a matter of defective aeration. 
 Thus, Main (1916 : 47) has shown that continuous cropping to 
 wheat with several irrigations has reduced the yield per acre at 
 Mirpurkhas from 759 pounds in 1908-09 to 372 pounds in 1913-14, 
 in spite of the use of fertilizers. 
 
 Table 28. — Water-saving experiments on wheat, 1916-17. 
 QUETTA. 
 
 No. of 
 
 Area 
 
 Total weight 
 
 Total weight 
 
 Yield of grain 
 
 
 waterings. 
 
 
 of produce. 
 
 of grain. 
 
 per acre. 
 
 
 
 acres. 
 
 Ihs. 
 
 m. 8. 
 
 m. 8. 
 
 p.ct. 
 
 1 
 
 3.99 
 
 10,367 
 
 52 6 
 
 13 2 
 
 
 
 3 
 
 2.65 
 
 6.620 
 
 25 15 
 
 9 23 
 
 26 
 
 GUNGAPUR, HARIPUR, AND SARGODHA. 
 
 
 
 Yield per acre. 
 
 Average yield per acre. 
 
 Total No. 
 of irrigations. 
 
 
 
 
 
 
 
 
 
 
 Gr< 
 
 lin. 
 
 Straw. 
 
 Grain. 
 
 Straw. 
 
 
 m. 
 
 
 m. s. 
 
 m. 8. 
 
 m. s. 
 
 1 
 
 12 
 
 19 
 
 20 10 
 
 
 
 1 
 
 8 
 
 31 
 
 19 14 
 
 9 34 
 
 21 17 
 
 1 
 
 8 
 
 12 
 
 25 27 
 
 
 
 2 
 
 18 
 
 
 25 8 
 
 
 
 2 
 
 15 
 
 21 
 
 23 16 
 
 16 11 
 
 25 5 
 
 2 
 
 15 
 
 12 
 
 26 32 
 
 
 
 3 
 
 14 
 
 25 
 
 18 
 
 
 
 3 
 
 16 
 
 8 
 
 26 4 
 
 15 11 
 
 22 2 
 
 Note. — In Mirpukhas one watering yielded 1,483 pounds, two yielded 1,471. 
 
 It appears almost certain that the common practice in irrigated 
 regions involves the use of too much water, with the consequent 
 economic losses. Not the least of these is the necessary restriction 
 of irrigation systems to a smaller territory than should be the case, 
 which results in a serious limitation of opportunity and production. 
 Throughout the western United States it is a fortunate system that 
 does not face an annual or periodic deficit, while the extent of new 
 reclamation projects must unfortunately be determined by the exist- 
 ing practice rather than by the optimum duty of water. As a con- 
 sequence, it would seem an indispensable task of every great system, 
 installed or to be installed, to determine the optimum use of water 
 and to take steps to see that everyday practice conforms to the 
 findings. Moreover, while annual crops quickly show the effect of 
 
110 AERATION AND AIR-CONTENT. 
 
 over-irrigation and defective aeration, perennial and especially woody- 
 plants exhibit damage less readily, with the result that they often 
 gradually develop diseases obscure in origin and impossible of refer- 
 ence to a specific parasite as a cause. In all such cases it is neces- 
 sary to consider as a possible cause the defective aeration that regu- 
 larly arises as a consequence of applying too much water. In older 
 districts the gradual rise of the water-table brings in its train the 
 evils of an oxygen deficit in the soil, and this must frequently be 
 the real cause of the troubles that are often referred to the pres- 
 ence of alkali. 
 
11. BOG XEROPHYTES AND ACID SOILS. 
 
 The true nature of bog and swamp plants that possess apparent 
 xerophytic structures has been a subject of discussion since Volkens 
 and Zingeler first observed the protected stomata of many hydrophytic 
 species of Carex. The concept of bog xerophytes was definitized by 
 Warming (1895) and by Schimper (1898), the latter regarding them 
 as outstanding examples of physiological drought. This interpreta- 
 tion was first questioned by Clements (1905, 1907), who showed that 
 the transpiration and growth of certain so-called bog xerophytes were 
 those of hydrophytes. Since this time a number of studies have 
 dealt with this problem (Yapp, 1909; Sampson and Allen, 1909; 
 Gates, 1914; Folsom, 1918; Dosdall, 1919; Bergman, 1920; Clements 
 and Goldsmith, 1921), with the result that the number of supposed 
 bog xerophytes has steadily decreased. In the series of investiga- 
 tions under way it is hoped to make a comparative study of the water- 
 relations of the majority of bog and swamp plants that have been 
 regarded as xerophytes, and consequently to ascertain the real 
 significance of their xeromorphic characters. 
 
 THE NATURE OF BOG XEROPHYTES. 
 
 Earlier views. — The first observations on the discrepancy between 
 structure and habitat were made by Volkens (1884 : 23; cf. Zingeler, 
 1873 : 127). It was found that certain species of Carex, e. g., C. 
 glauca, gracilis, limosa, maxima, panicea, and paniculata, possessed 
 stomata with papilliform projections that extended from the accessory 
 cells over the openings, forming a chamber protected from dry air. 
 Volkens stated that from all the analogies this adaptation must serve 
 as a device against excessive transpiration, and yet it was charac- 
 teristic of species without exception that thrive only in wet soil. 
 Moreover, this protective device was found to be absent in the 
 species of dry soil. He sought to explain why sedges growing in wet 
 soil should reduce water-loss by covering the stomata by the fact 
 that the ground-water sank in midsummer, thus causing a certain 
 amount of drought in the upper layers. 
 
 Warming (1888 : 125) showed that several swamp species of Carex 
 exhibited the same structure of the leaf as that found in pronounced 
 heath-plants, such as Carex nardina and Elyna hellardi. As a conse- 
 quence of the lack of harmony with the habitat, he was inclined to 
 regard the structure of the leaves as the common heritage of the 
 group MonostachycB, which was independent of the habitat. 
 
 From a study of the stomata of certain grasses and sedges Schwen- 
 dener (1889 : 76) concluded that the structural characters which are 
 
 111 
 
112 AERATION AND AIR-CONTENT. 
 
 to be regarded as adaptations to conditions without exception among 
 the endemic species of Carex do not always correspond to the present 
 habitats in the case of the derived ones. The steppe characteristics 
 shown by some grasses and grass-Hke plants of the flora have plainly 
 arisen in response to the great climatic extremes of their original 
 home, and not out of their present habitats in Germany. He re- 
 garded it as probable that this conclusion held also for the ericoid 
 leaf-forms, waxy coating, scale-hairs, and similar adaptations which 
 reduce transpiration and in particular protect the stomatal openings 
 and are apparently lacking among endemic plants. 
 
 In contrast to the views of Warming and Schwendener, Kihlmann 
 (1890:80, 105) sought the explanation of the protective devices of bog- 
 plants in the factors of the habitat. He cited the opinion of Hartig 
 (1880), who emphasized the similarity between the effects of winter- 
 killing and those of drying-out arising from a lack of water, and stated 
 that very many cases of winter-killing are really due to the drying of 
 leaves and shoots at a time when the absorption of water from the 
 frozen soil is impossible. He also repeated and confirmed Sachs's 
 results with plants whose roots were surrounded with ice, finding that 
 they wilted quickly and completely in the sun, while control plants 
 suffered not at all. Consequently, transpiration was regarded as 
 the most important factor in retarding tree-growth in the north. It 
 is not the mechanical force of the wind itself, the cold, the salt-con- 
 tent, or the humidity that sets a limit to the forest, but chiefly the 
 uninterrupted drying-out of the young shoots at the time of the year 
 when replacement of the water transpired is impossible. 
 
 While Kihlmann recognized the significance of the dry climate 
 of polar regions, and especially of the low humidity of the air, as 
 emphasized by Warming and others, he regarded the latter as not 
 sufficiently low in summer to alone explain the phenomena. In his 
 opinion, a more potent factor was the sudden and marked lowering 
 of the temperature of soil and air through the entire growing-period 
 by a sudden fall of snow or by an icy rain, while the strong winds 
 maintained transpiration at an active rate. The relatively small 
 snowfall in winter and its unequal distribution explain why the 
 drying-out of the plant-cover over wide stretches continues as in 
 summer and to an unusual degree. Consequently, the sHghtest dif- 
 ference in level can produce a sharp difference in vegetation. The 
 moisture of the underground ice is but slightly available, as a result 
 of the very slow melting in summer, and it can not protect the plants 
 from drying-out if they are not able to absorb and use the ice-cold 
 water. As a consequence, it is readily understood why so many 
 arctic plants, and among them the most universal and widely dis- 
 tributed, show a marked adaptation to drought, and especially to 
 dry air. 
 
BOG XEROPHYTES AND ACID SOILS. 113 
 
 Transpiration is dependent not only upon insolation, air-temper- 
 ture, and relative humidity, but also upon the strength of the wind, 
 while root activity is determined chiefly by soil-temperature. The 
 open swamps and morasses of polar regions are at once the windiest 
 and possess the coldest soils of all habitats on the earth's surface. 
 The temperature of the soil remains very low for a long time after 
 snow has disappeared, owing to the gradual melting of the subter- 
 ranean ice, and even in midsummer the uppermost layers of the wet 
 soil are almost always constantly and considerably colder than in the 
 drier habitats. Even while the root-system is still partially frozen 
 at least, some species, such as Eriophorum vaginatum, begin to form 
 shoots and leaves, often to expose them for a long time to the drying 
 breath of the polar winds. In spite of excessive water-content and 
 relatively high humidity, swamp-plants are thus exposed to severe 
 drying-out, and many of them require protection against this danger. 
 
 It is a well-known fact that in the high north many true swamp- 
 plants, such as Leduvi, Betula nana, Andromeda, and Myrtillus 
 uliginosa, grow also in dry, sunny habitats, which without doubt 
 must frequently be very dry. Further south, Calluna and Empe- 
 trum exhibit a similar behavior, in that they grow in high moors and 
 at the same time in a dry, sandy soil, where the transpiration is 
 much greater. The explanation of this hes in the fact that the ever- 
 green shrubs of the swamp are annually exposed for a considerable 
 time to marked water-loss, when the ground is frozen and the snow 
 insufficient to protect them. The great majority of the plants 
 of peat-moors and swamps are therefore of a type that can with- 
 stand drying-out in the air and must often be exposed to it. In 
 some the leaves are scale-like or needle-like, stiff, and strongly cuti- 
 nized {Lycopodium, Diapensia, Andromeda hypnoides), or they tend 
 to be succulent (Saxifragra oppositifolia, Eutrema, Rhodiola). The 
 stomata are sunken or inclosed in hollows {Andromeda tetragona, 
 Empetrum) or covered with a dense layer of hairs below {Ledum, 
 Dryas octopetala, Potentilla nivea, P. multifida, Loiseleuria procum- 
 bens, Phyllodoce). In other cases, the stomatal lower surface is cov- 
 ered with a thick coating of wax {Andromeda polifolia, Vaccinium 
 mtis-idcea, Salix glauca, and S. reticulata). Among the grass-hke 
 plants are a large number of northern species that must be placed in 
 the steppe type by virtue of the rolling, hardness, and strong cutini- 
 zation of the leaves {Hierochloe alpina, Festuca ovina, Nardus, Carex 
 rupestris, C. pedata). In addition, there are many other sedges and 
 rushes of xerophytic appearance, such as Scirpus ccespitosus, Carex 
 dioeca, chordorrhiza, liinosa, parallela, paucifiora, J uncus higlumis, 
 triglumis, and filiformis, as well as Equisetum fluviatile. It can not 
 be denied that some swamp-plants have no devices for reducing 
 transpiration. These are species with soft leaves, which neverthe- 
 
114 AERATION AND AIR-CONTENT. 
 
 less do not avoid the windiest and most unfavorable habitats. The 
 most notable are Ruhus chavicemorus, Pedicularis lapponica, Nar- 
 dosmia frigida, Ranunculus pallasii, and, even more sensitive, Hip- 
 puris, Caltha, Epilobium palustre and davuricum, Cardamine pratensis, 
 Com arum, etc. Their specific property seems to be a raising of the 
 functional ability of the tissue to a maximum of resistance against 
 cold. 
 
 Schimper (1890) stated that devices which indicate inadequate 
 water relations occur among plants of many habitats, where they 
 can be explained neither through low water-content nor inheritance. 
 His researches showed that in all cases where protective devices 
 against transpiration were found in the structure of plants a need for 
 such protection actually existed, but that this might be brought about 
 by very different causes. For example, such protective devices are 
 quite common in the case of halophytes, alpine plants, and ever- 
 green woody plants of the north temperate zone. Protection against 
 transpiration is necessary for halophytes on account of the greater 
 difficulty of absorption resulting from the high salt-content, and be- 
 cause concentrated solutions of salt hinder photosynthesis, while 
 still more concentrated ones result in the death of the organs. The 
 alpine flora of Java owes its highly peculiar impress not to low tem- 
 peratures, but to protective devices against transpiration. It is 
 clear that the rarefication of the air, together with its direct influence 
 upon transpiration and the indirect influence of the stronger inso- 
 lation, is to be regarded as the most important cause of timberline 
 and of the xerophyll character of these tropic alpine formations. The 
 flora of the solfataras has a pronounced xerophytic character, and 
 there can be no doubt that here, as in the case of the mangrove, the 
 chemical nature of the substratum makes protective devices against 
 transpiration a necessary condition of life. The retarding effect of 
 a lower temperature of the soil upon the water absorption of the 
 plant makes it also conceivable why alpine plants that grow in melted 
 snow, like Ranunculus glacialis, or on glacial streams, like Saxifraga 
 aizoides, exposed to the glowing rays of the alpine sun, are thick- 
 leaved or succulent, like the inhabitants of dry habitats. Moreover, 
 the peculiarities of polar plants, which show so many analogies with 
 those of deserts, may be related to similar causes. Biologically all 
 these peculiarities are wholly intelligible, and it is only necessary to 
 advance proof that such protective devices occur in all plants which 
 permanently or periodically have to contend against a lack of water, 
 whether the cause is to be sought in dryness of the air and soil, in 
 stronger insolation and rarefication of the air, in the salt-content of 
 the soil, or in the lower temperature of the latter. 
 
 Goebel (1891 : 11) investigated the discrepancy between habitat 
 and adaptation in the vegetation of the Paramos of Venezuela. The 
 greater humidity was regarded as causing the greater luxuriance in 
 
BOG XEROPHYTES AND ACID SOILS. 
 
 115 
 
 vegetation of the Paramos in contrast with that of the Punas. This 
 is all the more striking, since the vegetation of the first has in the 
 main an evident xerophilous character. This is due not only to the 
 amount of soil-water, but also to other conditions. Sachs has shown 
 long ago that the absorption of water from the soil was related to the 
 presence of a proper temperature. Plants can wilt in a soil rich in 
 water if the absorption of the roots on account of the low temper- 
 ature is less than the water-loss. In the Paramos the cooling-off 
 of the soil is significant and the change in temperature rapid, while 
 the warming-up through the sun lasts only a short time and is of little 
 effect in the wet spots. About 11 o'clock the Paramos are usually 
 shrouded in cloud and fog, and the sun indeed is often hidden before 
 this time. The roots, as a consequence, grow in a soil almost always 
 cold, and the absorption of water is relatively small. On the other 
 hand, transpiration is increased by the strong winds and the rarefied 
 air. These factors work together to explain the peculiar fact that 
 xerophilous vegetation occurs in habitats that are rather to be called 
 wet than dry. Thus, the thick woolly Espeletia and Culcitium were 
 not rarely found in the middle of swamps. Meigen (1894) sup- 
 ported the views of Kihlmann and Goebel as to the causes of the 
 xerophytic characters of swamp plants. 
 
 Warming (1895, 1896, 1909 : 193) enumerated the following swamp 
 species which exhibit xeromorphy to the extent that they are pro- 
 tected by certain devices from desiccation : 
 
 Hairy coating: 
 
 Ledum grcenlandicum. 
 palustre. 
 
 Salix glauca. 
 lanata. 
 
 Cassandra calyculata. 
 
 Nyssa uniflora. 
 
 Persea pubescena. 
 
 Magnolia viiginiana. 
 Stomatal papilla: 
 
 Carex limosa. 
 panicea. 
 rariflora. 
 
 Lysimachia thyrsiflora. 
 
 Polygonum amphibium. 
 Waxy coating: 
 
 Vaccinium uliginosum. 
 
 Andromeda polifolia. 
 
 Vaccinium oxycoccus. 
 
 Salix grcenlandica. 
 
 Acer rubrum. 
 
 Persea pubescens. 
 
 Waxy coating — con.: 
 
 Carex panicea. 
 
 Primula farinosa. 
 Thick cuticle: 
 
 Scirpus. 
 Leathery leaves: 
 
 Andromeda polifolia. 
 
 Vaccinium oxycoccus. 
 vitis-idsea. 
 
 Ledum palustre. 
 Mucilage: 
 
 Berchemia scandena. 
 
 Pieris nitida. 
 Ericoid leaves: 
 
 Erica tetralix. 
 
 Calluna vulgaris. 
 
 Empetrum nigrum. 
 Juncoid leaves and stems: 
 
 Equisetum limosum. 
 
 Junci genuini. 
 
 Scirpus coespitosus. 
 lacustris. 
 palustria. 
 
 Juncoid leaves and stems — con. 
 
 Eriophorum vaginatum. 
 
 Carex microglochin. 
 diceca. 
 
 chordorrhiza. 
 pauciflora. 
 Erect or equitant leaves: 
 
 Iris. 
 
 Narthecium. 
 
 Acorus. 
 
 Xyris. 
 
 Alisma plantago. 
 
 Sagittaria latifolia. 
 
 Butomus. 
 
 Typha. 
 
 Sparganium. 
 
 Ranunculus lingua. 
 
 Lathyrus nissolia. 
 Closure of leaves: 
 
 Carex goodenowii. 
 
 Warming regards it as evident that there must be a causal connec- 
 tion between the soil and the xeromorphic structures, i. e., the soil 
 must be physiologically dry, and hence some of the conditions under 
 which marsh-plants live compel them to conserve water. The va- 
 rious factors thought to be concerned and often to cooperate are: 
 
116 AERATION AND AIR-CONTENT. 
 
 (1) a transpiration optimum; (2) physiological dryness in wet, cold 
 soil; (3) poor soil-aeration; (4) water-retention in peat soil; (5) 
 chemical substances; (6) free humous acids and other dissolved sub- 
 stances that chemically affect the roots and are regarded as the chief 
 cause of physiological dryness. These are thought to depress the 
 root's acitivity and consequent absorption. Warming, however, 
 also recognizes the presence of hydrophytes in moors, which are not 
 in harmony with the supposed dryness of the habitat, such as Rubus 
 chammnorus, Caltha palustris, and Viola palustris. 
 
 Stenstrom (1895 : 117) discussed in an exhaustive fashion the re- 
 lation of species to different climates and habitats, and reached the 
 conclusion that Kihlmann's explanation of bog xerophytes was not 
 true. He cited Burgerstein's results with humus extract to support 
 the view that such substances explain the xerophily of bog-plants. 
 His explanation of the latter was based chiefly upon the transpira- 
 tion relation and the fixity of inherited structures. His views upon 
 transpiration seem to be unsound, and he himself admitted the 
 paradox involved in them (p. 184), stating further that the effects 
 supposed to be due to transpiration might well be caused by poor 
 aeration. As to the origin of unplastic or stable plants, such as 
 Ledum palustre and Pirola rotundifolia, he pointed out that they were 
 probably of great age phylogenetically, and that the evergreen char- 
 acter had persisted under widely different conditions, since the period 
 of tropical cHmates in high altitudes. 
 
 Schimper (1898, 1903) broadened the concept of non-available 
 water, and emphasized the distinction between physical drought, 
 in which the soil itself is dry, and physiological drought, where the 
 soil is wet but much of the water not available to the roots. He 
 mentioned soils rich in humous acids and those with temperatures 
 at or near freezing as examples of physiological dryness, which led 
 to xerophytic vegetation in such habitats. He also stated that many 
 plants that thrive in meadow-moors were completely absent from 
 high moors, apparently kept away by the great amounts of humous 
 salts in solution. The presence of xerophytes in swamps and bogs 
 was explained by the occurrence of humous acids, which hindered 
 the absorption of water by the roots and rendered the soil dry to 
 plants, and hence well-suited to xerophytes. Nothing is said as to 
 why humous acids have this effect, but it is possible that the assump- 
 tion was based upon a mistaken statement in Pfeffer's Physiology 
 (1897, 1:231; 1900, 1:249), to the effect that "transpiration is 
 decreased by the addition of small quantities of tartaric, oxaUc, nitric, 
 or carbonic acid to the soil, whereas it is increased by alkalies, such 
 as potash, soda, or ammonia, as Sachs has shown and Burgerstein 
 has since estabhshed more in detail." 
 
BOG XEROPHYTES AND ACID SOILS. 117 
 
 Later views. — Friih and Schroter (1904 : 14) stated that the con- 
 trolling factors in low moor and high moor were, the water-retaining 
 power of the peat, the low temperature, and the lack of oxygen, 
 which hindered the respiration of the roots and consequently all their 
 functions. These conditions make the absorption of water diffi- 
 cult, producing the physiologically dry soil of Schimper. The 
 xerophytic character of many species of the low moor, as shown in 
 the terete leaves of Cyperacece and Juncacece, stomatal protection and 
 roll-leaves in Carex and Poacece, equitant leaves in Iris and Tofieldia, 
 waxy coating in Primula farinosa, small leaves in Lysimachia, 
 Epilohium, Veronica, and Centaurea, and marked cutinization in 
 Scirpus, are not entirely clear in their significance. They bear some 
 relation to the greater difficulty of absorption, but probably in spite 
 of this the higher water-balance is made possible through the ina- 
 bility of all of these plants to close their stomata. Similar xerophytic 
 characters in the high moor, such as the ericoid leaf of Calluna, 
 Empetrum, and Oxycoccus, the leathery leaf of species of Vaccinium, 
 etc., are due partly to the more difficult absorption of water and partly 
 to the evergreen nature of these plants. Many typical inhabitants 
 of high moors can grow likewise in habitats very dry physically, but 
 most of the species of low moor are strict hydrophytes and do not 
 thrive in dry soil. 
 
 Clements (1905 : 126; 1907 : 169) first questioned the conclusion 
 of Schimper that bog xerophytes are due to the presence of humic 
 acids which inhibit absorption and aeration in the roots, and that 
 bogs and swamps are consequently physiologically dry. The fact 
 that weak solutions of organic acids usually increase transpiration was 
 regarded as making it improbable that small quantities of humic acids 
 should decrease absorption sufficiently to produce xerophytes in 
 ponds and bogs. Moreover, not a trace of acid was discovered in 
 many ponds and streams where Heleocharis, Scirpus, J uncus, etc., 
 grow. Plants with a characteristic hydrophytic structure through- 
 out are regularly found alongside of apparent xerophytes, and these 
 also show a striking contrast in size and vigor of growth where they 
 grow both upon dry gravel-banks and in the water, indicating that 
 the available water is much greater in the latter. The conclusion 
 was reached that the xerophytic features found in amphibious plants 
 are due to the persistence of stable structures, which were developed 
 when these plants were growing in xerophytic situations. 
 
 Clements grew Ranunculus sceleratus (1905 : 120, 156) and Sag- 
 ittaria latifolia (1907 : 169) in various water-contents under control, 
 and found that the amphibious form in swamps was a hydrophyte, 
 as shown by the differences between it and artificially produced 
 xerophytic forms, in growth, number of stomata, and transpiration. 
 Folsom (1918 : 809) has later grown Ranunculus sceleratus under 
 control with similar results. 
 
118 AERATION AND AIR-CONTENT. 
 
 Sampson and Allen (1909 : 49) studied the transpiration of Scirpus 
 lacusiris in comparison with that of Helianthus annuus, a pronounced 
 mesophyte, and found that the former lost water almost twice as 
 rapidly. Dosdall (1919 : 35) has shown that the transpiration of 
 Equisetum fluviatile is twice that of Helianthus annuus, and Clements 
 and Goldsmith (1921) have recently found that Typha transpires 
 several times as rapidly as the sunflower. 
 
 Transeau (1905 : 17) has studied the structure of a number of 
 representative bog-plants or bog-forms, namely, Eriophorum vir- 
 ginicum, Sarracenia purpurea, Oxy coccus macrocarpus, Andromeda 
 polifolia, Chamcedaphne calyculata, Chiogenes hispidula, Vaccinium 
 corymhosum, Salix sericea, Ledum grcenlandicum, Larix laricina, 
 Picea jnariana, and Pinus strobus. In general, they are character- 
 ized by a thick cuticle, waxy coatings and hairs, thick-walled epi- 
 dermal and hypodermal tissue, the presence of palisade tissue, and of 
 resinous bodies in the roots and leaves. There is a general reduction 
 in the size of the leaves, which are often revolute, and mycorrhizal 
 fungi are present in the roots of most. They resemble the xerophytes 
 of dry sandy plains in the reduced leaves, epidermal characters, and 
 palisade tissue, but differ greatly in root development and structure. 
 All of the woody plants listed possessed mycorrhiza, with the excep- 
 tion of Andromeda, Chamcedaphne, and Salix, and it was observed 
 Hkewise in Betula lutea, B. pumila, Oxycoccus oxycoccus, and Populus 
 tremuloides. In the case of Larix, it was found that mycorrhiza 
 developed only in poorly aerated substrata and that plants formed 
 normal roots under aeration, or in a soil naturally well-aerated. It 
 was concluded that acidity had nothing to do with the production of 
 mycorrhiza, since normal roots were developed in acid water-cultures. 
 
 Yapp (1909 : 309) has shown that there is a marked difference in 
 the evaporation at different levels in a marsh community. In vege- 
 tation 5 feet high, the evaporation was 100 just above, 32.8 in the 
 middle, and 6.6 at the soil level, while in that 2 feet high it was 100 
 just above, 56.2 just below, and 14.7 at the bottom. He concludes 
 that the mutual protection from excessive transpiration and the 
 mechanical effects of the wind, derived from the grouping of shoots, is 
 probably beneficial, even apart from the more obvious cases where the 
 chmate is exceptionally rigorous. The structural features of vege- 
 tation may thus be effectual in securing immunity from excessive 
 transpiration. Different species vary as to the depth of the root- 
 system, height of shoots, relative position of transpiring surface, and 
 length of vegetative period. Few of the species in a swamp-moor live 
 under precisely the same set of physical conditions. Thus, the argu- 
 ments of authors, who insist that the so-called xerophytic structures 
 of marsh-plants are not due to present-day conditions, because both 
 xerophytes and non-xerophytes often grow side by side, are entirely 
 inconclusive. However, this last statement would doubtless have 
 
BOG XEROPHYTES AND ACID SOILS. 119 
 
 been much modified if the author had determined the transpiration 
 at the different levels, since it is almost certain that this would have 
 shown Phragmites and Cladium to be hydrophytic in both transpira- 
 tion and stomatal behavior, as Scirpus and Typha are known to be. 
 
 Coville (1910) has demonstrated that the swamp blueberry {Vac- 
 cinium corymbosum) requires a well-aerated soil for active growth 
 and can not grow readily in soil saturated with water, Sandy soil 
 and drained fibrous peat offer satisfactory conditions, as well as live 
 moist sphagnum hummocks, which furnish permanent moisture and 
 thorough aeration. The rootlets of the blueberry contain an endo- 
 trophic mycorrhiza, probably belonging to the genus Phoma. It is 
 assumed as possible that this is able to assimilate atmospheric nitro- 
 gen much more actively than Clostridium, and also that it makes the 
 non-available nitrogen of the peaty soil available to the plant, thus 
 making up for the great lack of available nitrates, due to the inability 
 of nitrifying bacteria to thrive in the acid soil. The swamp blueberry 
 grows in peaty soils that contain acids or other toxic substances and 
 suppresses the root-hairs as a protection against these. As a conse- 
 quence, both absorption and transpiration are low, and many bog 
 shrubs show devices for retarding water-loss similar to those of desert 
 plants. Low absorption leads to insufficient nutrition, and the 
 danger of nitrogen starvation is especially great, owing to the lack of 
 nitrates. In the swamp blueberry the necessary nitrogen is secured 
 by means of the mycorrhizal fungus, and conveyed into the plant 
 without a large amount of the poisonous water. 
 
 Burns (1911 : 124) concluded that the bogs around Ann Arbor 
 contain xerophytic, hydrophytic, and even mesophytic areas. The 
 presence of definite communities in each zone is due chiefly to soil 
 conditions, especially temperature, and also to the position of the 
 water-table and to aeration. Of the 7 zones described, but 3, the 
 floating sedge, bog shrub, and tamarack, have dominants that are 
 xerophytic in nature. In the sedge community only the plants that 
 root deep in the floating mat are regarded as box xerophytes, while 
 those rooting in the surface are hydrophytes. The chief mat-form- 
 ing plants are Carex filiformis and C. oligosperma. Associated with 
 these, some playing an important part in mat formation, are Meny- 
 anthes trifoliata, Dulichium arundinaceum, Eriophorum viridi-cari- 
 natum, Drosera rotundifolia, Aspidium thelypteris, Onoclea sensibilis; 
 Equisetum limosum, Eupatorium purpureum, E. perfoliatum, Mentha 
 arvensis glabrata, Scutellaria galericulata, Utricularia, Calopogofi 
 pulchellus, Campanula aparinoides, Arethusa bulbosa, Galium trifidum. 
 Aster junceus, Potentilla palustris, Solidago serotina, Lysimachia ier^ 
 restris, etc. The characteristic plants of the bog-shrub zone in- 
 clude Chamcedaphne calyculata, Andromeda polifolia, Betula pumila, 
 Nemopanthes mucronata, Sarracenia purpurea, Vaccinium oxy coccus, 
 V. macrocarpum. The principal plants of the tamarack zone are 
 
120 AERATION AND AIR-CONTENT. 
 
 Larix laricina, Cornus stolonifera, Osmunda regalis, 0. cinnamomea, 
 Rhus vernix, and Aster junceus. 
 
 Rayner (1913, 1915) has shown that when seeds of Calluna vulgaris 
 are sterilized and germinated in sterile conditions, root-growth is 
 retarded and finally inhibited in the absence of mycorrhizal infection. 
 Pot cultures in soils favorable and unfavorable to the growth of the 
 plant in the field demonstrated that Calluna grew normally in the one 
 and abnormally in the other, as indicated by poor germination, the 
 stopping of growth in root and shoot, and the presence of bacterial 
 colonies on the roots about the tips. The relation between the plant 
 and its mycorrhiza seems to be obligate, and normal growth is de- 
 pendent upon early infection and the healthy growth of the fungus. 
 The usual preference for an acid soil is explained by the fact that lime 
 prevents the normal development of the fungus and promotes the 
 growth of the colonies of bacteria, interfering with the symbiotic re- 
 lations of the root and its proper functioning. Further studies of the 
 fungus showed that it is not confined to the roots, but is found also in 
 stems, leaf, flower, and fruit. Seedlings free from infection did not 
 form roots, but underwent complete cessation of growth, remaining 
 alive but rootless for months. It is regarded as possible that the 
 presence of the fungus in stem and leaf permits it to fix atmospheric 
 nitrogen. 
 
 Gates (1914 : 472) has shown that the summer transpiration of 
 Chamcedaphne is much less than that of such hydrophytes as Sagit- 
 taria latifolia and Carex filiformis, while Andromeda transpires nearly 
 as much as Potentilla palustris and somewhat more than Aspidium 
 thelypteris. The rate of conduction in the evergreen heaths was found 
 to be much lower than that of the hydrophytes. The latter exhibited 
 the highest rate of water-loss per unit area, and in general herbs 
 transpired more rapidly than shrubs. The more hydrophytic swamp- 
 shrubs transpired more vigorously than the typical bog-shrubs, and 
 the deciduous more than the evergreen. Water-loss in the deciduous 
 Larix laricina was noticeably greater than in the deciduous broad- 
 leaved Acer rubrum, and decidedly higher than for the evergreen 
 conifer, Picea mariana. The transpiration of the evergreen shrubs 
 was several to many times greater than that of the deciduous shrubs 
 during the winter, under both outdoor and indoor conditions. The 
 author states that some so-called xerophytic plants use as much or 
 more water than ordinary mesophytic plants, but they are xero- 
 phytic because they can not absorb a large amount of water in 
 proportion to that which they would otherwise transpire. This is 
 puzzling, and is in opposition to the later statement that the deter- 
 mination of the rate of transpiration per unit area of leaf-surface by 
 weighing is a satisfactory approach to a knowledge of the demands of 
 plants for water. 
 
BOG XEROPHYTES AND ACID SOILS. 121 
 
 Otis (1914 : 478) has determined the transpiration-rate of several 
 reed-swamp dominants and subdominants, namely, Pontederia cor- 
 data, Typha latifolia, Sagittaria latifolia, Scirpus validus, and Scirpus 
 americanus, in comparison with that of the water-Hly, Castalia odo- 
 rata and a free water-surface. The highest rate was shown by Scir- 
 pus and the lowest by Sagittaria. The maximum for one day in 
 cubic centimeters per square decimeter was as follows: Sagittaria, 
 1.05; Pontederia, 1.812; Typha, 2.129; Castalia, 2.28; free water, 
 2.281; Scirpus americanus, 3.198; S. validus, 3.690. It is significant 
 of the relation of the transpiring surface to the incident rays that 
 the water-lily and free water gave the maximum on the same day, 
 the two rushes on another day, and the three species with erect or 
 ascending leaves on still another day. 
 
 Dosdall (1919 : 29) has made a comprehensive study of the water- 
 relations of Equisetum, dealing with its transpiration, growth, echard, 
 etc. It was found that Equisetuin fluviatile wilted at a water-content 
 of 25 per cent. Ranunculus sceleratus at 13 per cent, Helianthus annuus 
 at 5.6 per cent, and Equisetum hiemale at 5 per cent. Experiments 
 with both plants in the same pot showed that Equisetum fluviatile 
 wilted in from 2 to 7 days in contrast to 10 to 12 days for Helianthus, 
 and 2 to 12 days in contrast to 17 to 19 for Phaseolus. Equisetum 
 arvense wilted in 5 days, while Helianthus required 12 days. The 
 transpiration-rate of Equisetum fluviatile was 25 per cent greater 
 than that of the hydrophytic Ranunculus sceleratus, about twice as 
 great as that of Helianthus, thrice higher than that of Phaseolus, and 
 10 times as great as in the xerophytic Bryophyllum calycinum. The 
 water-loss of E. hiemale was slightly greater than that of Helianthus 
 and much greater than in Phaseolus, while in E. arvense it about 
 equaled that of the latter. All three species of Equisetum guttated 
 vigorously in the greenhouse at night, as well as when placed under 
 bell-jars, showing that the roots have marked powers of absorption. 
 The growth of Equisetum fluviatile was much better in mud or when 
 submerged than in a water-content of 35 per cent, branches failing 
 to develop well in the latter, and its water-loss was also much higher 
 in mud. The stomata of both E. fluviatile and E. hiemale were found 
 to be constantly open, as in hydrophytes generally, and they were 
 more than three times as numerous in the more hydrophytic E. 
 fluviatile. The author concludes that the latter is a true hydrophyte, 
 its xerophily being only superficial and probably due to the persis- 
 tence of ancestral characters, while Equisetum hiemale and arvense 
 are more mesophytic in their tendencies. 
 
 The comparative water-relations of conifers are hardly sufficiently 
 known as yet to make their behavior in bogs entirely clear, but it 
 seems probable that species of Picea, Abies, and Pinus readily 
 withstand the low soil-temperatures found there in consequence of a 
 
122 AERATION AND AIR-CONTENT. 
 
 xerophily originally produced by winter cold. Several investigators 
 have shown that Larix has a higher transpiration than many broad- 
 leaved deciduous trees, and it is practically certain that it is not a 
 xerophyte at all. 
 
 Stopes (1907 : 48) has stated that the histological structure of 
 gymnosperms is incapable of allowing a rapid flow of water through 
 the wood, and hence the plants must set strict limits to leaf-surface 
 and transpiration. Even though growing with leafy deciduous trees 
 in a mesophytic community well supplied with water, the conduction 
 of the latter through the woody stem is insufficient for anything but 
 xerophytic foliage. Consequently, the xerophytic characters of coni- 
 fers are regarded in very many cases as not adaptations to xerophytic 
 conditions at present, or as inherited from the remote past as ves- 
 tigial characters, but as the result of physiological limitations of the 
 type of wood. A comparative study of transpiration in conifers and 
 deciduous trees in various habitats and altitudes is now under way 
 at the Alpine Laboratory, and it is hoped that this will throw new 
 light upon the nature and causes of xerophytism in conifers. 
 
 Groom (1910 : 251) concluded that the xerophytism of conifers was 
 partly architectural in nature, as shown by the fact that the aggre- 
 gate leaf-surface of the conifer is often much greater than that of 
 the dicotyl tree, although the individual leaf is small. Despite the 
 low rate of transpiration for a single leaf or a unit of its surface, at 
 least some north-temperate conifers expend and need as much water 
 as do some dicotylous trees. The aggregate leaf-surface of cold- 
 temperate conifers is such that even their xeromorphic and xero- 
 phytic leaves do not prevent numbers of species from succumbing 
 from desiccation or growing feebly in places where ordinary dicotyl 
 trees thrive. Such conifers are architectural xerophytes in which 
 the extensive aggregate surface of the tree makes it necessary for 
 the individual leaves to be xeromorphic in form and xerophytic in 
 structure. This enables them to live in regions where there is a sea- 
 son of physiological drought in situations varying from dry dunes 
 to moist forests and from arctic and alpine situations to tropical 
 sites. The tracheidal nature of their wood is not a bar to progress 
 and the adoption of the deciduous habit, for in the larch a rapid 
 transpiration current flows through it and the leaves transpire rap- 
 idly. In spite of the author's conclusions, however, conifers seem 
 to be chiefly winter xerophytes, and the great increase of total sur- 
 face for adequate photosynthesis in summer. 
 
 According to von Hohnel, the trees transpired as follows in grams 
 of water per unit of air-dry leaf- weight from April 1 to September 31, 
 1879: Larch, 1,150; linden, 1,030; beech, 860; birch, 845; elm, 755; 
 oak, 660; maple, 520; spruce, 210; Scotch pine, 105; Austrian pine, 
 100; fir, 75. In rate of movement in the stem, Groom found rates 
 of 204, 233, 240 cm. per hour in the larch in contrast to a maximum 
 
BOG XEROPHYTES AND ACID SOILS. 
 
 123 
 
 of 232 for beech. As to leaf-surface and water-loss, von Hohnel's 
 results were as given in table 29. 
 
 Table 29. 
 
 Tree. 
 
 Aggregate surface 
 in sq. cm. 
 
 Water lost June 1 
 
 to Sept. 1. 
 grams per sq. cm. 
 
 Picea excelsa 
 
 Pinua silvestris . . . 
 Abies pectinata . . . 
 Acer platanoides . . 
 Fraxinus excelsior . 
 Carpinus betulus. . 
 
 14,256 
 6,323 
 
 12,394 
 4,435 
 4,857 
 3,848 
 
 0.13 
 0.17 
 0.18 
 0.46 
 1.45 
 1.90 
 
 CAUSES AND INTERPRETATIONS OF BOG XEROPHYTES. 
 
 The early differences of opinion as to the effective causes of appar- 
 ent xerophytic adaptations in bog-plants still persist. However, a 
 critical study of the investigations in this field makes it evident that 
 some views have become invalid, while others are incomplete. 
 While further research is needed to determine quantitatively just 
 what species are xerophytes and what are the causal relations of the 
 various factors, it seems possible to anticipate some of these con- 
 clusions. In spite of the fact that none of the earlier studies measured 
 the various factors concerned, they advanced nearly all of the inter- 
 pretations among which choice must be made today. Although 
 frankly puzzled by finding xerophytic devices in bogs, Volkens 
 thought that these might be explained by periodic drought in the 
 upper layer of the soil. In his earher work. Warming regarded the 
 dry climate of polar regions as the major fatjtor in most cases, but 
 he explained the leaf-structure of certain swamp-sedges as a con- 
 sequence of inheritance, and hence independent of the habitat. 
 Schwendener hkewise appealed to a fixed heredity in explanation 
 of the leaf-structure of exotic grasses and sedges, and thought it 
 probable that the same interpretation would apply to the various 
 protective modifications found in bog-plants. 
 
 Kihlmann regarded low soil-temperatures and strong drying winds 
 as the primary factors in producing the xeroid adaptations of bog 
 and swamp plants, but admitted that some species of the most 
 exposed situations lacked such devices, assuming in consequence that 
 their tissues must possess a specific resistance against cold. 
 
 Stenstrom rejected Kihlmann's conclusion, and turned to fixity 
 of character in evergreen shrubs especially, and to transpiration 
 relations as affording the proper solution. On the contrary, Goebel 
 advanced the same explanation as Kihlmann, finding the cold soil, 
 strong winds, and rarefied air of the Paramos sufficient reason for 
 the presence of xerophilous species. 
 
124 AERATION AND AIR-CONTENT. 
 
 The controlling cause of xerophytic modifications in marine 
 swamps and solfataras was stated by Schimper to be the salt-content, 
 and in glacial alpine soils the low temperature. Warming consid- 
 ered free humous acids as the chief factors in producing bog xero- 
 phytes, as these were thought to depress the root's activity, but he 
 included low soil-temperatures, poor aeration, water-retention by 
 the peat, chemical substances, and a transpiration optimum as 
 factors playing some part in the result. In connection with the 
 concept of physiological dryness, Schimper laid the chief emphasis 
 upon the presence of humous acids and salts and upon low tempera- 
 tures as the causes of xerophytism in bogs and swamps, though it is 
 significant that he carried out no investigations with reference to the 
 former. It has already been suggested that this was due to a typo- 
 graphic error in the case of humous acids, while the retarding effect 
 of humous salts upon transpiration was accepted upon the evidence 
 furnished by Burgerstein, which Livingston, Transeau, and others 
 have since shown to be insufficient, as indicated later. 
 
 The error made in Pfeffer's Plant Physiology with respect to Bur- 
 gerstein's results has been so generally followed that it is desirable 
 to give a brief summary of these. 
 
 Burgerstein (1876 : 191) demonstrated that 0.15 per cent nitric 
 acid increased the transpiration of corn plants about 10 per cent, 
 and 0.3 per cent, about 30 per cent in comparison with distilled water. 
 Oxalic acid in solutions of 0.25, 0.5, and 1 per cent produced increases 
 of approximately 65, 200, and per cent in the water-loss of corn, 
 while the two higher concentrations effected an increase of 60 and 
 100 per cent in the transpiration of branches of Taxus. Tartaric 
 acid in 0.25 per cent solution increased water-loss in corn 20 per 
 cent, while the 0.5 per cent decreased it about 15 per cent. As has 
 been indicated earlier, carbonic acid usually increased transpira- 
 tion. Potassium and sodium hydroxid, and ammonia in concen- 
 trations of 0.02 and 0.1 per cent regularly decreased transpiration 
 from 15 to 40 per cent. A number of salts, viz, calcium nitrate, 
 potassium nitrate, acid potassium phosphate, magnesium sulphate, 
 ammonium nitrate, ammonium sulphate, sodium chloride, and po- 
 tassium carbonate, regularly promoted transpiration in strengths of 
 0.05 to 0.25 per cent and as regularly retarded it in solutions of 0.33 
 to 1 per cent. Nutrient solutions of 0.05 to 0.26 per cent decreased 
 water-loss from about 7 to 45 per cent, while humous extracts con- 
 taining about 0.06 per cent of solids depressed transpiration to about 
 the same degree. 
 
 It would appear that Friih and Schroter regarded the absence of 
 oxygen as the primary factor in bogs and the water-retaining power 
 of peat and the low soil-temperatures as contributing factors. 
 
 Clements questioned the views of Schimper as to the importance 
 of humous acids and pointed out that weak solutions of acid in the 
 
BOG XEROPHYTES AND ACID SOILS. 125 
 
 soil should promote rather than retard transpiration. He demon- 
 strated experimentally that Sagittaria latifx)lia and Ranunculus 
 sceleratus were hydrophytes, and concluded that this was true of 
 most bog and swamp species. Their xerophytic features were re- 
 garded as stable structures that had persisted since the much earlier 
 period when the habitat itself was xerophytic. 
 
 While Whitford (1901 : 314) regarded the accumulation of humous 
 acids and insufficient aeration as the probable factors concerned, he 
 emphasized the importance of the latter, owing to its preventing the 
 healthy growth of the root-system and the presence of nitrifying 
 bacteria. These are necessary to convert organic matter into ni- 
 trates, and hence organic decay is retarded or ceases in their absence, 
 with the consequent accumulation so characteristic of peat-bogs. 
 
 Livingston (1904 : 383) assumed that if physiological dryness were 
 due to humous acids or salts, these might check absorption physically 
 by high osmotic pressure or chemically by their toxic or stimulating 
 action. The osmotic pressure of bog-water from a number of lo- 
 calities was determined, and it was found that bog-waters do not have 
 an appreciably higher concentration of dissolved substances than 
 lakes and streams of the same region. Moreover, the same bog 
 showed practically no difference in the amount of dissolved material 
 found in the driest part of the summer and in winter. In further 
 studies (1905 : 348) the conclusion was reached that some bog- waters 
 contain chemical substances that produce the palmella stage of 
 Stigeoclonium, as do solutions of poisons and those of high osmotic 
 pressure, but such substances are not directly related to the acidity. 
 The response to bog-water and cold closely resembled that produced 
 in Rumex by Transeau. The stimulating substances were most 
 abundant in communities most definitely of the bog type, and the 
 amount was roughly porportional to the extent of xerophily in the 
 vegetation. However, the results are somewhat obscured by the 
 fact that some of the plants regarded as xerophytic, e. g., Eriophorum, 
 Typha, and Larix, are actually hydrophytes or mesophytes. 
 
 Transeau (1905 : 408) has given a comprehensive account of the 
 factors in bog habitats, which is of exceptional value because of the 
 large amount of experimental evidence. His results showed that 
 bog- water itself had no tendency to produce xerophytic modifications, 
 but that low soil-temperatures and lack of aeration caused a reduction 
 in the growth of several plant organs. When these two factors acted 
 together, the effect was very marked. Experiments with Rumex 
 acetosella showed that nearly all the characteristics of bog-plants could 
 be developed by lowering the soil temperature, preventing proper 
 aeration, or by growing plants in dry sand. Thus, while an undrained 
 peat substratum may cause xerophilous structures, this is due to lack 
 of aeration rather than acidity, which may, however, be a factor in 
 the competition between different species for possession of the bog 
 
126 AERATION AND AIR-CONTENT. 
 
 areas. While the temperatures in southern Michigan are regarded 
 as inadequate to render cold a factor in xerophily, it is probably a 
 powerful factor further north, an assumption borne out by the bogs 
 of northern Minnesota in which ice persists at certain depths until 
 midsummer or later. The high water capacity of peat is a factor of 
 some importance, in that it serves to prevent proper aeration of the 
 substratum. The activity of nitrifying bacteria is almost completely 
 inhibited in natural bog-soils, owing to the acidity, deficiency in 
 oxygen, and low temperatures, and denitrification often further 
 reduces the supply of available nitrogen. 
 
 Davis (1907) ascribed the xerophytic nature of bog-plants to the 
 tenacity with which peat retains water, as a consequence of which 
 plants wilt in it at 49.7 parts of water per 100 of dry soil, and crops 
 require more than 60 per cent of water in peat to be productive. The 
 drying-out of the peat in years of minimum rainfall was also regarded 
 as a factor of importance. It was noted that Betula and Salix en- 
 dured a foot rise in the water-level for 3 years, while more sensitive 
 plants perished. 
 
 Dachnowski (1908 : 134) has grown gemmseof Marchantiain bog- 
 soil extract, bog-water untreated, bog-water aerated, bog-water 
 neutral, obtained by shaking with dry calcium carbonate and filter- 
 ing, bog-water filtered, shaken with lamp-black and filtered, distilled 
 water in which bog-plants had been grown in culture, and spring- 
 water. In the case of water from the central zone, growth in the 
 aerated, neutral, and filtered cultures was greater than in spring- 
 water, while that in the extract and untreated water was much poorer. 
 Growth in water from the maple-alder zone was best in the aerated 
 solution, practically as good in the neutral and filtered, and but little 
 less in the untreated and bog-plant water. Similar cultures of a num- 
 ber of cultivated plants gave much the same results, though the 
 aerated solution was generally less favorable than the neutral and 
 filtered ones. This does not seem strange, as shaking and filtering 
 and the difi'usion of the dry particles should have brought about 
 much more effective aeration of these two cultures. The conclusion 
 is reached that the inhibiting factors of bogs are in part injurious 
 water-soluble substances, the toxicity of which can be corrected by 
 aeration and by the use of calcium carbonate and lampblack. The 
 author further says: 
 
 "It may readily be questioned whether part of the response arises from a 
 deficiency of oxygen in the soil. The evidence obtained by Bennett is against 
 aerotropism in roots. It follows, therefore, that results reported as due to 
 lack of aeration in the bog substratum are really due to toxicity. Under 
 natural conditions, the inhibiting effect is eliminated by aeration, a slow 
 process of oxidation preventing the accumulation of injurious plant excreta 
 in the soil. However, on account of the great demand for oxygen, this process 
 can be carried on efficiently only near the surface." 
 
BOG XEROPHYTES AND ACID SOILS. 127 
 
 It seems certain, however, that the lack of oxygen is the primary- 
 cause, and that the acids secreted by the roots and the deleterious 
 substances produced by anaerobic fermentation and decomposition 
 are the consequences of it. 
 
 In later studies (1909, 1910), Dachnowski has concluded that the 
 contaminated condition of the agricultural soils used and the de- 
 creased activity of plants in them indicates that xerophily can not be 
 due to acidity, lack of oxygen, or low temperatures, and that the 
 injurious substances present in bog- water and bog-soils are at least 
 in part the cause of xerophily and of decreased fertility. It is recog- 
 nized that fertility is restored through aeration, after sufficient time 
 has elapsed for the oxidation of the injurious bodies, and, in conse- 
 quence, decrease in toxicity always follows aeration and drainage. 
 Transpiration data obtained from wheat seedlings grown in solutions 
 inoculated with pure cultures of bog bacteria showed reduction in 
 varying degree, and this was confirmed by the growth, showing that 
 the residual products of many bacteria constitute in part the toxicity 
 of the bog habitat. 
 
 As indicated in earlier sections, Hesselmann (1910) has found 
 that the water of peat-bogs and swampy forests is often completely 
 free of oxygen and rarely contains more than a small amount, while 
 it is high in streams and lakes and often approaches saturation. In 
 consequence, he finds that swamped pine forests sufi'er greatly on ac- 
 count of the lack of oxygen, while those watered excessively by spring 
 brooks are remarkably luxuriant, and concludes that oxygen is the 
 paramount factor. 
 
 Burns (1911 : 105) accepted the view of Davis that xerophytic 
 areas in peat-bogs are chiefly due to the drying of surface layers and 
 the ability of peat to render large amounts of water non-available, 
 though he also regarded low temperature and low air-content as 
 secondary factors. 
 
 From a study of the bogs of Cranberry Island, Dachnowski 
 (1911 : 147) confirmed his earlier conclusion that the limiting factor 
 was not evaporation or temperature, but the toxicity of the sub- 
 stratum. The effect of the latter appears chiefly when the water- 
 supply has become stationary, through the action of bacteria espe- 
 cially. The edaphic aridity of the central zone reduces absorption 
 by the roots at the time when transpiration and growth are making 
 greater demands, and the dwarfing of the roots diminishes it still 
 further. In a later paper (1912 : 513), the character of the obligate 
 bacterial flora and the nature of the organic compounds produced in 
 bogs are considered to explain the xerophytic conditions. The or- 
 ganic products of decomposition play the controlling role, and cause 
 the infertility of peat-deposits, even when these have abundant soil- 
 air and water-content, and favoring temperature and humidity. 
 
128 AERATION AND AIR-CONTENT. 
 
 Gates (1914 : 483) concludes that the xerophytism of the ever- 
 green ericads is real, and that it has been brought about primarily 
 by winter conditions. The transpiration is lower the more xerophytic 
 the structure of the leaves, and this is greatest in the plants most 
 exposed to winter conditions. While the xerophytic structure also 
 reduces water demands in the summer, it seems to be unnecessary 
 then, for neither extreme drought in the field nor the extreme evapora- 
 ting conditions of a laboratory injured the many plants of Chamce- 
 daphne used. On the other hand, thousands of plants were killed 
 down to the snow-line during the continued severe dry winter-cold of 
 1911-12, though the average conditions of the preceding winter did 
 not have the slightest effect. During the winter the transpiration 
 and rate of conduction were much higher in the evergreens than in 
 deciduous plants, while in the summer they were much higher in the 
 herbs and deciduous woody plants than in the evergreens. 
 
 Dachnowski (1912) has brought together all of his previous results 
 in one comprehensive account, and reaffirms his conclusions as to the 
 fundamental role of toxicity and the paramount importance of 
 bacteria in producing it. He emphasizes the inadequacy of lack of 
 aeration and low soil-temperatures as causes of xeromorphy, but 
 assigns much importance to the presence of oxygen in decreasing 
 toxicity. The behavior of the roots of bog xerophytes is said not to 
 be due to low oxygen-content, but the growth is inhibited by the re- 
 ducing action of the substratum and the incomplete disintegration 
 of organic compounds, conclusions that are not supported by the 
 facts of anaerobic respiration. The quantity of the products of 
 bacterial decomposition are thought to constitute a toxic, physio- 
 logically arid habitat at one stage and an available supply of plant 
 nutrients at another stage of the process, and hence acidity, toxicity, 
 and reducing action represent merely a stage in the decomposition of 
 organic matter. 
 
 Rigg (1913 : 325) has found that Tradescantia shows stunted 
 root-hairs when grown in bog-water, but develops normal ones in 
 lake and spring water, as well as in water from drained or partly 
 drained bogs. The stunting effect disappears when bog-water is 
 diluted with an equal volume or even one-half of its volume of tap- 
 water, and it is increased by boiling the water down to a fraction of 
 its original amount. All of this is in harmony with the fact that 
 many typical bog-plants have no root-hairs. The toxin or toxins in 
 bog-water lose their effect with drainage. It is thought possible 
 that this toxin prevents mesophytes from growing in bogs by reducing 
 the amount of absorptive root surface. It is obvious that practically 
 all the effects observed can be correlated with the absence of oxygen. 
 The author's statement that the mere absence of air from water 
 does not render it toxic must be completely revised, if toxicity is to 
 be measured by injury and death. In a later paper it is shown that 
 
BOG XEROPHYTES AND ACID SOILS. 129 
 
 the products of the decay of Nymphcea rhizomes are injurious to 
 Tradescantia cuttings and to tomato, alfalfa, and corn, even in very 
 dilute solutions, as Sherff had shown earUer. Similar results for 
 Tradescantia were obtained from the decay of potatoes and turnips, 
 and of the rhizomes of Castalia odorata and Typha latifolia. 
 
 Rigg (1916) has summarized the data collected by Cox (1910) 
 with reference to temperature and frost in cranberry bogs in Wis- 
 consin and has reached the following conclusions : The temperatures 
 in both soil and air are less favorable in the bog than on adjacent 
 hard land. In so far as a difference of temperature between air and 
 soil is concerned, conditions in the bog are m.uch less favorable than 
 on land, frost sometimes remaining in the soil as late as the first of 
 July. With respect to relative humidity and wind velocity, the con- 
 ditions were less favorable to transpiration than on the neighboring 
 land. Rigg, Trumbull, and Lincoln (1916) have studied the osmotic 
 pressure of water from sphagnum bogs of the Puget Sound region 
 and Alaska, and they confirm the conclusions of Livingston and 
 Fitting that it is not a factor in the toxicity of bog-water or of the 
 very dilute solutions arising from the decay of Ny?nphoea rhizomes. 
 
 In a related paper (1916'), it is suggested that low osmotic pres- 
 sure of bog-water indicates that the material in solution in it is 
 probably in a colloidal state. The data presented are regarded as 
 confirming this view and warranting the suggestion that this col- 
 loidal matter is a large factor in the toxicity of the water. 
 
 Rigg (1916'*) has made a comprehensive summary of the theories 
 advanced to explain bog xerophytes, and indicates that part of the 
 discrepancy in opinion is due to various definitions of bog-plants. 
 While this is true in some degree, it seems unavoidable that the 
 views of Volkens, Warming, Schwendener, Kihlmann, Goebel, Sten- 
 strom, Schimper, and others must be examined in the light of the 
 species that they regarded as bog xerophytes, which are much more 
 numerous than those recognized by Rigg. 
 
 Clements (1916 : 90) has concluded that most of the xeroid species 
 of wet places are not xerophytic at all, but that a restricted group 
 characteristic of peat-bogs and heath-moors are actual xerophytes. 
 No final decision with respect to these was regarded as possible, 
 however, until their water requirements are studied experimentally 
 and their transpiration response known. The conclusion was reached 
 that lack of oxygen is the primary factor in bogs, and the acid a con- 
 sequence or concomitant. This view was later amplified (1920: 85), 
 and the presence of acids and bog toxins was attributed to the 
 direct activity of the roots and the bog flora under conditions of 
 deficient aeration. 
 
 Bergman (1920 : 13) has observed that growth is reduced in plants 
 with roots submerged in Sphagnum as compared with peat, and 
 ascribes this to the greater lack of oxygen in the former. When 
 
130 AERATION AND AIR-CONTENT. 
 
 aeration is provided, the development of plants is essentially as 
 good in bog- water as in nutrient solution. The oxygen-content of 
 bog-water decreases and the carbon dioxid increases from the Carez 
 to the Chamcedaphne- Andromeda and the Larix-Picea stages. The 
 adjustment of the roots of bog-plants to the water-level is due to the 
 need of securing a sufficient supply of oxygen. In a later paper 
 (1921 : 50), he has found that the injury of cranberry vines as a 
 result of flooding is due to the lack of oxygen, and that injury is most 
 frequent during cloudy weather, when the oxygen-content is the 
 lowest. 
 
 ACIDITY. 
 
 While the scope of the present treatment does not permit a com- 
 prehensive account of the numerous studies of soil acidity, it seems 
 desirable to deal with the more recent investigations, because of the 
 light they throw upon acidity in bogs and upon soil toxins. Eco- 
 logical studies of acidity have been few, and the paucity of experi- 
 mental results makes it impossible to determine whether acidity is 
 a cause or merely a concomitant (Fernald, 1907; Coville, 1910, 
 1913; Sampson, 1912; Wherry, 1920). Practically all of the quan- 
 titative and experimental studies of acid soils have been made by 
 chemists, and the value of the results has been somewhat obscured 
 by the general lack of physiological and ecological knowledge. In 
 spite of their great divergence as to the causes of acidity, nearly 
 all of them are valuable in helping to determine quantitative rela- 
 tions, and some of them are of the first importance to ecological 
 investigation. 
 
 The theory that soil acidity is due to the accumulation of insoluble 
 organic compounds, the so-called humic acids, has been generally 
 accepted until the last decade or two. Sprengel (1826) isolated a 
 substance from soil that he called humic acid, and Berzelius (1838) 
 obtained this acid and the related humin from the treatment of 
 soil with an acid. Mulder (1840) recognized 7 diff'erent organic 
 substances in the soil, namely, ulmic, humic, geic, apocrenic, and 
 crenic acids, regarded as successive steps in the decay of organic 
 matter in the soil, and humin and ulmin. Eggertz (1889) threw 
 doubt upon the existence of humic acids by showing their variability 
 of composition. Van Bemmeln (1888) contended that humic acids 
 were not definite compounds and that their formulae were without 
 value. He regarded humus substances as colloidal in nature and 
 the humates as adsorption compounds. This view has received 
 the support of many chemists, and it has apparently been con- 
 firmed for the acidity of sphagnum bogs by Baumann and Gully 
 (1910 : 47), who stated that no free humic acids are to be found in 
 peat-moss. They contend that the acidity of sphagnum and peat 
 soils generally is due to the colloids of the external walls of the sphag- 
 
BOG XEROPHYTES AND ACID SOILS. 131 
 
 num cells. This view has been vigorously assailed by Tacke and 
 Siichting (1911), as well as many other chemists, who maintain that 
 acid soils exhibit phenomena to be explained only by the presence of 
 true acids. Schreiner and Shorey (1909) have discussed the results 
 obtained by workers in this field, and lend their support to the con- 
 clusion of Cameron and Bell (1907) that the existence of humic 
 acids has never been demonstrated and no satisfactory descriptions 
 have ever been given of their physical or chemical properties or of 
 their salts or characteristic derivatives. Their effect upon plants, 
 whether harmful or otherwise, is considered to be as doubtful as 
 their constitution or composition. 
 
 Blair and Macy (1908) concluded that agencies are at work pro- 
 ducing acids in practically all cultivated soils, and these tend to be- 
 come permanently acid unless bases are present in sufficient quantity. 
 The acids may be the result of decomposing organic matter, of bac- 
 terial action, or of the breaking-up of commercial fertilizers. They 
 may exert a directly injurious effect upon the plants themselves, or 
 upon beneficial bacteria, or they may bring into solution mineral 
 compounds that are injurious to the plants or the bacteria. 
 
 Abbott, Conner, and Smalley (1913) have found that peat soils 
 typically dominated by huckleberries are excessively acid and poorly 
 supplied with lime. The water extract contained very large amounts 
 of aluminum salts, highly poisonous to corn seedlings. These facts 
 seem to warrant the conclusion that the unproductivity of such acid 
 marsh soils is due to the toxic action of soluble salts of aluminum, or 
 rather to the acid conditions that permit these salts to exist. 
 
 Ruprecht (1915 : 128) has shown- that aluminum sulphate is very 
 toxic to clover seedhngs in culture solutions containing more than 40 
 parts per million and that ferrous sulphate is toxic in concentrations 
 greater than 4 parts per million. The toxic effect of both can largely 
 be remedied by calcium carbonate up to a certain point, but not by 
 calcium sulphate. It is assumed that the carbonate precipitates the 
 aluminum and the iron as hydroxids, thus removing them from solu- 
 tion and counteracting their harmful effect, the difference between 
 the two being due to the different solubility of the hydroxids. The 
 toxic action is restricted to the outer layer or two of the apical region 
 of the root, thus retarding or arresting the growth of the latter. 
 
 Truog (1916) has found considerable amounts of manganese in 
 the soil solution of certain acid soils, and thinks that this element 
 may sometimes act injuriously as a result of high absorption. 
 
 Wilcox and Kelley (1912), White (1916), and Johnson (1917) have 
 similarly shown that manganese is toxic under certain conditions. 
 When lime is added to such soils, the amount of manganese in the 
 soil solution is inconsiderable, as the alkaline condition favors its 
 precipitation. 
 
132 AERATION AND AIR-CONTENT. 
 
 Ruprecht and Morse (1917) state that the positive presence of 
 soluble salts of iron, aluminum, and manganese in soils which have 
 been repeatedly dressed with ammonium sulphate without adding 
 Hme; the formation of one or more of these salts in soils that were 
 extracted with solutions of ammonium sulphate; and the positively 
 injurious action of manganese sulphate, iron sulphate, and aluminum 
 sulphate form a chain of facts which clearly indicates that the in- 
 jurious effects of sulphate of ammonia when used freely without the 
 accompaniment of lime are due to the formation of these soluble 
 salts in the soils of the fields so dressed. 
 
 Hart well and Pember (1918 : 276) have determined that the unlike 
 response of rye and barley to acid soil is due to active aluminum. 
 Treatment of an acid soil with phosphoric oxide or acid phosphate 
 reduced the amount of active aluminum in the soil, and large addi- 
 tions of the latter caused remarkable growth in plants in which pre- 
 viously growth was impossible. This was in spite of the fact that 
 the acid phosphate greatly increased the acidity of the soil ; however, 
 it much decreased the active aluminum. It is suggested that the 
 practical advantage of phosphate and lime may be due to the pre- 
 cipitation of active aluminum, quite as much as to the value of the 
 first as a nutrient and the second as a reducer of acidity. 
 
 In similar studies of the toxicity of aluminum, Mirasol (1920 : 153) 
 has shown that its salts are directly concerned in the unproductivity 
 of acid loam soils. In soils with sufficient calcium, toxic aluminum 
 salts may never be formed, but in acid soils, where the bases are 
 deficient, such salts are largely the end-products of sulfofication and 
 nitrification. The toxicity of aluminum salts is corrected by calcium 
 carbonate or by acid phosphate through their precipitation as in- 
 soluble calcium aluminate or aluminum phosphates. 
 
 In studies of an acid soil in Assam, Meggitt (1914) has concluded 
 that the acidity is partly due to specifically toxic organic compounds, 
 arising in consequence of reduced oxidation in the soil. Such toxic 
 organic compounds are destroyed by oxidation, and this is promoted 
 by lime, nitrates, or phosphates, whether it is carried on by the roots 
 themselves or by other organisms. 
 
 Harris (1914 : 14) concludes that the behavior of the soil toward 
 neutral salts is not due to insoluble humic acids or to the presence of 
 organic matter, but to inorganic compounds, probably hydrated 
 siHcates. The reaction of so-called acid soils of the sandy-loam type 
 is due to the selective adsorption by the soil of the basic constituents 
 of the neutral salt solution, and is not caused by double decomposi- 
 tion with adsorbed acids or insoluble "humic acids." The "acidity" 
 of soils of this type probably arises from the formation of soluble 
 salts through the interaction of weak acids (C2H2O4, CO2, etc.) in 
 the soil solution and the basic material naturally held absorbed by 
 
BOG XEROPHYTES AND ACID SOILS. 133 
 
 the soil, and their subsequent removal by leaching, thus leaving the 
 soil free to absorb more basic material from any source with which 
 it may come in contact. 
 
 Truog (1914 : 505) states that he is even more adverse to accept- 
 ing the colloid-absorption theory in explanation of soil acidity than 
 the so-called humic-acid theory. While the acidity of peat and 
 muck soils is undoubtedly due in part to organic acids, there are up- 
 land soils practically free from organic matter that react strongly 
 acid. It is assumed that this inorganic acidity is due to the reaction 
 of the water solution with the siHcates, which forms a soluble hydroxid 
 or salt taken up by plants or removed by drainage, and a compara- 
 tively insoluble acid silicate that accumulates in the soil to produce 
 acidity. By means of a special method it has been found that it 
 makes little difference whether calcium, barium or sodium hydroxid 
 is used to neutralize acidity, thus indicating strongly that this is due 
 to true acids and not colloids, since the latter should demand different 
 amounts. While there is no question that colloids exist in soils, 
 some absorbing bases and others acid-ions, the amount of absorption 
 in the case of the laboratory study of pure colloids is comparatively 
 very small, and the absorption capacity of these colloids is practi- 
 cally a negligible factor in soil acidity. 
 
 In a comprehensive discussion of sour soils and liming, Frear 
 (1915 : 81) has considered the various causes of soil acidity, appar- 
 ently assigning some value to all of them, but giving the primary role 
 to humous compounds. The other causes discussed are: (1) leaching 
 of basic material by waters containing carbonic acid; (2) acid sili- 
 cates; (3) iron pyrites; (4) furnace and coke-oven fumes; (5) alumi- 
 num salts; (6) decay of plant residues; (7) fermentation in the soil; 
 (8) fertilizers; (9) physiological acidity; (10) sulphur fungicides. 
 
 Truog and Sykora (1917 : 348) conclude that chemical reactions 
 probably play as important a role as such physical phenomena as 
 adsorption and possibly have even a greater effect. Calcium car- 
 bonate, acid kaolin, and other silicates are especially important in 
 this connection. The chemical and physical constitution of most 
 agricultural soils seems to be such that the injurious action of toxins 
 present or arising in various ways is entirely or largely prevented by 
 proper tillage and the use of Hme when needed. 
 
 Conner (1918 : 328) has shown that the acidity of soils varies 
 with the water-content, the acidity being greater in soils kept at 
 half than at fourth saturation. Soils high in organic matter showed 
 the greatest acidity at full saturation. The measurable acidity of 
 acid soils varied much under different conditions of moisture and 
 aeration, owing to chemical rather than physical changes. The water- 
 content of acid soils is acid in reaction as shown by determination 
 of the hydrogen-ions. Acidity in wet soils may be caused by the 
 
134 AERATION AND AIR-CONTENT. 
 
 leaching of basic elements in drainage-water, or the removal of bases 
 by crops, by the decay of carbonaceous and nitrogenous substances, 
 and by the hydrolysis of mineral compounds and organic matter. 
 
 Hoagland and Sharp (1918 : 139) define soil acidity as that con- 
 dition of the soil in which its aqueous solution contains H-ions in 
 excess of OH-ions. The H-ion concentration of suspensions of acid 
 soils is not markedly affected by increasing the content of carbon 
 dioxid up to 10 per cent, but it is sHghtly increased in alkaline soils, 
 and a notable increase occurs in soils containing alkali carbonates. 
 No treatment with carbon dioxid produced an alkahne reaction in 
 the suspension of an acid soil. When the original conditions were 
 restored, no permanent change in the soil reaction could be ascribed 
 to the carbon dioxid. 
 
 Sharp and Hoagland (1919 : 197) have found large inversion of 
 sugar only in soils of distinctly acid reaction, the greatest inversion 
 coinciding with the highest H-ion concentration of the soil suspen- 
 sion as well as of the water and the sugar extracts. Direct evidence 
 was also obtained that acid soils do give acid filtrates, the acid re- 
 actions of which were generally of a magnitude very similar to those 
 obtained with the suspensions. 
 
 Noyes and Yoder (1918 : 151) state that organic matter is acid in 
 reaction at certain stages of decay and that apparently no work 
 absolutely proves that this acidity is other than carbonic acid weakly 
 held by the organic matter. They find that acid soil increases in 
 acidity by standing in the greenhouse with one-half its water-holding 
 capacity satisfied. Cropping brought about a slight increase in 
 acidity, while applications of carbon dioxid increased it further, 
 constant treatment giving the greatest amount. Thus, carbon 
 dioxid added to cropped soil increased its acidity, whether it was 
 treated with lime alone or lime and fertihzer. The results support 
 the chemical theories of soil acidity, since different apphcations of 
 carbon dioxid gas, which is not only soluble in water but also com- 
 bines with it, yielding hydrogen-ions, caused differences in acidity. 
 
 Plummer (1918 : 30) has found that ammonium sulphate materially 
 increases the acidity of the soil as measured by the hydrogen-ion con- 
 centration, while the effect of potassium sulphate is somewhat less. 
 Sodium nitrate reduced acidity shghtly, acid phosphate seemed 
 without effect, and lime materially increased the OH-ion concentra- 
 tion of field plots. 
 
 Rice and Osugi (1918 : 354) state that "soil acidity" is the term 
 customarily applied when infertility of the soil can be corrected by 
 the use of a free base such as lime. There are many factors involved 
 in causing this condition in soils, the presence of real acid being but 
 one of them. The methods used for determining "soil acidity" 
 generally do not measure the acid, but depend upon properties of 
 the soil-mass unrelated to acidity. The power of a soil to catalyze 
 
BOG XEROPHYTES AND ACID SOILS. 135 
 
 the reaction of cane-sugar inversion is a measure of its acid, and is 
 probably the only method that can measure the acid bound up with 
 the solid soil phase. 
 
 Walker (1920) has found that more acidity was developed in muck 
 soil under water-soaked anaerobic conditions than when the moist 
 soil was kept aerated by stirring. This agrees with Conner's re- 
 sults to the effect that a peat soil develops more acid the more water 
 it contains. In this case, the drier soils are better aerated by diffu- 
 sion than the wetter ones, and the oxidation thus made possible 
 decreases the acidity. 
 
 Wherry (1920 : 164) regards soil acidity as probably a rather com- 
 plex phenomenon, and states that it is misleading to look to a single 
 substance or type of substances as the source of hydrogen-ion pro- 
 ducing it in all cases. It seems probable that comparatively few of 
 the possible sources of hydrogen-ion, and hence of acidity, coexist 
 in appreciable amounts in any one soil. A statement is given of the 
 various sources of hydrogen-ion, as follows : 
 
 Soil constituents yielding hydrogen-ion. 
 1. Directly (when treated with water alone). 
 
 A. Inorganic: 
 
 a. Strong, highly ionized acids, Hke hydrochloric, sulfuric, etc. 
 
 b. Weak, slightly ionized acids, especially carbonic. 
 
 c. Acid salts, like potassium acid sulphate, which may be moderately or slightly 
 
 ionized (as acids). 
 
 d. Salts of weak bases with strong acids, like aluminum chloride, ammonium sulphate, 
 
 etc., which are shghtly hydrolyzed and therefore yield a small amount of 
 hydrogen-ion. 
 
 B. Organic: 
 
 a. Strong, highly ionized acids, like oxalic. 
 
 b. Weak, slightly ionized acids, like acetic. 
 
 c. Acid salts, like potassium acid sulphate, which may be moderately or slightly 
 
 ionized (as acids). 
 
 d. Salts of weak bases with strong acids, hke aluminum citrate, ammonium oxalate, 
 
 etc., which are hydrolyzed as in A d. 
 
 e. Amino acids, like aspartic (aminosuccinic) acid, which are internal salts in the 
 
 sense that the acidity is neutrahzed by the amino group, and which may be 
 moderately or slightly ionized. 
 /. Humic acids, which if they exist at all are slightly ionized. 
 
 2. Indirectly (when treated with solutions op salts). 
 
 A. Inorganic, especially colloidal clay. 
 
 B. Organic, especially colloidal humus. 
 
 Gillespie (1916) has investigated the hydrogen-ion concentration 
 of 22 soils in water suspensions and found the range of H-ions to be 
 from pH 4.4 to pH 8.6. 
 
 Sharp and Hoagland (1916 : 123) have measured the acidity of 
 24 different soils by means of the hydrogen electrode, certain of them 
 giving an H-ion concentration as high as 0.2 X 10-' and hence pos- 
 sessing a considerable intensity of acidity. The hydrogen-ion 
 concentration of different soil suspensions varied within wide hmits, 
 
136 AERATION AND AIR-CONTENT. 
 
 from a condition of high acidity to one of high alkaHnity, namely, 
 from pH 3.7 to pH 9.7. The conclusion is reached that soil acidity 
 is due to the presence of an excess of hydrogen-ions in the soil solu- 
 tion, direct evidence of which fact is given by hydrogen-electrode 
 measurements. This confirms the view that soil acidity is fun- 
 damentally dependent upon the equilibria of reactions yielding an 
 excess of H-ions, and is not necessarily related to the phenomena 
 known as ' 'absorption" and "adsorption," in conformity with the 
 opinions of Loew, Hanley, Gillespie, and Truog. 
 
 Plummer (1918 : 19) has determined the hydrogen-ion concentra- 
 tion of soil suspensions of 68 samples of soils from the southeastern 
 United States. Excessive acidity was found in the Norfolk silt 
 loam and in the mucks, reaching values of 0.1 X 10-' and 0.2X10-^ 
 respectively. Truog and Loomis (1918) have found a range of 4.5 
 to 8.0 in the hydrogen-ion exponents for a number of the common 
 cultivated acid soils of Wisconsin. 
 
 Hoagland (1917 : 547) has grown barley seedhngs in partial nu- 
 trient solutions of like osmotic pressure, but with a considerable 
 range of H-ion and OH-ion concentrations, and found that the OH- 
 ion was relatively much more toxic. When the concentration of the 
 OH-ion was greater than 1.8X 10-^the effect was distinctly injurious, 
 and above 2.5X10-^ extremely toxic. With an H-ion concentra- 
 tion of approximately 0.7X10-^ growth was favored, but one of 
 0.3 X 10-' was very toxic. These results are not in accord with those 
 of Hartwell and Pember (1907), Breazeale and LeClerc (1912), 
 Dachnowski (1914), and Miyake (1914), all of whom found the H-ion 
 to be more toxic, but these are explained as due to the fact that dilute 
 solutions of potassium or sodium hydroxid do not give the effect of 
 the OH-ion concentration on plant-growth. 
 
 In later studies (1918 : 422), plants exhibited a strong tendency 
 to change the reaction of various potassium phosphate solutions 
 toward neutral, acid, and alkaline solutions, soon reaching a point 
 about equivalent to pH 7.0, and neutral ones remaining unchanged. 
 In extensive experiments with barley and beans, nutrient solutions 
 with acid reaction always reached a reaction approximately neutral 
 after varying periods of contact with plant roots. When barley 
 plants were grown in nutrient solutions and then transferred to solu- 
 tions of KCl, K2SO4, MgS04, K3PO4, NH4CI, and NaNOs, an excessive 
 concentration of OH-ion or H-ion was nowhere found, in spite of 
 active absorption. When an acid reaction was present, it was due 
 to slightly dissociated acids, usually carbonic, to acid salts in the 
 case of NH4CI solution, and possibly in some cases to organic acids. 
 In spite of the assumption that most crop plants require a slightly 
 alkaline soil solution, a reaction of pH 5.0 was found to be in nowise 
 injurious to barley seedlings or to beans, thus supporting the view 
 of Truog that acidity itself is not the limiting factor. In California 
 
BOG XEROPHYTES AND ACID SOILS. 137 
 
 peat soils that are decidedly acid (pH 5.4 to 4.5), excellent crops of 
 barley, oats, beans, potatoes, onions, corn, asparagus, etc., were 
 produced, showing that the acidity did not interfere with growth or 
 with the formation of nitrates. 
 
 Duggar (1920: 1) has studied the growth of seedlings of wheat, 
 corn, and field peas in relation to H-ion concentration and the com- 
 position of the nutrient solution. Theoretically, the solutions pos- 
 sessed a pH exponent of about 4.5, but solution B in particular 
 varied from pH 5.4 to pH 7.1. Under the most favorable condi- 
 tions the 3 solutions all gave excellent growth, but under extreme 
 conditions, producing high evaporation, it became important to 
 correct to the higher pH exponent. The sensitivity to high H-ion 
 concentration is in the order of wheat, corn, and peas. In general 
 it seemed that there was no single ''best" solution for the 3 plants 
 used, but there is a considerable range of salt or ion proportions 
 within the "optimum" concentration. If the initial pH of the cul- 
 ture solution is considerably less than neutral, there is a general 
 tendency for this to be shifted toward the neutral point. 
 
 In a series of papers, Truog (1918, 1919) has given an illuminating 
 discussion of the relation of acidity to the growth of plants, which is 
 of such importance as to warrant quoting the summary in its entirety: 
 
 "With a few exceptions agricultural plants grow best on soils well supplied 
 with readily available lime. To be readily available, lime may exist either 
 as the carbonate, as an easily hydrolyzable silicate or salt, or as a constituent 
 of easily decomposable organic matter. The classification of agricultural 
 plants as being lime-loving, lime-avoiding and indifferent, or as being acid- 
 intolerant, acid-tolerant and indifferent, leads to confusion and gives the 
 wrong impression regarding the relation of lime and soil acidity to plant 
 growth. The subject is very complex and, as has been indicated, soil acidity 
 has many indirect and general influences on soil fertility due to its effect on 
 physical, chemical, and biological conditions of the soil. 
 
 'Tt is well known that an acid condition is unfavorable to the highest 
 development of desirable physical and biological soil conditions. An acid 
 condition usually lowers the availability of nearly all the essential elements. 
 On the other hand, soil acidity usually favors the accumulation and solubility 
 of toxic organic and inorganic substances. Among these toxic substances 
 soluble aluminum salts have been noted by a number of investigators, and 
 possibly in some cases manganese salts should also be considered. In certain 
 unusual cases of soil acidity sufficient amounts of these toxic substances 
 may be present to be very harmful to some plants. The relation of soil 
 acidity and liming to malnutrition due to a lack of iron in the plant, to plant 
 diseases and to plant competition also need to be considered in a few special 
 cases, particularly in cases where soil acidity appears to be a favorable 
 condition. 
 
 "Besides the indirect influences which affect all plants, and some probably 
 to a considerable extent, soil acidity has a specific influence which affects 
 some plants like alfalfa and sugar beets very much more than others, like 
 cowpeas, potatoes, and oats. At various times this specific influence of soil 
 acidity has been ascribed to at least three causes: viz., (a) its effect on the 
 supply of available calcium needed by plants as direct plant-food material, 
 
138 AERATION AND AIR-CONTENT. 
 
 (6) its effect on the S5niibiotic nitrogen-fixing bacteria, (c) and its toxic or 
 destructive effect on the root tissues of plants. 
 
 "The supply of available calcium in all forms becomes less as soils become 
 acid, but usually there is still sufficient present to furnish that needed as 
 direct plant-food material. Since the symbiotic nitrogen-fixing bacteria live 
 in the nodules, soil acidity can not affect them directly, except before they 
 enter into symbiosis, when it may lessen their activity and delay the time of 
 infection. Since the relation of non-legumes to soil acidity runs parallel with 
 the relation of the legumes, it follows that the direct influence is not on the 
 legume bacteria but on the plants themselves. 
 
 "That this direct effect on the plants is not often due to a destructive 
 action of the acidity on the root tissues is evident from the fact that experi- 
 ments have shown that plant roots are unaffected by solutions of a higher 
 acidity than that of most acid soil solutions. This is further substantiated 
 by the fact that the acidity (H-ion concentration) of the sap of most plants 
 is of the same order as that of the soil solution of most acid soils, indicating 
 that similar processes are probably at work in the two cases, as a result of 
 certain analogous conditions which exercise a regulatory function in this 
 respect. It is undoubtedly largely proteins in the case of plants, and colloidal 
 organic and inorganic matter, especially silicates, in the case of soils which act 
 as "buffers" and thus bring about this regulation of acidity to a considerable 
 extent, preventing rapid changes and unusually high degrees of soluble 
 acidity. 
 
 "In most cases it thus appears that the main specific harmful influence of 
 soil acidity on certain plants is not due to any of the three suggested reasons, 
 but to its influences in preventing these plants from getting at a sufficiently 
 rapid rate the calcium as the carbonate or bi-carbonate which is needed 
 to neutraUze and precipitate certain acids in the plants themselves, which 
 are probably largely by-products, produced as the result of certain vital 
 reactions in the growth of plants. If calcium in these forms is not furnished 
 at a sufficiently rapid rate, then the rate of these reactions is lowered accord- 
 ingly as is also the rate of plant growth. 
 
 "Each species of plant has a certain Hme requirement which must be 
 satisfied for maximum plant growth and this Hme requirement is defined by 
 the writer as follows: The expression 'hme requirement' of a plant refers 
 to the actual hme needs of the plant itself, especially as to the ease and rate 
 at which lime must be secured from the soil by the plant for normal growth. 
 Thus if a plant has a high hme requirement, then the solution and dehvery 
 must be rapid and easy in order to meet the needs of the plant. 
 
 "The three main factors which determine the hme requirement of a plant 
 are : (a) hme content, (b) rate of growth, and (c) feeding power of the plant 
 for lime. The first two factors operate in one direction while the third operates 
 in the opposite direction. That is, the higher the hme content and the rate 
 of growth, the higher will be the Hme requirement, and conversely. Also, 
 the higher the feeding power for Hme, the lower will be the Hme requirement, 
 and conversely. The resultant of these three gives the Hme requirement 
 of the plant. A simple method of expressing these factors and obtaining the 
 resultant is described in this article. 
 
 "A table is also given in which are expressed the Hme requirements of 62 
 species of plants as obtained by this method. These Hme requirements are 
 compared with corresponding figures which represent the relative response 
 of these plants to the Hming of acid soils, or reciprocaUy to their abihty to 
 grow on acid soils. The comparison reveals a close correspondence and hence 
 substantiates the theory which has been proposed that, usually, the main 
 specific injury of soil acidity is that it prevents plants, especially those with 
 
BOG XEROPHYTES AND ACID SOILS. 139 
 
 high lime requirements and relatively weak feeding powers, from getting the" 
 lime from the soil at a sufficiently rapid rate to meet their needs. This is 
 further substantiated by the parallel relation found between the amount of 
 growth of alfalfa on acid soils and the amount of calcium which could be 
 extracted with carbonated water from these soils. These considerations are 
 especially important in formulating a practical system of using hme, especially 
 as regards the amount to be used which, as is discussed in detail, is dependent 
 on the Ume-requirement of the crop, the degree of acidity of the soil, and the 
 fertihty of the soil." 
 
 Truog and Meacham (1919 : 469) have found the acidity of the 
 cell-sap of various plants to vary from pH 6 to pH 4 and to be regu- 
 larly greater in the case of plants grown on unlimed acid soil than in 
 those on limed acid soil. The juice from plants cut in the morning 
 was more acid than that of plants cut later in the day, indicating an 
 accumulation of acids at night, and it was also more acid during warm, 
 dry weather than after a heavy rain. For each species there is a 
 certain acidity most favorable for the life processes, and in many 
 cases soil acidity affects the acidity of the sap by limiting the supply 
 of lime available. Lime and other bases are needed to neutralize the 
 acids formed during metabolism, some of which are mere by-products. 
 If the supply of bases is inadequate, the acidity of the sap rises to 
 a point where the accumulation of acids limits the processes that 
 produce them. While such a condition of self-regulation probably 
 prevents death from over-acidity, slow growth and a weakened con- 
 dition result in the case of plants of high lime requirement when 
 grown on acid soils. 
 
 Truog (1918 : 177) points out that the acidity of the plant-sap is 
 of the same order as that of the water extracts of acid soils, and 
 hence it is not to be expected that the acidity of such soils often be- 
 comes high enough to be directly of serious injury to plant roots. 
 
 Haas (1916 : 233) has demonstrated that the cell-sap of normal 
 cells is often decidedly acid (pH 3), contrary to the accepted view 
 that the cell-sap must be neutral or nearly so for normal physiologi- 
 cal functioning. Moreover, the blue coloring of living cells does not 
 indicate an alkaline reaction, but one decidedly acid to neutral or 
 barely alkaline (pH 3 to pH 8). The reaction of the cell may change 
 from pH 3 to pH 7 as it dies. 
 
 In reviewing Hart well and Pember's work, Crocker (1919) states 
 that the hydrogen-ion concentration found in acid soils by the gas 
 method is generally but a fraction of that necessary to reduce the 
 growth of plants in water or sand cultures. 
 
 The extent to which competition is a decisive factor in the appar- 
 ent preference of many native species for acid soils is indicated by the 
 behavior of sorrel, Rumex acetosella, which has perhaps received the 
 most attention experimentally. Tacke (1910) stated that sorrel 
 did as well on plats heavily limed as upon acid soils, while Wheeler 
 (1905) reported that lime seemed to be unfavorable to the growth of 
 
140 AERATION AND AIR-CONTENT. 
 
 sorrel, owing to the fact that it brought about conditions favorable 
 to clover and other plants, which were then successful in the com- 
 petition. 
 
 White (1915), in dealing with soil rendered very acid by am- 
 monium sulphate, showed that sorrel was largely replaced by clover 
 where limestone was present in slight excess, but that it also gave 
 the highest yield with the maximum amount of hmestone. The 
 calcium-content of sorrel in acid soil was but 10 per cent of that in 
 alkaline soil. The conclusion was reached that sorrel is not an 
 acid-loving plant, but that it is able to adapt itself to conditions 
 unfavorable to most field crops. It invaded the acid plat not 
 because of preference for an acid soil, but because of the opportunity 
 for establishment afforded by the failure of the clover. 
 
 Pipal (1916) has found that hme exerts no harmful effect upon the 
 growth of sorrel. Wherever acidity or other conditions unfavorable 
 to crop plants occur, sorrel takes possession at the expense of the 
 crop, but if the unfavorable factor is corrected by means of limestone, 
 manure, drainage, etc., crop plants are enabled to compete success- 
 fully with sorrel. Frear (1915 : 109) has discussed the results of 
 White and others with respect to sorrel and various crop plants. 
 
 Wherry (1920) has studied the distribution of plants around salt 
 marshes with respect to soil acidity, and concludes that the latter 
 is a factor of considerable if not fundamental importance. It is not 
 regarded as the only factor of importance, nor is it implied that the 
 acid or alkali acts directly upon the plant. It is thought that some 
 plants may require a soil of definite acidity or alkahnity for them- 
 selves or for symbiotic organisms, while others may be favorably 
 affected by some property of the soil that accompanies acidity, and 
 still others may find competition less severe in soils of a certain 
 degree of acidity. The presence of species in soils with an acidity 
 of 300 is assumed to be a matter of preference and not merely of 
 tolerance, since the majority of them are not known to grow in 
 much lower degrees of acidity. However, it is interesting and per- 
 haps significant that at least one-third of the species listed often 
 grow in soil but weakly acid or even somewhat alkaline. 
 
 Summary. — The causes of soil acidity are still a subject of earnest 
 debate, and much more investigation is needed before a final decision 
 is possible. However, there is a strong tendency to recognize multi- 
 ple causes, as has been done by Blair and Macy, Frear, and Wherry 
 especially, though there is great difference of opinion as to which of 
 these is paramount. Of all the authors considered, Frear is the only 
 one that regards humus as a factor in acidity. The view that acidity 
 is due to adsorption is maintained by Cameron, Harris, and Bogue, 
 and this process is regarded as one of the factors by Frear and by 
 Wherry. Veitch, Daikuhara, Conner, and Rice consider the re- 
 
BOG XEROPHYTES AND ACID SOILS. 141 
 
 placement of weak bases by strong ones, with resulting hydrolysis, 
 as primarily responsible for acidity, while Hanley, Truog, Gillespie, 
 Sharp and Hoagland, and Plummer ascribe it to the presence of 
 soluble acids, and Loew invokes both factors. 
 
 The manner in which acid soils work injury to plants still demands 
 much study, but at least three methods seem highly probable. In 
 muck soils and in bogs it appears certain that the lack of oxygen and 
 the accumulation of carbon dioxid are the primary factors, while the 
 organic acids and salts resulting from anaerobiosis probably play 
 some part also. In some soils acidity brings salts of aluminum, 
 manganese, or iron into solution, and the toxic effect is then exerted 
 by these. This has been demonstrated by Blair and Macy and by 
 Ruprecht and Morse for all three elements, by Abbott, Conner and 
 Smalley, Daikuhara, Frear, Ruprecht, Miyake, Funchess, Hartwell 
 and Pember, and Mirasol for aluminum, and by Wilcox and Kelley, 
 Truog, White, and Johnson for manganese. In general, the harm- 
 ful influence of acidity is explained by Truog as due to its effect in 
 preventing certain plants from getting the calcium needed for neutral- 
 izing the organic acids produced in metabohsm at a rate sufficiently 
 rapid, thus leading to the lowering of both metabolism and growth. 
 
 The frequent assumption that acidity itself is injurious is not borne 
 out by the results of Hoagland, who found that a large number of 
 plants gave excellent crops in soils with pH 5.4 to pH 4.5. Haas 
 found the acidity of cell-sap to be often as high as pH 3, and Truog 
 and Meacham as high as pH 4. The highest H-ion concentration 
 found in acid soils by Truog and Loomis was pH 4.5 and by Gilles- 
 pie, pH 4.4, while Sharp and Hoagland obtained a maximum of pH 
 3.7, and Plummer found greater concentrations only in muck soil. 
 Hence, the statement of Truog that the acidity of soils is not often 
 high enough to directly injure plant roots seems to be entirely war- 
 ranted. Even in muck soils it is probable that acidity is a conse- 
 quence of anaerobic respiration, and thus an effect rather than a 
 cause. 
 
 CONCLUSIONS. 
 
 Great divergence of opinion has prevailed with respect to the causes 
 of the xerophilous appearance of bog and swamp plants. The great 
 majority of the views have been derived from observation rather 
 than from the measurement of habitat factors and actual experi- 
 ment. With the increase of experimental study, the task of assign- 
 ing the proper value to each factor suggested has become much 
 easier, but further research is needed to determine the mutual re- 
 lations of the primary factors. The value assigned to humic acids 
 and humates by Warming, Schimper, and Whitford has not been 
 accepted by other workers, and little importance has been given to 
 periodic drought in bogs, as suggested by Volkens, Davis, and Burns, 
 
142 AERATION AND AIR-CONTENT. 
 
 or to the water-retaining power of peat, advanced by Warming, 
 Friih and Schroter, Davis, and Burns. Low temperature has gen- 
 erally been recognized as a primary factor in boreal, polar, and alpine 
 bogs, following the conclusions of Kihlmann, Schimper, Goebel, 
 Warming, Meigen, Friih and Schroter, Transeau, Burns, and others. 
 This is confirmed from another direction by Gates's demonstration 
 that the bog evergreens are winter xerophytes. 
 
 The oxygen-content of the soil has been regarded as the control- 
 ling factor by Hesselmann, Clements, and Bergman, and as a pri- 
 mary factor by Warming, Friih and Schroter, Whitford, Transeau, 
 Coville, and Burns. The importance of toxic compounds, which is 
 an outcome of the earlier view that humic acids were a factor, was 
 first suggested by Livingston, and has been advocated chiefly by 
 Dachnowski and by Rigg. Finally, Clements has maintained that 
 most so-called bog xerophytes are not xerophytes at all, but hydro- 
 phytes or rarely mesophytes that owe their peculiar impress to the 
 stability of ancestral characters. Similar views as to the significance 
 of fixed characters were early advanced by Warming for certain 
 sedges, by Schwendener for xeroid sedges and grasses, and by Sten- 
 strom for the evergreen bog plants. 
 
 The work of Clements, Sampson and Allen, Gates, Otis, Folsom, 
 Dosdall, Bergman, and Clements and Goldsmith has shown that a 
 large number of the so-called bog xerophytes are hydrophytes. It 
 clearly indicates that the greater number of the bog xerophytes listed 
 by Kihlmann and by Warming will prove to be hydrophytes when 
 their water-relations are determined. In fact, it appears probable 
 that the term "bog xerophyte" will finally be restricted to the broad- 
 leaved evergreen ericads and similar plants, whose xerophily has been 
 shown by Gates to be due to winter rather than to the direct action of 
 bog conditions. The species of supposed bog xerophytes as listed 
 by Warming and others that have been shown to be hydrophytes are 
 the following: Sagittaria latifolia, Ranunculus sceleratus, Scirpus 
 lacustris, Carex filiformis, Pontederia cordata, Scirpus americanus, 
 Typha latifolia, T. angustifolia, and Equisetum fluviatile. The results 
 now being obtained in continuation of this study indicate that prac- 
 tically all helophytic sedges, grasses, rushes, alismals, thin-leaved 
 dicotyl herbs, and deciduous shrubs will prove to be hydroj)hytic in 
 their water relations. 
 
 The universal need of oxygen by flowering plants, the nature and 
 products of anaerobic respiration, the regular presence of air-passages 
 or aerenchyma in amphibious and floating hydrophytes, as well as 
 of stomata constantly open, all indicate that the lack of oxygen is the 
 controlling factor in swamps and bogs, and the presence of toxic 
 substances a consequence of the resulting anaerobic respiration. In 
 this, all organisms that demand oxygen, and produce alcohol, organic 
 acids, or other deleterious substances in its absence, have a share, 
 
BOG XEROPHYTES AND ACID SOILS. 143 
 
 whether they be chlorophyllous plants, molds, bacteria, protozoa, 
 etc., but it is highly probable that bacteria and molds play the major 
 role. As a consequence, there is nothing fundamentally antagonistic 
 between the view that lack of oxygen is the chief factor in bogs, and 
 the view that toxic substances play the controlling part. Both must 
 be taken into account, but the place of first importance must be 
 accorded to lack of oxygen, since it not only affects plants pro- 
 foundly itself, but also because it bears a causal relation to the ac- 
 cumulation of carbon dioxid and other toxic substances. 
 
 Much additional investigation is required to determine the re- 
 spective shares of the lack of oxygen alone, the accumulation of 
 carbon dioxid, the presence of organic acids, and of other toxic com- 
 pounds in the characteristic effect of bog-water. The results of 
 many researches have shown that a deficient oxygen supply inhibits 
 absorption, photosynthesis, respiration, tropistic response, and 
 growth, while amounts of carbon dioxid ranging from 2 to 20 per cent 
 produce in addition a toxic effect, which further inhibits function- 
 ing, growth, and reproduction. While the results of Hesselmann 
 and of Bergman in particular indicate that the low oxygen-content 
 and the corresponding abundance of carbon dioxid are sufficient to 
 explain the effect of bog-water, studies of root excretion under anaer- 
 obic conditions, and of the effect of the organic acids excreted, make 
 it clear that these are sometimes concerned at least. 
 
 Wehmer (1891, 1906) has shown that the oxalic acid produced by 
 certain fungi is very toxic to plants, and Lovinson (1900 : 217) 
 has found that solutions of formates, acetates, or propionates hin- 
 dered germination, and decreased growth and functioning, as a con- 
 sequence of their effect upon the protoplasm and nucleus of the root- 
 cells. Stiehr (1903) has studied the effect of acetic, oxalic, and citric 
 acids upon root-hairs, and finds that very dilute solutions cause the 
 death of the root-tip and even the entire root. This result was 
 brought about by a solution of 0.02 per cent in the case of each acid 
 with a maximum exposure of 3 hours. 
 
 Aso (1906) has observed that sodium or calcium acetates or for- 
 mates exert an injurious effect upon higher plants, similar to that pro- 
 duced by free acetic or formic acid. It seems certain that bogs must 
 contain other toxic organic substances, similar to those isolated by 
 Schreiner, Reed, Shorey, and others, from various soils, while the 
 work of Kaserer (1905) shows that the hydrogen and methane de- 
 rived from the anaerobic fermentation of cellulose inhibit nitrifica- 
 tion, and may affect other aerobic processes. 
 
III. TOXIC EXUDATES AND SOIL TOXINS. 
 
 Early views.— The early observations upon root excretions were 
 made at a time when physiological knowledge was still of the vaguest, 
 and are merely of historical interest. The first mention of root 
 excretion seems to have been that of Hales (1727), who assumed that 
 albumen as well as carbon dioxid was secreted by roots. Duhamel 
 (1755) noted that the earth about the roots of old elm trees was 
 darker and more greasy than usual, and concluded that this was the 
 result of root secretion. Brugmans (1786) thought to see small drops 
 exuded from the ends of roots of Viola arvensis grown in sand, and 
 assumed that this peculiar sap was injurious to neighboring plants. 
 He ascribed to Lolium temulentuvi the power of corroding the roots 
 of nearby plants, and decided that it was a specific excretion that 
 made Cirsium arvense so harmful to oats. Euphorbia peplus and 
 Scahiosa arvensis to flax, Erigeron acris to wheat, Spergula arvensis 
 to buckwheat, and Inula helenium to carrots. 
 
 Senebier (1791) and Cotta (1806) supposed that roots excreted a 
 substance which often accumulated to the point of bringing about the 
 decomposition of bulbs in the soil, and Simon (cf. linger 1836) thought 
 that the roots of hyacinth served only for purposes of excretion. 
 Plenck (1795) believed that plants excreted refuse more or less after 
 the manner of animals, as shown by the drops exuded at night through 
 the ''openings" of the roots. He regarded this ''excrement" as partly 
 useful, partly injurious to the plant itself, as well as to its neighbors. 
 
 Sprengel (1812) regarded the moisture about the roots of grasses as 
 an excretion, and thought that it aided in increasing the fertility of 
 dune-sand, a view supported by the observations of E. Meyer (1830). 
 John (1819) found that the malic acid of hyacinth bulbs was ex- 
 creted into a solution, where it converted sodium carbonate into 
 sodium malate. 
 
 DeCandolle (1832 : 248) regarded the excretions of roots as of 
 great importance in their economy, and believed that cockscomb and 
 other weeds injured adjoining plants in consequence of harmful root 
 secretions. He followed Humboldt and Plenck in assuming that 
 such secretions were the basis of the supposed attraction and repul- 
 sion of plants as expressed in plant communities. He also ascribed 
 the benefits of crop rotation to root excretions, on the assumption 
 that the excretions of one crop would be harmful to the same crop, 
 but harmless or even beneficial to a different one. He supported 
 his conclusions by the results of Macaire (1832), who thought to 
 demonstrate that the roots of grains, grasses, and other plants 
 excreted gummy substances, calcium carbonate, etc. These excre- 
 tions were assumed to free the plant from substances that could not 
 
 144 
 
TOXIC EXUDATES AND SOIL TOXINS. 145 
 
 be assimilated or might prove injurious. He observed that peas 
 grew poorly in water in which peas had previously grown, but that 
 wheat grew readily in it. Perhaps his most striking experiment was 
 one in which one portion of the root-system of Mercurialis was 
 placed in a solution of lead acetate and the other in pure water. In 
 a few days the pure water was found to contain the salt, and he con- 
 cluded that it had been carried through the roots and excreted. 
 
 Roper (1833) called Macaire's results in question on the basis of 
 the difficulty of freeing the roots from soil without injuring them. 
 Unger (1836 : 147) shared this doubt, and believed that the results 
 might also have been due to capillary action. He employed Lemna 
 minor in order to avoid injury to the roots. Plants were placed in a 
 dilute solution of sugar of lead for 8 days, washed in distilled water, 
 and then kept in the latter for 3 days. The most sensitive tests of the 
 latter failed to show any traces of lead, and consequently showed that 
 no excretion had occurred. The experiment was then reversed by 
 placing plants of Lemna for various periods in a salt of ammonium 
 and subjecting them after thorough washing to a concentrated solu- 
 tion of lead acetate. In spite of proof that the salt had been ab- 
 sorbed, no trace of it could be found as an excretion into the solution. 
 
 Daubeny (1835) found that strontium nitrate absorbed by one- 
 half of a root was not excreted by the other half, but when potassium 
 chromate or ferrous sulphate was used, a trace seemed to be secreted 
 into the distilled water. In a later investigation that has become 
 classic (1845), he grew 18 different crops continuously on the same 
 plots and compared the yields with those of crops shifted so that each 
 crop was followed by one of a different kind. There was a gradual 
 decrease in nearly every case, and this was usually greater with con- 
 tinuous cropping. However, the differences between continuous 
 cropping and rotation were insufficient to justify the assumption of a 
 soil toxin. They were attributed to the more rapid removal of the 
 needed nutrients in the plots continuously cropped, and this was 
 borne out by soil and ash analyses, leading to the distinction between 
 available and non-available nutrients in the soil. 
 
 Braconnet (1839) also thought that Macaire's results were due to 
 capillarity or to a siphon-Hke action. His attempt to obtain opium 
 from soil in which poppies had grown was unsuccessful, as was also 
 that of Walser (1838). Both investigators showed, moreover, that 
 the solid excretions found by Brugmans were nothing but the exfoli- 
 ated outer layers of the root. ' 
 
 Boussingault (1844) reached the conclusion that roots do not nor- 
 mally excrete substances, though they may do so in water cultures. 
 
 Garreau and Brauwers (1858 : 186) were of the opinion that the 
 exfoliated matter left in the soil by the growth of roots served to 
 explain the antipathy of certain plants for others. 
 
146 AERATION AND AIR-CONTENT. 
 
 Cauvet (1861 : 320) concluded that roots physiologically sound did 
 not excrete poisonous or other substances absorbed by any portion 
 of the plant. He maintained that the theories of Macaire, Chatin, 
 and Bouchardat were not well grounded, and that the theory of 
 rotation advanced by DeCandolle and supported by Macaire and 
 Liebig was based upon error. He declared that the sterility of a 
 field after cultivation was not due to the deposit in the soil of material 
 injurious to plants of the same species. Differences in the amounts 
 of nutrients absorbed were ascribed to the selective power of the roots 
 rather than to the effect of root excretions. 
 
 The Woburn researches. — These have been carried on at the Woburn 
 Experimental Fruit Farm and at Ridgmont, England, since 1897, by 
 Bedford and Pickering. The results have been communicated in the 
 first, second, third, fifth, and thirteenth reports, for 1897, 1900, 1903, 
 1905, and 1911, respectively, and in various papers. A resume of 
 the first four reports has been given by Livingston (1907 : 10). A 
 summary of the investigations for the period of 16 years is given in 
 the report for 1911, and it appears desirable to repeat it in full: 
 
 "The action of grass on fruit trees is often so deleterious that it arrests 
 all growth, and even causes the death of the tree. In none of the experiments 
 on the subject, which have now extended over sixteen years, has any recovery 
 from the effect been noticed, except in cases where the roots began to extend 
 beyond the grassed area. > But trees which become grassed over gradually 
 during the course of several years, apparently accommodate themselves to 
 the altering conditions, and suffer much less than when the grass is actually 
 sown over their roots. It is partially due to this circumstance that the effect 
 of grass in commercial orchards is often less than that observed in the exper- 
 imental plots at the farm ; whilst another reason for differences in the results 
 is that the effect undoubtedly varies in intensity in different soils, though 
 the instances where the effect appears to have been nil are very rare. The 
 fact that a tree has become well-established in the ground before the land is 
 grassed does not, however, prevent it from suffering from the grass; trees 
 at the farm were grassed over four years after they had been planted, and 
 they were so much affected that many of them were nearly killed ; and other 
 trees — standards as well as dwarfs — when similarly treated twelve years 
 after planting, are behaving in the same way, though they did not suffer so 
 severely till the third or fourth season after grassing. 
 
 "Some varieties of apples — dependent, no doubt, on their vigour of growth — 
 evidently suffer less from grass than others, but very little difference has been 
 found between the effect on standards on the free stock, and dwarfs on para- 
 dise, and no explanation of the difference in the grass-effect in different 
 soils can be traced to the depth of good soil available for root-development. 
 The baleful effect of grass is by no means confined to apples; pears, plums, 
 and cherries were found to be affected by it in the same way, and to, probably, 
 nearly the same extent; though in the case of these trees the standards 
 suffered less than the dwarfs. 
 
 "It is possible that in some soils where the effect produced is not great, 
 grass might be advantageous from a commercial point of view, for the check 
 given to the growth of the tree tends to increase its cropping, and grass affects 
 the colouring matter of all parts of the tree, generally resulting in a high 
 
TOXIC EXUDATES AND SOIL TOXINS. 147 
 
 colouring of the fruit. Such results were obtained at Ridgmont when the 
 ground was grassed up to 5 or 6 feet from the stem of the tree. 
 
 "To what distance grass should be removed from a tree so as to have no 
 effect on it, must, naturally, depend on the nature and size of the tree, as 
 well as on the nature of the soil; with freshly planted standard apple trees, 
 in soil which was not specially favourable to the action of the grass, a very 
 considerable effect was produced when the grass was 4 feet away from the 
 stems: on the other hand, keeping a space free of grass extending only 6 
 inches from the stems of freshly planted dwarf trees was found to have some 
 beneficial effect, even in the Ridgmont soil. The proportion of roots extend- 
 ing into the grassed ground which are sufficient to make the grass-effect 
 apparent, is remarkably small, amounting in some cases examined to only 
 ^tf'ijffth of the weight of the whole tree. 
 
 "Forest trees appear to be affected by grass in the same way as fruit trees 
 when the grass is sown immediately after planting; six different kinds were 
 examined, both at Ridgmont and in some light sandy soil. The only differ- 
 ence in the behavior of them and of the fruit trees was, that, in the case of 
 conifers planted in Hght soil, the effect was much less than with other trees, 
 and some recovery occurred with them as time went on, instead of the effect 
 becoming intensified. 
 
 "The action of eighteen different grasses on apple trees was examined with 
 the general result that the action in all cases was considerable, but was 
 greater with the strong-growing grasses than with the weaker ones. Clovers 
 had a similar stunting effect, but the hghtness in the colour of the leaves, 
 conspicuous with trees under grass, was absent when clover was grown. 
 
 "The question of the action of grass being explicable by its affecting the 
 aeration of the soil, by its altering the amount of carbonic acid present, or 
 by its effect on the soil-temperature, was investigated some years ago, and 
 any explanation on such grounds was found to be inadequate. The question 
 of soil-moisture and of food-supply was also investigated, with a similar 
 result, and further evidence has much strengthened these conclusions. 
 
 "As regards soil-moisture, there are general grounds for regarding a defi- 
 ciency of such moisture as affording no explanation of the effect of grass on 
 trees, for this effect is produced in wet seasons as well as in dry ones, and trees 
 which are affected show none of the usual signs of suffering from drought; in- 
 deed, when vegetation suffers from drought, it is the grass which shows the 
 effect much sooner than the deeper rooted trees. Determinations were made 
 of the water contents of grassed and tilled soil at Harpenden at intervals 
 throughout a year, and it was found that the grassed soil was shghtly the 
 wetter of the two from the beginning of January till the end of March, after 
 which it became the dryer, but the water contents never fell below the hmit 
 which has been found to be favourable for plant-growth, although in this 
 very soil, when grassed, the trees were showing all the symptoms of grass- 
 injury. In the plots at Ridgmont, where dwarf apple trees have suffered 
 so much from grass, various determinations have all shown that the grassed 
 soil during the summer is actually wetter than the neighboring tilled ground. 
 What the explanation of this anomalous state of things may be, is not known, 
 but it effectually disposes of the view that the grass-effect there is due to lack 
 of moisture. In some experiments the moisture in the soil has been increased 
 to various extents by supplying the trees every week with water through 
 pipes under their roots, and, though such trees were slightly benefited by 
 this treatment, they still continued to show the effect of the grass very strong- 
 ly, and were far less vigorous than similar trees in tilled soil, though this was 
 much dryer. Still more conclusive experiments were made by growing trees 
 i n pots, and keeping the water contents up to the same point, by watering 
 
r state Coli«g« 
 
 148 AERATION AND AIR-CONTENT. 
 
 them two or three times a week; but even when the grass-roots were prevented 
 from coming into contact with the tree-roots by a layer of wire gauze, and 
 when the water was supphed from below, so that the tree got all that it wanted 
 first, the effect of the grass on it was nearly as great as ever. 
 
 "As to the food-supply, it is difficult to see how the tree can suffer from want 
 of nourishment so long as the soil is rich and the water-supply is sufficient. 
 The trees in the grassed plots have been manured annually just like those in 
 the tilled plots, and the grass crop is not removed, but is left to rot on the 
 ground: the soil of these grassed plots may be poorer by the amount of 
 material in the one crop which is actually growing on them, but in a series 
 of years this would represent a removal of food-material far smaller than that 
 removed by the vigorousl}'- growing and cropping trees in the tilled plots: 
 indeed, it is well known that grass crops, if properly manured, actually 
 enrich the soil, and it has been found by direct experiment that, when trees 
 are grown in soil taken from the grassed plots, they flourish better than in 
 soil taken from the tilled plots. Various other experiments have been made 
 on the subject, of which it is only necessary to allude to some pot experi- 
 ments, similar to those mentioned above, in which nourishment was supphed 
 with the water, without effecting any appreciable reduction in the action 
 of the grass, though the soil was thereby rendered richer than it was in the 
 pots without grass, where the trees were growing vigorously. It is evident, 
 therefore, that the grass-effect cannot be explained by any lack of nourish- 
 ment: if the immediate cause is starvation, it is starvation in a land of 
 plenty, due to some other factor which prevents the roots from availing 
 themselves of the food which is there. 
 
 "Amongst the possible causes of the action of grass, that of a physical 
 alteration in the soil has been examined, but with negative results. The 
 grass might either by mechanical or chemical means cause an accumulation 
 of very fine soil particles at a depth corresponding with that of the tree-roots, 
 and so interfere with the functioning of these. But mechanical analyses of 
 several grassed and tilled plots of ground failed to reveal any alteration in 
 the distribution of small soil particles which would account for the effect of 
 grass. Other experiments in which the soil was made alkaline, showed that 
 the grass-effect could not be attributed to alkahnity produced by the grass 
 in its growth. 
 
 "Incidentally, the physical alteration produced in soil by rendering it 
 alkaline with potassium carbonate was investigated and found to be sur- 
 prisingly small. 
 
 "The question of soil bacteria was also partially examined. The numbers 
 of such bacteria in some grassed soil in which trees had been suffering from 
 the grass-effect was found to be considerably greater than in the neighboring 
 tilled soil; but this could not account for the grass-effect, for such an effect 
 was equally apparent in the case of trees grown in sand, where the number of 
 bacteria present was found to be much less than in tilled soil. 
 
 "In connection with this question trees have been grown in soil which had 
 been partially sterilized by heating to different temperatures, and they have 
 been found to behave in the same way as other plants. The action of heat on 
 a soil results in the destruction of the greater part of the bacteria present in 
 it and the total destruction of certain protozoa, which feed on the bacteria; 
 the result of which is that, after a certain lapse of time, the bacteria left in 
 the soil multiply without check, and the soil becomes richer in bacteria, and 
 in the nitrates formed by them, than it was originally; such soil is specially 
 favourable to plant-growth; at the same time, however, the heating results 
 in the production of some substance which is actively toxic toward plant- 
 growth, and so long as this is present, plants will not flourish in it. But the 
 
TOXIC EXUDATES AND SOIL TOXINS. 149 
 
 toxin is rapidly oxidized by the action of air and moisture, and is destroyed 
 under cultivation in a few weeks. In soil which has been heated, therefore, 
 plants will not thrive at once, especially if the supply of air is restricted, 
 though after a time they grow better in it than in soil which has not been 
 heated at all. Thus plants may behave in diametrically opposite ways in 
 heated soil, according to the conditions under which they are grown. This 
 has been found to be the case with apple trees, as well as with grasses and 
 other plants. 
 
 "The toxic substance produced by heating soils was found to be toxic 
 toward the germination of seeds as well as toward the growth of plants, 
 retarding the germination and reducing the percentage of seeds which germi- 
 nate. In extreme cases seeds may take five or six times as long to germinate 
 in heated as in unheated soil. As experiments on seed-germination can be 
 carried out in a day or two, whereas those on plant-growth require many 
 weeks, during which the character of the soil may become materially altered, 
 the former offered a promising means for searching for the presence of toxic 
 matter in grassed soil. A considerable number of instances were taken in 
 which grassed and tilled soils within a few feet of each other were examined 
 as to their behaviour toward germinating seeds, and the examination was 
 conducted at three different seasons in the year; but the results in every case 
 showed, contrary to expectation, that the soil from the grassed ground was 
 shghtly more favourable toward germination than the tilled soil. These 
 results, of course, afford no direct evidence in favoiu- of the presence of a 
 toxic substance in grassed soils, though they are quite consistent with such 
 a view, for a toxic substance, if present, might, just as in the case of heated 
 soil, give rise, on decomposition, to conditions specially favourable toward 
 germination. It was noticed also that in most cases the soil which had been 
 under grass absorbed water much less readily than the neighbouring tilled 
 soil, a behaviour which is highly suggestive, inasmuch as the same character 
 is observed in heated soils, in contrast with unheated ones. 
 
 "Strong evidence of a positive character as to the formation of a toxic 
 substance during the growth of grass was finally obtained from various 
 series of experiments with trees grown in pots. It was found that such 
 trees, when watered with the leachings obtained from trays containing grasss 
 growing in sand, flourished more than when water alone was supphed; but 
 when the trays were placed on the surface of the soil (or sand) in which the 
 trees were growing, so that the washings from the grass reached the tree- 
 roots with practically no exposure to the air, they then had a very deleterious 
 effect, nearly, if not quite, as great as when the grass was grown above the 
 roots of the trees in the ordinary way. The trays containing the grass were 
 movable, and the sand in them, with the grass growing in it, was separated 
 from the medium in which the trees were growing by the perforated iron 
 bottoms of the trays and a sheet of wire gauze ; moreover, the contact between 
 the bottoms of the trays and the sand or soil beneath would be, at the best, 
 very imperfect, so that it is impossible to explain the action of grass in such 
 a case by the abstraction by the grass of anything from the soil (or sand) 
 below the trays, and it must be due to the passage of something from the 
 trays down to the trees. The experiments on this subject were numerous, 
 and the grass-effect was uniformly shown in all of them; and, it should be 
 mentioned, the trees without grass, with which the grassed trees were com- 
 pared, were grown with trays of sand above their roots, so as to exclude 
 the possibihty of explaining the results by the mere presence of the trays. 
 
 "The ready oxidisabiUty of the toxic matter formed by grass into some 
 substance which favours plant-growth will explain the previously observed 
 beneficial effect of grass-leachings in cases where these had been exposed 
 
150 AERATION AND AIR-CONTENT. 
 
 to air, and also why soil taken from grass-grown ground should be more 
 favourable to plant-growth than that from tilled ground. All this is in full 
 accordance with what has been estabHshed as to the behaviour of heated 
 soils towards plants, where toxic matter is formed by the heating, and in- 
 creased fertihty follows its destruction, and is in accordance, also, with the 
 results obtained with the germination of seeds in soil from grassed and un- 
 grassed ground, the time elapsing between, the drawing of the samples and 
 the germination of the seeds being sufficient for the conversion of any toxic 
 substance present into a beneficial substance." 
 
 Bedford and Pickering (1914) stated that every growing crop re- 
 sults in the formation of a substance toxic to the growth of other 
 plants, and still more so to itself. By oxidation this toxin loses its 
 properties and increases the fertility of the soil. There is no reason 
 for assuming that the toxin is excreted by the plant. The root 
 debris from the growing roots is probably sufficient to account for its 
 formation, or an alteration in the bacterial contents of the soil due 
 to the growth of the grass. In heated soils a toxin is formed by the 
 action of heat alone, and the subsequent oxidation of the toxin can 
 occur without the agency of bacteria. There is no reason to suppose 
 that changes in the organic debris of a growing crop may not equally 
 occur w'ithout the action of bacteria, though in all probability they 
 may be materially aided by them. 
 
 Pickering (1917 : 181) has carried out experiments for the purpose 
 of securing direct proof of the production of toxic substances by grow- 
 ing plants. Three flow^er-pots with mustard plants were fitted with 
 trays containing 5 inches of soil and with an aperture for the plants. 
 One tray contained a crop of mustard and had a perforated bottom, 
 so that water could pass through to the pot; a second had the 
 perforations closed, so that no water could reach the plants below; 
 and the third contained soil, but no plants. The mustard plants 
 below the last two trays grew normally, but those below the first 
 were reduced to a hundredth of the normal growth. It was regarded 
 as obvious that the leachings from the plants in the trays contained a 
 substance toxic to other plant-growth. By means of this method, 
 apples, cherries, plums, pears, 6 species of forest trees, mustard, to- 
 bacco, tomatoes, barley, clover, and 2 kinds of grasses, were found 
 to be susceptible to toxins, and apple seedlings, mustard, tobacco, 
 tomatoes, 2 kinds of clover, and 16 of grasses were found to produce 
 toxic effects. In pot experiments the effect varied from a reduction 
 in growth of 6 to 97 per cent, while in field experiments with trees 
 the effect ranged from slight to fatal. The possible factors elimi- 
 nated were protection and moisture, variations of temperature, alka- 
 linity and physical condition of the soil, carbon dioxid, and bacteria, 
 but it is disappointing not to have the details of these experiments. 
 As to the source of the toxin, it is said that while excretion from 
 the roots is possible, the dejecta left by the roots in the soil may 
 account for the toxic properties just as well as exudates. 
 
TOXIC EXUDATES AND SOIL TOXINS. 151 
 
 Researches of the Bureau of Soils. — The most extensive series of in- 
 vestigations have been carried on by the Bureau of Soils of the United 
 States Department of Agriculture from 1905 to 1915. The pioneer 
 study of this series was by Livingston, Britton and Reid (1905), 
 who grew wheat seedlings in untreated Takoma soil and its aqueous 
 extract, as well as in these when modified by various substances. 
 Native and cultivated plants growing on Takoma soil exhibited 
 structures similar to those of a soil subject to drought, and wheat 
 seedlings were much stunted, even though the water-content was kept 
 constant. When grown in aqueous extracts of the soil they made the 
 same kind of growth as in the soil itself. The dwarfing effect of both 
 soil and extract was reduced by the use of stable manure, pyrogallol, 
 calcium carbonate, etc. It was regarded as very well established that 
 Takoma soil contains some substance or substances toxic to wheat 
 plants, and as also indicated that bodies are given off by the roots of 
 growing wheat plants deleterious to them or to other wheat plants 
 following them. It is suggested that the so-called exhausted soils 
 are really poisoned and that crop rotation is beneficial because it 
 prevents the accumulation in the soil of the injurious excreta of any 
 one form of plant life. In further studies, Livingston (1907) stated 
 that injurious substances similar to those existing in soils are pro- 
 duced by the growth of wheat in water or sand cultures. 
 
 Schreiner and Reed (1907) maintained that the unfavorable con- 
 ditions brought about by root excreta may affect the succeedin g crop 
 if immediately planted. When wheat succeeds wheat the effect is 
 very marked, and it is also marked when wheat follows oats, but there 
 is little or no effect when it follows cowpeas or corn. They tested 
 the effect of more than 30 different organic soil constituents on wheat 
 seedlings, and found that the majority of them caused injury in con- 
 centrations ranging from 1 to 50 parts per milKon. The toxic solu- 
 tions were markedly improved by treatments similar to those that 
 benefit the extracts of unproductive soils. When succeeding crops 
 were grown in the same soil, certain fertilizers were found to act very 
 beneficially upon soils containing the toxic excreta of plant roots. 
 Methods of cultivation that promote the aeration of the soil and the 
 growth of micro-organisms may aid in destroying soil toxins, and their 
 undue accumulation may be prevented by proper crop rotation. 
 
 Schreiner and Reed (1907^) have discussed the role of the toxic 
 excreta of roots, and suggested that they may be of importance in 
 plant succession, as well as in determining the composition of plant 
 communities, such as the characteristic "oak openings." The ex- 
 creta of growing roots are also regarded as one of the main causes of 
 the low yields obtained in improper crop rotations. While the pro- 
 duction of toxic excretions by the roots of plants is undoubtedly a 
 factor of importance in soil fertihty, they probably do not accumu- 
 late to a harmful extent in soils kept in good tilth. Proper aeration 
 
152 AERATION AND AIR-CONTENT. 
 
 will do much to destroy them, by favoring the decay of organic matter 
 through the activities of soil organisms and the processes of oxidation. 
 
 Jensen (1907 : 872) has tried the effect of tree seedlings on the 
 growth of wheat in paraffined wire pots, in which the water-content 
 was maintained by frequent watering. The results showed a de- 
 crease in the green weight of the wheat grown in the pots with the 
 tree seedlings. It is pointed out that this can not be due to shade, 
 to water-content, or to the nutrient content, and it is assumed that 
 the retarding effect is due to substances excreted by the tree roots. 
 The conclusion is reached that tree seedlings of the tulip-tree, dog- 
 wood, maple, cherry, and pine retard the growth of wheat, when the 
 roots of the latter are in close physical relation with the tree roots. 
 The retardation differs with the species and is greatest while the tree 
 seedlings are most active physiologically. The final conclusion is to 
 the effect that the injurious action of trees upon wheat appears to 
 be due to the excretion of substances by the tree roots, which are toxic 
 to the growth of wheat. 
 
 Schreiner and Shorey (1909) isolated four organic compounds from 
 soil, of which two, picoline carboxyhc acid and dihydroxystearic acid, 
 were found to be harmful to wheat seedlings, the second in all con- 
 centrations, and the first in that of 100 parts per million. The results 
 were stated to furnish simple tangible proof that injurious organic 
 compounds exist in unproductive soils and to lay the foundation for 
 the rational study and improvement of unfavorable conditions. In 
 further studies of dihydroxystearic acid, Shreiner and Skinner (1910) 
 have found that it retards the growth of wheat plants when present 
 in solution in pure distilled water at the rate of 50 parts per million. 
 It was hkewise harmful in the presence of nutrient and fertilizer salts 
 in all ratios of P2O3NH3, and K2O, but least harmful in the ratios best 
 suited to plant growth. The direct effect of the acid is to darken the 
 root-tips, stunt root development, and inhibit strongly the oxidizing 
 power of roots. The fertilizer combinations that tend to increase 
 root oxidation are also those that minimize the harmful effects. 
 Schreiner and Lathrop (1911) have examined 60 soils from 18 States, 
 and have found dihydroxystearic acid in half of the 35 soils classed as 
 poor and in but 2 of the 25 good soils. This acid is regarded as a 
 direct factor in the low productivity of soils by virtue of its harmful 
 effect on growing crops, or as an indirect factor, serving to indicate 
 other harmful compounds or conditions. 
 
 Schreiner and Skinner (1912) have isolated a number of nitroge- 
 nous constituents from the soil, and tested their effect upon wheat 
 seedlings. The majority of these, such as nucleic acid, xanthine, 
 guanine, creatine, choline, etc., exert a beneficial effect and are able 
 to replace nitrate in its effect, while others, such as picoline carboxylic 
 acid and guanidine are harmful. It is thus clear that the soil con- 
 tains both beneficial and harmful compounds, and the predominance 
 
TOXIC EXUDATES AND SOIL TOXINS. 153 
 
 of the one or the other depends upon soil conditions, composition, 
 drainage, plants, etc., all of which are affected by tilling, cultivation, 
 draining, liming, fertilizing, and rotation. 
 
 Shorey (1913) has carried further the study of organic soil con- 
 stituents, and has isolated the following compounds from widely 
 separated soils : oxalic, succinic, saccharic, and acryhc acids, lysine, ade- 
 nine, choHne, trimethylamine, salicyUc aldehyde, mannite, rhamnose, 
 trithiobenzaldehyde, nucleic acid, and an unidentified aldehyde. 
 This brings the number of compounds isolated to 35, of which 13 are 
 organic acids, 9 organic bases, 3 sugars, 2 aldehydes, and 2 alcohols. 
 
 Schreiner and Reed (1909) have confirmed the results of Molisch 
 (1888), Czapek (1896), and Raciborski (1905) as to the oxidizing 
 power of the roots of growing plants, finding this to be greatest in the 
 root-hair region. The oxidizing power is greater when plants are 
 grown in an extract of productive soil than in one of an unpro- 
 ductive soil. Oxidation was usually favored by adding an absorbing 
 agent to the extract, as well as by the addition of nitrates, phosphates, 
 and calcium salts. Toxic organic substances in solution were ex- 
 tremely injurious to the oxidizing power, which was able to reduce 
 the toxicity, however, especially in the presence of nitrates. Oxida- 
 tion by roots is due largely if not entirely to the acitivity of a peroxi- 
 dase produced by them. This enzyme is most active in neutral or 
 slightly alkaline solutions, and its action may be inhibited by acids 
 as well as by putrefaction processes. Oxidation by roots is of agri- 
 cultural interest, since the promotion of oxidation is an important 
 factor in tillage and cultivation. 
 
 Schreiner and SulHvan (1910) have studied oxidation in the soil 
 and conclude that it plays an important part in the organic and in- 
 organic changes that occur. It appears not to be enzymotic, but the 
 result of interaction between inorganic constituents and certain kinds 
 of organic matter. It may also be brought about by organic matter 
 in a state of autoxidation and by inorganic oxygen-carriers, such as 
 manganese and iron. Oxidation was increased by the addition of 
 salts of manganese, iron, aluminum, calcium, and magnesium. Some 
 kinds of organic matter, such as dihydroxystearic acid, inhibit soil 
 oxidation, but in general a plentiful supply of organic material pro- 
 motes oxidation. Excessive oxidation, however, is harmful to vege- 
 tation. The oxidative power of the soil is regarded as a sympton of 
 its condition, so that whatever decreases oxidation tends also to bring 
 about the conditions that decrease growth, while the factors that 
 favor oxidation are those that promote productivity. 
 
 Sullivan and Reid (1912) have shown that soils posse-s the power 
 to decompose hydrogen peroxid, and that this is greater in soil than 
 in subsoil and in strong than in weak soils. In general, the catalytic 
 power of soils seems to be due not to an enzyme, such as catalase, 
 but rather to the separate or joint activity of the inorganic and 
 
154 AERATION AND AIR-CONTENT. 
 
 organic matter. Strong catalytic power in a soil may be taken as 
 evidence that the many factors of soil fertility will be prominent and 
 the soil will be productive. 
 
 Skmner (1913 : 342) concluded that soil which had grown sesame 
 contained substances that were harmful to cabbage plants, but not 
 to wheat seedlings. Field observations showed that the soil in which 
 sesame had grown was injurious to cabbage, while in the same soil 
 without sesame, cabbage grew well. It is assumed that plants are 
 affected by the remains of previous vegetation or plant growth, and 
 that the effects are more or less specific, injuring one species and not 
 another. Since the plants grew much better in soil solutions shaken 
 up with carbon black, it seems possible that a lack of oxygen or an 
 abundance of COo was responsible. 
 
 Other researches. — Jones and Morse (1903) have observed an ap- 
 parent antagonism between the butternut and Potentilla fruticosa, 
 and attribute this to the root relations rather than to the shade. The 
 invasion of the soil by the vigorous roots of the butternut near the 
 surface is thought to interfere with the nutrition of the cinquefoil 
 in some manner. It seems probable that this is a combined light and 
 water relation, since "it is stated that the shrubby cinquefoil is 
 quickly killed by tree-growth of any kind," especially since it is more 
 or less hydrophytic. "If the stock is fenced out of a field the trees 
 will soon come in and the cinquefoil weaken and die out as the trees 
 overshadow it." 
 
 Hedrick (1905) observed that young peach trees shed their leaves 
 and matured quickly when oats were planted in pots with them. 
 Potatoes or tomatoes wrought less injury to the trees, mustard and 
 rape had but slight effect, and beans and crimson clover none at all. 
 The leaves turned yellow before falling, indicating drought resulting 
 from the competition. The effect of a grass sod upon apple trees 
 was later investigated (1910). The grass was cut once or twice each 
 year during the 5 years, while the tilled plot was plowed each spring, 
 cultivated 4 to 7 times until late July, and then planted to a cover- 
 crop. The average yield on the sod plot for 5 years was 72.9 barrels 
 and on the tilled plot 109.2 barrels per acre, while the average weight 
 per apple was 5 and 7 ounces, respectively. The average gain in trunk 
 diameter was 1.1 inches in sod and 2.1 inches under tillage, and the 
 average leaf weight was 9.7 gm. and 11.5 gm., respectively. The 
 average annual growth of branches for sodded trees was 1.9 inches 
 and 4.4 inches for tilled trees, while the average number of laterals per 
 branch was 3.4 for one and 6.7 for the other. These differences are 
 ascribed to water-content, the latter being highest in the tilled plot. 
 Aeration and bacterial activity are regarded as playing some part in 
 the results. 
 
TOXIC EXUDATES AND SOIL TOXINS. 155 
 
 The effects of various methods of culture upon the growth and pro- 
 duction of the apple have been investigated by Green and Ballou 
 (1906). An orchard was divided into four plots, each of which was 
 given different treatment. In the cover-crop plot, the ground was 
 plowed or disked early in the spring, cultivated until late in July, 
 and then sown to a cover-crop. The plot with clean culture was 
 treated similarly, except that cultivation continued through the 
 growing-season and no cover-crop was employed. The sod-culture 
 method consisted in planting the trees directly in sod and in culti- 
 vating a circle of 3 or 4 feet around the tree throughout the season. 
 The grass was cut several times each season and allowed to lie. In 
 the sod-mulch plots, the treatment was the same, except that the 
 circular area was mulched with straw and the cut grass used to main- 
 tain the mulch. Continuous clean culture was abandoned after four 
 seasons, owing to the washing of the soil and the removal of the vege- 
 table matter. The trees made the heaviest and most uniform growth 
 in the sod-mulch plot, in comparison with good growth in the cover- 
 crop plot, and much poorer in the sod-culture one. The average 
 diameter of the trees in the three plots was 10.56, 9.71, and 8.55 
 inches, respectively. The greater growth under sod-mulch was 
 ascribed to the greater supply of food-material under the mulch. 
 
 Dandeno (1909 : 24) assumed from field observations that grain 
 grew better when associated with Canada thistle, and tested this 
 experimentally by planting oats, barley, wheat, buckwheat, and flax 
 separately in pots, as well as with a vigorous underground shoot of 
 Canada thistle in a second series of pots. In another series young 
 elm trees were planted singly in 6-inch pots, and oats grown with 
 them. With the exception of buckwheat, all the plants grew as well 
 or better with the Canada thistle as alone, the stimulating effect 
 being most pronounced at 22 days after planting. On the contrary, 
 the elm tree had an injurious effect, as all species grew more poorly 
 with it. It was suggested that the results were due on the one hand 
 to the excretion of substances that stimulate growth or release plant 
 food, and on the other to the excretion of harmful substances. 
 
 Howard (1910, 1915) has found that grass has a marked effect 
 upon most species of fruit trees at Pusa. The leaves are few, very 
 small, and pale yellow; the leaves and flowers appear much later than 
 normally, and the leaves fall early. Very little new wood is formed 
 and the growth in height is much less. The fruit from trees under 
 grass is smaller, less abundant, as well as less juicy and of poorer 
 flavor. The effect of grass is greater with small trees than with large 
 ones, and Cynodon dactylon is more injurious than Imperata arun- 
 dinacea. This difference in the two grasses seems to be explained by 
 the fact that Cynodon requires more air, and correspondingly reduces 
 the supply to the tree roots. Moreover, the Pusa soil packs to such 
 
156 AERATION AND AIR-CONTENT. 
 
 an extent that the accumulation of carbon dioxid is favored, and it is 
 suggested that this may be the toxin concerned. 
 
 Russell (1912 : 111) grew six crops of rye in succession in sand to 
 which._only nutrie nt sa lts werej^dded to keep the food material qpn- 
 ^tant- A seventh crop was then grown at the same time as one on 
 perfectly fresh sand on which no crop had ever grown, though it was 
 also supphed with an equal amount of the same nutrients. Similar 
 experiments were made with buckwheat and spinach, and a parallel 
 series was carried out in soil cultures. There was no significn,nt dif- 
 ference in the two crop yields, except in the case of buckwheat ig^sand, 
 an exceptional result that could not be confirmed. If the rye, buck- 
 wheat, or spinach excreted any toxin, the amount accumulated dur- 
 ing the growth ofjhe six successive^rops was insufficient to depress 
 th^ yield, oTIhfijiex£crop appreciably. ThuSj, no lasting toxic effect 
 at least was produced by any of these crops, and the toxin hypothesis 
 fails to explain the advantages of rotation where there is always a 
 lengthy interval between crops. It is concluded that there is no 
 evidence of soluble toxins in normally aerated soils sufficiently sup- 
 plied with plant-food and with calcium carbonate, but toxins may 
 occur on sour soils badly aerated and lacking in calcium carbonate, or 
 in other exhausted soil. There is no evidence of any plant excre- 
 tions conferring toxic properties on the soil, but the Woburn results 
 show that a growing plant may affect its neighbor. 
 
 Sherff (1912 : 428) observed that Sagittaria was able to invade 
 ponds of NymphoBa from the reed-swamp only when Nymphoea was 
 nearly or quite absent. The rhizomes of the latter are usually de- 
 cayed well toward the growing apex, and generally when the rhizomes 
 of Sagittaria penetrate the decayed parts, they also begin to decay. 
 Where the decayed Nymphcea rhizomes lay nearer the surface, 
 Sagittaria had grown underneath without harm. 
 
 Hall, Brenchley and Underwood (1913, 1914) have studied the 
 growth of plants in soil solutions with especial reference to the theory 
 of Whitney and Cameron, and have obtained the following results: 
 
 "We may now consider how far these results bear on the theory that crops 
 leave behind in the soil specific toxins which depress the growth of succeed- 
 ing crops of the same kind. In Series I, wheat and barley yielded almost 
 exactly the same weight of plant, whether they grow in solutions from the 
 wheat or the barley soils. As a rule the wheat plants were a little heavier 
 when grown in the solutions from the barley soils than when grown in solu- 
 tions from the corresponding wheat soils, but the barley plants were similarly 
 heavier in the solutions from the barley soils. The ratio of root to shoot is 
 very close in the two sets. Again, wheat and barley grown in the same 
 solution yield weights agreeing witliin the range of error of such experiments. 
 These facts alone would dismiss the hypothesis that the wheat soils contain 
 any soluble toxin injurious to wheat but not to barley, and vice versa, not- 
 withstanding the 60 years' repeated growth of these crops on the same soils. 
 In Series II the demonstration was pushed a stage further by including in 
 the comparison an artificial culture solution made from pure salts and con- 
 
TOXIC EXUDATES AND SOIL TOXINS. 157 
 
 taining phosphoric acid and potash in the same proportions as the solutions 
 from the completely manured plots. Another set of the soil solutions was 
 boiled before use, since boiling had been reputed to destroy the toxin and 
 would at any rate kill off any bacteria that might be factors in the result. 
 Lastly in another set the solutions were evaporated, the residue ignited and 
 dissolved afresh in a minimum quantity of hydrochloric acid, then diluted 
 to the original volume. 
 
 "In this series boiUng was without effect, whether the solutions contained 
 added nutrients or not; the residue left on evaporation, after ignition and 
 re-solution, gave generally lower results, in some cases to a marked degree. 
 The soil solutions from completely manured plots gave higher yields than the 
 artificial solutions of corresponding strength. In order to ascertain whether 
 the results were limited in any way by the nature of the plant (it might be 
 objected as regards Series I that barley and wheat are so closely akin as to 
 excrete the same toxin) the experiments in Series II were repeated with 
 sunflowers, white lupins, and buckwheat. 
 
 "These plants are far from being so suitable for experiment as barley, 
 and the results are somewhat erratic (e. g., white lupins gave almost their 
 maximum yield in the solution from the unmanured plot, indicating that 
 growth had been mainly sustained on the original food-store in the seed), 
 but they in no way indicate the presence of a toxin in the soil solutions which 
 depresses the growth of barley, but ex hypothesi is without effect on plants 
 of another order. Finally in Series III, both barley and peas grew as freely 
 in the soil solutions from the completely manured plots and in the solutions 
 from the incompletely manured plots after repair of the deficiency by adding 
 salts, as in the artificial solutions made up with pure salts. Indeed the 
 superiority, though hardly large enough to be significant, lay with the plants 
 grown in the soil solutions. Thus the experiment yielded no evidence of the 
 existence in soils on which a particular plant had been growing for 60 years 
 and upwards, of a soluble 'toxin' having a depressing effect upon the growth of 
 that plant." 
 
 Lyon and Bizzell (1913 : 38) have conducted a series of experi- 
 ments on the stimulating influence of plants on each other. These 
 were made by planting primary plants or crops, followed by secon- 
 dary ones at a somewhat later time. In the case of greenhouse soil 
 and of nutrient solutions in crushed quartz, the yield of the primary 
 crop in mixtures was greater than when it was grow^n alone, in just 
 the same number of cases as it was less. When the primary crops 
 and mixtures were grown on field soil, the yield of the primary crop 
 and the mixture w^as greater in 11 cases and less in 4. Moreover, it 
 was found that the so-called stimulus is stronger during the early 
 part than during the later part of the life of the plant. This is indi- 
 cated by the field experiment, in which nearly all of the primary crop 
 harvested at bloom gave a larger yield in combination than alone, 
 while similar mixtures allowed to mature, gave opposite results. 
 
 Bottomley (1914 : 531) has found that "bacterized" peat acts as 
 a stimulant to growth, and that phosphototungstic and silver frac- 
 tions derived from it show the effect of the accessory food factors of 
 Hopkins and to some extent of the vitamines of Funk. He concludes 
 that the nutrition of a plant may depend upon the presence of these 
 
158 AERATION AND AIR-CONTENT. 
 
 accessory food substances, as well as upon mineral nutrients, and 
 thinks that the very small amounts necessary are at first supplied 
 by the seed and later by the humus of the soil. Bacterized peat 
 results from the action of certain aerobic soil organisms at 26° C. 
 which decompose it and convert a large amount of the humic acid 
 present to soluble ammonium humate. 
 
 Bergen (1915 : 491) observed that an exceptional rainfall in July 
 led to much greater growth in perennial mesophytes growing along- 
 side of a belt of deciduous trees. The stems of Aster novce-anglice, 
 Asclepias tuber osa, and Helianthus grosse-serratus were about twice 
 as tall and much more robust than during the ordinary season. A 
 suppressed plant of Chelone glabra grew luxuriantly and flowered 
 freely. The dwarfing in ordinary seasons was ascribed to the lower 
 water-content, due to the demands made by the trees. 
 
 Amos (1918), in a study of the causes of clover sickness, finds scant * 
 evidence that it is due to the excretion of toxic substances by the pre- 
 ceding clover crop. 
 
 Hart well, Pember and Merkle (1919) have conducted experiments 
 on the effect of one crop upon another, in which five different crop 
 plants were grown for 2 to 3 years in the same soil, and then fol- 
 lowed by a particular crop plant. When onions were grown after 
 each of the individual crops, the yield was least with buckwheat and 
 mangels, larger with rye and onions, and best with redtop. When 
 buckwheat was the succeeding crop its yield increased after crops in 
 about the following order: redtop, buckwheat, mangels, rye, and 
 onions. The divergent effect of crops on those that follow seems not to 
 be due, principally at least, to the amount of nutrients removed, since 
 the smallest yield may not occur after the crop removing the largest 
 amount of the nutrient most needed. Soil acidity was affected 
 differently by the various crops, and generally the best yield of 
 onions, which are sensitive to the conditions accompanying acidity, 
 followed the crops giving rise to the least acidity. This relation was 
 supported by the fact that the effects of the crops on the following one 
 were much less divergent when acidity was reduced by liming. 
 
 CONCLUSIONS. 
 
 The original assumption of Livingston, Schreiner, and their asso- 
 ciates, and of Bedford and Pickering as well, to the effect that plant 
 roots excrete substances toxic to the plants themselves and to other 
 plants, seems no longer to be accepted even by them. While 
 Livingston (1918 : 93) regards the general hypothesis that unpro- 
 ductiveness in agricultural soils is frequently due to soil toxins as well 
 estab fished and generally accepted, he states that: 
 
 "The evidence that crop plants do actually excrete toxic substances into 
 the soil is not veiy strong in any of this work. Better than to assert that they 
 are so excreted is to state that there is evidence that the soil frequently 
 
TOXIC EXUDATES AND SOIL TOXINS. 159 
 
 contains toxins, and that these sometimes result, directly or indirectly, from 
 the growth of higher plants. As to the manner in which these poison sub- 
 stances arise in the soil, no definite statement can yet be made, but they are 
 surely not generally excreted as such from the plant roots. That such poisons 
 are present in many soils has now been extablished without question by 
 Schreiner and his coworkers, and also that their deleterious effects may often 
 be removed by oxidation, or by the addition of proper substances." 
 
 Pickering (1917) says: "But though their excretion from the roots 
 is possible, there is no need for imagining such an occurrence; all 
 plants in growing leave much root-detritus in the soil, and such de- 
 jecta may account for toxic properties just as well as ejecta." 
 
 However, if he is right in ruling out deficient aeration and carbon 
 dioxid as causes, then both of these statements appear incorrect, 
 since it has been shown over and over again that under normal con- 
 ditions roots excrete no other toxic substances than carbon dioxid, 
 and the aerobic fermentation of plant material rarely produces toxins. 
 There seems to be no doubt that roots do not excrete other toxins 
 than carbon dioxid, except under anaerobic conditions, and the re- 
 sults drawn from cultures in solutions are either to be explained by 
 deficient aeration or by the limitations of the method itself, as indi- 
 cated by Stiles (1915), Hoagland (1919), Jordan (1920), and Davis 
 (1921). 
 
 The statement of Bedford and Pickering (1914) that every growing 
 crop results in the formation of a substance toxic to the growth of 
 other plants, and still more so to itself, would seem to require that 
 the fruit trees of an orchard or the trees of a grove or forest should 
 be more toxic to each other than to grass or grass to them. However 
 this may be, the results of other investigators warrant Howard's 
 suggestion (1915 : 23) that carbon dioxid should not be finally dis- 
 missed as the toxin concerned, without repeating the Woburn experi- 
 ments dealing with this gas. He finds that the results obtained by 
 Bedford and Pickering with tobacco are exceedingly like those 
 observed at Pusa when tobacco is water-logged or grown on heavy 
 lands that have been green manured. Since tobacco requires a 
 great deal of air and green manuring produces much carbon dioxid 
 in the soil, it seems probable that the Woburn results, in which grass 
 washings injured tobacco, may be due after all to the inhibiting effect 
 of carbon dioxid. This may be the toxin about wliich so much is 
 written, and it may prove to be the cause of the effect cf grass on 
 trees, as well as of one crop on another. In many parts of England 
 grass is grown under fruit trees without particular damage, but in 
 most of these the soil is very porous, and the carbon dioxid diffuses 
 without doing harm. The soils at Woburn and Pusa are not porous, 
 and in such dense soils the effect of carbon dioxid should be far greater 
 than in the porous soils of Kent. 
 
 Hole (1918 : 439) has also pointed out the probable significance of 
 defective aeration for the problem at Woburn. A dense growth of 
 
160 AERATION AND AIR-CONTENT. 
 
 grass is correlated with an accumulation of dead roots, leaves, and 
 other debris in the surface soil, which promotes the activity of soil 
 organisms. Rain-water percolating through such a layer of grass 
 would tend to lose its oxygen and to become heavily charged with 
 CO2. Pickering notes that when such ''toxic matter is exposed to 
 the air for 24 hours its toxic property is found to have entirely 
 disappeared." Exposure to the air would tend to make good a 
 deficiency of oxygen and to dissipate an accumulation of CO2 by 
 diffusion. Such an accumulation of carbon dioxid about plant roots 
 has been demonstrated by Leather for a number of plants. 
 HKing (1908 : 626) criticized at some length the assumption that 
 crops excrete and leave in the soil toxins which are the chief cause of 
 reduced yields and worn-out lands, and that rotation, manuring, and 
 fertihzing owe their good effects to destroying or removing the toxins 
 rather than to their ability to supply nutrients. The amount of 
 nutrients carried by the soil was discussed, and the conclusion reached 
 that it is impossible that a mere rotation of crops, coupled with good 
 tillage and adequate water-content, should indefinitely maintain 
 high yields, when the whole crop above ground is regularly and con- 
 tinually removed from the field. Figures were given to show the ex- 
 treme variation of the soil solution, and these indicated that there is 
 no good foundation for the contention that all soil solutions have 
 essentially the same composition and concentration, viewed from the 
 standpoint of their function in plant-growth. Evidence was also 
 given to show that there was a regular and corresponding increase 
 in the yield with each increase in the amount of nitrogen, phos- 
 phorus, and potassium recoverable from the soil, and Rothamsted 
 results were cited to prove that the soluble salt-content of soils is not 
 constantly maintained at a point sufficient to give good crop yields. 
 Experiments in which the toxic effect of organic compounds was de- 
 termined on the basis of transpiration or green weights were regarded 
 as misleading and indecisive, and particular objection was raised to 
 the short term, the small amount of solution, and the generally ab- 
 normal conditions of experimentation. In short, it is concluded 
 that nothing yet published by the Bureau of Soils or by others should 
 in any sense be regarded as proof that toxic excreta play an important 
 role in rendering soils unproductive. 
 
 Hall, Brenchley, and Underwood (1913) have tested the conten- 
 tions of Whitney and Cameron, and reach the following conclusions 
 with respect to the soil solution and the growth of plants in it: 
 
 "The composition of the natural soil solution is not constant as regards 
 phosphoric acid and potash, but varies significantly in accord with the com- 
 position of the soil and its past manurial history. Within wide limits the rate 
 of growth of a plant varies with the concentration of the nutritive solution, 
 irrespective of the total amount of plant food available. When other condi- 
 tions, such as the supply of nitrogen, water, and air, are equal, the growth 
 
TOXIC EXUDATES AND SOIL TOXINS. 161 
 
 of the crop will be determined by the concentration of the soil solution in 
 phosphoric acid and potash, which in its turn is determined by the amount 
 of these substances in the soil, their state of combination, and the fertilizer 
 appUed. The net result of these investigations is to restore the carUer theory 
 of the direct nutrition of the plant by fertiUzers." 
 
 In conformity with the view that roots do not excrete toxic sub- 
 stances, it must be recognized that the value of crop rotation does not 
 depend upon getting rid of the toxic exudates of a particular crop. 
 Where such toxins as dihydroxystearic acid are present as the result 
 of the partial decomposition of organic matter, the effect w^ould be 
 produced by all crops that leave residues in the soil. As a conse- 
 quence, fallowing, tillage, or fertilizing, alone or in combination, 
 would suffice to get rid of the toxin, regardless of the crop sequence. 
 The fact that the absence of rotation for 60 years does not result 
 in the appearance of specific toxins in normal cultivated soil has been 
 proved by Hall and his associates (1913, 1919), in connection wdth 
 the growth of continuous crops of wheat and barley on the Rothamsted 
 plots. 
 
 Russell has also shown that the growth of 6 successive crops was 
 insufficient to cause appreciable reduction in the yield of the next 
 crop, and concludes that this rules out toxins as an explanation of the 
 advantages of rotation, when there is a lengthy interval between 
 crops. As King and Hall maintain, rotation still appears to rest 
 upon the different nutrient and tillage relations of the succes- 
 sive crops, though in many soils the relation of the various crops to 
 acidity may be an important factor, as indicated by Hartwell and 
 his associates. 
 
 The early 'assumption that root secretions were a factor in plant 
 communities and in succession is no longer vahd, but apparently this 
 is still thought of as a possibiHty in connection with soil toxins. In 
 all successions, except for the early stages of the hydrosere, the 
 amount of organic matter in the soil steadily increases, but the ab- 
 sence of any toxic effect is demonstrated by the fact that the number 
 of individuals and often the number of species also increases to 
 the subclimax or climax stage. Since a plant community regularly 
 returns its material to the soil, the question of nutrients enters only 
 in cases of intense competition, though their increasing availability 
 is a factor in succession. Field studies of germination and growth 
 and of community development, as well as competition cultures 
 under control, have show^n that w^ater-content, air-content, nutrient- 
 content, and temperature are normally the primary factors, more or 
 less modified by competition. Innumerable seedlings have been 
 found to grow as well in parent communities as in those of other 
 species, and consocies of annuals have been known to maintain them- 
 selves for 10 to 20 years, and to yield only when invasion became 
 overwhelming. 
 
162 AERATION AND AIR-CONTENT. 
 
 Many successional stages and climaxes have been under detailed 
 observation in Nebraska and Colorado since 1896, without the slight- 
 est evidence that toxins are in any manner concerned in their con- 
 dition. Some of these are more luxuriant than when first seen, and a 
 close study of their growth from year to year has shown that it varies 
 only in relation to the rainfall and the resulting water-relations. It 
 seems certain that most climaxes have occupied their habitats for 
 thousands of years, or even longer, and that their present growth 
 and composition make the depressing effect of toxins unthinkable. 
 In short, they extend the Rothamsted results with soils continuously 
 cropped from 60 years to thousands of years. 
 
 Soil toxins are probably to be definitely related to deficient aera- 
 tion and to anaerobic conditions, as has been indicated by Schreiner, 
 Hall, Russell, and others. This is also shown by the fact that they 
 are readily oxidized, and soon disappear under proper tillage. Hence, 
 they appear to be due to essentially the same conditions and processes 
 as obtain in bogs, the relationship being especially well exhibited by 
 muck soils. In both, the primary causes of toxicity are the direct 
 lack of oxygen and its indirect effect in permitting the accumulation 
 of carbon dioxid in harmful amounts and in producing injurious 
 organic acids and other compounds. In many cases probably the 
 first two alone are concerned, but in sour soils and muck soils at least, 
 all of them must have a part, though the lack of oxygen plays the 
 primary role. Since carbonic and other acids are the products of 
 respiration under such conditions, a considerable part of soil acidity 
 may be ascribed to them, though it must be recognized that toxic 
 effects may arise from acidity otherwise produced, as shown in the 
 preceding section. In conclusion, the present facts appear to war- 
 rant the statement that organic toxins are excreted by roots or pro- 
 duced in soils only as a consequence of the anaerobic respiration of 
 plant roots and of micro-organisms, and that inorganic toxins may 
 arise as a result of chemical processes or of adsorption. 
 
 n. C State College 
 
BIBLIOGRAPHY. 
 
 Abbott, J. B., S. D. Connor, and H. R. Smalley. 1913. The reclamation of an unpro- 
 ductive soil of the Kankakee marsh region. Soil acidity, nitrification, and the tox- 
 icity of soluble salts of aluminum. Purdue Univ. Agr. Exp. Sta. Bull. 170. 
 
 Aeroboe, F. 1893. Untersuchungen uber den directen und indirecten Einfluss des 
 Lichtes auf die Athmung der Gewiichse. Forsch. Geb. Agr. Phys. 16:429; Bot. 
 Cent. 59:182. 
 
 Aim£, G. 1841. Note sur les gaz d^gag^s paries plantes marines. Ann. Chim. 2:535. 
 
 Alway, F. J. 1920. Report of Golden Valley peat experimental fields. Minn. Agr. Exp. 
 Sta. Bull. 104. 
 
 Amm, a. 1893. Untersuchungen uber die intramoleculare Atmung der Pflanzen. Jahrb. 
 Wiss. Bot. 25:1; Bot. Cent. 56:86. 
 
 Amos, A. 1918. The difficulties of growing red clover. Clover sickness and other causes 
 of failure. Jour. Roy. Agr. Soc. 79:68; Bot. Abs. 4:1. 
 
 Allan^ R. G. - 1918. Soil drainage. Pusa Agr. Res. Inst. Bull. 85. 
 
 ANDitEws, F. M. 1912. Protoplasmic streaming in Mucor. Bull. Torr. Bot. Club 
 39:473. 
 
 Apileman, C. O. 1914. Chemical and physiological study of the rest period in the tu- 
 bers of Solanum tuberosum. Md. Agr. Exp. Sta. Bull. 183:223. 
 
 . 1916. Relation of oxidases and catalase to respiration in plants. Am. Jour. 
 
 Bot. 3:223. 
 
 . 1918. Respiration and catalase activity in sweet corn. Am. Jour. Bot. 5:207. 
 
 Arker, J. 1901. Die Beeinflussung des Wachstums durch das umgebende Medium. 
 Diss. Erlangen 1900; Bot. Cent. 87:433. 
 
 Aso, K. 1906. Injurious action of acetates and formates on plants. Bull. Coll. Agr. 
 Tokyo 7:13. 
 
 Atwood, W. M. 1914. A physiological study of the germination of Avena fatm- Bot. 
 Gaz. 57:387. 
 
 Aubert, E. 1892. Recherches sur la respiration et I'assimilation des plantes grasses. 
 Rev. Gen. Bot. 4:203. 
 
 AuDOYNAUD, A., and B. Chauzit. 1879. De passage de I'eau et de I'air dans la terre 
 arable. Ann. Agron. 393; Forsch. Geb. Agr. Phys. 3:19. 
 
 Babcock, S. M. 1912. Metabolic water: Its production and role in vital phenomena. 
 Wis. Agr. Exp. Sta. Res. Bull. 22. 
 
 Balls, W. L. 1912. The cotton plant in Egypt. 
 
 Barthelemy, a. 1873. De I'exhalation aqueuse des plantes dans I'air et dans I'acide 
 carbonique. Comp. Rend. 77:1080. 
 
 . 1874. De la respiration et de la circulation des gas dans les v6g6taux. Ann. 
 
 Sci. Nat. 5:19:131. 
 
 Bartholomew, E. T. 1913. Black heart of potatoes. Phytopath. 1:180. 
 
 . 1915. A pathological and physiological study of the black heart of potato tubers. 
 
 Centb. Bakt. 2:43:609. 
 
 Batten, Lily. 1918. Observations on the ecology of Epilobium hirsutum. Jour. Ecol. 
 6:161. 
 
 Bauman, a., and E. Gully, 1910. II: Untersuchungen uber die Humussauren. II: Die 
 freien Humussauren des Hochmoores. Ihre Natur, ihre Beziehungen zu den Sphagnen 
 und zur Pfianzenernahrung. Mitt. Bayr. Moorkulturanstalt. 4:47, 154. 
 
 Beauverie, J. 1901. Influence de la pression osmotique du milieu sur la forme et la 
 structure des v^g^taux. Comp. Rend. 132:226. 
 
 Becker, H. 1912. Ueber die Keimung verschiedenartiger Fruchte und Samen bei dersel- 
 ben Species. Beih. Bot. Cent. 29:21. 
 
 Becquerel, a. C. 1833. Premier memoire sur I'application des forces 6lectro-chimiques 
 k la physiologic v^g^tale. Archiv. Bot. 1 : 400. 
 
 Bedford, and S. U. Pickering. 1903. The effect of grass on trees. Rep. Woburn Exp. 
 Farm 3. 
 
 , . 1911. The effect of grass on trees. Rep. Woburn Exp. Farm 13. 
 
 , . 1914. The influence of one crop upon another. Jour. Agr. Sci. 6:136. 
 
 163 
 
164 AERATION AND AIR-CONTENT. 
 
 Beijerinck, M. W. 1897. Emulsions- und Sedimentfiguren bei beweglichen Bakterien. 
 
 Centb. Bakt. 2:3:40. 
 Bentzen, G. E. 1882. Die Kohlensaure in der Grundluft. Zeit. Biol. 18:446. 
 Bennett, M. E. 1904. Are roots aerotropic? Bot. Gaz. 37:247. 
 Bi;KARD, J. E. 1821. M6moire sur la maturation des fruits. Ann. Chim. 16:152. 
 Bergen, J. Y. 1915. Dwarfing effect of trees upon neighboring plants. Bot. Gaz. 
 
 60:491. 
 Bergman, H. F. 1920. The relation of aeration to the growth and activity of roots and 
 
 its influence on the ecesis of plants in swamps. Ann. Bot. 34:13. 
 . 1921. The effect of cloudiness on the oxygen content of water and its significance 
 
 in cranberry culture. Am. Jour. Bot. 8:50. 
 Bernard, C. 1878. La fermentation alcoolique. In Berthelot Rev. Sci., 1878. 
 
 . 1883. Lec^ons sur les effets des substances toxiques et medicamenteuses. 
 
 Bernbeck, — ., 1914. Beitriige zur Physiologie des Bodens. Forstw. Centralbl. 58:26. 
 Bert, P. 1873. Recherches experimentales sur I'influence que les changements dans la 
 
 pression barometrique exercent sur les phenomenes de la vie. Comp. Rend. 76:443; 
 
 77:531. 
 . 1875. Influence de I'air comprime sur les fermentations. Comp. Rend. 80:1579; 
 
 Ann. Chim. Phys. 5:7:145; Just. Bot. Jahrb. 4:116. 
 
 . 1878. La pression barometrique k Paris. 
 
 Berzelius, J. J. 1838. Untersuchung einiger Sauren welche mit Schwefelsaure aus 
 
 organischen Stofifen gebildet werden. Poggend. Ann. Phys. Chem. 44:368. 
 
 . 1839. Lehrbuch der Chemie. 3 Aufl., 8:389. 
 
 Birge, E. a., and C. Juday. 1911. The inland lakes of Wisconsin; the dissolved gases 
 
 of the water and their biological significance. Wis. Surv. Bull. 22. 
 Birner, H., and B. Lucanus. 1866. Wasserkulturversuche mit Hafer. Landw. Ver- 
 
 suchst. 8:160. 
 Blair, A. W., and E. J. Macy. 1908. Soil studies II: Acid soils. Fla. Agr. Exp. Sta. 
 
 Bull. 93. 
 BoHM, J. 1873. Ueber die Respiration von Landpflanzen. Sitzb. Akad. Wiss. Wien. 
 
 67:1,219. 
 . 1873^. Ueber das Keimen von Samen in reinem Sauerstoffgase. Sitzb. Akad. 
 
 Wiss. Wien. 68:132. 
 . 1873^ Ueber den Einfluss der Kohlensaure auf das Ergriinen und Wachstum der 
 
 Pflanzen. Sitzb. Akad. Wiss. Wien. 68:171. 
 • . 1874. Ueber die Starkebildung in den Keimblattern der Ivresse, des Rettigs und 
 
 des Leins. Sitzb. Akad. Wiss. Wien 69:163. 
 . 1875. Ueber die Respiration von Wasserpflanzen. Sitzb. Akad. Wiss. Wien. 
 
 71:694. 
 . 1875^. Ueber die Gahrungsgase von Sumpf und Wasserpflanzen. Sitzb. Akad. 
 
 Wiss. Wien. 71:481. 
 . 1881. Ueber die Ursache des Absterbens der Gotterbaume und ilber die Meth- 
 
 ode der Neubepflanzung der Ringstrasse im Wien. Monbl. Wiss. Clubs Wien. 2: 
 
 . 1893. Ueber die Respiration der Kartoffeln. Verh. Zool. Bot. Ges. Wien. 42:47. 
 
 Boerhaave, H. 1724. Elementa chemica. 
 
 Bogue, R. H. 1915. The adsorption of potassium and phosphate ions by typical soils 
 
 of the Connecticut Valley. Jour. Phys. Chem. 19:665. 
 BoLLEY, H. L. 1911. Interpretation of results noted in experiments upon cereal cropping 
 
 methods after soil sterilization. Science 33 :229. 
 Bonnier, G., and L. Mangin. 1884. Recherches sur la respiration et la transpiration 
 
 des champignons. Ann. Sci. Nat. 6:17:210. 
 Borodin, J. P. 1875. Sur la respiration des plantes. 
 
 . 1876. Phy.siologische Untersuchungen uberdie Atmung blattertragender Sprosse. 
 
 Bosio, — . 1893. Sulla conservazione della birra per mezzo dell'acido carbonico. Riv. 
 
 Igiene San. Pub. 4:61. 
 BoTTOMLEY, W. B. 1914. The significance of certain food substances for plant growth. 
 
 Ann. Bot. 28:531. 
 BouiLHAC, R. 1896. Sur la fixation de I'azote atmosph^rique par I'association des algues 
 
 et des bactdrics. Comp. Rend. 123:828. 
 and GujsTiNiANi. 1903. Sur une culture de sarrasin en presence d'un melange 
 
 d'algues et de bact^ries. Comp. Rend. 137:1274. 
 
BIBLIOGRAPHY. 165 
 
 BotJSSiNGATJLT, J. B. 1844. Sur la respiration des plantes. Comp. Rend. 19:945. 
 
 . 1865. Sur les fonctions des feuilles. Comp. Rend. 61:005. 
 
 . 1868. De la v6g6tation dans I'obscurit^. Ann. Chim. Phys. 4:13:219. 
 
 . 1868. Agrononiie, chimie agricole et physiologic. Ann. Chim. Phys. 4:269. 
 
 , and E. Lewy. 1853. Mdmoire sur la composition de I'air confine dans la terre 
 
 vegetale. Ann. Chim. Phys. 37:5. 
 Boyle, R. 1660. New experiments physico-mechanical touching the spring of air and 
 
 its effects. 
 Braconnet, H. 1839. Recherches sur I'influence des plantes sur le sol. Ann. Chim. 
 
 Phys. 72:27. 
 Breazeale, J. F., and J. A. LeClerc. 1912. The growth of wheat seedlings as affected 
 
 by acid or alkaline conditions. U. S. Dept. Agr., Bur. Chem. Bull. 149. 
 Brefeld, O. 1874. Untersuchungen uber die Alkoholgiihrung. Arb. Bot. Inst. Wurz- 
 
 burg 4:517. 
 
 . 1874. Ueber Gahrung. Landw. Jahrb. 3:65, 4:405, 5:281. 
 
 . 1876. Ueber Gahrung. III. Vorkommen und Verbreitung der Alkoholgahrung 
 
 im Pflanzenreiche. Bot. Zeit. 34:381. 
 Bristol, B. M. 1916. On the remarkable retention of vitality of moss protonema. New 
 
 Phyt. 15:137. 
 
 . 1919. On the retention of vitality by algse from old stored soils. New Phyt. 18:92. 
 
 . 1920. On the alga-flora of some desiccated English soils: An important factor 
 
 in soil biology. Ann. Bot. 34:35. 
 Brizi, U. 1905. Ricerche suUa malattia del riso detta "brusone." Ann. Istit. Agr. 
 
 Ponti 5:79. 
 . 1906. Ulteriori ricerche intorno al brusone del riso. Ann. Istit. Agr. Ponti 
 
 6:61. 
 Brown, H., and F. Escombe. 1902. The influence of varying amounts of carbon dioxid 
 
 in the air on the photosynthetic processes of leaves and on the mode of growth of 
 
 plants. Proc. Roy. Soc. 70:397; Bot. Cent. 90:293. 
 Brown, P. E. 1917. The importance of mold action in soils. Science 46; Agr. Jour. 
 
 India 13:529. 
 Brtjgmans, S. J. 1785. Dissertatio de solis eusque varia specie noxa et usu. 
 . 1789. De mutata humorum in regno organico indole a vi vitali vasorum derivan- 
 
 da. 
 Buchner, E. 1897. Alkoholische Gahrung ohne Hefezellen. Ber. Deut. Chem. Ges. 
 
 30:117. 
 Buckingham, E. 1904. Contributions to our knowledge of the aeration of soils. U. S. 
 
 Dept. Agr., Bur. Soils. Bull. 25. 
 Burgerstein, a. 1876. Untersuchungen liber die Beziehung der Nahrstoffe zur Trans- 
 piration der Pflanzen. Sitzb. Akad. Wiss. Wien 73:191. 
 Burns, G. P. 1911. A botanical survey of the Huron river valley. VIII. Edaphic con- 
 ditions in peat bogs of southern Michigan. Bot. Gaz. 52:105. 
 Butler, O. 1919. The effect of environment on the loss of weight and germination of 
 
 seed potatoes during storage. Jour. Am. Soc. Agron. 11:114; Bot. Abs. 3:20. 
 Cahours, A. 1864. Sur la respiration des fruits. Comp. Rend. 69:466. 
 Calvert, F. C, and E. Ferrand. 1844. Memoire sur la vegetation considerde sous le 
 
 point de vue chimique. Ann. Chim. Phys. 11:477. 
 Cameron, F. K. 1912. The soil solution. 
 , and J. M. Bell. 1905. The mineral constituents of the soil solution. U. S. 
 
 Dept. Agr., Bur. Soils Bull. 30. 
 Cannon, W. A. 1915. Soil aeration and root-growth. Carnegie Inst. Wash. Year 
 
 Book 14:63. 
 . 1915. On the relation of root-growth and development to the temperature and 
 
 aeration of the soil. Am. Jour. Bot. 2:211. 
 . 1916. On the relation between the rate of root-growth and the oxygen of the 
 
 soil. Carnegie Inst. Wash. Year Book 15:74. 
 . 1917. Root-growth of Prosopis velutina and Opuntia versicolor under conditions 
 
 of a small oxygen supply in the soil. Carnegie Inst. Wash. Year Book 16:82. 
 . 1918. Root-growth in desert plants and the o.xygen supply of the soil. Carne- 
 gie Inst. Wash. Year Book 17:81. 
 
166 AERATION AND AIR-CONTENT. 
 
 Cannon, W. A. 1919. Influence of soil aeration upon growth of shoots. Carnegie 
 
 Inst. Wash. Year Book 18:71. 
 . 1919^. Reactions of roots of species with dissimilar habitats to different amounts 
 
 of carbon dioxid in the soil. Carnegie Inst. Wash. Year Book 18:92. 
 . 1920. Effects of a diminished oxygen supply in the soil on the rate of growth 
 
 of roots. Carnegie Inst. Wash. Year Book 19:59. 
 , and E. E. Free. 1917. The ecological significance of soil aeration. Science 
 
 45:178. 
 , . 1919. Soil aeration experiments with helium. Carnegie Inst. Wash. 
 
 Year Book 18:85. 
 , . 1920. Anaerobic experiments with helium. Carnegie Inst. Wash. Year 
 
 Book 19:61. 
 , . 1920-. Root adaptation to deficient soil aeration. Carnegie Inst. 
 
 Wash. Year Book 19:62. 
 Cauvet, D. 1861. fitudes sur le role des racines dans I'absorption et I'excr^tion. Ann. 
 
 Sci. Nat. 4:15:323. 
 . 1880. Deu.xi^me note sur le degagement de I'acid carbonique par les racines des 
 
 plantes. Bull. Soc. Bot. France 27:113. 
 Celakowsky, L. 1892. Ueber die Aufnahme lebender und todter verdauHcher Korper 
 
 in die Plasmodien der Myxomyceten. Flora 76:182. 
 . 1898. Ueber den Einfluss des Sauerstoffsmangels auf die Bewegungen einiger 
 
 aeroben Organismen. Bull. Acad. Sci. Prag 5:51. 
 Chamberlin, T. C. 1911. Soil producitivity. Science 33:225. 
 Chambers, C. O. 1912. The relation of algae to dissolved oxygen and carbon dioxid, 
 
 with special reference to carbonates. Mo. Bot. Card. Rep. 23:171. 
 Chapin, P. 1902. Einfluss der Kohlensaure auf das Wachstum. Flora 91:348. 
 Christenson, p. 1908. Chemical researches on bulbs in the later phases of the rest 
 
 period. Bull. Acad. Roy. Danemark 44. 
 Chudiakow, N. 1894. Beitrage zur Kenntniss der intramolekularen Athmung. Landw. 
 
 Jahrb. 23:333; Bot. Cent. 59:283. 
 
 . 1894-. Untersuchungen liber die alkoholische Gahrung. Landw. Jahrb. 23:391. 
 
 . 1896. Zur Lehre von der Anaerobiose. 
 
 Clark, J. 1888. Ueber den Einfluss niederer Sauerstoffpressungen auf die Bewegung 
 
 des Protoplasmas. Ber. Deut. Bot. Ges. 6:273. 
 Clausen, H. 1890. Beitrage zur Kenntniss der Atmung der Gewachse und des pflanz- 
 
 lichen Stoffwechsels. Landw. Jahrb. 19:893. 
 Clements, F. E. 1905. Research methods in ecology. 
 
 . 1907. Plant physiology and ecology. 
 
 . 1916. Plant succession. Carnegie Inst. Wash. Pub. 242. 
 
 . 1920. Plant indicators. Carnegie Inst. Wash. Pub. 290. 
 
 Clouston, D., and A. R. P. Aiyer. 1918. The physical texture of soils in its relation to 
 
 agriculture. Agr. Jour. India 13:89. 
 Coleman, D. A. 1916. Environmental factors influencing the activity of soil fungi. 
 
 Soil Science 2:1. 
 Conn, H. J. 1916. Relative importance of fungi and bacteria in the soil. Science 44:857. 
 Connor, S. D. 1918. Soil acidity as affected by moisture conditions of the soil. U. S. 
 
 Dept. Agr., Jour. Agr. Res. 15:321; Bot. Abs. 2:67. 
 Corenwinder, C. 1867. fitude sur les functions des racines des v6g6taux. Mem. Soc. 
 
 Sci. Lisle. 3:4; Ann. Sci. Nat. 5:9:63, 1868. 
 Correns, C. 1892. Ueber die Abhangigkeit der Reizerscheinungen hoherer Pflanzen von 
 
 der Gegenwart freier Sauserstoffes. Flora 75 :87. 
 CoRTi, B. 1774. Osservazione microscopiche sulla Tremella e sulla circolazione del 
 
 fluido in una pianta acquajuolo. 210. Ref. in Meyen Neues System der Pflanzen- 
 
 Physiologic 2: (1832). 
 Costantin, J. 1885. Recherches sur la structure de la tige des plantes aquatiques. 
 
 Ann. Sci. Nat. 6:19:287; Bot. Cent. 24:141. 
 CoTTA, H. 1806. Naturbeobachtungen liber die Bewegung und Funktion des Saftes in 
 
 den Gewachsen. 
 Coventry, B. O. 1917. Correlation between the light and soil requirements of a species 
 
 for its natural regeneration. Indian Forester 43:186. 
 
BIBLIOGRAPHY. 167 
 
 CoviLLE, F. V. 1910. Experiments in blueberry culture. U. S. Dept. Agr.. Bur PI Ind 
 
 Bull. 193. 
 . 1911. The use of acid soil for raising seedlings of the mayflower, Epigaea repens. 
 
 Science 33:711. 
 
 . 1913. The agricultural utilization of acid lands by means of acid tolerant crops. 
 
 U. S. Dept. Agr. Bull. 6. 
 
 . 1915. Directions for blueberry culture. U. S. Dept. Agr. Bull. 334. 
 
 Cox, H. J. 1910. Frost and temperature conditions in the cranberry marshes of Wis- 
 consin. Weather Bur. Bull. T. 
 Crocker, W. 1906. Role of seed-coats in delayed germination. Bot. Gaz. 42:273. 
 
 . 1907. Germination of seeds of water plants. Bot. Gaz. 44:375. 
 
 . 1916. Mechanics of dormancy in plants. Am. Jour. Bot. 3:99. 
 
 . 1919. Soil acidity. Bot. Gaz. 67:519. 
 
 , and W. E. Davis. 1914. Delayed germination in seeds of Alisma plantago. Bot. 
 
 Gaz. 58:285. 
 , and G. T. Harrington. 1918. Catalase and oxidase content of seeds in relation 
 
 to their dormancy, age, vitality, and respiration. U. S. Dept. Agr., Jour. Agr. Res. 
 
 15:137. 
 Crump, W. B. 1911. The wilting of moorland plants. Rep. Brit. Assoc. 1911:12. 
 CuGiNi, G. 1876. Sulla presenza costante dcU'idrogeno trai prodotti della fermentazione 
 
 alcoolica. Just. Bot. Jahrb. 4:116. 
 CzAPEK, F. 1895. Untersuchungen iiber Geotropismus. Jahrb. Wiss. Bot. 27:243. 
 
 . 1896. Zur Lehre von den Wurzelausscheidungen. Jahrb. Wiss. Bot. 29:321. 
 
 Dachnowski, A. 1908. The toxic property of bog water and bog soil. Bot. Gaz. 46:130. 
 
 . 1909. Bog toxins and their effect upon soils. Bot. Gaz. 47:389. 
 
 . 1910. Physiologically arid habitats and drought resistance in plants. Bot. Gaz. 
 
 49:375. 
 . 1910^. The bacterial flora as a factor in the unproductiveness of soils. Ohio 
 
 Nat. 10:137; U. S. Dept. Agr., Exp. Sta. Rec. 24:529. 
 
 , 1911. The vegetation of Cranberry Island. Bot. Gaz. 52:1. 
 
 . 1912. The relation of Ohio bog vegetation to the chemical nature of peat soils. 
 
 Bull. Torr. Bot. Club 39:53. 
 . 1912. Peat deposits of Ohio, their origin, formations and uses. Geol. Surv. 
 
 Ohio BuU. 16. 
 . 1912. The nature of the absorption and tolerance of plants in bogs. Bot. Gaz. 
 
 54:503. 
 . 1914. The effects of acid and alkaline solutions upon the water relation and the 
 
 metabolism of plants. Am. Jour. Bot. 1:412. 
 , and R. Gormley. 1914. The physiological water requirement and the growth 
 
 of plants in glycocoU solutions. Am. Jour. Bot. 1 :174. 
 Daikuhara, G. 1914. Ueber saure Mineralbodcn. Bull. Imp. Agr. Exp. Sta. Japan 2:1. 
 Dandeno, L. B. 1909. Mutual interactions of plant roots. Rep. Mich. Acad. Sci. 11 :24; 
 
 Exp. Sta. Rec. 15:780. 
 D'Arsonval, a. 1891. Emploi de I'acide carbonique liquefi^ pour la filtration et la stdiil- 
 
 ization rapidcs des hquides organiques. Comp. Rend. 112:667. 
 Daubeny, C. 1835. Memoir on the degree of selection exercised by plants, with regard 
 
 to the earthy constituents presented to their absorbing surfaces. Trans. Liim. Soc. 
 
 17(1837) :253. 
 . 1845. Memoir on the rotation of crops, and on the quantity of inorganic matters 
 
 abstracted from the soil by various plants under different circumstances. Phil. 
 
 Trans. 179. 
 Davis, A. R. 1921. The variability of plants grown in water cultures. Soil Science 11:1. 
 Davis, C. A. 1907. Peat, essays on its origin, uses and distribution in Michigan. Mich. 
 
 Geol. Surv. Rep. 1906. 
 Davis, W. E., and R. C. Rose. 1912. The effect of external conditions upon after ripen- 
 ing of the seeds of Cratcegus mollis. Bot. Gaz. 54:49. 
 Davy, H. 1821. Elements of agricultural chemistry. Third ed. 
 Day, G, E. 1906. Experiments on aeration. Ann. Rep. Ont. Agr. Coll. 31:37. 
 
 . 1907. Aeration of soils. Ann. Rep. Ont. Agr. Coll. 32:36. 
 
 DeCandolle, a. p. 1832. Physiologic v6g€tale. Excretions des racines. 1:248; 
 
 3:1474. 
 
168 AERATION AND AIR-CONTENT. 
 
 Deherain, p. p. 1892. Chimie agricole. p. 8. 
 
 . 1896. Sur la travail du sol. Ann. Agron. 22:468. 
 
 , and Ed. Landrin. 1874. Recherches sur la germination. Ann. Sci. Nat. 
 
 5:19:383. 
 , and H. Moissan. 1874. Recherches sur I'absorption d'oxygtee et remission 
 
 d'acide carbonique par les plantes maintenues dan I'obscurit^. Ann. Sci. Nat. 
 
 5:19:321. 
 , and J. Vesque. 1876. Recherches sur la respiration des racines. Ann. Sci. 
 
 Nat. 6:3:326. 
 , . 1877. Recherches sur Tabsorption et remission des gaz par les racines. 
 
 Comp. Rend. 84:959. 
 Delf, E. M. 1912. Transpiration in succulent plants. Ann. Bot. 26:409. 
 De Luca, S. 1878. Recherches chimiques tendant a demontrer la production de I'alcool 
 
 dans les feuilles, les fleurs et les fruits de certaines plantes. Ann. Sci. Nat. 6:6:286. 
 Demoor, J. 1894. Contribution a la physiologic de la cellule. Individualite fonctionelle 
 
 du protoplasma et du noyau. Bull. Soc. Beige Micro. 20:36; Archiv. Biol. 13:163; 
 
 Bot. Cent. 59:24. 
 Detmer, W. 1875. Physiologisch-chemische Untersuchung iiber die Keimung olhaltiger 
 
 Samen. 
 
 . 1880. Vergleichende Physiologic des Keimungsprocesses der Samen. 
 
 . 1881. Ueber die Entwirkung verschiedener Gase, inbesondere des Stickstoff- 
 
 oxydulgases auf Pflanzenzellen. Sitzb. Jena Ges. Naturw. 7; Landw. Jahrb. 11:213. 
 
 . 1883. Lehrbuch der Physiologic. 
 
 . 1893. Beitrage zur Kenntniss des Stoffwechsels keimender Kartoffelknollen. 
 
 Ber. Deut. Bot. Ges. 11:149. 
 Devaxjx, H. 1889. Du m^canisme des ^changes gazeux chez les plantes aquatiques sub- 
 
 merg^es. Ann. Sci. Nat. 7:9:115, 177. 
 1890. Les ^changes gazeux d'un tubercule repr^sentes schematiquement par un 
 
 'appareil physique. Bull. Soc. Bot. France. 37:257. 
 . 1891. Porosit6 du fruit des Cucurbitacees. Rev. Gen. Bot. 3:49; Beih. Bot. 
 
 Cent. 1:27. 
 . 1899. Asphyxie spontan^e et production de I'alcool dans les tissus profonds des 
 
 tiges ligneuses poussant dans les conditions naturelles. Comp. Rend. 128:1346. 
 Devries, H. 1884. Ueber die Periodicitat in Sauregehalte der Fettpflanzen. Med. Ak. 
 
 Wet. Natur. 1:58. 
 DiAKONow, N. W. 1886. Ueber die sogennante intramolekulare Atmung der Pflanzen. 
 
 Ber. Deut. Bot. Ges. 4:411. 
 . 18862. Intramoleculare Atmung und Garthatigkeit der Schimmelpilze. Ber. 
 
 Deut. Bot. Ges. 4:2. 
 . 1894. Typische Reprasentanten des Lebenssubstrates. Arb. St. Petersburg. 
 
 Nat. Ges. 23; Bot. Cent. 59:132. 
 Dobereiner, J. W. 1822. Zur phyto-chemie. Gilbert Ann. Phys. 22:430. 
 . 1832. E.xperiences sur la germination dans I'air condense ou rarifi6. Bibl. 
 
 Univ. Geneve 22:121. 
 DosDALL, Louise. 1919. Water requirement and adaptation in Equisetum. Plant World 
 
 23:1. 
 Dude, M. 1903. Ueber den Einfluss des Sauerstoffentzuges auf pflanzliche Organismen. 
 
 Flora 92:205. 
 DuGGAR, B. M. 1920. Hydrogen ion concentration and the composition of nutrient 
 
 solutions in relation to the growth of seed plants. Ann. Mo. Bot. Gard. 7:1. 
 Duhamel, H. L. 1755. Traite des arbes et arbustes. 
 DuMONT, — . 1819. Sur des experiences faites dans le but de connaitre Taction du gaz 
 
 acide carbonique sur les fruits. Ann. Chim. 10:50. 
 Dutrochet, H. 1837. Memoires pour servir k I'histoire anatomique et physiologique 
 
 des v^g^taux et des animaux. 
 . 1837. De la tendance des vegetaux h se diriger vers la lumiSre et de leur ten- 
 dance k la fuir. Comp. Rend. 4:486. 
 . 1838. Observations sur la circulation des fluides chez le Chara fragihs, Desvaux. 
 
 Ann. Sci. Nat. 2:9:5, 65. 
 . 1840. Recherches sur la chaleur propre des etres vivants i basse temperature. 
 
 Ann. Sci. Nat. 2:13:5. 
 
BIBLIOGRAPHY. 169 
 
 Duval, J. W. 1904. The vitality and germination of seeds. U. S. Dept. Agr., Bur. PI. 
 
 Ind. Bull. 58. 
 Ebermayer, E. 1873. Die physikalischen Einwirkungen des Waldes auf Luft und Boden. 
 
 . 1878. Mittheilungen iiber den Kohlensauregehalt der Waldluft und des Wald- 
 
 bodens im Vergleich zu einer nicht bevvaldeten Flache. Forsch. Geb. Agr. Phys. 1 :158. 
 
 . 1885. Die Beschaffenheit der Waldluft. 
 
 . 1890. Untersuchungen iiber die Bedeutung des Humus als Bodenbestandtheil, 
 
 und iiber den Einfluss des Waldes verschiedener Bodenarten und Bodendecken auf 
 die Zusammensetzung der Bodenluft. Forsch. Geb. Agr. Phys. 13:15. 
 Eggertz, C. G. 1889. Studien und Untersuchungen uber die Humuskorper der Acker 
 
 und Moorerde. Biedermann's Centb. Agr. Chem. 18:75. 
 Ehrenberg, p. 1906. Einige Beobachtungen iiber Pflanzenbeschadigungen durch 
 
 Spiiljauchenberieselung. Zeit. Pflanzenk. 16:193. 
 EsMARCH, F. 1910. Beitrag zur Cyanophyceen-Flora unserer Kolonien. Jahrb. Ham- 
 burg Wiss. Anst. 28:62. 
 
 . 1914. Untersuchungen iiber die Verbreitung der Cyanophyceen auf und in ver- 
 
 schiedenen Boden. Hedwigia 55:224. 
 EwART, A. J. 1894. Observations on the vitality and germination of seeds. Trans. Liver- 
 pool Biol. Soc. 8:207. 
 
 . 1894^. O.xtropic irritability of roots. Trans. Liverpool Biol. Soc. 8:238; 
 
 10:190. 
 
 . 1896. Additional observations on the vitality and germination of seeds. Trans. 
 
 Liverpool Biol. Soc. 10:185. 
 Faivre, E., and V. Dupre. 1866. Recherches sur les gaz du mtlricr et de la vigne, les 
 parties qui les renferment, les changements que la v6g6tation y determine. Comp. 
 Rend. 62:778. 
 Fankhauser, F. 1919. Zur Kenntnis der Larche. Zeit. Forst. Jagdw. 51:289; Bot. 
 
 Abs. 4:63. 
 Fernald, M. L. 1907. The soil preferences of certain alpine and subalpine plants. 
 
 Rhodora 9:149. 
 Fleury, G. 1865. Recherches chimiques sur la germination. Ann. Chim. Phys. 4:4:38. 
 FoDOR, J. 1881. Hygienische Untersuchungen iiber Luft, Boden und Wasser. 
 FoLSOM, D. 1918. The influence of certain environmental conditions, especially water 
 
 supply, upon form and structure in Ranunculus. Physiol. Res. 2:209. 
 FoTH, G. 1889. Ueber den Einfluss der Kohlensaure auf das Wachstum und die Gahr- 
 thatigkeit der Hefe und ihre Bedeutung fur die Conservirung des Bieres. Wochens. 
 Brauer. 6:263. 
 Frank, B. 1872. Ueber die Veranderung der Lage der Chlorophyllkerner und des Proto- 
 plasmas in der Zelle. Jahrb. Wiss. Bot. 8:270. 
 
 . 1889. Krankheiten der Pflanzen. 2d ed. 1 :307. 
 
 Frankel, C. 1889. Die Einwirkung der Kohlensaure auf die Lebensthatigkeit der Mik- 
 
 roorganismen. Zeit. Hygiene 5:332. 
 Frankland, p. F. 1889. Ueber den Einfluss der Kohlensaure und anderer Gase auf die 
 
 Entwickelungsfjihigkcit der Mikroorganismen. Zeit. Hygiene 6:13. 
 Frear, W. 1915. Sour soils and liming. Pa. Dept. Agr. Bull. 261. 
 Fred, E. B., and A. R. C. Haas. 1919. The etching of marble by roots in the presence of 
 
 bacteria. Jour. Gen. Phys. 1:631. 
 Freudenreich, E. 1892. Ueber die Widerstandsfahigkeit der Bakterien gegen hohen 
 Druck combinirt mit einer Erhohung der Temperatur. Mitt. Naturf. Ges. Bern.; 
 Beih. Bot. Cent. 4:457. 
 Freyberg, E., and A. Mayer. 1879, Ueber die Atmungsgrosse bei Sumpf- und Wasser- 
 
 pflanzen. Landw. Versuchst. 23:463. 
 Frtjh, J., and C. Schroter. 1904. Die Moore der Schweiz mit Berucksichtigiing der 
 
 gesammten Moorfrage. 
 FuNCHESS, M. J. 1918. The development of soluble manganese in acid soils as influenced 
 
 by certain nitrogenous fertilizers. Ala. Agr. Exp. Sta. Bull. 201. 
 Garreau, L. 1851. De la respiration chez les plantes. Ann. Sci. Nat. 3:15:5^ 
 
 . 1851. Nouvelles recherches sur la respiration des plantes. Ann. Sci. Nat. 3:16: 
 
 271. 
 
 , and Braxjwers. 1858. Recherches sur les formations cellulaires, I'accroisse- 
 
 ment, et I'exfohation des extr^mites radiculaires et fibrillaires des plantes. Ann. 
 Sci. Nat. 4:10:186, 191. 
 
170 AERATION AND AIR-CONTENT. 
 
 Gasparrini, G. 1856. Ricerche sulla natura dei succiateri e la escrezione delle radici. 
 Gates, F. C. 1914. Winter as a factor in the xerophily of certain evergreen plants. Bot. 
 
 Gaz. 57:445. 
 Gautier, a., and R. Drouin. 1888. Recherches sur la fixation de I'azote par le sol et les 
 
 v6g6taux. Comp. Rend. 106:754. 
 Gerber, C. 1896. Recherches sur la maturation des fruits charnus. Ann. Sci. Nat. 8:4:1. 
 . 1903. Respiration des fruits parfum^s lors de leur maturation compl6te, 
 
 quand on les place h V6ta.t vert et non parfumds dans de I'air enrichi en oxygene. 
 
 Comp. Rend. Soc. Biol. 55:269. 
 GiGLiOLi, J. 1879. Resistenza dei semi, e specialmente dei semi di medica, all'azione 
 
 prolungata di agenti chimici, gassosi e liquidi. Gazz. Chim. Ital. 9:477. 
 
 . 1895. Latent vitality in seeds. Nature 52:544. 
 
 Gillespie, L. J. 1916. The reaction of soil and measurements of hydrogen-ion concen- 
 tration. Jour. Wash. Acad. Sci. 6:7. 
 . 1920. Reduction potentials of bacterial cultures and water-logged soils. Soil 
 
 Science 9:199. 
 Gluck, H. 1905. Biologische und morphologische Untersuchungen iiber Wasser- und 
 
 Sumpfgewachse. 
 Godlewski, E. 1873. Abhangigkeit der Sauerstoffausscheidung der Blatter von dem 
 
 Kohlensjiuregehalt der Luft. Arb. Bot. Inst. Wiirzburg 1:343. 
 
 . 1882. Beitrage zur Kentniss der Pflanzenathmung. Jahrb. Wiss. Bot. 13:491. 
 
 . 1892. Studien iiber das Wachstum der Pflanzen. Abhandl. Cracow Akad. 
 
 Math-Naturw. Kl.; Bot. Cent. 55:34. 
 . 1904. Ein weiterer Beitrag zur Kenntniss der intramolekularen Athmung der 
 
 Pflanzen. Bull. Acad. Sci. Cracovie; Bot. Cent. 99:535. 
 , and F. Polzeniusz. 1897. Ueber Alkoholbildung bei der intramolekularen 
 
 Athmung hoherer Pflanzen. Bull. Acad. Sci. Cracovie 267. 
 , . 1901. Ueber die intramolekularen Atmung von in Wasser gebrachten 
 
 Samen und iiber die dabei stattfindende Alkoholbildung. Bull. Acad. Sci. Cracovie 
 
 227. 
 Goebel, K. 1886. Ueber die Rhizophoren vegetation. Archiv Ver. Meckl. 40:xxiv. 
 
 . 1886. Ueber die Luftwurzeln von Sonneratia. Ber. Deut. Bot. Ges. 4:249. 
 
 . 1887. Bemerkung zu der Abhandlung von L. Jost. "Ein Beitrag zur Kenntniss 
 
 der Atmungsorgane der Pflanzen." Bot. Zeit. 45:717. 
 . 1891. Pflanzenbiologische Schilderungen: IV. Die Vegetation der Venezolan- 
 
 ischen Paramos. 
 GoLA, G. 1905. Sulla respirazione intramolecolare nelle piante palustri. Att. Ace. 
 
 Reale Sci. Torino 40:880. 
 Graves, A. H. 1915. Root rot of coniferous seedlings. Phytopath 5:213. 
 Green, W. J., and F. H. Ballou. 1906. Orchard culture. Ohio Agr. Exp. Sta. Bull. 171. 
 Grischow, C. C. 1819. Physikalisch-chemische Untersuchungen iiber die Athmungen 
 
 der Gewachse und deren Einfluss auf die gemeine Luft. 
 Groom, P. 1910. Remarks on the ecology of the Coniferse. Ann. Bot. 24:241. 
 GRtJss, J. 1907. Enzymewirkungen aus Wundrand der KartoffelknoUe. Zeit. Pflanzenk. 
 
 17:69. 
 Haas, A. R. 1916. The acidity of plant cells as shown by natural indicators. Jour. Biol. 
 
 Chem. 27:233. 
 
 . 1919. Respiration after death. Bot. Gaz. 67:347. 
 
 Hales, S. 1727. Vegetable staticks. 
 
 Hall, A. D. 1906. The solvent action of roots upon the soil particles. Sci. Prog. 1:51. 
 
 . 1908. Theories of manure and fertilizer action. Science 28:625. 
 
 . 1919. The book of the Rothamsted experiments. 2d ed. 
 
 , W. E. Brenchley, and L. M. Underwood. 1913. The soil solution and the 
 
 mineral constituents of the soil. Phil. Trans. Roy. Soc. Lond. B. 204:179; Jour. Agr. 
 
 Sci. 6:278 (1914). 
 Hanley, J. A. 1914. Estimation of the surface of soils. U. S. Dept. Agr., Jour. Agr. 
 
 Sci. 6:58. 
 Harris, J. E. 1914. Soil acidity. Mich. Agr. Exp. Sta. Tech. Bull. 19. 
 
 . 1914^. Soil acidity and methods for its detection. Science 40:491. 
 
 Harrison, W. H., and P. A. S. Aiyer. 1913. The gases of swamp rice soils: Their com- 
 position and relationship to the crop. Mem. Dept. Agr. India, Chem. Ser, 3:65. 
 
BIBLIOGRAPHY. 171 
 
 Harrison, W. H., and P. A. S. Aiyer. 1914. The gases of swamp rice soils: Their utili- 
 zation for the roots of the crop. Mem. Dept. Agr. India, Chem. Ser. 4:1. 
 
 , . 1916. The gases of swamp rice soils: A hydrogen oxidizing bacterium 
 
 from these soils. Mem. Dept. Agr. India, Chem. Ser. 4:135. 
 
 , • The source of the gaseous soil nitrogen. Mem. Dept. Agr. India, Chem 
 
 Ser. 5:1. 
 
 Hartig, R. 1894. The diseases of trees. English edition. 
 
 . 1880. Ueber die Einwirkung des Frostes auf die Pflanzen. 
 
 Hartwell, B. L., and F. R. Pember. 1918. The presence of aluminum as a reason for 
 the difference in the effect of so-called acid soil on barley and rye. Soil Science 
 6:259. 
 
 , . 1907. The relation between the effects of liming and of nutrient solu- 
 tions containing different amounts of acids upon the growth of certain cereals. 
 R. I. Agr. E.xp. Sta. Rep. 20: 358. 
 
 -, and G. E. Merkle. 1919. The influence of crop plants on those which 
 
 follow: II. R. I. Agr. Exp. Sta. Bull. 176. 
 
 Hasselbring, H. 1918. Effect of different oxygen pressures on the carbohydrate metab- 
 olism of the sweet potato. U. S. Dept. Agr., Jour. Agr. Res. 14:273; Bot. Abs. 1 :222. 
 
 , and L. A. Hawkins. 1915. Physiological changes in sweet potatoes during 
 
 storage. U. S. Dept. Agr., Jour. Agr. Res. 3:331. 
 
 1915. Respiration experiments with sweet potatoes. U. S. Dept. Agr., 
 
 Jour. Agr. Res. 5:509. 
 Hauptfleisch, p. 1892. Untersuchungen uber die Stromung des Protoplasmas in be- 
 
 hauteten Zellen. Jahrb. Wiss. Bot. 24:173. 
 Hedrick, U. G. 1905. The relationship of plants in the orchard. Proc. Soc. Hort. Sci. 
 
 1905:72. 
 . 1910. Comparison of tillage and sod mulch in an apple orchard. N. Y. Agr. 
 
 Exp. Sta. Bull. 314. 
 Heiden, J. C. 1879. Lehrbuch der Diingerlehre. 2 Aufl. 
 
 Heintz, a. 1873. Atmung der Riibenwurzel. Zeit. Ver. Rubenind. 23:196; Just 1:358. 
 Hesselmann, H. 1910. Ueber den Sauerstoffgehalt des Bodenwassers und dessen Einwir- 
 kung auf die Versumpfung des Bodens und das Wachstum des Waldes. Skogsfor. 
 
 Tids. 1910: 177; Silva 4:65; For. Quart. 9:481. 
 . 1917. On the effect of our regeneration measures on the formation of saltpetre 
 
 in the ground and its importance in the regeneration of coniferous forests. Medd. 
 
 Stat. Skogsfor. 13:923. 
 . 1917^. Studien uber die Verjiingungsbedingungen der norrlandischen Kiefern- 
 
 heiden. Medd. Stat. Skogsfor. 17:1221; Skogsfor. Tids. 17:29. 
 Hill, G. R. 1913. Respiration of fruits and growing plant tissues in certain gases with 
 
 reference to ventilation and fruit storage. Cornell Agr. Exp. Sta. Bull. 330. 
 HoAGLAND, D. R. 1917. The effect of hydrogen and hydroxyl-ion concentration on the 
 
 growth of barley seedhngs. Soil Science 3:547. 
 . 1918. The relation of the plant to the reaction of the nutrient solution. Science 
 
 48:422. 
 . 1919. Notes on the technique of solution culture experiments with plants. 
 
 Science 49:360. 
 , and L. T. Sharp. 1918. Relation of carbon dioxid to soil reaction as measured 
 
 by the hydrogen electrode. U. S. Dept. Agr., Jour. Agr. Res. 12:139. 
 Hoffmann, J. F., and S. Sokolowski. 1910. Comparative respiration experiments with 
 
 different varieties of potatoes. Exp. Sta. Rec. 24:541; Zeit. Spiritusind. 391. 
 HoFMEiSTER, W. 1867. Die Lehre von der Pflanzenzelle. 
 HoHNEL, F. 1879. Ueber die Transpirationgrossen der forstlichen Holzgewachse. Mitt. 
 
 Forst. Versuch. Oester. 2:47. 
 . 1880. Weitere Untersuchungen iiber die Transpirationgrossen der forstlichen 
 
 Holzgewachse. Mitt. Forst. Versuch. Oester. 2:275. 
 Hole, R. S. 1911. On some Indian forest grasses and their oecology. For. Dept. India, 
 
 For. Bot. Ser. 1. 
 . 1918. Recent investigations on soil aeration: II. With special reference to 
 
 forestry. Agr. Jour. India. 13:430. 
 
 , and P. Singh. (Ecology of sal (Shorea robusta). 
 
 , . 1914. Part I: Soil-composition, soil-moisture, soil-aeration. Indian 
 
 Forest Records 5:4:1. 
 
172 AERATION AND AIR-CONTENT. 
 
 Hole, R. S., and P. Singh. 1916. Part II: Seedling reproduction in natural forests 
 and its improvement. Indian Forest Records 5:43. 
 
 , . 1916. Part III: Soil-aeration and water-cultures. Indian Forest Records 
 
 5:S7. 
 HoMBERG, W. 1699. Exp(5rienccs sur la germination des plantes. Mem. Acad. Paris, 
 
 p. 69. 
 HovELER, W. 1892. Ueber die Verwerthung des Humus bei der Ernahrung der chloro- 
 
 phyllfuhrenden Pflanzen. Jahrb. Wiss. Bot. 24:283. 
 Howard, A. 1910. Second report on the fruit experiments at Pusa. Pusa Agr. Res. 
 Inst. Bull. 16. 
 
 . 1916. Soil erosion and surface drainage. Pusa Agr. Res. Inst. Bull. 53. 
 
 . 19162. Soil aeration in agriculture. Pusa Agr. Res. Inst. Bull. 61. 
 
 . 1918. Recent investigations on soil aeration: I. With especial reference to 
 
 agriculture. Agr. Jour. India 13:416. 
 
 . 1920. The improvement of crop production in India. Jour. Roy. Soc. Arts. 
 
 68:555. 
 and G. L. C. Howard. 1915. Soil ventilation. Quetta Fruit Exp. Sta. Bull. 3. 
 
 . 1915-. First report on the improvement of indigo in Bihar. Pusa Agr. 
 
 Res. Inst. Bull. 51. 
 
 . 1915^. Soil ventilation. Pusa Agr. Res. Inst. Bull. 52:23. 
 
 . 1917. Irrigation of alluvial soils. Quetta Fruit Exp. Sta. Bull. 7. 
 
 . 1917^. Economic significance of the root development of agricultural 
 
 crops. Agr. Jour. India 12:17. 
 
 . 1917'. Report of the imperial economic botanists, 1916-17. 
 
 . 1918. Methods suitable for the study of root-development. Agr. 
 
 Jour. India 13:36. 
 
 . 1918^. Report of the imperial economic botanists, 1917-18. 
 
 . 1919. Drainage and crop production in India. Agr. Jour. India 14:377. 
 
 . 1919^. The agricultural development of Baluchistan. Quetta Fruit 
 
 Exp. Sta. Bull. 11. 
 
 . 1919'. The saving of irrigation water in wheat growing. 2d ed. 
 
 Quetta Fruit Exp. Sta. Bull. 4. • 
 
 . 1919^ Report of the imperial economic botanists, 1918-19. 
 
 . 1920. Some aspects of the indigo industry in Bihar. Mem. Dept. 
 
 Agr. India, Bot. Ser. 11:1. 
 HuBER, C. F., and J. Senebier. 1801. Memoires sur I'influence de I'air et de diverses 
 
 substances gazeuses dans la germination de diff6rentes graines. 
 Humboldt, A. 1793. Flora fribergensis. 
 
 . 1794. Aphorismen. p. 68. 
 
 Hunter, C. 1912. Some observations on the effect of soil aeration on plant growth. 
 
 Proc. Univ. Durham Phil. Soc. 4:183. 
 Hutchinson, C. M. 1916. A modified method of green-manuring. Pusa Agr. Res. 
 
 Inst. Bull. 63. 
 Hutchinson, H. B." 1918. The influence of plant residues on nitrification and on losses 
 of nitrate in the soil. Jour. Agr. Sci. 9:92; Bot. Abs. 2:97. 
 
 , and A. C. Thaysen. 1918. The non-persistence of bacterio-toxins in the soil. 
 
 Jour. Agr. Sci. 9:43. 
 HuYGENS, C, and D. Papin. 1674. Experiences du vuide. 
 
 , . 1675. Experiences du vuide. Phil. Trans. 
 
 Ingenhousz, J. 1779. Experiments upon vegetables, discovering their great power of 
 purifying the common air in the sunshine, and of injuring it in the shade and at night. 
 
 . 1796. Nutrition of plants and fertility of the soil. 
 
 Ihaklionow, p. p. 1912. Ueber den Einfluss des Warmbads auf die Atmung und Keimung 
 
 der ruhenden Pflanzen. Jahrb. Wiss. Bot. 51:515. 
 IvANOFF, L. 1911. Ueber die sogenannte Atmung der zerriebenen Samen. Ber. Deut. 
 
 Bot. Ges. 29:563. 
 IWANOWSKY, D. 1894. Untersuchungen iiber Alkoholgarung. 
 Jaccard, p. 1893. Influence de la pression des gaz sur le d^veloppement des v^g^taux. 
 
 Rev. Gen. Bot. 5:289. 
 Jensen, G. H. 1907. Some mutual effects of tree roots and grasses upon soil. Science 
 
 25:871. 
 Jentys, S. 1888. Ueber den Einfluss hoher Sauerstoffpressungen auf das Wachsthum der 
 Pflanzen. Unters. Bot. Inst. Tubingen 2:419. 
 
BIBLIOGRAPHY. 173 
 
 Jentys, S. 1892. Sur rinfluence de la pression partiale de I'acide carbonique dans I'air 
 
 sousterrain sur la v6g6tation. Bull. Acad. Sci. Cracovie 306; Bot. Cent. 52:93. 
 JoDiDi, S. L., and A. A. Wells. 1911. Influence of soil factors on the decomposition of 
 
 soil organic matter. Iowa Agr. Exp. Sta. Res. Bull. 3:135. 
 . 1914. Ueber den gegenwartigen Stand der Bodenchemie mit besonderer Beriick- 
 
 sichtigung der organischen Verbindungen. Landw. Versuchst. 85:359. 
 JoDiN, V. 1897. Recherches sur la germination. Ann. Agron. 23:433. 
 JoHANNSEN, W. 1885. Ueber den Einfluss hdher Sauerstoffspanning auf die Kohlen- 
 
 saureausscheidung einiger Keimpflanzen. Unters. Bot. Inst. Tubingen. 1 :686. 
 . 1887. Ueber Fortdauer der "Athmungs-Oxydation" nach dem Tode. Bot. 
 
 Zeit. 45:762. 
 John, J. F. 1819. Ueber die Ernjihrung der Pflanzen. 
 Johnson, J. 1916. The determination of carbonic acid, combined and free in solution, 
 
 particularly in natural waters. Jour. Am. Chem. Soc. 38:947. 
 Jones, L. R., and W. J. Morse. 1903. The shrubby cinquefoil as a weed. Vt. Agr. 
 
 Exp. Sta. Rep. 16:187. -■ — 
 
 Jordan, W. H. 1920. The influence of fertilizers and plant growth upon soil solubles. 
 
 N. Y. Agr. Exp. Sta. Bull. 473. 
 JoST, L. 1887. Ein Beitrag zur Kenntniss der Athmungsorgane der Pflanzen. Bot. 
 
 Zeit. 45:601, 639. 
 
 . 1893. Wachstum wiihrend der Ruheperiode. Bot. Zeit. 51:100. 
 
 JtJMELLE, H. 1892. Recherches physiologiques sur les Lichens. Rev. Gen. Bot. 4:49. 
 Kabsch, W. 1862. Ueber die Einwirkung verschiedener Gase und des luftverdiinnten 
 
 Raumes auf die Bewegungserscheinungen im Pflanzenreiche. Bot. Zeit. 20:341. 
 Kaserer, H. 1905. Ueber die Oxydation des Wasserstoffes und des Methans durch 
 
 Mikroorganismen. Zeit. Landw. Vers. Oester.; Bot. Cent. 101:71. 
 Keller, I. A. 1890. Ueber Protoplasmastromung im Pflanzenreich. 
 Kellerm.\n, K. F., I. G. McBbth, and F. M. Scales. 1912. Fermentation of cellulose. 
 
 Science 35:199. 
 Kenkel, J. 1913. Ueber den Einfluss der Wasserinjektion auf Geotropismus und Helio- 
 
 tropismus. 
 KiDD, F. 1914. The controlling influence of carbon dioxid in the maturation, dormancy, 
 
 and germination of seeds. I and II. Roy. Soc. Proc. B. 87; III. Roy. Soc. Proc. B. 
 
 1915. 
 . 1919. Laboratory experiments on the sprouting of potatoes in various gas mix- 
 tures. New Phyt. 18:248; Bot. Abs. 5:135. 
 , and C. West. 1917. The controlling influence of carbon dioxid: IV. On the 
 
 production of secondary dormancy in seeds of Brassica alba following treatment with 
 
 carbon dioxid. Ann. Bot. 3:457. 
 Kihlmann, a. O. 1890. Pflanzenbiologische Studien aus Russisch Lappland. Ein 
 
 Beitrag zur Kenntniss der regionalen Gliederung an der polaren Waldgrenze. Acta 
 
 Soc. Fauna Flora Fenn. 4:107. 
 King, F. H. 1908. Toxicity as a factor in the productive capacity of soils. Science 
 
 27:631. 
 Knop, W. 1864. Chemische Untersuchungcn uber die Erniihrung der Pflanze. Ann. 
 
 Chem. Pharm. 129:298, 330. 
 . 1861. Ueber ein elementar-analytisches Verfahren der Kohlenstoffbestimmung. 
 
 Chem. Centb. 6:17. 
 Knttdson, L. 1920. The secretion of invertase by plant roots. Am. Jour. Bot. 7:371. 
 Kny, L. 1889. Ueber die Bildung des Wundperiderms an Knollen in ihrer Abhangigkeit 
 
 von ausseren Einflussen. Ber. Deut. Bot. Ges. 7:163. 
 Koch, G. P. 1915. Activity of soil protozoa. U. S. Dept. Agr., Jour. Agr. Res. 5:477. 
 KoLKwaxz, R. 1901. Ueber die Athmung ruhender Samen. Ber. Bot. Deut. Ges. 19:285. 
 Kosaroff, p. 1897. Einfluss verschiedener ausserer Factoren auf die Wasseraufnahme 
 
 der Pflanzen. Diss. Leipzig; Naturw. Rundschau 12:604; Bot. Cent. 74:351; 
 
 Sorauer 1909:99. 
 . 1900. Die Wirkung der Kohlensaure auf den Wassertransport in den Pflanzen. 
 
 Bot. Cent. 83:138. 
 KossowiTSCH, p. 1892. Durch welche Organe nehmen die Leguminosen den freien 
 
 Stickstoff auf . Bot. Zeit. 50:698. 
 . 1894. Untersuchungen iiber die Frage, ob die Algen Stickstoff fixiren. Bot. 
 
 Zeit. 52:98. 
 
174 AERATION AND AIR-CONTENT. 
 
 KosTYTSCHEW, S. 1907. Zur Frage der Wasserstoffbildung bei der Atmung der Pilze. 
 
 Ber. Deut. Bot. Ges. 25:178. 
 . 19072. Ueber anaerobe Atmung ohne Alkoholbildung. Ber. Deut. Bot. Ges. 
 
 25: 188. 
 . 1908. Zweite Mittheilung iiber anaerobe Atmung ohne Alkoholbildung. Ber. 
 
 Deut. Bot. Ges. 26a:167. 
 . 1908. Ueber den Zusammenhang der Sauerstoffatmung der Samenpflanzen mit 
 
 der Alkoholgfirung. Ber. Deut. Bot. Ges. 26:565. 
 . 1913. Ueber das Wesen der anaeroben Atmung verschiedener Samenpflanzen. 
 
 Ber. Deut. Bot. Ges. 31:125. 
 Kraus, G. 1884. Ueber die Wasservertheilung in der Pflanze. IV. Die Aciditat des 
 
 Zellsaftes. Abh. Naturf. Ges. Halle 16:151. 
 . 1885. Ueber Stoffweschel bei den Crassulaceen. Abh. Naturf. Ges. Halle 
 
 16:393. 
 . 1897. Ueber das Verhalten des Kalkoxalats beim Wachsen der Organe. Flora 
 
 83:54. 
 KiJHNE. 1864. Untersuchungen iiber das Protoplasma und die Kontraktilitat. 
 . 1897-8. Ueber die Bedeutung des Sauerstoffs fiir die vitale Bewegung. Zeit. Biol. 
 
 35:43, 36:425. 
 KuNZE, G. 1906. Ueber Saureausscheidung bei Wurzel und Pilzhyphen und ihre Bedeu- 
 tung. in Schleichert Neuere Untersuchungen iiber Wurzelhaare und ihre Secrete. 
 
 Naturw. Wochens 22:93:1907. 
 Lau, E. 1906. Beitrage zur Kenntniss der Zusammensetzung der im Ackerboden befind- 
 
 Uchen Luft. Diss. Rostock. 
 Leather, J. W. 1915. Soil gases. Mem. Dept. Agr. India, Chem. Ser. 4:85. 
 Lechartier, G. 1867. Sur la movement des gaz dans les plantes aquatiques. Comp. 
 
 Rend. 65:1987. 
 . 1881. Sur la fermentation et la conservation des fourrages verts in silo. Ann. 
 
 Agron. 7:481. 
 
 , and F. Bellamy. 1869. De la fermentation des fruits. Comp. Rend. 69:466. 
 
 , . 1872. De la fermentation des fruits. Comp. Rend. 75:1202. 
 
 , . 1874. De la fermentation des pommes et des poires. Comp. Rend. 
 
 79:949. 
 
 , . 18742. De la fermentation des fruits. Comp Rend. 79:1006. 
 
 Lefebure, E. A. 1801. Experiences sur la germination des plantes. 
 
 Lehmann, E. 1911. Zur Kenntniss des anaeroben Wachstums hoherer Pflanzen. Jahrb. 
 
 Wiss. Bot. 49:61. 
 Lesage, p. 1891. Contributions k la physiologic de la racine. Comp. Rend. 112:109. 
 Letts, E. A., and R. F. Blake. 1900. The carbonic anhydride of the atmosphere. Proc. 
 
 Roy. Soc. Dublin 9:214. 
 Lewakoffski, N. 1873. Ueber den Einfluss des Wassers auf das Wachstum der Stengel 
 
 und Wurzeln einiger Pflanzen. Just Bot. Jahrb. 594. 
 LiEBiG, J. 1858. Ueber einige Eigenschaften der Ackerkrume. Ann. Chem. Pharm. 
 
 105:109. 
 Link, H. F. 1848. Ueber Aussonderungen der Wurzelspitzen. p. 591. 
 LiNOSSiER, G. 1888. Influence de I'oxyde de carbone sur la germination. Comp. Rend. 
 
 Soc. Biol. 40:565. 
 . 1889. A propos de Taction de I'oxyde de carbone sur la germination. Comp. 
 
 Rend. 108:820. 
 Livingston, B. E. 1904. Physical properties of bog water. Bot. Gaz. 37:383. 
 
 . 1905. Physiological properties of bog water. Bot. Gaz. 39:348. 
 
 — c-rrri_J.fiQI-— Further studies on the properties of unproductive soils. U. S. Dept. 
 
 Agr., Bur. Soils Bull. 36. 
 
 . 1918. Palladin's Plant Physiology, Eng. ed. 
 ■ , J. C. Britton, and F. R. Reed. 1905. Studies on the properties of an unpro- 
 ductive soil. U. S. Dept. Agr., Bur. Soils Bull. 28. 
 , and E. E. Free. 1915. The relation of soil aeration to plant growth. Carnegie 
 
 Inst. Wash. Year Book 14:60. 
 , . 1916. Relation of soil aeration to plant growth. Carnegie Inst. Wash. 
 
 Year Book 15:78. 
 
 1917. The effect of deficient soil oxygen on the roots of higher plants. 
 
 Johns Hopkins Univ. Circ. 182. 
 
BIBLIOGRAPHY. 175 
 
 LoEW, O. 1913. Studies on acid soils of Porto Rico. Porto Rico Agr. Exp. Sta. Bull. 13. 
 LoPRioRE, G. 1895. Ueber die Einwirkung der Kohlensaure auf das Protoplasma der 
 
 lebenden Pflanzenzelle. Jahrb. Wiss. Bot. 28:531. 
 LoviNSON, O. 1900. Ueber Keimungs- und Wachsthumsversuche an Erbsen in Losungen 
 
 von fettsauren Salzen unter Abschluss von Mineralsauren. Bot. Cent. 83:217. 
 LovEN, H. 1891. Nagra ron om algernas anding. Bih. Sven. Vet. Akad. Handl. 17:3:3. 
 LuKAS, F. 1886. Versuche iiber die Keimung und das Wachstum von Pflanzen im Luft- 
 
 verdunnten Raum. Lotos 7; Bot. Cent. 28:298. 
 Lyon, T. L., and James A. Bizzell. 1913. Is there a mutual stimulation of plants through 
 
 root influence? Jour. Am. Soc. Agron. 5:38. 
 Macaire, H. 1832. M^moire pour servir k I'histoire des assolemens. Mem. Soc. Phys. 
 
 Gen^vee 5:282. (Cf. DeCandolle Physiologic V6g6tale). 
 . 1833. Denkschrift zur Geschichte der Wechselwirtschaft. Ann. Pharm. 8:78. 
 
 (Cf. Ann. Chim. Phys. 42:325.) 
 McKenna. 1918. The importance of soil ventilation. Agr. Jour. India 13:148. 
 Magness, J. R. 1920. Composition of gases in intercellular spaces of apples and po- 
 tatoes. Bot. Gaz. 70:308. 
 Main, T. H. 1916. Rotation and Tillage. Dept. Agr. Bombay Ann. Rep., p. 47. 
 
 . 1918. Irrigation in Sind. Agr. Jour. Ind. 13:653. 
 
 Malpighi, M. 1687. Opera omnia. 
 
 Mangin, L. 1895. Sur I'aiiration du sol dans les promenades et plantations de Paris. 
 
 Comp. Rend. 120:1065. 
 . 1896. Sur la v6g6tation dans une atmosphere vici6e par la respiration. Comp. 
 
 Rend. 122:747. 
 . 1896. Etude sur la v6g6tation dans ses rapports avec I'adration du sol. Recher- 
 
 ches sur les plantations des promenades de Paris. Ann. Sci. Agron. 2:1. 
 Maquenne, L. 1894. Sur la respiration des feuilles. Comp. Rend. 119:100. 
 Marcet, F. 1829. Note relative k Taction des champignons sur I'air et sur I'eau. Ann. 
 
 Chim. Phys. 40:318. 
 . 1834. Recherches sur les modifications qu'6prouve I'atmosphere par le contact 
 
 de certains v^g6taux d^pourvus de parties vertes. Bibl. Univ. Geneve 57:393. 
 Martins, C. 1866. M^moire sur les racines a^riferes ou vessies natatoires des espSces 
 
 aquatiques du genre Jussicea. Mem. Acad. Montpellier 6:353. 
 Mayer, A. 1875. Ueber die Bedeutung der organischen Sauren in den Pflanzen. Landw. 
 
 Versuchst. 18:428. 
 . 1878. Ueber die Sauerstoffausscheidung einiger Crassulaceen. Landw. Ver- 
 suchst. 21:277. 
 . 1884. Heine Beitrage zur Frage der Sauerstoffausscheidung in den Crassulaceen- 
 
 blattern. Landw. Versuchst. 30:217. 
 . 1887. Die Sauerstoffausscheidung einiger dickblattrigen Pflanzen, bei Abwesen- 
 
 heit von Kohlensaure und die physiologische Bedeutung dicser Erscheinung. Landw. 
 
 Versuchst. 34:127. 
 Mayow, J. 1668(1674). Tractatus quinque medico-physici. 
 Maze, P. 1899. Signification physiologique de I'alcool dans la rSgne vdg(5tale. Comp. 
 
 Rend. 128:1608. 
 . 1900. Recherches sur le role de I'oxygen dans la germination. Ann. Inst. Pas- 
 teur 350. 
 Meggitt, a. a. 1914. Studies of an acid soil in Assam. Mem. Dept. Agr. India, Chem. 
 
 Ser. 3:9. 
 Meigen, F. 1894. Biologische Beobachtungen aus der Flora Santiagos de Chile. Engler 
 
 Jahrb. 18:394. 
 Meyen, F. 1838. Neues System der Pflanzenphysiologie. 2:62, 524. 
 Meyer, E. 1830. De plantis labradoricis. p. 29. 
 
 Mirasol, J. J. 1920. Aluminum as a factor in soil acidity. Soil Science 10:153. 
 Mirbel, C. F. 1815. Physiologic veg^tale. 
 Missaghi, G. 1875. Sulla emissione dell'idrogeno nella vegetazione delle muffe. Gazz. 
 
 Chim. Ital. 5:419; Just Bot. Jahrb. 4:116, 1876. 
 Mitscherlich, E. a. 1910. Ein Beitrag zu Kohlensaure Diingung. Landw. Jahrb. 
 
 39:158. 
 Miyake, K. 1914. Ueber die Wirkung von Sauren, Alkalien und einiger Alkali auf dem 
 
 Wachstum der Reispflanzen. Trans. Sapporo Nat. Hist. Soc. 5:91. 
 
176 AERATION AND AIR-CONTENT. 
 
 MiTAKE, K. 1916. The toxic action of soluble aluminum salts upon the growth of the 
 
 rice plant. Jour. Biol. Chera. 25:23. 
 MoHL, H. 1838. Untersuchung uber die Wurzelausscheidung. 
 
 . 1851. Grundzuge der Anatomie und Physiologic der vegetabiUschen Zelle. 
 
 MoiLLARD, M. 1914. Sur la secretion par les racines de substances toxiques pour les 
 
 plantes (note pr^hminaire.) Bull. Soc. Bot. Fr. 
 MoLDENHAWER, J. J. 1812. Beitragc zur Anatomie der Pflanzen. 
 MoLiscH, H. 1884. Ueber die Ablenkung der Wurzeln von ihrer normalen Wachstum- 
 
 richtung durch Gase (Aerotropismus.) Sitzb. Akad. Wiss. Wien. 89:111; Ber. Deut. 
 
 Bot. Ges. 2:160. 
 . 1887. Ueber Wurzelausscheidungen und deren Einwirkung auf organische 
 
 Substanzen. Sitzb. Akad. Wiss. Wien. 96:84. 
 . 1893. Zur Physiologic des Pollens mit besonderer Rlicksicht auf die chemotropis- 
 
 chen Bewegungcn der Pollenschleuche. Sitzb. Akad. Wiss. Wien. 102:423. 
 MoLLER, H. 1884. Ueber Pflanzenatmung. Ber. Deut'. Bot. Ges. 2:35. 
 MoLLER, J. 1878. Ueber die freic Kohlensaure im Boden. Mitt. Forst. Vers. Oester. 
 
 2:121; Ref. Forsch. 1:162, 1878. Forsch. Geb. Agr. Phys. 2:329, 1879. 
 MoNTEMARTiNi, L. 1894. Sull I'influcnza di atmosfere ricche di biossido di carbonic 
 
 sopra lo sviluppo e la struttura delle foghe. Att. 1st. Bot. Pa via 3:83. 
 Moore, G. T., and J. L. Karrer. 1919. A subterranean algal flora. Ann. Mo. Bot. 
 
 Gard. 6:281. 
 MoRREN, A., and C. F. A. Morren. 1841. Recherches physiologiques sur les hydro- 
 phytes de Belgique. Troisieme memoire, contenant les recherches physiologiques, 
 
 zoologiques, botaniques, et chimiques sur I'influence qu'exercent la lumi^re, les 
 
 algues et les animalcules de couleur verte ou rouge, contenus dans les eaux stagnantes 
 
 et courantes, sur la quantite et la quality des gaz que celles-ci peuvent contenir. 
 Mulder, G. J. 1840. Untersuchungen liber die Humussubstanzen. Jour. Prakt. Chem. 
 
 21:203,321. 
 MiJLLER-THURGAtr, H. 1882. Beitrag zur Erklarung der Ruheperiode der Pflanzen. 
 
 Landw. Jahrb. 11:751. 
 
 . 1885. Landw. Jahrb. 14:859. 
 
 , and Schneider-Orelli. 1910-12. Beitrage zur Kenntniss der Lebensvorgange 
 
 in ruhender Pflanzen. Flora 101:309; 104:386. 
 MtJNTZ, M. A. 1876. Recherches sur les fonctions des champignons. Ann. Chim. Phys. 
 
 5:8:67. 
 . 1876. Recherches sur la fermentation alcoohque intracellulaire des v^g^taux. 
 
 Ann. Chim. Phys. 5:13:558. 
 Musschenbroek, p. 1729. Elementa physica. 
 Nabokich, a. J. 1900. Ueber anaerobes Wachstum der Wurzeln. Jour. Exp. Landw. 
 
 1:660. 
 . 1901. Wie die Fahigkeit der hoheren Pflanzen zum anaeroben Wachstum zu 
 
 beweisen ist. Ber. Deut. Bot. Ges. 19:222. 
 . 1901. Ueber Bedetung des Zuckers fiir anaerobes Wachstum. Versam. Naturf. 
 
 Petersburg. 
 
 . 1902. Zur Physiologic des anaeroben Wachstums. Naturf. Ges. Warsaw. 
 
 . 1902^. Ueber die Ursachen der Absterbungserscheinungen im sauerstoffreien 
 
 Medium und ueber die anaerobe Salpeterreduktion durch Samen. Naturf. Ges. 
 
 Warsaw. 
 . 1903. Zur Physiologic des anaeroben Wachstums der hoheren Pflanzen. Beih. 
 
 Bot. Cent. 13:272. 
 . 1909. Temporare Anaerobiose hohere Pflanzen. Landw. Jahrb. 38:51; Zeit. 
 
 Bot. 731. 
 Nabokich, O. 1904. Ueber anaerobe Zellteilung. Ber. Deut. Bot. Ges. 22:62. 
 Nakano, H. 1917. Untersuchungen iiber die Entwicklungs- und Ernahrungsphysiologie 
 
 einiger Chlorophyceen. Jour. Coll. Sci. Univ. Tokyo 40:66. 
 Neller, J. R. 1920. The oxidizing power of soil from limed and unlimed plots and its 
 
 relation to other factors. Soil Science 10:29. 
 Newcombe, F. C. 1902. The rheatropism of roots. Bot. Gaz. 33:343. 
 Nobbe, F. 1865. Ueber die Zu- und Abnahmc des Starkegehalts der Kartoffelknolle. 
 
 Landw. Versuchst. 7:451. 
 Notes, H. A. 1914. Effect on plant growth of soil taturated with carbon dioxid. 
 
 Science 40:792. 
 
BIBLIOGRAPHY. 177 
 
 Notes, H. A., and L. Yoder. 1918. Carbonic acid gas in relation to soil acidity changes. 
 
 Soil Science 5:151. 
 Ortloff, II. 1900. Der Einfluss der Kohlensaure auf die Garung. Cent. Bakt. 6:753. 
 Otis, C. H. 1914. The transpiration of immersed water plants: Its measurement and 
 
 relationship. Bot. Gaz. 58:457. 
 OuDEMANs, A. C, and N. W. P. Rauwenhoff. 1858. De scheikundige Verschynselen 
 
 by de Kieming der Planten-Zaden. Jahrber. Chem., p. 493; Linnaea 30:213 (1859). 
 Overton, J. B. 1911. Studies on the relations of the living cells to the transpiration and 
 
 sap-flow in Cyperus. Bot. Gaz. 51:28. 
 Paal, a. 1912. Analyse des geotropischen Reizvorgangs mittelst Luftverdvinnung. 
 
 Jahrb. Wiss. Bot. 50: 
 Pack, D. A. 1921. After-ripening and germination of /um'pcri^s seeds. Bot. Gaz. 71:32. 
 Palladin, W. 1886. Atmung und Wachstum. Bcr. Deut. Bot. Ges. 4:322. 
 . 1886. Bedeutung des Saucrstoffs fur die Pflanzen. Bull. Soc. Imp. Nat. Mos- 
 
 cou. 3:44; Bot. Cent. 33:102. 
 . 1888. Ueber Eiweisszersetzung in den Pflanzen bei Abwesenheit von freiem 
 
 SauerstofT. Ber. Deut. Bot. Ges. 6:205. 
 . 1888. Ueber Zersetzungsproducte der Eiweissstoff im den Pflanzen bei Abwesen- 
 heit von freiem Sauerstoff. Ber. Deut. Bot. Ges. 6:296. 
 . 1894. Die Bedeutung der Kohlehydrate fiir die intramoleculare Athmung der 
 
 Samenpflanzen. Arb. Naturf. Charkow. Bot. Cent. 59:243. 
 . 1897. Influence de diverses .substances et influence de I'oxygfine sur la formation 
 
 de la chlorophylle. Comp. Rend. 125:827; Bot. Cent. 74:21. 
 , and S. Kostytschew. 1906. Anaerobe Atmung, Alcoholgarung, und Aceton- 
 
 bildung bei den Samenpflanzen. Hoppe-Seyler Zeit. Phy.siol. Chem. 48:214. 
 , . 1907. Ueber Anaerobe Atmung der Samenpflanzen ohne Alkoholbil- 
 
 dung. Ber. Deut. Bot. Ges. 25:51. 
 Parker, E.G. 1913. Selective adsorption by soils. U. S. Dept. Agr., Jour. Agr. Res. 1:179. 
 Pasteur, L. 1861. Influence de I'oxyg^ne sur la ddveloppement de la levtire et sur la 
 
 fermentation alcoolique. Bull. Soc. Chim. 
 
 . 1872. Note sur la production de I'alcool par les fruits. Comp. Rend. 75:1064. 
 
 . 1872. Faits nouvaux pour servir 6. la connaissance de la th6orie des fermenta- 
 tions proprement dites. Comp. Rend. 75:784. 
 
 ■ . 1876. fitude sur la hihre. 
 
 Patten, A. J. 1910. Organic nitrogenous compounds in peat soils. Mich. Agr. Exp. 
 
 Sta. Rep. 23:509. 
 Payer, J. B. 1842. M^moire sur la tendance des tiges vers la lumilre. Comp. Rend. 
 
 15:1194. 
 Pearsall, W. H. 1920. The aquatic vegetation of English lakes. Jour. Ecol. 8:163. 
 Pember, F. R. 1917. Studies by means of both pot and solution cultures of the phos- 
 phorous and potassium requirements of the barley plant during its different periods 
 
 of growth. R. I. Agr. Exp. Sta. Bull. 169:25. 
 Perseke, K. 1877. Formveranderung der Wurzel in Wasser und Erde. Diss. Leipzig. 
 Peterson, J. B. 1915. Studier over Danske aerofile alger. Dan. Vidselsk. Skrift. 12:269. 
 Pettenkoffer, M. 1871. Ueber Kohlensaurcgehalt der Grundluft im Gcrollboden von 
 
 Miinchen in verschiedenen Tiefen und zu verschiedenen Zciton. Zeit. Biol. 7:395. 
 . 1873. Ueber den Kohlensaurcgehalt der Grundluft im GeroUboden von Miinchen 
 
 in verschiedenen Tiefen und zu verschiedenen Zeiten. Zeit. Biol. 9:250. 
 Pfeffer, W. 1871. Die Wirkung farbigen Lichtes auf die Zersetzung der Kohlensaure 
 
 in Pflanzen. Arb. Bot. Inst. Wiirzburg. 1 :33. 
 . 1878. Das Wesen und die Bedeutung der Atmung in der Pflanze. Landw. 
 
 Jahrb. 7:805. 
 
 . 1885. Ueber intramoleculare Athmung. Unters. Bot. Inst. Tubingen 1:636. 
 
 . 1889. Beitrage zur Kenntniss der Oxidationsvorgange in lebenden Zellen. 
 
 . 1897. Pflanzenphysiologic. 2 Aufl. 
 
 . 1900. The physiology of plants. A treatise upon the metabolism and sources 
 
 of energy in plants. 2d ed. (Ewart). 
 Pickering, S. U. 1917. The effect of one plant on another. Ann. Bot. 31:181; Bot. 
 
 Abs. 2:44. 
 
 . 1919. The action of one crop on another. Jour. Roy. Hort. Soc. 43:372. 
 
 PiPAL, F. J. 1916. Red sorrel and its control. Ind. Agr. Exp. Sta. Bull. 197. 
 Plenck, J. J. 1795. Physiologic und Pathologie der Pflanzen. 
 
178 AERATION AND AIR-CONTENT. 
 
 Plummer, J. K. 1918. Studies in soil reaction as indicated by the hydrogen electrode. 
 
 U. S. Dept. Agr., Jour. Agr. Res. 12:19. 
 PoLowzow, W. 1901. Untersuchungen uber die Atmung der Pflanzen. Abh. Acad. 
 
 Wiss. Petersburg 8:12:62. 
 Potter, R. S., and R. S. Snyder. 1918. The production of carbon dioxid by molds 
 
 inoculated into sterile soil. Soil Science 5:359. 
 Pringsheim, N. 1888. Ueber Inanition der griinen Zelle und den Ort ihrer Sauerstofif 
 
 Abgabe. Ber. Deut. Bot. Ges. 5:249. 
 PtJTTER, A. 1904. Die Wirkung erhohter Sauerstoflfspannung auf die lebendige Substanz. 
 
 Zeit. Allgem. Physiol. 3:363; Bot. Cent. 101:172. 
 Raciborski, M. 1905. Oxydirende und reducirende Eigenschaften der lebenden Zelle: 
 
 I. Ueber die oxydirende Fahigkeit der Resorptionflache der Wurzel der Bluthen- 
 
 pflanzen. Bull. Acad. Sci. Cracovie, p. 338. 
 Ramann, E. 1911. Bodenkunde. 3 Aufl. 
 Ray, J. 1686. Historia Plantarum. 
 Rayner, M. C. 1913. The ecology of Calluna vulgaris. New Phyt. 12:59. 
 
 . 1915. Obligate symbiosis in Calluna vulgaris. Ann. Bot. 29:97. 
 
 , and W. N. Jones, 1911. Preliminary observations on the ecology of Calluna 
 
 vulgaris on the Wiltshire and Berkshire Downs. New Phyt. 10:227. 
 Reinke, J. 1887. Zur Kenntniss der Oxydationsvorgange in der Pflanzn. Ber. Deut. 
 
 Bot. Ges. 5:216. 
 Renk, F. G. 1878. Untersuchungen der Grundluft im Krankenhause Miinchen. Ann. 
 
 Krankenh. Miinchen. 1: 
 Rice, F. E. 1916. Studies on soils: I. Basic exchange. Jour. Phys. Chem. 20:214. 
 , and S. OsuGi. 1918. The inversion of cane sugar by soils and allied substances 
 
 and the nature of soil acidity. Soil Science 5:333. 
 Richards, E. H. 1917. Dissolved oxygen in rain-water. Jour. Agr. Sci. 8:331. 
 Richards, H. M. 1896. The respiration of wounded plants. Ann. Bot. 10.531. 
 RiGG, G. R. 1913. The effect of some Puget Sound bog waters on the root hairs of Trades- 
 
 cantia. Bot. Gaz. 55:314. 
 
 . 1916. Physical conditions in Sphagnum bogs. Bot. Gaz. 61:159. 
 
 . 19162. The toxicity of bog water. Am. Jour. Bot. 3:436. 
 
 . 1916^ Decay and soil toxins. Bot. Gaz. 61:295. 
 
 . 1916*. A summary of bog theories. Plant World 19:310. 
 
 . 1918. Growth of trees in Sphagnum. Bot. Gaz. 65:359; Bot. Abs. 4:37. 
 
 , H. L. Trumbull, and Mattie Lincoln. 1916. Physical properties of some toxic 
 
 solutions. Bot. Gaz. 61:408. 
 RiscHAwi, L. 1876. Einige Versuche iiber die Athmung der Pflanzen. Landw. Versuchst. 
 
 19:321. 
 
 . 1877. Zur Frage iiber die Atmung der Pflanzen. Just. Bot. Jahrb. 5:721. 
 
 RiTTER, G. 1899. Die Abhangigkeit der Plasmastromung und der Geiselbewewung voo 
 
 freien Sauerstoff. Flora 86:329. 
 RoBBiNS, W. W. 1912. Algaj in some Colorado soils. Col. Agr. Exp. Sta. Bull. 184. 
 RoLLO, J. 1798. Experiences et Observations sur le Sucre. Ann. Chim. 25:37. 
 Roper, J. A. C. 1833. De Candolle's Pflanzenphysiologie, 2:219. 
 RosANOFF, S. 1871. Ueber den Bau der Schwimmorgane von Desmanthus natans. 
 
 Willd. Bot. Zeit. p. 829. 
 Rose, D. H. 1915. A study of delayed germination in economic seeds. Bot. Gaz. 59:435. 
 RuPRECHT, R. W. 1915. Toxic effect of iron and aluminum salts on clover seedlings. 
 
 Mass. Agr. Exp. Sta. Bull. 161. 
 , and F. W. Morse. 1917. The cause of the injurious effect of sulfate of ammonia 
 
 when used as a fertilizer. Mass. Agr. Exp. Sta. Bull. 176. 
 Russell, E. J. 1912. Soil conditions and plant growth. 
 , and A. Appleyard. 1915. The atmosphere of the soil: Its composition and 
 
 causes of variation. Jour. Agr. Sci. 7:1, 240. 
 , . 1917. The influence of soil conditions on the decomposition of organic 
 
 matter in the soil. Jour. Agr. Sci. 8:385. 
 , and E. H. Richards. 1919. The amount and composition of rain falling at 
 
 Rothamsted. Jour. Agr. Sci. 9:309. 
 Sabline, V. 1903. L'influence des agents externes sur la division des noyaux dans les 
 
 racines de Viciafaba. Rev. Gen. Bot. 15:488. 
 
BIBLIOGRAPHY. 179 
 
 Sachs, J. 1860. Physiologische Mittheilungen verschiedenen Inhaltes: II. Auflos- 
 
 ung des Marmors durch Maizwurzeln. Bot. Zeit. 18:117. 
 . 1865. Corrosion von Mineralien durch Wurzeln. Handbuch der Experimental- 
 
 Physiologie der Pflanzen. 
 Sachsse, R. 1872. Ueber einige chemische Vorgange bei der Keimung von Pisum sativum. 
 
 Chem. Centb. 3:137. 
 Sadebeck, R. 1881. Beobachtungen und Untersuchungen liber die Pilzvegetation in der 
 
 Umgebung von Hamburg. Verb. Ges. Bot. Hamburg 2: 
 Saikewicz, a. 1877. Recherches physiologiques sur la respiration des racines. Trav. 
 
 Hist. Nat. Univ. Charkoff; Ann. Agron. 7:476, 1881. 
 Salger, C. 1882. Boden Untersuchungen mit besonderer Berucksichtigung des Ein- 
 
 flusses der Ventilation auf die Kohlensiiuremenge. Diss. Erlangcn. 
 Samassa, p. 1900. Ueber die Einwirkung von Gasen auf die Protoplasmastromung und 
 
 Zelltheilung von Tradescantia, sowie auf die Embryonalentwickelung von Rana und 
 
 Ascaris. Verb. Natur. Ver. Heidelberg 4:1; Bot. Cent. 84:320. 
 Sammet, R. 1905. Untersuchungen liber Chemotropismus und verwandte Erscheinungen 
 
 bei Wurzeln, Sprossen und Pilzfaden. Jahrb. Wiss. Bot. 41 :621 ; Bot. Cent. 101 :575. 
 Sampson, A. W. 1912. The relation of soil acidity to plant societies. Proc. Soc. Am. 
 
 For. 7:51. 
 , and L. M. Allen. 1909. Influence of physical factors on transpiration. Minn. 
 
 Bot. Studies 4:33. 
 Saussure, T. 1804. Recherches chimiques sur la v^g^tation. 
 ScHAEFFER, G., 1919. Facteurs acccssoircs de la croissancc et dcl'dquiUbre. Vitamines; 
 
 auximones. Bull. Inst. Pasteur 17:1. 
 ScHAiBLE, F. 1900. Physiologische Experimente liber das Wachstura und die Keimung 
 
 einiger Pflanzen unter verminderten Luftdruck. Fiinfstuck Beit. Wiss. Bot. 4:93; 
 
 Bot. Cent. 82:52. 
 ScHEELE, C. W. 1777. Chemische Abhandlungen von Luft und Feuer. 
 ScHENCK, H. 1889. Ueber die Luftwurzein von Avicennia tomeniosa und Laguncularia 
 
 racemosa. Flora, 72:83. 
 . 1889. Ueber das Aerenchym, ein dem Kork homologes Gewebe bei Sumpf- 
 
 pflanzen. Jahrb. Wiss. Bot. 20:526. 
 ScHiMPER, A. F. W. 1890. Ueber Schutzmittel des Laubes gegen Transpiration, beson- 
 
 ders in der Flora Javas. Sitzb. Akad. Wiss. Berlin 1890:1045. 
 
 . 1898 Pflanzengeographie auf physiologischer Grundlage. 1903. EngUsh ed. 
 
 Schley, E. O. 1920. Geo-prescntation and geo-reaction. Bot. Gaz. 70:69. 
 ScHLOESiNG, T. 1873. fitude de la nitrification dans les sols. Comp. Rend. 77:203. 
 
 . 1889. Sur la'atmosphere confine dans le sol. Comp. Rend. 109:618. 
 
 . 1889=. Sur I'atmosphere contenue dans le sol. Comp. Rend. 109:673. 
 
 ScHREiNER, O. 1913. The organic constituents of soils. U. S. Dept. Agr. Bur. Soils 
 
 Circ. 74. 
 , and H. S. Reed. 1907. Some factors influencing soil fertility. U. S. Dept. 
 
 Agr., Bur. Soils Bull. 40:190. 
 , . 1907^. Certain organic constituents of soils in relation to soil fertility. 
 
 U. S. Dept. Agr., Bur. Soils Bull. 47. 
 , . 1907^ The production of deleterious excretions by roots. Bull. Torr. 
 
 Bot. Club 34:271. 
 , . 1909. The role of oxidation in soil fertility. U. S. Dept. Agr., Bur 
 
 Soils Bull. 56. 
 , and E. C. Shorey. 1909. The isolation of harmful organic substances from 
 
 soils. U. S. Dept. Agr., Bur. Soils Bull. 53. 
 , and J. J. Skinner. 1910. Some effects of a harmful organic soil constituent. 
 
 U. S. Dept. Agr., Bur. Soils Bull. 70. 
 , . 1912. The toxic action of organic compounds as modified by fertilizer 
 
 salts. U. S. Dept. Agr., Bur. Soils Bull. 84:31. 
 , . 19122. Nitrogenous soil constituents and their bearing on soil fertility. 
 
 U. S. Dept. Agr., Bur. Soils Bull. 87. 
 , and M. X. Sullivan. 1910. Studies in soil oxidation. U. S. Dept. Agr., Bur. 
 
 Soils Bull. 73. 
 , . 1911. Reduction by roots. Bot. Gaz. 51:121. 
 
180 AERATION AND AIR-CONTENT. 
 
 ScHREiNER, O., and E. C. Lathrop. 1911. Examination of soils for organic constitu- 
 ents, especially dihydroxystearic acid. U. S. Dept. Agr., Bur. Soils Bull. 80. 
 
 , . 1912. The chemistry of steam-heated soils. U. S. Dept. Agr., Bur. 
 
 Soils Bull. 89. 
 
 , and B. E. Brown. 1912. Occurrence and nature of carbonized material in soils. 
 
 U. S. Dept. Agr,, Bur. Soils Bull. 90. 
 ScHiJTZENBURGER, P., and E. QuiNQUAND. 1873. Sur la respiration des v6g6taux aqua- 
 
 tiques immerges. Comp. Rend. 77:272. 
 ScHWARZ, F. 1883. Die Wurzelhaare der Pflanzen. Unters. Bot. Inst. Tubingen. 1:135; 
 
 Bot. Cent. 15:337. 
 ScHWENDENER, S. 1889. Die Spaltoffnungen der Graminien und Cyperaceen. Sitzb. 
 
 Preuss. Akad. Wiss. Berlin 1:65. 
 Scott, D. H., and H. Wager. 1888. On the floating roots of Sesbania aculceta. Pers. 
 
 Ann. Bot. 1:307. 
 Selmi, F. 1874. Osservazioni sullo sviluppo d'idrogeno nascente dalle muffe e dei funghi. 
 
 Rend. Accad. Sci. Bologna; Just Bot. Jahrb. 4:116, 1876. 
 Sen, J. N. 1918. The influence of potsherds on nitrification in the Indian alluvium. 
 
 Jour. Agr. Sci. 9:32; Bot. Abs. 2:97. 
 Senebier, J. 1785. Physikalisch-chemische Abhandlungen. 
 
 . 1791. Physiologic vegdtale. In Encyclopedic Methodique 1:7, 125. 
 
 Sharp, L. T., and D. R. Hoagland. 1916. Acidity and adsorption in soils as measured 
 
 by the hydrogen electrode. Jour. Agr. Res. 7:123. 
 
 , . 1919. Notes on recent work concerning acid soils. Soil Science 7:197. 
 
 Sherff, E. E. 1912. The vegetation of Skokie Marsh, with special reference to sub- 
 terranean organs and their relationships. Bot. Gaz. 53 :415. 
 Shorey, E. C. 1913. Some organic soil constituents. U. S. Dept. Agr. Bur. Soils Bull. 88. 
 Shull, C. a. 1914. The role of oyxgen in germination. Bot. Gaz. 57:64. 
 . 1911. The oxygen minimum and the germination of Xanthium seeds. Bot. 
 
 Gaz. 52:453. 
 Simon, S. 1906. Untersuchungen iiber das Verhalten einiger Wachstumsfunktionen, 
 
 sowie der Atmungstatigkeit der Laubholzer wahrend der Ruheperiode. Jahrb. 
 
 Wiss. Bot. 43:1. 
 Skene, M. 1915. The acidity of Sphagnum and its relation to chalk and mineral salts. 
 
 Ann. Bot. 29:65. 
 Skinner, J. J. 1913. Illustration of the effect of previous vegetation on a following crop. 
 
 Plant World 16:342. 
 Smirnoff, S. 1903. Influence des blessures sur la respiration normale et intramol^culaire 
 
 (fermentation) des bulbes. Rev. Gen. Bot. 15:26. 
 Smith, T. 1895. Ueber die Bedeutung des Zuckers in Kulturmedien ftir Bakterien. Cent- 
 
 Bakt. 18:1. 
 Smolenski, PT' 1877. Ueber den Kohlensauregehalt der Grundluft. Zeit. Biol. 13:383. 
 Snow, L. M. 1905. The development of root-hairs. Bot. Gaz. 40:12. 
 Sorauer, p. 1873. Handbuch der Pflanzenkrankheiten. 2d ed., 1886; 3d ed., 1909. 
 
 . 1895. A popular treatise on the physiology of plants. 
 
 Sprengel, C. 1826. Ueber Pflanzenhumus, Humus-Saure und humussaure Salze. Ar- 
 
 chiv. Ges. Naturl. 8:145. 
 Sprengel, K. 1812. Von dem Bau und der Natur der Gewachse. 
 Stahl, E. 1894. Einige Versuche iiber Transpiration und Assimilation. Bot. Zeit. 52: 
 
 117. 
 Stenstrom, K. O. E. 1895. Ueber das Vorkommen derselben Arten in verschiedenen 
 
 Klimaten an verschiedenen Standorten, mit besonderer Beriicksichtigung der xero- 
 
 phil ausgebildeten Pflanzen. Eine kritische pflanzenbiologische Untersuchung. 
 
 Flora 80:117; Nachtrag lb. 421. 
 Stewart, F. C., and A. J. Mix. 1917. Blackheart and the aeration of potatoes in stor- 
 age. N. Y. Agr. Exp. Sta. Bull. 436. 
 Stich, C. 1891. Die Atmung der Pflanzen bei vermindeter Sauerstoffspannung und bei 
 
 Verletzungen. Flora 74:1. 
 Stiehr, G. 1903. Ueber das Verhalten der Wurzelharchen. Diss. Kiel. 
 Stiles, W. 1915. On the relation between the concentration of the nutrient solution and 
 
 the rate of growth of plants in water culture. Ann. Bot. 29:89. 
 
BIBLIOGRAPHY. 181 
 
 Stoklasa, J., J. Jelinek, and E. Vitek. 1903. Der anaerobisch Stoffwechsel der hoheren 
 Pflanzen und seine Beziehung zur alkoholischen Gahrung. Beit. Chem. Phys. 
 Path. 3:460. 
 
 , and A. Ernest. 1905. UeberdenUrsprung, dieMengeundBedeutungdesKohlen- 
 dioxids im Boden. Centb. Bakt. 14:723. 
 
 , . 1908. Beitrage zur Losung der Frage der chemischen Natur des Wurzel- 
 
 sekretes. Jahrb. Wiss. Bot. 46:55. 
 
 and K. Chocensky. 1906. Ueber die glykolytischen Enzymen in 
 
 Pflanzenorganisraus. Zeits. Physiol. Chem. 50:303. 
 , , . 1906. Ueber die anaerobe Atmung der Samenpflanzen und 
 
 iiber die Isolierung der Atmungsenzymen. Ber. Deut. Bot. Ges. 24:542. 
 Stopes, M. C. 1907. The xerophytic nature of the Gymnosperms. New Phyt. 6:46. 
 Strohmer, F. 1902. Ueber die Atmung der Zuckerrubenwurzel: Ein Beitrag zur 
 
 Kenntniss der Ursachen des Zuckerverlustes der Zuckerriiben wiihrend ihrer Auf- 
 
 bewahrung. Oest. Ungar. Zeit. Zuckerind. Landw. 31:933. 
 Sullivan, M. X. 1914. The passage of nucleic acid from plant to medium. Science 
 
 39:958. 
 , and F. R. Reid. 1912. Studies in soil catalysis. U. S. Dept. Agr., Bur. Soils 
 
 Bull. 86. 
 Tacke, B., and StJCHxiNG, H. 1911. Ueber Humussauren. Landw. Jahrb. 41:717. 
 Takahashi, T. 1905. Is germination possible in absence of air? Bull. Coll. Tokyo. 
 
 6:439. 
 Transeau, E. N. 1904. The development of palisade tissue and resinous deposits in 
 
 leaves. Science 19:866. 
 
 . 1905. The bogs and bog flora of the Huron River valley. Bot. Gaz. 40:351, 41 :17. 
 
 Traube, M. 1874. Ueber das Verhalten der Alkoholhefe in sauerstoffgasfreien Medien. 
 
 Ber. Deut. Chem. Ges. 7:872. 
 Truog, E. 1912. Soil acidity and methods for its detection. Science 42:505. 
 . 1916. The utilization of phosphates by agricultural crops, including a new theory 
 
 regarding the feeding power of plants. Wis. Agr. Exp. Sta. Res. Bull. 41. 
 . 1918. Soil acidity: Its relation to the growth of plants. Soil Science 5:169; 
 
 Bot. Abs. 1:221. 
 , and J. Sykora. 1917. Soil constituents which inhibit the action of plant toxins. 
 
 Soil Science 3:333. 
 , and M. R. Meacham. 1919. Soil acidity: II. Its relation to the acidity of the 
 
 plant juice. Soil Science 7:469. 
 Unger, F. 1836. Einfluss des Bodens auf die Vertheilung der Gewachse. 
 . 1854. Beitrage zur Physiologic der Pflanzen. I. Bestimmung der in den Inter- 
 
 cellulargangen enthaltenen Luftmenge. Sitzb. Akad. Wiss. Wien 12:367. 
 Vageler, P. 1907. Ueber Boden temperature im Hochmoor und iiber die Bodenluft in 
 
 verschiedenen Moorformen. Mitt. Bayr. Moorkulturanstalt. 1:1. 
 Van Ameijden, U. P. 1917. Geotropism and phototropism in the absence of free oxy- 
 gen. Rec. Trav. Bot. Neerl. 14:149. 
 Van Bemmelen, J. M. 1888. Die Absorptionsverbindungen und das Absorptionsver- 
 
 mogen der Ackererde. Landw. Versuchst. 35:69. 
 Van Slyke, L. L., and A. W. Bosworth. 1908. The effect of treating milk with carbon 
 
 dioxid gas under pressure. N. Y. Agr. E.xp. Sta. Bull. 292. 
 Van Tieghem, P., and G. Bonnier. 1882. Recherches sur la vie latente des graines. Bull. 
 
 Soc. Bot. France 29:25. 
 Veitch, F. p. 1904. Comparison of methods for the estimation of soil acidity. Jour. 
 
 Am. Chem. Soc. 26:637. 
 Vines, S. H. 1886. Lectures on the physiology of plants. 
 
 VocHTiNG, H. 1902. Ueber die Keimung der Kartoffelknollen. Bot. Zeit. 60:87. 
 Volk, R. 1906. HamburgischeElb-Untersuchung: VIII. Studien uber die Einwirkung der 
 
 Trockenperiode in Sonamer 1904 auf die biologische Verhaltnisse der Elbe bei Ham- 
 burg. 
 VoLKENS, G. 1884. Zur Kenntniss der Beziehungen zwischen Standort und anatomischen 
 
 Bau der Vegetationsorgane. Jahrb. Bot. Gart. Mus. Berlin 3:23. 
 VONHAUSEN, W. 1877. Einfluss des Luftwechsels im Boden auf die Entwickelung der 
 
 Pflanzen. Forstl. Blat. 14:361; Just 5:724. 
 Wacker, J. 1898. Die Beeinflussung des Wachstums der Wurzeln durch das umgebende 
 
 Medium. Jahrb. Wiss. Bot. 32:71. 
 
182 AERATION AND AIR-CONTENT. 
 
 Waksman, S. A. 1916. Soil fungi and their activities. Soil Science 2:103. 
 
 . 19162. Do fungi actually live in the soil and produce mycelium? Science 44:320. 
 
 . 1917. Is there any fungus flora of the soil? Soil Science 3:565. 
 
 Walker, S. S. 1920. The effect of aeration and other factors on the lime requirement of 
 
 a muck soil. Soil Science 9:77. 
 Walser, E. 1838. Untersuchungen ueber die Wurzelausscheidungen. 
 Warburg, O. 1886. Ueber die Bedeutung der organischen Sauren zu den Lebensprozess 
 
 der Pflanzen, speziell der sog. Fettpflanzen. Unters. Bot. Inst. Tubingen. 2:53. 
 . 1886. Einfluss kohlensaurereicher Luft auf die Assimilation bei Bryophyllum. 
 
 Unters. Bot. Inst. Tubingen. 2:122. 
 Warming, E. 1888. Cm Gronlands Vegetation. Medd. Gronland 12. 
 
 . 1895. Plantesamfund. Grundtraek af den Okologiske Plantegeografi. 
 
 . 1896. Lehrbuch der okologischen Pfllanzengeographie. 
 
 . 1909. (Ecology of plants. 
 
 Wehmer, C. 1891. Entstehung und physiologische Bedeutung Oxalsaure in Stoflfwechsel 
 
 einiger Pilze. Bot. Zeit. 49:233. 
 . 1906. Die Bildung freier Oxalsaure durch Aspergillus niger. Ber. Deut. Bot. 
 
 Ges. 24:381. 
 Wheeler, H. J. 1905. On the causes of unproductivity in a Rhode Island soil. R. I. 
 
 Agr. Exp. Sta. Rep. 18:286. 
 . 1905. Plant peculiarities as shown by the influence of sodium salts. R. I. Agr. 
 
 Exp. Sta. Bull. 104. 
 Wherry, E. T. 1920. Plant distribution around salt marshes in relation to soil acidity. 
 
 Ecology 1 :42. Soil acidity and a field method for its measurement. Ecology 1 :160. 
 Whipple, C. C., and H. N. Parker. 1902. On the amount of oxygen and carbonic acid 
 
 in natural waters and the effect of these gases upon the occurrence of microscopic 
 
 organisms. Trans. Amer. Mic. Soc. 23:103. 
 White, J. W. 1916. Concerning the growth and composition of clover and sorrel as in- 
 fluenced by varied amounts of hmestone. Pa. Agr. Exp. Rep. 1913-14:46. 
 Whitford, H. N. 1901. The genetic development of the forests of northern Michigan; 
 
 a study in physiographic ecology. Bot. Gaz. 31:314. 
 WiDTSOE, J. A. 1914. The principles of irrigation practice. 
 Wiegand, K. M. 1910. The relation of hairy and cutizined coverings to transpiration. 
 
 Bot. Gaz. 49:430. 
 Wiegmann, a. F., and L. Polstorff. 1842. Ueber die anorganischen Bestandtheile der 
 
 Pflanzen. 
 WiELER, A. 1883. Die Beeinflussung des Wachsens durch verminderte Partiarpressung 
 
 des Sauerstoffs. Unters. Bot. Inst. Tubingen 1:189. 
 . 1898. Die Function der Pneumathoden und des Aerenchyms. Jahrb. Wiss. 
 
 Bot. 32:503. 
 
 . 1903. Wachstum ohne Sauerstoff. Beih. Bot. Cent. 13:431. 
 
 WiESNER, J. 1871. Experimental-Untersuchungen iiber die Keimung der Samen. 
 
 Sitzb. Akad. Wiss. Wien 64:415; Halle Zeit. Gesammt. Naturw. 40:120 (1872). 
 
 . 1877. Die Entstehung des Chlorophylls. 
 
 . 1878. Die heliotropischen Erscheinungen im Pflanzenreiche. 
 
 Wilcox, E. V., and W. P. Kelley. 1912. The effect of manganese upon pineapple 
 
 plants. Hawaii Agr. Exp. Sta. Bull. 28. 
 Wille, N. 1889. Ueber die Blasen der Fucaceen. Biol. Foren Stockholm Forhand. 1:63; 
 
 Bot. Cent. 42:110. 
 Wilson, W. P. 1882. Ueber Atmung der Pflanzen. Flora 40:93. 
 . 1889. The production of aerating organs on the roots of swamp and other plants. 
 
 Proc. Acad. Sci. Phil.; Bot. Cent. 43:148; Just. 1889:682. 
 Winkler, L. W, 1891. Die Loslichkeit der Gase im Wasser. Ber. Deut. Chem. Ges. 
 
 24:3206. 
 Winogradsky, S. 1890. Recherches sur les organismes de la nitrification. Ann. Inst. 
 
 Pasteur 4:213. 
 WiSMEWSKi, p. 1912. Beitrage zur Kenntnis der Keimung der Winterknospen der Was- 
 
 serpflanzen. Bull. Acad. Sci. Cracovie, Ser. B. 7:1045; Bot. Cent. 122:395. 
 Wolf, W. 1870. Untersuchungen iiber den Ernahrungsprocess der Pflanzen. Jahrb. 
 
 Agr. Chem. 13:2:131. 
 
BIBLIOGRAPHY. 183 
 
 WoLFFHtJGEL, G. A. 1879. Die Grundluft und ihr Kohlensauregehalt. Aertzl. Intelli- 
 genzbl. 
 
 . 1879*. Ueber den Kohlensauregehalt im Gerollboden von Munchen. Zeit. 
 
 Biol. 15:98. 
 
 WoLKOFF, A., and A. Meyer. 1874. Beitrage zur Lehre iiber die Athmung der 
 Pflanzen. Landw. Jahrb. 3:481. 
 
 WoLLNT, E. 1880. Untersuchungen iiber den Einfluss der Pflanzendecke und der 
 Beschattung auf dem Kohlensauregehalt der Bodenluft. Forsch. Geb. Agr. Phys. 
 3:1. 
 
 . 1880*. Untersuchungen iiber den Kohlensauregehalt der Bodenluft. Landw. 
 
 Versuchst 25:373; 36:197. 
 
 . 1886. Untersuchungen uber den Einfluss der physikaUschen Eigenschaften dea 
 
 Bodens auf dessen Gehalt an freier Kohlensaure. Forsch. Geb. Agr. Phys. 9:165. 
 
 . 1889. Untersuchungen iiber den Einfluss der Farbe des Bodens auf dessen 
 
 Feuchtigkeitsverhaltnisse und Kohlensauregehalt. Forsch. Geb. Agr. Phys. 12:385. 
 
 . 1889^ Die Durchluftung der Ackererde. Fuhhngs Landw. Zeit. 48:377, 402. 
 
 . 1890. Der Einfluss der Menge der im Boden befindhchen organisehen StofiFe 
 
 auf dem Kohlensauregehalt der Bodenluft. Landw. Versuchst, 36:210; Forsch. 
 Geb. Agr. Phys. 13:233. 
 
 . 1896. Untersuchungen iiber den Einfluss der Pflanzendecken auf den Kohlen- 
 sauregehalt der Bodenluft. Forsch. Geb. Agr. Phys. 19:151. 
 
 . 1897. Die Zersetzung der organisehen Stoffe. 
 
 WoRTMANN, J. 1879. Ueber die Beziehungen der intramolekularen zur normalen Ath- 
 mung der Pflanzen. Diss. Wiirzburg. 
 
 . 1882. Ueber die Beziehungen der intramolecularen zu normalen Athmung 
 
 der Pflanzen. Arb. Bot. Inst. Wiirzburg 2:500. 
 
 . 1884. Studien ueber geotropische Nackwirkungerscheinungen. Bot. Zeit. 
 
 42:705. 
 
 Yapp, R. H. 1908. On the hairiness of certain marsh plants. Rep. Brit. Assoc. 1907:691. 
 
 . 1909. On stratification in the vegetation of a marsh, and its relations to evapora- 
 tion and temperature. Ann. Bot. 23:275, 301. 
 
 ZiEGENBEiN, E. 1893. Untersuchungeu iiber den Stoffwechsel und die Athmung kei- 
 mender Kartoffelknollen, sowie andcrer Pflanzen. Jahrb. Wiss. Bot. 25:563. 
 
 ZiNGELER, C. 1873-74. Die Spaltoffnungen der Cariccs. Jahrb. Wiss. Bot. 9:127. 
 
 ZoLLEB, p. H. 1863. Vegetationsversuche in Boden welche die pflanzlichen Nahrstoffe 
 in physiogischer Bindung (absolvirt) enthielten. Landw. Versuchst. 5:40. 
 
) 1 3 '40