hook niTF on the last date stamped below SOUTHERN BRANCH, UNIVERSITY OF CALIFORNIA, LIBRARY, *-OS ANGELES, CALIF. DISEASE IN PLANTS DISEASE IN PLANTS BY H. MARSHALL WARD, Sc.D., F.R.S. FELLOW OF CHRIST'S COLLEGE VKRSITY OF CAMBRIDGE MACMILLAN AND CO., LIMITED NEW YORK : THE MACMILLAN COMPANY 1901 All rights Reserved 57825 . Wetensch, te Amsterdam, 1898. Koning's paper is in Zettschr. f. Pflanzenkrank., Vol. IX., 1899, p. 65. See also Nature, Oct. n, 1900, p. 576. CHAPTER XX. SYMPTOMS OF DISEASE (Continued}. Spotted leaves The colours of spots White, yellow, drown, and black spots on leaves Parti-coloured spots The browning, etc., of leaves. Discoloured spots or patches on the herbaceous parts of plants, especially leaves, furnish the prominent symptoms in a large class of diseases, due to many different causes, and although we cannot maintain this group of symptoms sharply apart from the last, as seen from the considerations on albinism, it is often well marked and of great diagnostic value. By far the greater number of spot-diseases are due to fungi, but this is by no means always the case. The most generally use- ful method of subdividing the classes, though artificial like all such classifications, will be accord- ing to the colour of the spots or flecks, which, moreover, are usually found on the leaves. It is necessary to note, however, that various conditions may modify the colour of spots on leaves. Many 1 86 SYMPTOMS OF DISEASE. 187 fungi, for instance, induce different coloured spots according to the age of the leaf or other organ attacked, or according to the species of host, the weather, etc. Moreover the spots due to these parasites are frequently yellow when young and some other colour, especially brown or black, when older. Scale is the name given to the characteristic shield-like insects (Mytilaspis, Aspidiotus, etc.) which attach themselves to branches of Apples, Pears, Oranges, Camellias, and numerous other plants, and suck the juices. It is the female insect which has the body broadened out into the " scale," under which the young are brought up. Enormous damage has been done by some forms e.g. the San Jose* scale in the United States. The superficial resemblances of the patches of eggs of some Lepidoptera to Aecidia and other fungi may be noted in passing e.g. Bombyx neustria on Apple twigs, Aporia Crataegi. White or greyish spots are the common symptom marking the presence of many Peronosporeae and Erysipheae in or on leaves, e.g. Peronospora Tri- foliorum, P. parasitica on Crucifers, etc., and Sphaerotheca on Hops; also Septoria piricola, Cysto- pus, Entyloma Ranunculi, etc. White spots are also caused by insects such as Tetranychus (red spider) on Clover and other plants. Yellow, or Orange-coloured Spots. In cases where these occur on leaves, and in the case of grasses, etc., on the leaf sheaths as well, they i88 DISEASE IN PLANTS. commonly indicate the presence of Uredineae, and sections under the microscope will show the mycelium in the tissues beneath. Species of Uromyces, Puccinia, etc., in the Uredo state have the spots powdery with spores ; Aecidia show the characteristic " cluster cups," and so forth. These spots are often slightly pustular, and in some cases markedly so. Other fungi also induce yellow spots on leaves e.g. Phyllosticta on Beans, Exoascus on Poplars, Cluster osp or ium on Apricot leaves, Synchytrium Succisae on Centaurea, etc. Yellow spots are also a frequent symptom of the presence of Aphides, of Red Spider, etc. Thus the minute golden yellow spots sometimes crowded on Oak leaves are due to Phylloxera punctures. Yellow patches are formed on the large leaves of A risarum by a species of parasitic Alga, Phyllo- siphon, which lives in the mesophyll. Many tropical leaves are spotted yellow by epiphytic Algae e.g. Cephaleuros. It must be noticed that many fungi produce yellow spots or flecks in the earlier stages, which turn brown or black as the fructifications appear, e.g. Dilophia graminis, Rhytisma acerinum. The yellow-spotted leaves of Farfugium grande (Senecio Kaempferi] are so like those of Petasites attacked with Aecidimn in its early stages, that an expert might be deceived until the microscopic analysis was completed. Red spots, varying from rusty or foxy red to bright crimson, are the symptomatic accompaniment SYMPTOMS OF DISEASE. 189 of several fungi, the former often characterising the teleutospore or aecidium stage of Uredineae e.g. Aecidium Grossulariae the latter sometimes in- dicating the presence of Chytridiaceae. Red spots are also caused by Gloeosporium Fragariae on Strawberry leaves, Poly stigma rubrum on Plums. Crimson spots on Apple and Pear leaves are also due to Phytoptus : they turn brown later. Brown spots or flecks, varying in hue from dull slaty brown to deep red browns, are a common symptom of Fungus and Insect diseases, the colour often indicating the death of the tissues, rather than any special peculiarity of the action of the parasite. Good examples are furnished by the Potato-disease, and by Peronospora viticola, Sphae- rella vitis and other disease-fungi of the Grape Vine. The teleutospore stage of many Uredineae also occurs in deep brown spots. Black spots and flecks are exceedingly common symptoms of the presence of fungi, e.g. Fusidadium on Apples and Pears, and the pycnidial and ascus stages of many Ascomycetes e.g. Phyllachora graminis. The teleutospore stages of species of Puccinia y Phragmidium, etc., are also so deep in colour as to appear almost black. Scab on Pears is due to the presence of Fusi- cladium, which indurates the outer skin of the fruit causing it to crack under pressure from within, and to dry up, the deep brown to black patches of fungus persisting on the dead surface. Black spots on grasses and sedges are caused 190 DISEASE IN PLANTS. by Ustilagineae, and are commonest in the grain, the soot-like powdery spores (Smut) being very characteristic. Ustilago longissima induces black streaks on the leaves. Many of these fungi cause distortions or pustules on leaves and other organs. Brown and black leaf spots are frequently fur- nished with concentric contours arranged round a paler or other coloured central point e.g. Cerco- spora on Beans, Ascochyta on Peas. Brown spots with bright red margins are formed in young Beans by Gloeosporium. Species of Fumago, Herpotrichia, etc., may cover the entire surface of the leaf with sooty patches, or even weave the leaves together as if with black spider-webs. Mai nero of the Vine is a particular case of black spotting and streaking of the leaves for which no satisfactory explanation is as yet to hand. As with Chestnuts, Walnuts, and other plants containing much tannin, the dark spots appear to be due to this substance, but whether the predisposing cause is a lack of some in- gredients in the soil, or some temperature reaction, or fungi at the roots, is as yet unknown. The most recent explanation puts the disease down to the action of bacteria, but the results obtained by different workers lead to uncertainty. The " dying back " of leaves, especially of grasses, from the tip, is usually accompanied by a succession of colours yellow, red, brown, to black and is a common symptom of parching from summer drought ; and spots of similar SYMPTOMS OF DISEASE. 191 colours, frequently commencing at the margins of leaves,, are characteristic symptoms of the injurious action of acid gases in the air. Brown and blackish spots on Pears are caused by a species of Thrips. In many cases the minute spots of Rust-fungi on one and the same leaf are bright orange yellow (uredo), deep brown, or almost purple-black (teleutospores), foxy-red brown (older uredospores), or dead slaty black where the old teleutospores have died off e.g. Uromyces Fabae on Beans, U. Pisi on Peas, etc. Parti-coloured leaves. The leaves sometimes start shrivelling with red edges, while yellow, red, and finally brown and black blotches appear on the lamina, from no known cause e.g. Vines. In other cases similar mimicry of the autumnal colouring of leaves results from the action of acid gases. Burning is a common name for all cases where the leaves turn red or red-brown in hot, dry weather, and many varieties are distinguished in different countries and on different plants, because species react dissimilarly. The primary cause is usually want of water drought. Foxy leaves are a common sign of drought on hot soils, and the disease may usually be recognised by the gradual extension of the drying and fox-red colour proceeding from the older to the younger leaves, and from base to apex e.g. Hops. Coppery leaves. The leaves of the Hop, etc., may show yellow spots and gradually turn red-brown 192 DISEASE IN PLANTS. copper-coloured as they dry ; the damage is due to Tetranychus, the so-called Red Spider. These cases must of course be carefully dis- tinguished from the normal copper-brown of certain varieties of Beech, Beet, Coleus, etc. Silver-leaf. The leaves of Plum, Apple, and other fruit trees often obtain a peculiar silvery appearance in hot summers, the cause of which is unknown. Discolorations in the form of confluent yellow and orange patches, etc., resembling variegations, are not infrequently due to the ravages of Red Spider and mites e.g. on Kidney Beans. Sun-spots. Yellow spots, which may turn brown or black according to the species of plant affected and the intensity of the action, are often caused by the focussing of the solar rays by lens-like thickenings due to inequalities in the glass of green- houses, or by drops of water on them or on other leaves, e.g. Palms, Dracaena, etc. The action is that of a burning glass, and extends throughout the leaf- tissues. Young grapes, etc., may also be injured in this way. Water-drops on the glass can only act long enough to produce such injuries if the atmosphere is saturated. The old idea that a drop on a leaf can thus focus the sun's rays into the tissues beneath is not tenable. Here again we see that the disease-agencies concerned in producing the symptoms described in this chapter, agree for the most part in so far that the principal effect is generally the disturbance of chlorophyll action in the spots or flecks on SYMPTOMS OF DISEASE. 193 the leaves, and the rendering useless of these areas so far as providing further food-supplies is con- cerned. The effects may be due merely to the shading action of a parasite e.g. epiphytic fungi or to actual destruction of the tissues invaded e.g. by endophytic fungi or the tissues may be burnt, poisoned, etc. In so far the results are again quantitative and cumulative, and the amount of damage depends on the number and size of the spots or other areas affected, and the proportion of foliage involved, as well as the length of time the injurious action is at work. But, again, it must be remembered that several symptoms may co-exist, and matters may be complicated by the spread of the destructive agent, or its consequences, to other parts, and in some cases we are quite uninformed as to the true nature of the disease. NOTES TO CHAPTER XX. Further information regarding these "leaf-diseases" will be found in special works dealing with the fungi and insects which cause them. In addition to works already quoted, the reader may also be referred for Fungi .to Massee, A Text- book of Plant-diseases caused by Cryptogamic Parasites, London, 1899; or Prillieux, Les Maladies des Plantes Agri- coles, 1895. See also Marshall Ward, Coffee-leaf Disease, Sessional Papers, XVII., Ceylon, 1881, andjourn. Linn. Soc., Vol. XIX., 1882, p. 299. The question of" Sun-spots" has been dealt with by Jonnsonin Zeitschr.f. Pflanzenkrankh., 1892, p. 358. CHAPTER XXI. ARTIFICIAL WOUNDS. The nature of wounds and of healing processes Knife wounds Simple cuts Stripping Cuttings Branch-stumps and pruning Stool-stumps Ringing Bruises. Wounds. All the parts of plants are exposed to the danger of wounds, from mechanical causes such as wind, falling stones or trees, hail, etc., or from the bites of animals such as rabbits, worms, and insects, and although such injuries are rarely in themselves dangerous, they open the way to other agencies water, fungi, etc., which may work great havoc; or the loss of the destroyed or removed tissues is felt in diminished nutrition, restriction of the assimilative area, or in some other way. We have seen that living cells die when cut, bruised, or torn ; and that the cells next below in a layer of active tissue are stimulated by the exposure to increased growth and division, and at once pro- 194 ARTIFICIAL WOUNDS. 195 duce a layer of cork, the impervious walls of which again protect the living cells beneath. This is found to occur in all cell-tissues provided the cells are still living, and it matters not whether the wound occurs in the mesophyll of a leaf, the storage paren- chyma of a Potato-tuber, the cortex of a root or stem, or in the fleshy parts of a young fruit, the normal effect of the wound is in all cases to call forth an elongation of the uninjured cells beneath, in a direction at right angles to the plane of the injured surface, which cells then divide by successive walls across their axis of growth : the layers of cells thus cut off are then converted into cork, by the suberisa- tion of their walls. Further changes may then go on beneath the protective layer of wound-cork thus produced, and these changes vary according to the nature of the cells beneath : the cambium forms new wood, the medullary rays similar rays, cortex new cortex, and so on. Knife-wounds. Artificial cuts in stems are easily recognised and soon heal up unless dis- turbed. Several cases, differing in complexity, are to be distinguished. The simplest is that of a longitudinal, oblique, or horizontal short cut in which the point of the knife severs all the tissues of the stem down to the wood. The first effect usually observed is that the wound gapes, especially if longitudinal, because the cortex, tightly stretched on the wood cylinder, contracts elastically. This exposes the living cortex, phloem and cambium to the air, and such tissues at once behave as already described above : the cells actually cut die, 196 DISEASE IN PLANTS. those next below grow out under the released pressure, and these give rise to cells which become cork. As the growth and cell-division continue in the cells below this thin elastic cork-layer, they form a soft herbaceous cushion or callus looking like a thickened lip to each margin of the cut. Each lip soon meets its opposite neighbour, and the wound is closed over, a slight projection with a median axial depression alone appearing on the surface. The depression contains the trapped- in callus-cork squeezed more and more in the plane of the cut as the two lips of callus press one against the other, and sections across the stem and perpendicular to the axis of the cut show that this thin cork, like a bit of brown paper, alone intervenes between the cambium, phloem and cortex respectively of each lip, as each layer attempts to bridge over the interval. If the healing proceeds normally, these layers, each pressing against the trapped cork-film, and grow- ing more and more in thickness, shear the cork- layer and tear its cells asunder, and very soon we find odd cells of the cambium of one lip meeting cambium cells of the other, phloem meeting phloem, and cortex cortex, and the normal thickening of the now fused layers previously separated by the knife goes on as if nothing had happened, the only external sign of the wound being a slight ridge-like eleva- tion, and, internally, traces of the dead cells and cork trapped here and there beneath the ridge. When the conjoined cambium resumes the develop- ARTIFICIAL WOUNDS. 197 ment of a continuous layer of xylem and phloem, no further trace of the injury is observable, unless a speck of dead cells remains buried beneath the new wood, and indicates the line where the knife point killed the former cambium and scored the surface of the wood in making the wound. Stripping. Now suppose that, instead of a mere slit with the knife-point, a strip of bark is removed down to the wood. Exactly the same processes of corking and lip-like callus formation at the edges of the wound occur, but of course the occlusion of the bared wood-surface by the meet- ing of the lips occupies a longer time. Moreover, the living cells of the medullary rays exposed by the wound on the wood-surface also grow out under the released pressure, and form protruding callus pads on their own account. In course of time the wood is again completely covered by the coming together over its face of these various strips of callus, but two important points of differ- ence are found, as contrasted with the simpler healing of the slit-wound. In the first place the exposed wood dries and turns brown, or it may even begin to decay if moisture and putrefactive organisms act on it while exposed to the air ; and, in the second place, the normal annual layer of wood or layers, as the case may be formed by the cambium only extends over that part of the stem where the cambium is still intact, and is entirely wanting over the exposed area. Thus, if it takes two years for the cambium to extend 198 DISEASE IN PLANTS. across the wound, a layer of wood will be formed all round the intact part of the stem, from lip to lip of the cut tissues during the first year ; then a second annual layer outside this will be formed during the second year, but extending further over the edges of the wound, and nearly complete, because the cambium has now crept further across the wounded surface to meet the opposite lip of cambium ; and during the third year, when the cambium has once more become continuous over the face of the wound, the annual wood layer will be complete. But, of course, this last layer covers in the edges of the two previously developed incomplete wood-layers as well as the exposed and brown, dry, or rotten dead face of the wood. It also covers up the trapped-in brown cork and any debris that accumulated in the wound, and this " blemish," though buried deeper and deeper in the wood during succeeding annual deposits of wood-layers, always remains to remind us of the existence of the wound, the date of which can be fixed at any future time by counting the annual rings developed subsequently to its formation. Obviously, also, the deficiency of wood at this" place makes itself visible on the outside by a depression. Cuttings. When a cutting of Pelargonium, Willow, or other plant is made, we have a typical knife-wound, the behaviour of which is very in- structive in illustration of plant-surgery, and ma)' be most easily seen by keeping it in damp air instead of plunging it into sand or soil. ARTIFICIAL WOUNDS. 199 All the living cells actually cut or bruised turn brown and die as before ; those beneath e.g. the living pith, medullary rays, cambium, phloem, and cortex, grow out under the released pressure and form a callus, the outermost layer of which becomes cork, while those below, abundantly supplied with food-materials, proceed to spread, as if flowing over the surface of the cut wood, and rapidly occlude the wound. Meanwhile new roots are formed adventitiously from the cambium just above the plane of section, and push out through the cortex into the damp air, and if the cutting had been in soil it would now be capable of independent existence. It is important to keep cuttings upright, as the roots only spring from the lower end. Such cuttings can be obtained not only from stems, but also from roots and even leaves. Callus-formation is not confined to the basal end of a cutting; it has nothing to do with position, but is a reaction to the wound stimuli, independent of light, gravitation, etc. As time goes on, however, the internal organisation of the erect cutting usually reacts on the callus at either end, and roots only rise from the lower one, while shoot-buds may form in the upper one, though it is possible to bring about the formation of buds from the lower end also. Branch stumps. A more complex example is furnished by a branch cut off short some distance say a foot from the base, where it springs from the trunk. As before, the immediate effect of the 200 DISEASE IN PLANTS. section is the formation of a callus from the cam- bium, phloem and cortex, which begins to rise as a circular occluding rim round the wood. The transpiration current in the trunk, however, is not deflected into the 1 2 inches or so of amputated branch, because there are no leaves to draw the water up it, and so the stump dries up and the cortex and cambium die back to the base, leaving the dead wood covered with shrivelled cortical tissues only. This dead stump gradually rots under the action of wet, fungi, and bacteria, and since the pith and heart-wood afford a ready passage of the rot-organisms and their products into the heart of the trunk, we find in a few years a mere stump of touch-wood and decayed bark, which falls out at the insertion like a decayed tooth, leaving a rotten hole in the side of the trunk. If, however, instead of allowing the basal part of the amputated branch to protrude as a stump, we cut it off close to the stem, and shave the section flush with the normal surfaceof the latter, the callus formed by the cambium, etc., rapidly grows over the sur- face, and soon forms a layer of cambium continuous with that of the rest of the stem. The wound heals, in fact, much as if it were a strip-wound, and beyond a slight prominence for a year or two no signs are visible from the outside after the occlu- sion. Of course these matters depend on the relative thickness of branch and stem, and if much wood is exposed the dangers of rot and a resulting hollow in the stem are increased. It is interesting to note how much thicker the callus lips are at the ARTIFICIAL WOUNDS. 201 sides of the wound than above and below, owing to differences in the distribution of the nutrient materials. Stool-stumps. When a tree is felled, the stump may, if the section is close to the ground and kept moist, begin to form a thick rim-like callus round the wood, in which adventitious buds soon make their appearance, and grow out into so-called Stool-shoots. The products of assimilation of these, and the stores accumulated in the stump, often suffice to feed the callus sufficiently to enable it to grow over and completely occlude the wound, if the wood surface is not too large, or so long exposed that rotting processes have meanwhile set in. Ringing. If the strip of cortical tissues and cambium is removed all round the stem, exposing the wood in a form of a ring, complications may ensue owing to the following circumstances. A well-marked callus appears at the upper edge of the wound, because, the transpiration current up the young wood not being stopped, plenty of water and salts from the soil can reach the leaves; but the nutritive materials supplied by the latter are accumulated at the upper lip of the wound owing to the stoppage there of their descent in the phloem, cortex, etc. No such callus-lip appears at the lower margin of the wound owing to want of these supplies. Consequently the occlusion and healing of the ring-wound only takes place from above downwards, and if the ring of cortical tissues removed is a broad one, the healing may be a long process, or may even be 202 DISEASE IN PLANTS. indefinitely delayed, a thicker and thicker callus projecting over from above. For similar reasons no annual wood layers are formed below, but only above the wound, and thus the branch or tree may die. The latter contingency is the more likely the further up the tree the ringing takes place, owing to the risk of drying up which threatens the exposed wood, and to the consequent interruption of the transpiration current, and the likelihood that lateral shoots below the wound may divert the water to their own leaves. If the ringing occurs low down on a stem, and the environment remains damp, the upper thick callus may put out new roots ; the part above the wound then behaves like a cutting. If the ringing is done on a young and vigorous branch of an old tree, the lower lip may receive supplies from the leaves of branches below the wound, or from shoots which spring from adventitious buds close to it, and the wound may heal over normally. Such healing may be rendered more certain by keeping the wounded surface moist e.g. by means of damp moss, and so encouraging the formation of callus- bridges from the medullary rays. If on ringing a tree or a branch the young wood is removed as well as the cambium and cortical layers, the death of the parts above the wound is almost certain, owing to the stoppage of the tran- spiration current : the exceptions to this rule de- pend simply on the existence of other channels of communication, such as internal phloems, very thick sap-wood, and so forth. ARTIFICIAL WOUNDS. 203 Bruises. If a branch or woody stem is struck sharply, with a hammer, for instance, the bruised cortex, phloem and cambium are killed by the blow, and the general effect is as if these tissues had been removed at that spot by the knife, but with the following complications. The bruised cortical tissues rapidly dry as they perish, and may adhere to the wood below. Consequently the still sound parts bordering on the wound are not released from pressure, but, on the contrary, have to advance towards each other over the surface of the wood under still greater pressures, in part due to the tightening of the whole cortex as the dead parts dry and contract, and in part due to the above-mentioned adherence of the latter to the wood. It results from this that such wounds heal very slowly and badly, and when the killed patch at last ruptures, wound-fungi, insects, and other injurious agencies may get in and do irreparable damage, as has been found to occur in cases where such wounds have been made in striking trees to shake down insects, fruit, etc. NOTES TO CHAPTER XXI. The essential facts regarding wounds and healing by occlusion are given in Marshall Ward, Timber and some of its Diseases, 1889, chapters viii. and ix., and in Laslett, Timber and Timber Trees, 1894, chapters iv. and v. More detailed treatment will be found in Frank, Krankh. d. PJlanzen, B. I. cap. 2, where the special literature is collected. The reader may also consult Hartig, Diseases of Trees, Engl. ed. 1894, pp. 225-269. CHAPTER XXII. NATURAL WOUNDS. Burrows and excavations. Bark-boring Wood-boring Wood fungi Leaf-miners Pith flecks Erosions. Skeleton leaves Irregular erosions Shot holes. Frost cracks Strangulations Spiral grooving. NATURAL wounds are produced in a variety of ways during the life of the plant, and, generally- speaking, are easily healed over by the normal process if the area destroyed is not too large, and the parts remaining uninjured are sufficiently pro- vided with foliage, or with supplies of food- materials stored up in the roots, rhizomes, medul- lary rays, etc., to feed a vigorous callus. The nature of such wounds and the mode of healing are explained by what we know of arti- ficial wounds, and it only remains to point out that the principal danger of ordinary wounds is not so much the direct traumatic action, because the simpler organisation of the plant does not involve matters connected with shock, loss of 204 NATURAL WOUNDS. 205 blood, etc., as in animals ; the danger consists, rather, in their affording access to other injurious agents, especially fungi, and the treatment of wounds frequently resolves itself into cutting or pruning in order to get clean surfaces which can heal readily. Wounds on leaves imply loss of foliar surface i.e. of chlorophyll action and the remarks on page 193 apply. Burrows may be taken as comprising all kinds of tunnel-like excavations in the various organs of plants, including those cases where insects burrow into hollow stems of grasses, etc., as indicated by the perforations they make in the outer tissues. Bark-boring is done by many species of beetles, especially Scolytidae, which excavate characteristi- cally formed branching passages tangentially in the inner bark of Conifers and other trees. Some of them also bore down to the surface of the sap wood (e.g. Tomicus bidentatus) or even burrow right into the latter (e.g. T. lineatum). It com- monly happens that the external apertures show up clearly, owing to the brown dust and excre- ment, sometimes accompanied by turpentine, which exude from them. Many of these Bark beetles only attack trees which are already injured by fire, lightning, etc. ; possibly they cannot bore through a cortex which swamps them with sap, as a vigorous one might do. Wood-boring is also done by many of the bark- beetles as well as by Longicorns, e.g. Saperda in 206 DISEASE IN PLANTS. Poplars and Willows, the young shoots of which often show characteristic swellings with lateral holes indicating the points of exit. From the ex- ternal apertures comminuted wood, like saw-dust, is frequently ejected in quantity and betrays the presence of the insects. Certain wood-wasps (Sirex) and the larvae of moths (Cossus) also make large perforations in the wood of Willows and other trees, often destroying it completely. In the case of these larger borers, whose tunnels may be as broad as the little ringer, the foul smell as well as abundant " saw-dust " betray the evil. Excavations in wood are by no means caused only by insects : several of the larger Hymenomy- cetes Stereum, Thelephora,Polyporus, etc. tunnel the timber in characteristic ways and often after a fashion very suggestive of insects. They usually obtain access through fractures. Tunnels in leaves are invariably due to the activity of miners belonging to the smaller moths and beetles e.g. Tinea, Orchestes, etc. the larvae of which eat out the mesophyll but leave the cover- ing epidermis or cuticle untouched, and since the insect bores forwards only, in an irregular track, and leaves its excrement in the winding passage, the effect is very characteristic. Whitish leaf tunnels in Peas are excavated by Phytomyza. Characteristic foxy-red tunnels are mined in the leaves of Apples by Lyonettia, Coleophora, etc. Falling of fruit, of Apples, Plums, Apricots, etc., before they are ripe, is frequently due to insects, of NATURAL WOUNDS. 207 which the various species of Grapholitha or Carpo- capsa are conspicuous : the fallen fruits show a small hole leading by a labyrinth of passages to the " core " or " stone," and in which the grub and its excrement are visible. The cuttting off of the vascular bundles and disturbance of the water supply only partly explain the premature fall. Pith-flecks are minute brown specks or patches found in the wood-layers of many trees, and consist of dead parenchymatous thick-walled cells, reminding one of the structure of pith. They are explained as due to the borings of minute insects, Diptera or Beetles, the larvae of which pierce the cortex and phloem and bore their way into the cambium. The latter then occludes the tunnels by filling them up with cells, and continuing its wood-forming activity gradually buries them deeper and deeper in the wood. Such pith-flecks are common in Willow, Birch, Alder, Sorbus, etc. It is possible that they may be due to other causes also in other trees. Erosions or irregular wounds on leaves are caused by large numbers of grubs and caterpillars and other insects, such as earwigs, as well as slugs, snails, and other animals ; but it must by no means be assumed that all marginal leaf wounds, for instance, are caused by animals, since many fungi which rot the tissues, as explained below (p. 208), also cause such erosions, the putrescent parts falling out e.g. the Potato disease. Skeleton leaves frequently result from the 208 DISEASE IN PLANTS. ravages of caterpillars, which leave the coarser ribs and veins untouched, but much finer skeletons with the minute veins almost intact may be found on plants infested with certain insects e.g. Selandria on Cherries. Skeletonised patches on Cherry leaves, often pink or brown-pink, are eaten out by this grub. Shot-holes are perforations in leaves presenting the appearance, from their more or less rounded shape, of gunshot wounds. They may be due to insects which bore through the young leaves while still folded in the bud e.g. Willow Beetle or which gnaw out the tissue e.g. the Beech Miner. Similar but usually more torn and irregular holes are eaten out by many cater- pillars e.g. the Cabbage Moth. Shot-holes on Peas may be the work of Thrips. Leaf perforations are commonly caused by severe hail-storms, the hail-stones beating right through the thin mesophyll. Certain chemicals used for spraying have also been known to cause shot-holes by killing the tissue beneath the stand- ing drops. There is, however, a class of shot-holes in thin leaves which are due to the action of minute fungi, the mycelium of which so rots the tissues in a more or less circular area round the point of infection, that, in wet weather, the decomposing mass falls out and leaves a round hole e.g. certain Chytridiaceae, Peronosporeae, Gloeosporiitni, Rxoascus, etc. If dry weather supervenes these holes frequently dry at the edges, and the leaves appear as if eaten out. NATURAL WOUNDS. 209 Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to Phyllosticta, in Cherry leaves also to Clasterosporium, and in Potato leaves to Haltica. Frost-cracks. The trunks of trees exposed to the north-east, and occasionally with other aspects, are apt to show longitudinal ridges which realise on a larger scale the features of healed wounds scored with a knife. These wounds are due to the outer layers of wood losing water from their cell-walls as it congeals to ice in their lumina, more rapidly than do the warmer internal parts of the trunk ; as this drying of the wood causes its shrinkage, especially in the tangential direction, the effect of a sudden frost and north-east wind is to rend the wood, which splits longitudinally with a loud report, as may often be heard in severe winters. Since the cortex and bark are ruptured at the same time the total effect resembles that of a deep knife-cut, and the same healing processes result on a larger scale when the wood swells and closes up the wound again in spring. But this recently-closed lesion is evidently a plane of weakness, and if a similarly severe winter follows the wound reopens and again heals, and so on, until after a succession of years a prominent Frost-ridge results, which may finally heal completely if milder winters ensue or the tree be eventually protected. Strangulations. We are now in a position to understand the so-called strangulations which result when woody climbers, telegraph wires, etc., o aio DISEASE IN PLANTS. kill or injure trees by tightly winding round them. If strong wire is twisted horizontally round a stem, the growth in thickness of the latter causes the trapping of the cortex and cambium, etc., between the wire and the wood, and a ringing process is set up in consequence of the death of the compressed tissues. A callus then forms above the wound, as in the case of true ringing by means of a cut, and eventually bulges over the upper side of the wire : in the course of years this overgrowth may completely cover in the wire, and, pressing on to the lower lip of the wound, may at length fuse with the cambium below. Hereafter the thickening rings of wood are continuous over the buried wire. The process is obstructed by all the impediments referred to in dealing with ringing, and of course the stem thickens more above than below the wire. If the sapwood is thin, and the bark is so thick as to put great obstacles in the way of the junction of the upper and lower cambiums, death may result the tree is permanently ringed. (See p. 201.) Spiral grooves are frequently met with where Wood-bine or other woody climbers have twined round a young stem or branch, the upper lip of the groove always protruding more than the lower. If a kink or a crossing of two plants or branches of the twiner results in a complete horizontal ring, the results are as in the above cases of ringing and strangulation. Naturally grooved walking sticks are often seen. NATURAL WOUNDS. 211 Buried letters, etc. These processes of healing by occlusion enable us to understand how letters of the alphabet, cut into the wood of trees, come to be buried deep in the timber as successive annual rings cover them in more and more. Chains, nails, rope, etc., have frequently been found thus buried in wood. NOTES TO CHAPTER XXII. In addition to the notes to the last chapter, the reader may be referred to Fisher in Vol. IV. of Schlich's Manual of Forestry, Chap. VI., for an account of Hess' excellent work on Boring Beetles, etc. The authority on Wood-fungi is Hartig, see especially his Zersetzungs-erscheinungen des Holzes, the principal results of which are condensed in his Diseases of Trees already referred to. As regards " Pith-flecks," the reader should consult Frank, Krankh. der Pflanzen, B. I., p. 212 : the subject needs further investigation. CHAPTER XXIII. EXCRESCENCES. Herbaceous excrescences, or galls Erineum Intumes cences Corky warts, etc. Pustules Frost-blisters Galls and Cecidia Root nodules. Excrescences, or out-growths of more or less abnormal character from the general surface of diseased organs, are very common symptoms, and widely recognised. They are due to hypertrophy of the tissues while the cells are young and capable of growth, and may be induced by a variety of causes, among which the stimulus of insect-punctures and of the presence of insect eggs are best known ; but that of fungi, though less widely recognised, plays an equally important part, and, as we shall see, galls and other ex- crescences may be due to widely different agents. Galls or Cecidia are protuberances of the most varied shapes, colours, and sizes found on her- baceous parts attacked by insects, fungi, etc. In the simplest cases the insects only pierce and EXCRESCENCES. 213 suck the young cellular tissue e.g. Phytoptus, Aphides, etc. but in others the stimulus to hyper- trophy starts by the puncture of the embryonic tissue of a leaf, root, etc., by the ovipositor of the female insect, which then lays an egg e.g. Cynips, Ceddomyia, etc. the presence of which appears to intensify the irritating action, or such only occurs when the young larva escapes. Our knowledge of the primary cause of gall- formation amounts to very little. Generally speak- ing, only embryonic or very young cellular tissue reacts, and galls on adult leaves and branches have usually been initiated long before. The same gall-insect may induce totally different galls on different plants, or even on different parts of the same plant, and different insects call forth different galls on any one plant. These facts point clearly to the co-operation of both plant and insect in the gall-formation, and the best hypothesis yet to hand is to the effect that a gall is a hypertrophy of cells, the normal nutrition, growth, and division of which have been disturbed owing to the action of some poison or other irritant derived from the insect, or fungus, or other organism. Attempts have been made to reproduce galls by injecting the juices of similar galls into the tissue, but as yet without success, and this may point to the co-operation of mechanical irritation during the hypertrophy in normal gall-formation. Galls, in the broad sense, are not always pre- ceded by a wound, however. Insects on the outside of young tissues may cause such irritations 2i 4 DISEASE IN PLANTS. that the parts in contact with the animal are arrested in their growth, while those further away grow more rapidly e.g. where Mites, etc., cause puckers and leaf- rolling. In true galls the hyper- trophy may consist merely in the enlargement of cells already present, and no new cell-divisions and, still less, changes in the nature of the tissues result e.g. some pocket galls on Viburnum, Pyrus, etc., and the hairy outgrowths of the epidermis known as Erineum. In other cases there is not only hypertrophy of existing cells, but new cell-divisions are instituted : these cell- divisions may be confined to the direction per- pendicular to the epidermis, and the tissues grow only in the direction of the surface, producing puckerings e.g. the Aphis galls on Ribes, Phy- toptus galls of Salvia, leaf galls on Tilia, Acer, Alnus, etc., and the curious galls on Plums due to Cecidomyia Pruni, and which must not be con- founded with the " pocket plums " and similar galls due to Exoasci. In a third series of cases, cell-divisions occur parallel to the surface of the leaf, and galls are formed which grow in thickness, and develop the most extraordinary and complicated new tissues proteid-cells surrounding the egg or larva deposited inside, followed by a protective layer of sclerenchyma encasing this food layer, and around this again softer tissues which may assume the structures and functions of respiratory tissues, water-storing tissues, starch reservoirs, assimilatory, or protective tissues of various kinds, EXCRESCENCES. 215 and over all may be a well-marked epidermis, with stomata, or cork with lenticels. The chief seat of these hypertrophies and what is more remarkable development of new tissue elements not found elsewhere in the leaves, or even in the species, is the mesophyll, and various speculations and hypothesis have been founded on these curious phenomena. Erineum. The simplest excrescences on plants are certain hair-like developments of epidermal cells due to the irritation of species of Phytoptus, and similar insects which rise in clusters on the surfaces of leaves and by their colours, consistence, arrangement in patches, spots, etc., so simulate fungi that Persoon was deceived by them and gave them the genus name Erineuin. They occur on most of our trees, e.g. Poplar, Lime, Oak, and are very common in the Tropics. Usually pale or even white at first, they turn brown as the hair-like outgrowths die and lose their sap, but since the latter may be bright coloured yellow, red, purple, the patches are sometimes very conspicuous objects on smooth leaves. In many cases these hairs exactly resemble in shape and other characters the abnormal root- hairs found on roots exposed to the effects of poisonous reagents, or of unsuitable food-materials, or the rhizoids developed from wounded Algae, etc. Intiunescences are similar trichomatous out- growths not associated with insects or fungi, and due to some disturbance of the balance 216 DISEASE IN PLANTS. between transpiratory and assimilatory functions of their leaves, as indicated by the less localised occurrence and by their non-appearance when the plant is under favourable cultural conditions. Structures not unlike these have been artificially induced by exposure to particular lights, and also by painting spots with dilute corrosive sublimate, indicating that poisons may impel the epidermis cells to grow out abnormally. Corky warts. Several forms of disease are known in which the pathological condition is expressed by the formation of cork in unwonted places and quantities. The Scab or Scurf of Potatoes is a case in point. The tissue of the lenticels absorbs water and the outermost cells are cut off by cork and die : the cells below them burst the dead bark-like masses thus formed, and again cork is formed and cuts off the outer masses, and the rough cork warts Scab or Scurf- are the result. The causes predisposing to scab have been variously assigned to dampness, want of lime, action of bacteria and fungi e.g. Sorosporium, Oospora, Spongospora, the latter making their way into the ruptured tissue of the lenticels and irritating the cells to further growth. It seems probable that several different kinds of scab exist in Potatoes, as well as in roots e.g. Beets, and the whole subject needs further investigation. The scab-like rough scaly bark of Pear trees in dry districts may also be mentioned here. EXCRESCENCES. 217 Cork-wings are well known on the young branches of Elms, Maples, etc., some varieties of which have received specific names on this account. Corky excrescences on leaves occur occasionally in the Gooseberry, Holly and other plants, for which no cause has been discovered. Lenticels are also formed on some leaf-galls, and are remarkable as being structures not normal on leaves. Pustules. This term may be employed gener- ally for all slight upheavals of the surfaces of herbaceous organs, which subsequently burst and give egress to the spores, etc., of the organism causing them, or merely fray away at the top if no organism is discoverable. They are often due to fungi e.g. Synchytriuin, Protomyces, Cystopus, and Ustilagineae, and we may extend the use of the general term also to those cases where the stroma of the fungus itself bursts through the cortex of older parts and forms the principal part of the pustule e.g. Monilia, forming white or grey pustules on Apples, Roestelia and other ^Ecidia, forming yellow or orange pustules on leaves, etc. ; Cucurbitaria and Nectria (red) breaking through the cortex of trees, and Phoma and numerous other Ascomycetes which form black cushions. Pustules on the leaves of Lysimachia, Ajuga, etc., are due to the parasitic Alga Phyllobium. Cylindrical stem swellings are caused by Calyptospora : they are due to the hypertrophy of the cortex of Bilberry stems permeated by 2i8 DISEASE IN PLANTS. the hyphae. Epichloe, which clothes the sheaths and halms of grasses with its stroma, at first snowy white and later ochre-yellow as the peri- thecia form, is another example. The cylindrical layer of eggs of a moth such as Bombyx on a twig must not be confounded with these cases. Frost-blisters are pustule-like uprisings of the cortex, where the living tissues below have formed a callus-like cushion into the cavity beneath the dead outer parts of the cortex which were killed by the frost ; they occur on the stems of young Apples, Pears, etc. Galls in the narrower sense are tissue out- growths usually involving deeper cell-layers. They are so varied and numerous that classification is difficult. For symptomatic purposes we may divide them as follows : Leaf-galls. A well-marked type is that of the pocket-galls or bladders in which the whole thick- ness of the leaf is as it were pushed up like a glove-finger at one spot, so that if the upper surface of the leaf forms the outside of the gall the lower surface is its lining. Such galls are common on Limes (Phytoptus], Glechoma (Ceci- domyia), Elms ( Tetraneura\ etc. Similar localised extension of the leaf surface, compelling it to rise up like a pocket, are caused by fungi e.g. Taphrina on Poplars, Exoascus on Birches, etc., Exobasidium on Bilberries, Rhododendrons, etc. Another type is that of the Gall-apple, so well known on Oaks, where the spherical swelling is EXCRESCENCES. 219 solid except for the inner cavity containing the eggs Neurotus, Cynips, Hormomyia, etc. These are comparable in general characters to the nodules on roots. Fungus galls with similar external features when young are found on Maize (Ustilago Maydis), and betray their nature by the black powdery spores as they mature. Bud galls on Willows are due to Cecidomyia, which causes several internodes to swell out into a greenish barrel-shaped mass, from which leaves may spring. Small irregular excrescences on Willow stems are referred to Phytoptus, and another species of the same insect induces similar swellings on Pines which are not surcharged with resin. American Blight, or Woolly Aphis, on Apples especially, causes the tumour-like swellings covered with sticky white fluff, which is a waxy excretion of the insect. Galls on Pilea, in Java, are due to an Alga Phytophysa. Root-nodules or nodosities are frequently caused by insects e.g. Centhorhynchus, a beetle which attacks Crucificers, Cynips and allied " gallflies " of Oaks, and the notorious Phylloxera. But similar root-galls are produced by Nematode worms, Heterodora, on Beets, Tomatoes, Cucumbers and numerous other plants, and by the Slime fungus Plasmodiophora, and it is not always easy to distinguish such cases from the fungus-galls (Myco- cecidia) on the roots of Alders, Juncus, and Legumi- noseae where the symbiosis of bacteria or fungi with 220 DISEASE IN PLANTS. the roots are of benefit to the plant. Urocystis Leimbachii forms similar nodules at the collar of young plants of Adonis. Heterodora javanica passes into the cortex of sugar-cane roots through fissures, and makes its way to the place where a young rootlet is about to emerge ; here it sticks its beak into the growing- point and remains fixed. Molliard has shown that in the roots of Melons, Co/eus, etc., Heterodora causes the cells in immediate contact with its head, and which would normally become vessels of the xylem, to swell up into huge giant-cells, with their walls curiously folded, and containing large supplies of proteids and numerous nuclei, reminding us of the food-layer of insect galls and of the tapetal layer of pollen-sacs. While the stimulus exerted by the Nematode thus induces hypertrophy and storage with food- substances of these cells, those of the next layers undergo reticulate thickenings of their walls. Again instances of the evolution of new tissue elements by the action of the foreign organism. So far as galls on leaves are concerned the amount and kind of damage done are in pro- portion to the area of chlorophyll action put out of play for the benefit of the plant, and the remarks already made on p. 193 apply here also. Where buds are destroyed the effects may of course extend further, but it rarely happens that leaf-galls are so abundant as to maim a tree per- manently. Nevertheless we must remember that cases like Phylloxera are notorious. EXCRESCENCES. 221 Far more dangerous, however, are the root-galls due to such insects, because here the damage is not so local : the water-supplies are cut off, and injurious consequences result from the absorption of the products of decomposition in the soil. NOTES TO CHAPTER XXIII. In addition to the literature on galls quoted in the Notes to Chapter XIV., the reader should consult Dale " On certain Outgrowths (Intumescences) on the green parts of Hibiscus" Proc. Cambr. Phil. Soc., Vol. X., 1899, p. 192, and Brit. Ass. Rep., Bradford, 1900. The detailed study of the anatomy and histology of Galls has been recently undertaken by Kiister, " Beitrage zur Kenntniss der Gallenanatomie? Flora, B. 87, 1900, p. 117, where the principal references will be found. On the root-galls due to Nematodes see Atkinson in Science Contributions from the Agric. Expt. Station, Alabama, Vol. I., p. I, 1889; Percival, "An Eel-worm disease of Hops" in Natural Science, Vol. VI., 1895, p. 187 ; and Molliard in Revue generale de Botanique, Apl., 1900, p. 157, where the histology is dealt with. The nodules of the roots of Leguminosae are not part of the subject of this work : the literature is collected in Science Progress, 1895, Vol. III., p. 252, and Dawson, Phil. Trans., 1900. CHAPTER XXIV. EXCRESCENCES (continued). Cankers Burrs Sphaeroblasts> and other excrescences of woody tissues Witches' 1 Brooms. Cankers are irregular excrescences due to the perennial struggle between tissues attempting to heal up a wound, and some organism or other agent which keeps the lesion open. A canker always originates in a wound affecting the cam- bium, and usually in a small wound such as an insect puncture or frost nip ; if undisturbed the dead parts would heal over by cork and callus, but if recurring frost-cracks break open the coverings, or if insects or fungi penetrate the callus and invade the cambium, irregularities of growth due to the occluding tissue on the one hand, and continued growth of the still unim- paired cambium on the opposite side of the injured shoot on the other, result in the canker. Frost cankers occur on fruit-trees, Vines, Beeches, etc. EXCRESCENCES. 223 Cankers due to insects are found on Apples, the cortex of which is punctured by the woolly Aphis (Schizoneura) while the twigs are young, and the wound is kept open by the insects nestling in crevices in the occlusion tissues. Species of Coccus, Lachnus, and Chermes also produce cankers on forest trees. Cankers due to fungi usually originate in a wound primarily due to an insect puncture or bite, or to frost, the invading fungus hyphae making their way into the wounded tissues and gradually extending more and more into the cambium and the occluding callus. Among the best known of these wound fungi which cause cankers are Dasyscypha Willkommii the peziza of Larch disease, Nectria ditissima and N. cucurbitula on Beech and Conifers ; less common are Scleroderris on Willows, Aglaospora on Oaks and some others. P eridermiuDi Pini and Aecidium elatinum also cause cankers under certain conditions, as also does Gymnosporangitim, but in these cases the fungi are more truly parasitic. In some cases e.g. Ash, Pine, Olives bacteria are concerned as associated organisms in the cankering of trees. Burrs or Knauers are irregular excrescences, principally woody, with gnarled and warted sur- faces. They are frequently due to some previous injury, such as the crushing or grazing of cortical tissues by cart-wheels. The excitation of the tissues thus wounded results in the development 224 DISEASE IN PLANTS. of shoots from adventitious or dormant buds at the base of old tree trunks, or in the starting of the same process where a branch has been broken off. The new bud begins to develop a shoot, but soon dies at its tip owing to paucity of food- supplies to the weak shoot, while new buds at its base repeat the process next year with the same result, and each of these again in turn, and so on. The consequence is an extremely complex nest of buds, all capable of growing in thickness and putting on wood to some extent, but not of growing out in length. In course of time this mass may attain dimensions measurable by feet, forming huge rounded and extremely hard- knotted burrs, the cross-section of which shows the vascular tissues running irregularly in all directions, and, owing to the very slow growth, extremely dense and hard. The dark spots in such sections e.g. Bird's-eye Maple are the cut bud-axes all fused together, as it were. On old Elms such burrs are common at heights on the stem which preclude the assumption of any coarse mechanical injury, and similar structures occur on the boles of other forest trees suddenly exposed to light by the felling of their companions, which suggests that these epicormic shoots result from some disturbance due to the action of light. Witches' Brooms are irregular tufts of twigs often found among the branches of trees such as Birches, Hornbeam, etc., where they look like crows' nests, and similar structures are to be found on Silver Firs and other conifers. In the former case they EXCRESCENCES. 225 are due to Exoascus, in the latter to Aecidium, fungi which are perennially parasitic in the shoots, and stimulate the twiggy development of a number of buds which would normally have remained in abeyance, or not have been formed at all, and only do so now in a fashion different from that of normal branches. Rosette-like formations, depending on similar disturbing causes on the part of insects, occur in conifers e.g. Gastropacha Pint. Dense tufts of twiggy shoots may be developed on many trees by pruning in such a way as to stimulate the shooting out of basal buds which would otherwise remain dormant, e.g. Elm, Ash, and thus it occurs that injuries such as frost, insect bites, etc., may induce the production of such tufts in a tree crown. The dense nests of stool-shoots thrown up from felled tree-stumps are of essentially the same nature partly adven- titious and partly dormant buds being enabled to grow out because they can now be supplied with materials previously carried beyond them while the trunk was still there. Suckers, if repeatedly cut down, may also behave similarly. Wood-nodules or Sphaeroblasts are curious marble- like masses of wood which protrude with a cover- ing of bark from old trunks of Beeches, etc., and can be readily dug out with a knife. The nodule has arisen by the slow growth of the cambium of a dormant bud, the base of which separated at an early date from the wood beneath ; the cam- bium then closed in over the base and laid on p 226 DISEASE IN PLANTS. thickening rings all round the axis of the bud except at the extreme apex. When the separa- tion occurred the cambium of the wood beneath covered over the previous point of junction, and thus the woody bud was pushed out with the bark, and now protrudes covered with a thin layer of the latter. Similar nodules are occa- sionally found on Apple trees. NOTES TO CHAPTER XXIV. For further information on Cankers the student should read Marshall Ward, Timber and some of its Diseases, Chapter X. Further, the discussion as to the causes of canker in Frank, Krankheiten der Pflanzen, B. I., p. 207, and B. III., pp. 167 and 172, and various papers in Zeitschrift fur Pflanzen- krankheiten. CHAPTER XXV. EXUDATIONS AND ROTTING. Tumescence Rankness Bursting of fruits, etc. Root rot Rot of fruits Bulb diseases Flux Honey - dew Slime flux Resinosis Gummosis Manna. I PUT together in one artificial class a varied group of diseases, the principal symptom of which is the escape of fluids from the tissues, under circumstances which betray an abnormal state of affairs, often obvious, but sometimes only to be inferred. In many of these cases bacteria abound in the putrefying mass, and some evidence exists for connecting these microbes causally with the disease in a few of the more thoroughly investi- gated cases, but in no case has this been sufficiently demonstrated ; and considering the ease with which bacteria gain access via wounds caused by insects and fungi, as well as by other agents, the necessity for rigid proof must be insisted upon before we can accept such alleged examples of Bacteriosis. 227 228 DISEASE IN PLANTS. Tumescence. It occasionally happens that herbaceous parts of plants pass into a condition of over-turgescence from excess of water in the tissues, an abnormal state which indicates patho- logical changes resulting from various causes, often not evident and therefore regarded as internal. Such disease was formerly termed CEdema or Dropsy. This disease is frequently due to the excessive watering of pot plants with large root systems and deficient foliage, in hot- houses with a saturated atmosphere : it is, therefore, primarily referable to diminished transpiration. It can sometimes be brought about by covering potato plants, for instance, with a bell-jar in moist, hot weather ; and this, and the prevalence of the disease in hot-houses as compared with plants grown out of doors, point to the above explanation. Similar phenomena do occasionally occur out of doors in hot, moist situations or during wet seasons, however, and the watery shoots of rank vegetation are merely particular cases of the same class. Moreover, the well- known tendency to succulence of sea-side varieties of plants which have thin herbaceous leaves when growing inland, points to the action of the environment in these matters, excess of salts being no doubt one factor in such cases. Rankness affords another example where super- fluity of water is concerned, though it does not involve simply this, because the plant may also contain excessive quantities of nitrogenous and mineral matters taken up by the roots. EXUDATIONS AND ROTTING. 229 Rankness is, in fact, in many respects analogous to etiolation in so far as the tissues are soft and surcharged with water, but it differs fundamentally in the deep green of the chlorophyll : this may lead to abundant assimilation if free access of air and drier conditions can be gradually brought about. Any sudden drying, however, may be fatal to the tender tissues. Rankness commonly depends on excess of food materials, especially nitrogenous manures, as may be seen in meadows and cornfields where the manure heaps have remained on the ground and saturated it to excess as compared with the rest of the soil ; this may often be observed with weeds, etc., in the neighbourhood of farm- buildings. If the period of rank growth is accompanied and followed by days of suitably bright sunshine and dry air, the increase of vege- tative structures usually results in increased flowering, heavy crops, or strong wood ; but if the rankness continues too long, or is accompanied by wet and dull weather, the watery tissues are peculiarly susceptible to attacks of fungi and insects, and to damage by sudden frosts or chilly winds. Rankness affords, in fact, a typical illustration of predisposition to disease. Damping off, When seedlings are too closely crowded in beds kept too damp, or in moist weather, they are very apt to rot away, with all the symptoms spreading from a centre, con- tagious infection, mycelia on and in the tissues, etc. of a fungus attack. The commonest agent 230 DISEASE IN PLANTS. concerned is one of the species of Pythium, the propagation of which is favoured by the rank, over-turgid, and etiolated conditions of the plants. Species of Mucor, Botrytis, and other fungi, may also be met with. Bursting of fleshy fruits, such as Tomatoes, Grapes, etc., is due to over-turgescence in rainy weather or excessively moist air. But the pheno- menon is by no means confined to such organs. Hot-house plants when oedematous not infre- quently put out watery blisters from the cortex or leaves, which rupture ; and the stems of fleshy fasciated (e.g. Asparagus) or blanched and forced plants (e.g. Celery, Rhubarb) are particularly apt to crack here and there from the pressure of the turgescent tissues on the strained epidermis. Beets, Turnips, and other fleshy roots show the same phenomena in wet seasons. That these ruptures and exposures of watery tissues afford dangerous points of entry for parasites and moulds will be obvious e.g. Edelfaiile, a rotten condition of the grapes in the Moselle district. Root-rot is a common disease in damp, sour clay soils after a continuance of wet weather e.g. Wheat, especially if root-drawn and exposed to thaw water. In the disease known as Beet-rot, the roots turn black at the tip, where the tissues shrivel and become grooved and wrinkled extensively. Inside the flesh also blackens and finally rots. In earlier stages, only the vascular bundles are brown and blocked with gum-like substances. EXUDATIONS AND ROTTING. 231 In advanced stages there is much gummy material in the lumina, and even large cavities filled with this gum may be found. The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to several fungi, of which Botrytis, Monilia, Mucor, Penicilliuni, and Asper- gillus are the chief. The fruit may be attacked while still on the tree, but very often fungi and bacteria gain access to the tissues, through bruises, cracks, etc., formed in the fruit lying in the storage baskets or on the shelves. Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of Botrytis or Sclerotinia, followed by moulds, yeasts, and bacteria in the stores. Sour-rot in Grapes, and other fleshy fruits which need much sun to ripen them, is probably a usual result of continued cold, wet weather at the cropping season, setting in when the fruits are beginning to swell. Flux. It is a common event to see fluids of various kinds issuing from wounds in trees, or congealing in more or less solid masses about them ; and owing to the prevailing tendency to compare plant diseases with those of animals, we find such expressions as Gangrene, Ulcer, and so forth, applied to these " open sores." In so far as such outflowings frequently indicate diseased states of injured tissues which are incapable of healing up, the analogy is perhaps a true one ; but it must be remembered that very different structures and processes in detail are concerned. Moreover, 232 DISEASE IN PLANTS. liquid excretions more or less indicative of diseased states are by no means confined to wounds or definitely injured tissues, in which case such terms are wholly misapplied. Honey-dew. The leaves, or other organs, of many plants are sticky in hot weather, owing to the excretion of a sweet liquid containing sugar, the consistency and colour of which vary accord- ing to circumstances. This honey-dew must not be confounded with the normal viscidity of certain insectivorous plants e.g. Sundew or with the sticky secretion on the internodes of species of Lychnis, etc., where it plays the part of a protec- tion against minute creeping things. Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of these cases the honey-dew is excreted by Aphides, which suck the juices of the leaves and pour out the saccharine liquid from their bodies. The sweet fluid is in its turn sought after by ants, and also serves as nutritive material for various epiphytic fungi e.g. sooty mould, Capnodium, Fumago, and Antennaria which give the leaves and honey-dew a brown or black colour. Certain Coccideae also excrete honey-dew, especially in the tropics. At least one case is known where honey-dew is formed as the result of the parasitic action of a fungus, namely Claviceps purpurea in its conidial stage on the stigmas of cereals, and this may be compared with the sweet odorous fluid excreted by the spermogonia of certain Aecidia. In both EXUDATIONS AND ROTTING. 233 cases the sweet fluid attracts insects which disperse the spores. Honey-dew may also be formed without the agency of fungi or insects, when hot and dry days are followed by cool nights, with a saturated atmosphere, e.g. Caesalpinia, Calliandra and other trees in the tropics, which are called rain trees owing to the numerous drops of fluid which drip from the leaves under the abnormally turgescent conditions referred to. Cuckoo-spit. The leaves of Willows, Meadow grasses and herbs, etc., are often seen with froth on them, in which is a green insect, Aphrophora, which sucks the juices from the tissues and excretes the frothy watery cuckoo-spit from its body. SUme-flux. The trunks of trees may some- times be observed to pour out a slimy fluid from cracks in the bark, or from old wounds, or branch scars. In some cases, e.g. in Oaks, the slime has a beery odour and white colour, and abounds in yeasts and other fungi to the fermentative activity of which the odour and frothiness are due. In other cases the slime is red e.g. Hornbeam; or brown e.g. Apple and Elm ; or black e.g. Beech, the colour in such cases being due to the mixture of yeasts, bacteria, and fungi with which these slimes abound. The phenomenon appears to be due to the exudation of large quantities of sap under pressure root pressure and is primarily a normal phenomenon comparable to the bleeding of cut trees in spring : the fungi, etc., are doubtless 234 DISEASE IN PLANTS. saprophytes, but their activity is concerned with the putrefactive processes going on in the diseased wood, and which may lead to rotting of the timber. The origin of the wounds in the bark and cortex, and which extend into the wood and other tissues as the putrefactive and fermentative pro- cesses increase, appears to be in some cases at least due to lightning. Resin-flux or Resinosis. The stems of Pines and other conifers are apt to exude resin from any cut or wound made by insects, or by the gnawing of other animals ; but in many cases the flow is due to fungi, e.g. Peridennium, the hyphae of which invade the medullary rays and resin canals and thus open the way to an outflow through cracks in the bark. Agaricus melleus not only invades the resin passages, but stimulates the tree to produce abnormal quantities of resin, which flows down to the collar and roots, and exudes in great abundance at the surface of the soil. Various other plants also exude resin from wounds, and in some cases the flux seems to be increased by degeneration of the tissues, e.g. Copaifera. Gummosis. Cherries, Apricots, Acacias, and many other trees are apt to produce abnormal quantities of gum, which flows from any wound or exudes through cracks in the bark. Degeneration of the wood-cells, and especially of the cell-walls of a soft wood formed by abnormal activity of the cambium, points to its origin being due, in some EXUDATIONS AND ROTTING. 235 cases at any rate, to a conversion of the cellulose, and fungi are sometimes found in the masses of gum ; but beyond the fact that gummosis is a pathological phenomenon we know very little of the disease. With regard to such gumming, it is significant how frequently pruned trees Cherries, Oranges, Lemons, Plums, etc. suffer. Manna flux. Certain trees, such as the Manna Ash, species of Tamarisk, etc., yield manna from wounds, and in some cases the latter are due to insects, e.g. Cicada. The Potato-disease is best known by the pale whitish fringe, giving an almost meally appearance to the margins of the brown to black patches in damp weather. In dry weather the brown patches shrivel and dry, and as they are apt to be at the edges and tips of the leaflets, these curl up. The young disease spots are yellowish, and the leaves of badly affected plants are apt to be sickly yellow throughout. This Potato-disease due to Phytophthora must be distinguished from the curling and puckering, with wilting and browning of the leaves and yellow glassy look of the stems, due to the invasion of the vessels by a fungus which lurks in the tubers, and gains access thence to the shoots. In the disease traceable to Phytophthora the stock remains green and the leaves plump and plane, and only the brown patches slough out in wet or shrivel in dry weather, and are bordered by the pale whitish zone of conidiophores. 236 DISEASE IN PLANTS. In the leaf-curl the yellow and flaccid appear- ance of all the leaves of a stalk, or even of the plant, is the striking symptom, and the stem soon droops and blackens just above the soil, a white mould appearing also at the black spots. Subse- quently black spots appear higher up, and bacteria gain an entrance. The stolons rot, and eventually the roots and the leaves wither. The tubers appear sound, but are small ; they are apt to rot in the store, the vascular zones turning brown. This leaf-curl has been ascribed to Pleospora, Polydesmus, Vertici Ilium, and other parasites, as well as to excessive manuring and other agencies, but it still needs explanation. Rot of Potato tubers in the soil, or in store, may be brought about by very different agents. If Phytophthora has obtained access, the fungus hyphae spread between the cells, starting from the haulm, and cause the flesh to turn yellowish and then brown in patches. On the exterior are discoloured patches, depressed, with the flesh beneath brown and soft. The mycelium spreads mostly in the outer layers, which though they turn deep brown remain firm. Wet rot of potatoes may be due to various fungi, and, in excess of water, to putrefactive bacteria (e.g. Clostridiuni), which destroy the cell- walls. The flesh becomes soft, then soup-like, and finally putrefies to a liquid mass with a vile smell of butyric acid, etc., in which the starch grains may be seen floating. Tubers are often found with the cork burst and EXUDATIONS AND ROTTING. 237 peeling in shreds, the flesh more or less converted into a putrid and stinking pulp, with a spotted brown boundary of partly destroyed but firmer tissue between the dark utterly rotten and the white and still firm healthy flesh. The principal agent in the destruction of the tissues is Clostridium, an anaerobic bacillus which consumes the cell-walls but leaves the starch intact. Hence a thoroughly decomposed tuber consists of a cork bag full of starch and foetid liquid. In the dried condition the flesh shows a brown marbling ; this passes into a soft soupy starchy part, and here and there may be violet grey cavities lined with Spicaria, Hypomyces, etc., the white stromata of the latter often appearing externally. The excavations are filled with loose starch grains, and bounded by cork and cambium formed in the peripheral cells. The cell-walls eventually undergo slimy decom- position. Spicaria, Fusisporium, various moulds, and bacteria may all be associated with wet-rot. Dry-rot of Potatoes is also due to various fungi and bacteria, but the destructive action goes on slowly, owing to there being no more moisture than the tissues afford. The flesh becomes ex- cavated here and there, owing to the slow destruction of the cell-walls by Clostridium : the destroyed tissues are brown, and the uninjured starch grains powder them all over. Finally the whole shrunken mass has a crumbly consistency. When the flesh remains white, but assumes a powdery consistency and dry-rot, with the cork 238 DISEASE IN PLANTS. destroyed here and there, Frank refers the damage to Phellomyces. Where the dry-rot is due to Fusarium the chalk-white stromata may often be detected breaking through the periderm ; but it must be remembered that the soil-contaminated, broken skin of a potato-tuber is a favourable lurking spot for many fungi, and Periola, Acros- tatqgmtts, and others have been detected therein. Brown spots, depressed into the flesh, some- times result from the ravages of Tylenchus, the minute worms being found in the diseased tissues. In some cases the flesh turns watery and soft, grey, almost glass-like, starting at the haulm end, and this may be owing to the invasion of Rhizoctonia. NOTES TO CHAPTER XXV. The rotting of bulbs, roots, etc., has been much discussed during the last few years in the pages of the Gardeners' Chronicle, Zeitschrift filr Pflanzenkh., and elsewhere. The principal references to Bacteriosis the rot in which bacteria are stated to be the primary agent causing these and similar diseases may be found in Massee, Diseases of Plants, pp. 338-342, and more fully in Russell, Bacteria in their Relation to Vegetable Tissue, Baltimore, 1892 ; and in Migula,A>zVz'.sv:/^ Uebersicht derjenigen Pflanzen-krankheiten, welche Ange- blich durch Bakterien verursacht uierden, Semarang, 1892. The most convincing accounts, however, are since that date; see Smith, " Pseudomonas Campestris," Cent./. Bakt., B. III., 1897, p. 284, and Arthur and Bolley, Bacteriosis of Carnations, Perdue University Agr. Expt. Station, 1896, Vol. VII., p. 17. Woods has lately shown that this disease is due to Aphides only, the bacteria having nothing to do with the disease primarily, Stigmonose, Bull. 19, U.S. Dept. EXUDATIONS AND ROTTING. 239 Agr., 1900; but it is necessary to bear in mind that actual penetration of the cell-walls from without must be proved, as De Bary proved it for germ-tubes of fungi, before the evidence that Bacteria are truly parasitic in living plants can be called decisive. This is a difficult matter, but until it is settled we do not know whether these organisms are really parasitic in the sense that Phytophthora is, or merely gain access by other means I have traced them through dead fungus-hyphae to the vessels, dead cell-walls, etc. The proof of infection via water pores and vessels is given for one species by Harding, " Die Schwarze Faulnis der Kohls," etc., Cent. f. Bakt., Abh. II., B. VI., 1900, p. 305, with literature. Concerning the " Damping off" of seedlings, see Marshall Ward, " Observations on the Genus Pythium," Quart. Journ. Microsc. Soc., Vol. XXIII., 1883, p. 485, and Atkinson, Bull. 94 of Cornell University Agric. Expt. Station, 1895, P- 233. On Bacteriosis in Turnips, see Potter, Proc. R. S. 1901, Vol. LXVII., p. 442. CHAPTER XXVI. NECROTIC DISEASES. Patches Frost-patches Bruising due to hail, shot, etc. Fire Sun-burn or scorching Sun-cracks. Dying- back Frost Fungi Wound fungi Defoliation by insects Defoliation by hand Staghead. Necrosis. This is a general term for cases where the tissues gradually turn brown or black in patches which die and dry up, the dead area sometimes spreading slowly and invading the usually sharply demarcated healthy tissues around. It is a com- mon phenomenon on the more slender stems ,or branches of trees, especially those with a thin cortex, and the terms Brand or Scorching some- times applied signify the recognised resemblance between burnt patches and these dead areas of necrotic tissue. Necrosis is often due to frost, which kills the cortex of Pears, Beech, etc., in patches of this kind. The dead cortex and cambium stick to the wood beneath and contract as they dry. The living 240 NECROTIC DISEASES. 241 cambium and cortex around them then begin to push in callus towards the centre of the necrotic area ; but since this callus is formed under the pressure of the cortical tissues it does not form a thick lip or margin to the healing wound, as it does in a Canker, but insinuates itself with thinned- off edges between the wood and the dead tissue, or at most traps a little of the latter in the final closing up of the wound. It is easy to see how such an area of Necrosis may become a Canker if the dead tissues split or slough off, arid fungi or insects ob- tain access to the callus at the margins of the area, setting up the disturbances described on p. 222. As matter of fact many Cankers e.g. those of the Larch disease, and those due to Nectria, or Aphides, etc. often begin as flattened or de- pressed areas of Necrosis started by frost, and many small necrotic patches would eventually become Cankers if not healed up by the callus. Necrosis may also be due to the bruising of the tissues by large hailstones, to gun-shot wounds, or to any form of contusion which kills the living cells of cortex and cambium. Necrosis is a natural and common result of fire, and it frequently happens after forest-fires which have run rapidly through the dry underwood, fanned by steady winds, that the lower parts of the boles are scorched on one side only. The killed cambium and cortex then dry up in black necrotic patches, which may eventually heal up by intrusion of callus from the uninjured parts. Sun-burn or Scorching. If thin-barked trees, Q 242 DISEASE IN PLANTS. such as Hornbeam, Beech, Firs, etc., which have been growing in partial shade owing to dense planting, are suddenly isolated by thinning, the impingement of the sun's rays on the south-west side during the hottest part of summer days may kill the cambium, and produce necrosis of the cortical tissues, and such necrotic patches heal very slowly or not at all, because the dead tissues have contracted so tightly on to the wood below that the callus cannot readily creep between. Sun-cracks are due to intense insolation on the south side of trees in clear weather in early spring, causing the drying and contraction of the wood and its coverings down that side of the tree : the contracted tissues consequently split, as in the case of frost-cracks, the healing up of which is very similar. Dying-back. All that is true of the necrosis of cortical tissues in small patches also applies to cases where the whole of the outer tissues of thin twigs and branches die of inanition owing to a premature fall of leaves e.g. after a severe attack of some insect or fungus pest. The consequent arrest of the transpiration current and the proper supply of nutriment to the cambium and cortex explain the phenomena. The younger branches of Coffee trees suffering from severe attacks of leaf-disease are often denuded of leaves and die back from the causes mentioned, the whole of the outer tissues becoming necrotic, and drying up tight on to the wood, because other branches with functionally active leaves on them divert the NECROTIC DISEASES. 243 transpiration current, and drought and inanition supervene. Dying-back is frequently also a direct effect of early frosts, which kill the thin twigs before the " wood is ripened," as gardeners say. Dying-back is also a frequent result of direct frost action on thin watery shoots or "unripe wood," and is apt to occur every year in certain varieties of Roses, for instance, in particular situations, such as "frost-beds," or aspects exposed to cutting winds, and so forth. The necrosis which results may affect all the tissues, or only the cortex and cambium, and the frequent accom- paniment of all kinds of saprophytic Ascomycetes and moulds or other fungi is in no way causal to the phenomenon. Dying-back may also be caused by fungi, and not necessarily parasites, for cases are often observed where saprophytes only are to be found in the necrotic tissues of the cortex, having made their way in through minute cracks, lenticels, etc. A simple case is often seen in Chrysanthemums, Roses, etc., chilled and wetted to danger point, but not frozen, during the nights of autumn. The lowered resistance of the chilled tissues enables fungi like Botrytis cinerea to gain a hold, and the peduncles die-back with all the symptoms of Necrosis, the fungus gaining power more and more as its myceliun spreads in the dead tissues. Many other cases are known where wound- fungi, such as Nectria, Cucurbitaria, Phoma, etc., in themselves incapable of true parasitism, gain a hold 244 DISEASE IN PLANTS. on the necrotic tissue of a wounded twig, and having laboriously accumulated a vigorous my- celium saprophytically, extend into other parts. In many of these cases the dying-back of the twigs is expedited owing to the mycelium invading the medullary rays and wood vessels, and so obstruct- ing the transpiration current. The much more rapid spread of the hyphae up into the parts thus killed sufficiently indicates the fundamentally saprophytic character of such fungi. Dying-back in all its forms is a common result of defoliation by insects, e.g. caterpillars, especially if it occurs when the wood is depleted of reserve materials, and thus cannot supply the auxiliary buds and enable the twigs to clothe themselves with a new flush of foliage, a common danger in Conifers. Any form of defoliation e.g. excessive plucking of tea and mulberry leaves, browsing of animals, etc. exposes the twigs to the dangers of dying- back, the accessory phenomena being similar to those already described. Stag-head. Old trees, though vigorous and in full foliage throughout the crown generally, fre- quently lose the power of bearing leaves on their topmost branches and twigs, which stand out bare and brown, and fancifully resemble the antlers of a stag : hence the forester's name "stag-head." This "top-dry" condition is frequently due to the re- moval of litter, or to excessive draining, or to the roots having gradually penetrated into unsuitable soil. The consequence is that some dry summer NECROTIC DISEASES. 245 the drought causes the breakage of the water columns above, and the twigs die back. Tropical trees may also become stag-headed owing to the attacks of Loranthus and other parasites, the portions above the point of attach- ment dying back from inanition. Cases also occur in the tropics where the stag- head condition is due to the persistent roosting of frugiferous bats " flying foxes " which tear the bark and foliage with their claws, and befoul the twigs generally. NOTES TO CHAPTER XXVI. The principal literature as regards frost is given in the works of Frank, Sorauer, and Hartig already referred to. An excellent summary will be found in Hartig's Diseases of Trees, p. 282, and in Fisher " Forest Protection," Vol. IV. 01 Schlich's Manual, p. 423. CHAPTER XXVII. MONSTROSITIES AND MALFORMATIONS. Monstrosities Teratology Atrophy of organs Shank- i n f grapes Barren fruit trees Dwarfing Distortions and malformations Fasciations Flattened roots Torsions Curling and puckering Leaf rolling So-called "spontaneous" teratological changes. Monstrosities. In a wide sense this term is applicable to many cases here treated under other headings, and signifies any departure from the normal standard of size, form, arrangement, or number of parts, and so forth, due to arrest of growth, excessive growth of parts, or of the whole organs, etc. Such teratological conditions are however by no means always pathological: that is to say, they may be variations which do not threaten the existence of the plant. In some cases they are clearly due to exuberant nutrition, and although they may occasionally predispose to disease, in 246 MONSTROSITIES AND MALFORMATIONS. 247 others they show no evidence of doing so. The whole practice of horticulture and agriculture abounds in examples of teratological sports or varieties which are transmissible by seeds, bud- ding and grafting, and other means e.g. double flowers, hypertrophied floral organs (cauliflowers), seedless grapes and 'oranges, crested ferns, etc.; and even when such varieties could not live as such in a state of nature, there is evidence to show that many of them readily revert to the original seed-bearing or single condition, and adapt themselves to the altered environment. Every part of the plant may exhibit teratological changes, and I shall for the most part select cases in illustration which indicate approach to patho- logical states, and group with them cases known to be pathological in origin. Atrophy is a common phenomenon denoting dwindling or reductions in size of organs due to insufficient nutrition, or arrest of growth from various causes. Atrophy of leaves is a common result of the attacks of parasitic fungi, even when the latter induce local hypertrophy i.e. excessive growth of particular parts, e.g. Synchytrium on Dandelions and Anemones. Puccinia suaveolens causes partial atrophy of the leaves of Thistles, Aecidium Euphorbiae of those of Euphorbia. The carpels of Anemone are atrophied in plants attacked by Aecidium, and the whole flower is suppressed in Cherries infested with Exoascus Cerasi, while other fungi e.g. Cystopus, Exoasci, 248 DISEASE IN PLANTS. etc. cause atrophy of the seeds, and numerous instances of atrophied grain occur in plants in- fested with Ustilagineae. Atrophy of the grains of cereals is sometimes due to the direct attack of animals, e.g. eel-worms (Tylenchus] eat out the grains of Corn; weevils and other beetles (Curculio, Bruchus, etc.) simi- larly devour the contents of grain and nuts, the flowers of Peas and Apples, and so forth, inducing atrophy of the parts left. Still more striking cases are afforded by small insects which bore into the halms of cereals, and cause atrophy of the whole ear e.g. Cephus in Wheat and Rye. Barley occasionally withers after flowering, the grain atrophying from no known cause, terms like consumption given to the disease conveying no information. Atrophy of young fruits is commonly due to the flowers not setting i.e. some agent has inter- fered with the normal transference of the pollen to the stigma. This may be due to excessive rain washing out the pollen (e.g. Vine), to a lack of the necessary insects which effect pollination, often seen in greenhouse plants ; to the stamens being barren e.g. certain varieties of Vine or to the premature destruction of the stigmas by frost, as in Cherries, Pears, etc., or by insects, as in Apples, or fungi, e.g. the infection of bilberries with Sclerotinia ; or even by poisonous gases, as is sometimes seen in Wheat, etc., growing near alkali works. Drought is also a common cause of atrophy of young Plums. MONSTROSITIES AND MALFORMATIONS. 249 Shanking of Grapes is a particular case of atrophy and drooping of the immature fruits, due to the supplies being cut off by some agency. It may arise from very various causes which bring about disease in the leaves or roots, and should always be looked upon as a sign of weakness in the Vine, the structure of which is affected, e.g. poor wood or the functions interfered with, e.g. water supplies deficient owing to paucity of roots. Barren Apple, Pear, Plum, and other flowers are often found to have been bored through the petals while in bud, and the whole " heart " of the flower eaten out by the grubs of A nthonomus, leaving the unopened buds brown and dead, as if killed by frost or drought, and often erroneously supposed to be so. The wilting and shrivelling of Clover is some- times due to Sderotinia, the mycelium of which pervades the roots and stock, on which the sclerotia may be found. Lucerne is similarly killed in Europe by the barren mycelium of Leptosphaeria, which may be found as a purple mat on the roots. Dwarfing consists in partial atrophy of all the organs, and is a common result of starvation in poor, dry, shallow soils, as may often be seen in the case of weeds on walls or in stony places. Dwarfs which are thus developed in consequence of perennial drought are not, however, necessarily diseased, in the more specific sense of the word ; their organs are reduced in size proportionally 250 DISEASE IN PLANTS. throughout in adaptation to the conditions, and simply carry out their functions on a smaller scale. Dwarfing is frequently a consequence of the lack of food materials, or of some particular ingredient in the soil, and in such cases is a diseased condition of some danger ; similar results may ensue in soils containing the necessary chemical elements, but in unavailable forms. Dwarfing may also be brought about by repeated maiming, nipping off the buds, pruning, etc., as in the miniature trees of the Japanese; and the case of trees continually browsed down by cattle, or of moor plants perennially dwarfed by cutting winds, are further illustrations in the same category, as are also those of certain alpine and moraine plants, whose only chance of survival depends on their adapting themselves to the repeated prunings suffered by every young shoot which rises into the cutting winds, since there is no question of lack of food-materials in these cases. The practice of the Japanese is to pinch out the growing tips of the shoots wherever they wish to prune back, and it is by the judicious use of this heading in, and suitable pot-culture, that the dwarfs are made, 6-20 inches high at from 30-80 years old. Dwarfing is often brought about by grafting on a slow-growing stock, and this method is employed in practice, as are also heading in, pruning of roots, and confinement in pots. MONSTROSITIES AND MALFORMATIONS. 251 Dwarfing may also be due to poor or shrivelled partially atrophied seeds or such as have had their endosperms or embryos injured by insects or fungi, and although it is possible to nurse such dwarfs into normal and vigorous plants with good culture, they do not usually recover under natural conditions in competition with more vigorous plants. Distortions or Malformations may be defined as abnormalities in the form of organs which concern all, or nearly all the parts, and do not refer merely to swellings or excrescences on them or excava- tions, etc., in them. Fasciation. Shoots of Asparagus, Pine, Ash, and many other plants are occasionally expanded into broad ribbon-like structures often studded with more than the normal number of buds or leaves, etc., such as would be found on the usual cylin- drical shoots. Such fasdations are due to several buds fusing laterally under compression when young and the whole mass growing up in common, or, in a few cases, to the unilateral overgrowth of one side of the terminal bud. Fasciations appear to depend on excessive nutri- tion in rich soils. They may spread out above in a fan-like manner, exaggerating the abnormality, or they may revert to the original form. Some cases are more or less fixed by heredity e.g. Celosia. Fasciated stems are frequently curved like a crozier, owing to one edge growing more rapidly than the other. Cauliflowers are really cultivated monstrosities. 252 DISEASE IN PLANTS. Fasciated Dandelions, Crepis, monstrous Chrysan- themums, peloric Linaria, five-leaved Clovers, spiral Teazels, etc., may all, if grown with care, be kept more or less constant in the monstrous state. That is to say, the particular kinds of variation here manifested can be maintained in proportion as the external conditions controlling the variation are maintained. Such conditions are chiefly rich supplies of food-stuffs, plenty of water and air, suitable temperature and lighting, etc. Mutilations, favouring the development of abnormal buds may also induce fasciations. Torsions or spiral twistings of stems also frequently arise among plants grown in rich soils, and are often combined with fasciations e.g. Asparagus, Dipsacus; and De Vries has shown that the peculiarity is not only transmissible by seed, but may be more or less fixed by appro- priate culture. Contortions of stems are often due to the unequal growth on different sides of the stems owing to the presence of fungi e.g. Caeoma on Pines, Aecidium on Nettles, also Puccinia on petioles of Mallow, Cystopus on inflorescences of Capsella, etc. Distortions of roots may be brought about in various ways by the hindrances afforded by stones. Spiral roots occur occasionally in pot plants. Flattened roots usually result from compression between rocks, the young root having penetrated into a crevice, and been compelled to adapt itself later. The distortions of stems by constricting MONSTROSITIES AND MALFORMATIONS. 253 climbers, wire, etc., have been described, and fruits e.g. Gourds are easily distorted by means of string tied round them when young. Distortions of leaves are very common, and are sometimes teratological i.e. due to no known cause e.g. the pitcher-like or hood-like cucullate leaves of the Lime, Cabbage, Pelargonium, etc., and of fused pairs in Crassula. Also coherent, bifurcate, crested, displaced and twisted leaves occasionally met with, and in some cases fixed by cultivation, may be placed in this category. Puckers must be distinguished from pustules, since they consist in local upraisings of the whole tissue, not swellings e.g. the yellowish green pockets on Walnut leaves, due to Phyllereum. Puckered leaves in which the area of mesophyll between the venation is increased by rising up in an arched or dome-like manner are sometimes brought about by excessive moisture in a confined space. Leaf-curl is a similar deformation caused by fungi, such as Exoascus on Peaches. Wrinkling or puckering of leaves is also a common symptom of the work of Aphides e.g. Hops. Characteristic curling and puckering, with yellow and orange tints, of the terminal leaves of Apples, Pears, etc., are due to insects of the genera Aphis, Psylla, etc. Small red and yellow spots with puckerings and curlings of the young leaves of Pears, the spots turning darker later on, are due to Phytoptus. 254 DISEASE IN PLANTS. Leaf-rolling. The leaves of Beeches, Poplars, Limes, and many other plants, instead of opening out flat, are often rolled in from the margins, or from the apex, by various species of Phytoptus, Cecidomyia, or other insects, which puncture or irritate the epidermis in the young stages and so arrest its expansion in proportion to the other tissues. According as the lower or upper surface is attacked the rolling is from the morphologically upper surface downwards, or vice versa. Very often the mesophyll is somewhat thickened where rolled and Erineum-Vfae hairs may be developed e.g. Lime. Many caterpillars also roll leaves, drawing the margins inward to form shelters e.g. Tortrix viridana, the Oak leaf-roller. Certain beetles Rhynchitis also roll up several leaves to form a shelter in which the eggs are laid. Webs are formed among the mutilated leaves of Apples by the caterpillars of Hyponomeuta. It must be borne in mind that instances can be found of teratological change of every organ in the plant e.g. stamens transformed into carpels or into petals ; anthers partly polliniferous and partly ovuliferous ; ovules producing pollen in their interior, and so on, being simply a few startling examples of what may happen. Such abnormali- ties are frequently regarded as evidence of internal causes of disease, and this may be true in given cases ; in a number of cases investigated, however, it has been shown that external agents of very definite nature bring about just such deformations as those sometimes cited as examples of teratology MONSTROSITIES AND MALFORMATIONS. 255 due to internal causes, and the question is at least an open one whether many other cases will not also fall into this category. The study of galls has shown that insects can induce the formation of not only very extraordinary outgrowths of tissues and organs already in existence, but even of new formations and of tissue elements not found elsewhere in the plant or even in its allies ; and Solms" investigations on Ustilago Treubii show that fungi can do the same, and even compel new tissues, which the stimulating effects of the hyphae have driven the plant to develop, to take part in raising and distributing the spores of the fungus i.e. to assume functions for the benefit of the parasite. Molliard has given instances of mites whose irritating presence in flowers causes them to undergo teratological deformations, and Peyritsch has shown that the presence of mites in flowers induces transformations of petals into sepals, stamens into petals. Similarly De Bary, Molliard, Magnus, Mangin, and Giard have given numerous cases of the transformation of floral organs one into another under the irritating action of fungi, of which the transformation of normally unisexual (female) flowers into hermaphrodite ones, by the production of stamens not otherwise found there, are among the most remarkable. These and similar examples suffice to awaken doubts as to whether any teratological change really arises " spontaneously," especially when we learn how slight a mechanical irritation of the growing point may induce changes in the flower ; e.g. Sachs 256 DISEASE IN PLANTS. showed that a sunflower head is profoundly altered by pricking the centre of the torus, and Molliard got double flowers by mechanical irritation. NOTES TO CHAPTER XXVII. For the details and classification of the multitude of facts, the student is referred to Masters' Vegetable Teratology, Ray Society, 1869, and the pages of the Gardeners' Chronicle since that date. Concerning torsions, etc., the student should read De Vries, " On Biastrepsis in its Relation to Cultivation," Ann. of Bot., Vol. XIII., 1899, p. 395, and " Hybridising of Monstrosities," Hybrid Conference Report, Roy. Hort. Sac., 1900, Vol. XXIV., p. 69. The reader will find an excellent account of the abnormali- ties in flowers due to the action of parasitic insects and fungi in Molliard, " Cecidies Florales," Ann. des Sc. Nat., Sen VIII., Bot., T. i, 1895, p. 67. CHAPTER XXVIII. PROLIFERATIONS. Proliferations Vivipary Prolepsis Lammas shoots Dormant buds Epicormic shoots Adventitious buds -Apospory and apogamy. Proliferation consists in the unexpected and abnormal on-growing or budding out of parts stems, tubers, flowers, fruits, etc. which in the ordinary course of events would have ceased to grow further or to bear buds or leaf-tufts directly. Thus we do not expect a Strawberry the swollen floral axis to bear a tuft of leaves terminally above the achenes, but it occasionally does so, and similarly Pears may be found with a terminal tuft of leaves, Roses with the centre growing out as a shoot, Plantains (Plantago} with panicles in place of simple spikes, and so on. We regard such cases as teratological, because they are exceptional for the particular species, and as pathological because they appear to be con- nected with over-feeding in soils with excessive R 257 258 DISEASE IN PLANTS. supplies of available food-materials ; but it should be noted that conditions quite comparable to proliferation are normal in the inflorescences of Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of proliferations come under the head of injurious actions of fungi, insects, and other agents. Proliferation of tubers is sometimes seen in Potatoes still attached to the parent plant in wet weather following a drought. The eyes grow out into thin stolons, or forthwith into new tubers sessile on the old tuber. Similarly in store we sometimes find the eyes transformed directly into new tubers, and cases occur where the growth of the eye is directed backwards into the softening tuber, and a small potato is formed inside the parent one. Threading is also occasionally met with in the " sets " when ripened too rapidly in hot dry soils. Vivipary is a particular case of proliferation, in a certain sense, where the seeds appear to ger- minate in situ, and we have small plants springing from the flowers, reminding us of wheat which has sprouted in the shocks in damp weather. In reality, however, the grains are here replaced by bulbils which sprout before they separate from the inflorescence. In varieties of Poa, Polygonum, Allium, Gagea, etc., this phenomenon is constant in plants growing in damp situations. Proplesis. It frequently happens that branches or whole plants are suddenly defoliated in summer, PROLIFERATIONS. 259 e.g. by caterpillars or other insects at a time when considerable stores of reserves had already been accumulated during the period of active assimilation. In such cases the axillary buds, which would normally have passed into a dormant condition over the winter had the leaves lived till the autumn-fall, suddenly shoot out into proleptic shoots (also termed Lammas shoots), and re- clothe the tree with foliage. The wood of the year in which this occurs may exhibit a double annual ring, and the vigour of the tree is likely to suffer in the following season and no fruit be matured. Proleptic branches may also be due to the shooting out of accessory buds i.e. extra buds found in or near the leaf-axils of many plants, such as Willow, Maples, Cercis, Robinia, Syringa, Aristolochia, etc. which do not normally come to anything, or do so only if a surplus of food materials is provided. Dormant buds, or preventitious buds, are such as receive no sufficient supply of water and food materials to enable them to open with the other buds in ordinary years, for in most trees only the upper buds on the branches develop into new shoots. The lower buds do not die, however, but merely keep pace with the growth in thickness of the parent branch, and may be elongated sufficiently each year to raise the minute tips level with the bark, their proper cambium only remaining alive but not thickening the bud. 260 DISEASE IN PLANTS. When, by the breaking of the branch above the insertion of the dormant bud or by pruning, defoliation by insects, etc. the transpiration current and supplies of food materials are in any way deflected to the minute cambium and growing points of the dormant buds, they are stimulated to normal growth, and may grow out as epicormic shoots or " shoots from the old wood." In many cases such epicormic shoots are stimulated to grow out by suddenly exposing an old tree to more favourable conditions of root-action and assimilatory activity, owing to the felling of competing trees which previously hemmed it in from light and air, and restricted the spread and action of its roots in the soil. This is often seen in old Elms, Limes, etc. It is by such means as the above that substitu- tion branches are obtained when a leader is broken or cut away. Adventitious buds are such as are newly formed from callus or other tissues in places not normally provided with buds, as is often seen on occluding wounds e.g. stool shoots. They may also be developed on roots, a fact utilised in propagating Bouvardias, Horse-radish, etc., by means of root- cuttings, and the suckers of Plums and other fruit trees are shoots springing from adventitious buds on roots. Adventitious buds are also common on leaves (e.g. Bryophyllum, Ferns, etc.), and are frequently induced on them by wounds e.g. Gesneria, Gloxinia, etc. Even cut cotyledons may develop PROLIFERATIONS. 261 them, and pieces of leafless inflorescence (Hyacinth), hypocotyl (Anagallis), and in fact practically any wounded tissue with a store of reserve materials may be made to develop them : thus they have been found arising from the pith of Sea-kale, and are commonly developed from the cut bulb scales of Hyacinths. Apospory and Apogamy are particular cases of the production of vegetative buds on the leaves in place of sporangia in Ferns (Apospory), and on prothallia in place of Archegonia (Apogamy), in the latter case induced by dry conditions and strong illumination. NOTES TO CHAPTER XXVIII. In addition to the literature quoted in the notes to Chapter XXVII., the student should consult the works on Forest Botany for the scattered information regarding adventitious buds. A good account may be found in Busgen, Bau und Leben unserer Waldbdume, Jena, 1897. For Apospory and Apogamy, see Lang " On Apogamy and the Development of Sporangia upon Fern Prothalli," Phil. Trans., vol. 190, 1898, p. 187, where the literature is collected. CHAPTER XXIX. GRAFTS. Grafting Comparison with cuttings Effects of environ- ment Relations between scion and stock Variation in grafts 'Grafting and parasitism Infection Pollination Grafts -hybrids Predisposition of Natural grafts Root-fusions. GRAFTING is a process which consists in bringing the cambium of a shoot of one plant into direct union with that of another, and is practised in various ways, the commonest of which is as follows : One plant the stock rooted in the ground, is cut off a short distance above the surface of the soil, and a shoot from the second plant the scion cut off obliquely with a sharp knife, is inserted into a cleft in the stock, so that the two cambiums (and sometimes the cortex and pith of each as well) are in close contact : the scion is then tied in position, the wounds covered with grafting wax, and the whole left until union of the tissues is completed. This union depends on the for- 262 GRAFTS. 263 mation of callus at the cut surfaces, and the intimate union of the ingrowing cells from each callus. The development of the callus follows the course described for wounds, cuttings, etc., and the union is exactly comparable to the union of the two lips of a healing callus over a wound (see P- 197)- Grafting was known and practised far back in the ages. Virgil was well acquainted with the process, and Theophrastus compared it with propagation by cuttings. The scion differs from a cutting, however, in having no roots of its own : it is parasitic upon, or rather is in symbiosis with the stock, the root and tissues of which intervene between it and the soil. Consequently the selective absorption, size and number of vessels, and innumerable other physio- logical and anatomical peculiarities of the stock determine what and how much shall go up into the scion, while the latter supplies the former with organic materials and rules what and how much food, enzymes, and other secretions, etc., it shall receive to build up its substance. Surely, then, if such factors as the nature of the soil, the water and mineral supplies, the illumination, and the various climatic factors of altitude can cause variations on a plant direct, these and other factors are still more likely to be effective on stock and scion, and each must affect the other. Nevertheless opinions have differed much as to whether any important effect is to be seen, and on 264 DISEASE IN PLANTS. no point more than on whether the scion can affect the stock, in spite of such examples as Cytisus Adami, Carrey a on Aucuba, Sunflower on Jerusalem Artichoke, etc. Recent results, especially of ex- periments with herbaceous plants, show that not only can the stock affect the scion (and vice versa] directly, but the effect of the changes may be invisible on the grafted plant and only show itself in the progeny raised from the seed of the grafted plant. In other words, variation occurs in grafts either directly, as the results of the effects of the environment on the graft, or owing to the interaction of scion and stock, showing as changes in general nutrition in the tissues concerned, etc., owing to special reactions of the protoplasm of the uniting cells one on the other, and of the results of the further protoplasmic secretions, sortings, and so forth, on the cells developed as descendants of these in the further growth of the graft : or indirectly, in that some of these changes so alter the nature of the special protoplasm put aside for reproductive purposes, that the resulting embryo in the seed transmits the effects, and they show as variations in the seedling. If these results are confirmed they should meet all objections that have been urged against the transmission of acquired characters. In fact there are analogies between grafting and parasitism which cannot be overlooked, and should not be underestimated, their commonest expression appearing in the alterations in stature, habit, period of ripening, and so forth. These analogies GRAFTS. 265 are easily apprehended when we compare parasites like the Mistletoe, Loranthus, or even such root- parasites as the Broom-rapes and theRhinanthoideae with grafts ; but they also exist in the case of many fungus-parasites, and we might almost as accurately speak of grafting some fungi on their hosts as of infecting the latter with them, especially when it is borne in mind that the effect of the scion on the stock is by no means always to the benefit of the latter, and that there are reasons for regarding the action of some such unions as that of a sort of slow poisoning of the stock by the scion. Why do we not here say that the stock has been infected by the scion ? The resemblances between pollination and the infection by fungus hyphae may also be insisted upon. If we take into account Darwin's remark- able experiments showing that in " illegitimate unions " the pollen exerts a sort of poisonous action on the stigmas or ovules, it is possible to arrange a series of cases starting with perfectly legitimate pollinations where the pollen tube feeds as it descends the style on materials provided by the cells, and proceeding to cases where the pollen is more and more merely just able to penetrate the ovary and reach the ovules, to the extreme cases where no union at all is possible. Side by side with such series could be arranged analogous cases where fungus spores can enter and infect the cells of the host, and live symbiotically with or even in them, or can penetrate only with 266 DISEASE IN PLANTS. difficulty, or with poisonous effects, and finally cannot infect the plant at all. Less obviously, but nevertheless existing, are gradations in grafting to be observed, where one and the same stock may be successfully combined with a scion which improves it or which is improved by it or the scion may unite but acts injuriously on it, or, finally, cannot be induced to unite. But we may go further than this in these comparisons. Just as the results of pollination frequently induce far-reaching effects on distant tissues e.g. the swelling of Orchid ovaries, and rapid fading of the floral organs so also the effects of hyphae in the tissues may induce hypertrophies, deflection of nutrient materials, and the atrophy of distant parts e.g. the curious phenomena observed in Euphorbia attacked by Uromyces and some of the distant actions in grafts may be compared similarly. Going still further, we may compare the effects of cross-breeding or of hybridisation, where the progeny show that changes have resulted from the mutual interactions and re- actions of the commingled protoplasm, with Daniel's results, in which he obtains proof of such interactions of the commingled protoplasmic cell-contents of grafts in the seedling progeny ; although there is no probability we may even say possibility in this latter case that the effects are due to nuclear fusions, but only that the germ-plasm of the seed-bearing plant has been GRAFTS. 267 affected by the changes in the cell-protoplasm which nourishes it when the reproductive cells are forming. In the case of graft-hybrids the matter appears to be somewhat different, and we may well suppose, with Strasburger, that the commingling of characters observed in flowers, fruits, foliage, etc., on shoots borne after grafting are due to the occurrence of nuclear fusions during the union of the grafted tissues ; though it is by no means impossible that what has really happened is profound alterations in the nuclear substance (germ-plasm) owing to its being nourished by cell-protoplasm (somato-plasm) which has been itself affected by the interchanges of substance between scion and stock, and therefore itself furnishes a different nutrient medium from the unaltered cytoplasm of either. But even here we can find parallels among the ordinary phenomena of plant reproduction. Maize plants with white endosperm containing starch, if crossed by pollen from other plants with purple endosperm containing sugar, bear seeds with purple endosperm containing sugar, and such Xenia may be compared to graft-hybrids in many respects. I know of no case among fungus infections which could be compared directly with these examples, and it is not at all likely that we shall meet with any instance of a fungus-hypha handing over nuclear substance to an egg-cell, and so affecting the latter that an embryo results. 268 DISEASE IN PLANTS. But the case is not hypothetically impossible, although the distant relationships of the two groups of organisms render it extremely im- probable among the higher plants. It is by no means so improbable, however, that further research may show cases where the egg-cell of a lower cryptogam e.g. another fungus may be affected either directly, or indirectly, by the protoplasm of a parasitic or symbiotic hypha, as suggested by the extraordinary phenomena of symbiosis. Some of the variations in grafted plants are found to predispose the plant to disease, or the reverse, and cases may be cited where the resulting shoots, foliage, or fruits, or seedlings more readily fall a prey to, or resist, parasitic fungi and insects than the ungrafted plants. Daniel gives instances of such e.g. among other examples, Peas grafted on Beans yield seeds which suffer more from Erysipheae than the normal seedlings. But the best known cases are those of Vines in their relations to Phylloxera, already referred to (P- 155). Several instances are also known where grafted plants show more or less resistance to such factors of the environment as low temperatures ; grafted or budded Roses often suffer much from Erysipheae, and so forth. Much research is still needed to determine how far these matters depend on real alterations in the nature of the graft, or are only true for the localities in which the experiments have been made, a point which has, I think, been over- looked by all observers. GRAFTS. 269 Grafted plants are apparently very much ex- posed to injury by slugs, insects, and the invasions of parasites during the healing of the callus and the fusion process. Here again it must not be overlooked that the callus is, so to speak, a tit-bit of luscious, thin-walled, succulent tissue ; and, like all wounds, the graft affords entrance to parasites such as Nectria and Ascomycetes of various kinds, under circumstances very favourable to their in- vasion. Natural Grafts. It is by no means an un- common event to find the branches of Beeches, Limes, and other trees which have been acci- dentally brought into contact during growth, joined where they cross. As they press one against the other, they become naturally grafted, by that form of the process known as inarching : except that in artificial inarching the operator cuts off the cortical tissues of the two branches and brings their cambial surfaces together, whereas in nature the cambiums only come into con- tact after the destruction by pressure, or slight abrasion, of the entrapped intervening tissues. The fusion occurs, in fact, exactly as in the burying-in of a nail or wire, referred to on p. 2 1 1 . Natural grafts are very common among the roots of trees, and possibly explain some queer cases of the apparent revivification of stumps of trees not usually given to forming abundant stool shoots. It is regarded as probable in some old forests that the majority of the roots of trees of the same species are linked up together by such 270 DISEASE IN PLANTS. natural grafts, a probability not diminished by the fact that such roots cross at many points, and are easily grafted. NOTES TO CHAPTER XXIX. The student should read Bailey, The Nursery Book, 18.96, for details regarding the practice of grafting, and facts in abundance can be obtained from the pages of the Gardeners' Chronicle. Concerning graft-hybrids and the variations of grafted plants see Jouin, Can Hybrids be obtained by Grafting? and especially Daniel, " La Variation dans la Greffe," in Ann. des Sc. Naturelles, S. VIII., Vol. 8, 1898, p. i, and the literature there collected. The whole subject is largely con- troversial, and much work remains to be done. CHAPTER XXX. LIFE AND DEATH. Protoplasm Hypothesis as to its structure and be- haviour Assimilation Groivth Respiration Metabolism Action of the environment Nuclear protoplasm Pollination Grafting Parasitism Graft-hybrids Life Death Variation Disease. WE have seen that all the essential phenomena of disease concern only the living substance the protoplasm of the plant, and that how- ever complex the symptoms of disease may be, the occurrence of discolorations, lesions, hyper- trophies, and so forth are all secondary matters subsidiary to the fundamental alterations of structure and function constituting the disease. It remains to see if we can adopt any hypothesis as to the nature of this physical basis of life the protoplasm which shall help us to understand still more clearly in what must reside those processes which, so long as they proceed har- moniously and uninterruptedly, constitute life and 271 272 DISEASE IN PLANTS. health, and which when interfered with result in disease and death. The protoplasm of the living plant-cell looks like a slimy translucent mass which has been superficially compared in appear- ance to well-boiled sago or clear gum. Fifty years of observations and experiments with it have convinced physiologists that it is not a mere solution or emulsion, however, or even a chemical compound in the ordinary sense of the term, although chemical analysis gets little out of it beyond water, proteids, carbohydrates and fats, and traces of certain mineral salts ; for living protoplasm does not respond to the laws of physics and mechanics in obeying them, simply as do ordinary solutions and liquids. On the other hand, the most delicate chemical manipu- lation fails us, because when killed it is no longer protoplasm. Nor does the microscope advance matters far, beyond convincing us that this marvellous material must have a structure far more intimate than anything visible to the highest magnifying powers at our disposal. Nevertheless, some information is forthcoming from the comparative examination of the pro- toplasm of numerous different kinds of organisms, for we have learnt that certain ingredients and no others are necessary for its composition namely, carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, calcium, 1 magnesium, potassium and it is as a rule of no use trying to foist on to it any substitute for any one of these. Moreover, these 1 See note at end of chapter. LIFE AND DEATH. 273 chemical elements must be given in certain definite proportions and forms : for instance it is of no use to offer the carbon and sulphur in such a form as carbon disulphide, or the nitrogen and hydrogen in that of hydrocyanic acid, but the carbon must be given to the protoplasm in the form of a car- bohydrate or in some similar form, the nitrogen as an ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so forth, and thus water, air, carbohydrates, and the nitrates, sul- phates, and phosphates of potassium, calcium, and magnesium become the chief natural sources of the essential ingredients. Again, we have learnt that while there are different forms of protoplasm in the cell, and that these react on each other, and go through cycles of arrangement and rearrangements, the intimate structure must be of that kind termed molecular beyond the region of vision, just as is the microscopic structure of a crystal ; but, while like the latter affording evidence of order and sequence when properly examined, the structural arrangements and changes must be infinitely more complex. All these, and numerous other results of enquiry, have led to the conclusions that we must regard living protoplasm as a complex made up of very large molecular units, each containing atom-groupings of the elements named ; and, partly on account of the large number of atoms they contain, and partly due to the vibrations of absorbed heat, these units must be extremely labile. Moreover, they are linked up into an 274 DISEASE IN PLANTS. invisible and intricate meshwork, bathed in a watery liquid held in the interstices somewhat as water is held in a sponge. In this imbibed liquid are dissolved the substances, consisting of the same elements, which are to serve as food, and which are to be taken up into the molecular framework and built up into the structure of new molecular units or, as they may be shortly termed, molecules of protoplasm : in the bathing liquid are also dispersed the fragments again con- taining the elements named which have resulted from the breaking asunder of some of the complex protoplasm molecules, and which are partly de- stined to be used up again, partly to be burnt off in respiration, and partly to be put aside as metabolic products such as reserves, secretions, permanent structure, etc. Among the elements carried into this liquid and dissolved in it the free oxygen of the air also plays an important part. As new molecules are formed, by mutual combinations of the food-materials selected by molecular attractions, they are taken up into the protoplasmic framework, and built in between those already in existence, thus distending the whole, and we say that the protoplasm Assimilates food-materials and Grows. When distended beyond a given degree, or disturbed in various other ways, the molecular framework breaks, and some of the molecules are shattered, and as they fall to pieces certain of their constituent parts containing carbon and hydrogen forcibly combine at the moment of liberation with the oxygen in LIFE AND DEATH. 275 the fluid around and are burnt off in the form of carbon-dioxide and water, heat being of course evolved. This is the fundamental process of Respiration. It is probably the alternation of these pro- cesses of Assimilation the building up into the protoplasmic structure of new complex labile molecules and Destruction the shattering of such molecules with redistribution, oxidation, etc., of their fragments which constitute the funda- mental process of life. Different authorities attempt to explain the details of these processes in various ways, but there is practical agreement on the one point, that life consists in the alternate building up of new protoplasm from the food-materials Assimilation and the breaking down of the molecular complexes to simpler ones Disintegration, or D is -assimilation, as we may call it. During the periods when assimilation prevails, and the protoplasm increases in mass, we recognise Growth, and since this is usually associated with the vigorous imbibition of water, owing to the powerful osmotic attractions for that liquid exhibited by some of the products, and with consequent further stretching of the invisible molecular plexus, the growth may be so evident in increased size, that we are accustomed to look upon the visible increase in volume alone as growth; but it is essential to understand that growth of the protoplasm is always proceeding during life, even when as many older molecules are being shattered and dispersed as new ones 276 DISEASE IN PLANTS. are being formed by assimilation, and when, there- fore, no visible permanent enlargement occurs. Similarly, during periods when disintegration of the molecules prevails, we must not assume that the assimilation of new molecules is not occurring and that growth is not proceeding. The two processes are always going on during the active life of the protoplasm : in fact life consists in the play of these processes, as already said. That numerous chemical rearrangements of the atom-complexes take place outside the proto- plasmic molecules both of those left unemployed in assimilation and of those rejected during the destructive processes will be readily understood : many of the bye-products found in plants, such as vegetable acids, alkaloids, colouring matters, crystalline bodies, etc., etc., are due to these, so to speak, fortuitous combinations and re-com- binations. The part played by respiration has often been misunderstood. It consists in the burning off of some of the carbon and hydrogen of the shattered protoplasm molecules, by means of the oxygen of the air, which finds its way into the fluids around the protoplasm, and when it is active every act of combustion which is here an explosion leads to the shattering of more protoplasm molecules, and consequently to more respiratory combustion of the products. If the supply of oxygen is limited the breaking down of the molecules of protoplasm does not cease, but the carbon and hydrogen which would otherwise have been LIFE AND DEATH. 277 oxidised are now in part left to form other compounds in the surrounding liquid, and thus incompletely oxidised bodies, such as vegetable acids, alcohols, etc., accumulate. Even in the complete absence of atmospheric oxygen the protoplasm may go on breaking down and accumulating various compounds containing re- latively much carbon and hydrogen so-called intramolecular respiration ; but in ordinary plants this process soon comes to an end, because the blocking up of the molecular plexus leads to obstruction and interferes with the normal assimi- lation and dis-assimilation, and, if prolonged, leads to pathological conditions, and eventually death. Here, then, we meet with a cause of disease, or of predisposition to disease. The deprivation of oxygen interferes with the normal processes of building up and breaking down of the proto- plasmic molecules, and bodies we term poisonous accumulate and may lower the vitality or even bring life to an end. During normal life other products of the dis- ruption of the protoplasm molecules are nitrogenous bodies, such as proteids, and these we have reason to believe are used up again, acting as the nuclei, so to speak, of the new molecules, and so being built up again with fresh food -materials into the plexus, to be again set free, and again used up, and so on. Others are the carbohydrates, such as cellulose, which pass out of the molecule into an insoluble form, and are accumulated outside the protoplasm in the form of cellulose membranes, and so forth. 278 DISEASE IN PLANTS. It is these formed products of metabolism (Meta- bolites), especially cellulose and bodies which result from its subsequent transformation, which constitute the main permanent mass of the ordinary plant. We are now in a position to see how another fundamental cause of disease or predisposition to disease exists in the deprivation of the protoplasm of any of the elements needed to supply in the food-materials the place of those which have been permanently put aside in the form of cell- walls, or burnt off in respiration, passed out as excretions, or in other ways lost. It is clear that the indispensability of an element must mean that the protoplasmic molecule cannot be completed without it : the same conclusion is supported by the experimental proof that these elements cannot be replaced by chemically similar elements. It does not follow, however, that the protoplasm molecule must always have the same number of atoms of these elements, and grouped always in the same atom-complexes before being assimilated ; nor that the protoplasm molecule, when once built up, always breaks down in exactly the same way. On the contrary, while the protoplasm of corre- sponding parts of a daisy and of a rose must contain all the elements named, we must believe that the atom groupings are different in the protoplasm molecule in each case ; and though the molecules of the cell-protoplasm, of the nucleus, of the chlorophyll-corpuscles, etc., of one LIFE AND DEATH. 279 and the same plant must have all these elements, the atom groupings and modes of building up and breaking down may be very different in each case. Again, the cell-protoplasm, bathed by the sap taken in by roots from the soil or fed directly by that derived from the leaves, must be exposed to very different stimuli and modes of nourishment, etc., from those incurred by the protoplasm of the nucleus which it encloses : and similar con- clusions must apply in turn to the protoplasm of the root in the dark moist soil and of the leaf in the light dry air, or to that of the super- ficial epidermis cells as contrasted with that of the deeply immersed pith, and so on. It is no doubt in these directions that we must seek for the explanation of many life-phenomena at present quite beyond explanation. Thus, it is tolerably easy to modify the action of the cell- protoplasm of a plant, by exposing it to differences of illumination, temperature, moisture, and so forth, within certain limits ; at least, since the changes in stature, tissue differentiation, cell-secretions, flower- ing capacity, etc., of plants affected by such factors of the environment e.g. alpine plants brought into the plains must be due to changes in the mode of activity of the protoplasm, we must assume that the above factors affect the latter. But it is ex- tremely difficult to reach the nuclear-protoplasm directly by such stimuli, as proved by the experi- ence that even where we allow the factors to act for a long time, no permanent change can be 2 8o DISEASE IN PLANTS. detected in the behaviour of the nuclear-proto- plasm the essential material in the reproductive organs and reproductive process. At least we must infer that no change has been permanently stamped on this nucleo-plasm from such facts as the characters of the seedlings of the progeny of the plain-raised plants : if they are again sown in an alpine situation they forthwith behave again as alpines. Must we not conclude, then, that this difficulty of reaching the nuclear-protoplasm is owing to the fact that it is nourished and influenced directly only by the cell-protoplasm? That the cell-proto- plasm is its environment, and not so directly the outer world? We may influence the cell-proto- plasm we may make it work harder or less actively, respire vigorously or slowly, build up and break down in various different ways, or at different rates, and so forth, within limits ; but it is nevertheless cell-protoplasm of its specific kind, with its own range of molecular variations and activities within these limits, and it supplies the nuclear-protoplasm with what it wants so long as these limits are not exceeded. Consequently, while it is very easy to make the cell- protoplasm vary within the limits of its range, it is not easy to induce it to vary its Effects on the nuclear- protoplasm to such an extent or in such a way that the latter is permanently or materially altered in constitution. Nevertheless it would appear that cases do occur where the nuclear-protoplasm is reached and LIFE AND DEATH. 281 affected by external stimuli, as evinced by some of the phenomena of hybridisation and of cross- and self-fertilisation, because we find the results expressed in the mingling of the characters of parents, in strengthened or enfeebled progeny, and even in the appearance of unexpected pro- perties, which, from the facts of Reproduction, we know must have taken their origin in some alteration of the nuclear substance of the embryo. Here, however, we know in most cases that the principal agent which has reached the nuclear- protoplasm, is another portion of nuclear-proto- plasm. In hybridisation, one which has been fed and influenced by cell-protoplasm of a very different plant ; in cross-fertilisation, one fed and influenced by the cell-protoplasm of a different plant of the same species, and in self-fertilisation, one fed and influenced by the same cell-proto- plasm. That somewhere, and somehow, such nuclear- protoplasm as induces the changes in the char- acters of hybrids, etc., has been influenced by its immediate environment the cell-protoplasm of the plant appears to be a conclusion from which there is no escape. We may obtain similar evidence from the experience of grafting. It is relatively easy to influence the cell-protoplasm of a scion by a suitable stock, obviously because the latter, while handing on to the former all necessary materials from the soil, presents the indispensable elements and compounds in somewhat different proportions, dilutions, etc., from those which its 282 DISEASE IN PLANTS. own roots would have done, and probably mingles with them a certain amount of its own peculiar products, as well as affects the modes of working and interaction of both by the molecular impetus impressed on them. Consequently the cell-proto- plasm of the scion, while obtaining from the stock all it needs within the limits of its own variations of structure and activity, nevertheless builds up and breaks down in ways or at rates slightly different from those hitherto normal to it, and perceptible variations result when the sequences and correlations of these material and mechanical changes have affected a sufficiently large mass for the accumulation of visible effects. The limits to grafting suggest not that an in- appropriate stock does not offer to the protoplasm of the scion the right materials, but that it pre- sents them in proportions and in forms which are unsuitable for the assimilable powers of the latter, or, possibly, mingled with substances poisonous in themselves or capable of becoming so in con- junction with bodies in the scion. What has been said of the action of stock on scion, will also be true, mutatis mutandis, of the reciprocal action of scion on stock. Here again we may have causes for disease, or predisposition to disease. It occasionally happens, however, that the nu- clear protoplasm of the stock or scion is affected in grafting, and we infer from the difficulty of modifying it in any other way in ordinary repro- duction than by means of other nuclear protoplasm LIFE AND DEATH. 283 e.g. in hybridisation that in such cases a fusion of the nuclei of stock and scion has occurred during the grafting, and a graft-hybrid has resulted e.g. Cytisus Adami. It is not impossible however that the nuclear protoplasm has in such graft-hybrids been sub- sequently modified by the differences in nutrition to which it has been subjected, in the modified cell-protoplasm affected by the mingling of the juices, etc., of scion and stock ; for it is quite conceivable that such materials may affect the protoplasm far more profoundly than anything derived directly from the environment. If Daniel's researches are confirmed, however, it appears that in some cases, at any rate, the nuclear- protoplasm is so altered by the grafting that when the new embryo is developed, after fusion with nuclear substance from another plant of the same species, the results are apparent only in the pro- geny, and the effects of alteration in the cell- protoplasm have been transmitted to the nuclear protoplasm of the germ-cells i.e. acquired characters have been transmitted and fixed by heredity. Should this prove true the importance of the results can hardly be over-estimated. The matter is too problematical for further discussion here, but we see that any such action may profoundly affect the " constitution " of the resulting plant. Turning now to the case of fungi or other organ- isms which obtain access to the cell-protoplasm. At the one extreme we have cases where the proto- plasm of the diseased plant is rapidly and directly 284 DISEASE IN PLANTS. poisoned and destroyed, as in the killing off of seedlings in " Damping Off" : near the other ex- treme we have cases where the foreign protoplasm of the parasite, although it gains complete access to that of the host, merely stimulates the latter to greater activity and itself works for its own ends in conjunction with it e.g. Plasmodiophora. In such instances we must figure to ourselves the cells of the root of the Crucifer handing on food-materials to both masses of protoplasm that of the Plas- modiophora and that of the cell into which it penetrates ; and it is immaterial whether both ob- tain the food-materials directly, or, what seems more likely, the fungus only at second hand and by the medium of the host's protoplasm. In any case, the latter is for a long time at least not poisoned or maimed, or in any perceptible way injured' by excreta from the fungus-protoplasm, although it is evident that each must excrete various metabolites which may soak into and be taken up by the other : on the contrary the host-protoplasm grows larger, attracts more food supplies, makes larger cells, and is evidently stimulated to greater activity for the time being, its behaviour reminding us of the stimulation of cells by means of slight doses of poison referred to previously. We must therefore assume that the general course of building up and breaking down of its protoplasm-molecules go on as usual or nearly so in both the host cell and the invader ; and that the assimilatory, respiratory, excretory and other functions are carried on in LIFE AND DEATH. 285 the former as in the normal cell, or are but slightly modified to an extent which does no immediate injury to its life. But we must further assume that the same is also true of the invading protoplasm, and that the Plasmodiophora is also supplied with suitable atom-complexes to build up its protoplasm molecules, as fast as they are shattered and the rejecta burnt off in respiration. A step further, and we come to instances of Symbiosis, where the commingled masses of proto- plasm of host and invader continue this har- monious action during life. Clearly there are resemblances between these latter cases and suc- cessful grafts, and between both and successful sexual unions where the resulting embryo-cell gives rises to a vigorous and healthy plant ; and the more these resemblances are examined in the light of what we know of symbiosis the more they support our contention. Such considerations as the foregoing suggest, then, that life consists in the regular and pro- gressive building up and breaking down of the complex protoplasm molecules, and is necessarily accompanied by the influx of the indispensable food-elements in certain combinations and atom- complexes for assimilation, and by the combustion of some of the debris of the shattered molecules, which combine with the oxygen in respiration and so afford explosions which raise the temperature and enhance the lability of existing molecules, and act as stimuli to the shattering of further molecules. The results of these rhythmical buildings 286 DISEASE IN PLANTS. up (assimilation) and shatterings (dis-assimilation) of the protoplasm molecules are the growth of the protoplasm, with further intercalations of water and new food-supplies, etc., on the one hand, and the formation of metabolic products (proteids, cellulose, sugars, fats, etc.), some of which are again used up, others respired, others deposited as stores, cell-walls, etc., on the other. That the building-up process depends on the action of molecular forces comparable to those by which a growing crystal goes on selecting atom- complexes of its particular kind from the solution around seems highly probable, and this being the case we can understand how under certain cir- cumstances substitutive selections may occur. That is to say, just as a crystal will sometimes build up into its structure atom-complexes of a kind different from its normal molecules, so, given the proper conditions, a protoplasmic molecular unit will build up into its structure atom-complexes somewhat different from those it had hitherto taken up i.e. assimilated with consequent modifications of its behaviour. If this occurs, the modes of further building up and breaking down will be affected by the subsequent action of these slightly modified protoplasm units, and it may well be that the whole significance of variation turns on this. Whether the resulting variation makes for the welfare or otherwise of the organism will then be decided by the struggle for existence, and the natural selection which ensues. Such a view also implies that the LIFE AND DEATH. 287 energy concerned is primarily what is usually termed chemical energy, and that every compound entering into the protoplasm carries in a supply of this, available in various ways. Death, on the contrary, is the cessation of these rhythmical processes of building up and breaking down of the protoplasm molecules. It does not imply the cessation of chemical changes of other kinds, but that these rhythmical con- structions of the complex and labile protoplasm molecules breaking down on stimulation to bodies partly re-assimilable, partly combustible in respira- tion, and partly excretory, etc., have ceased, and that further chemical changes in the material are thenceforth simpler and different in kind and degree, eventually leading to total disintegration so that no units are left capable of restoring the rhythm. If these ideas are correct, we may define Disease as dangerous disturbances in the regularity, or interference with the completeness or range of the molecular activities constituting normal Life i.e. Health and it is evident that every degree of transition may be realised between the two extremes. Now, if we further assume, as I think we must do, that a considerable range or " play " must exist in the molecular activities of the protoplasm constituting life, we obtain a sort of expression of what we mean by limits of variation. The fact that life can go on in a given plant at temperatures between from i-5 and 35-4O C., or in lights of different intensity, or within 2 88 DISEASE IN PLANTS. considerable ranges of water supply, concentration of salts, partial pressure of oxygen, etc., implies that the molecular activities of the protoplasm are of the normal kind all the time, though they may differ in rapidity, and even in quantitative and qualitative respects within certain limits ; and the meaning of the optimum temperature, illumination, oxygen pressure, etc., is, from this point of view, not that the molecular activities differ in kind from those nearer the minima and maxima, so much as that they are running at the best rates for the welfare of the plant i.e. for permanent health. If we transcend the cardinal points limiting the range of this play, however, and we get variations in the kind as well as rates of molecular con- structions and disruptions, then we pass by imper- ceptible gradations into ill-health i.e. Disease. And similarly in relation to other protoplasm. That of the right kind of pollen grain from another plant of its own species, stimulates the contents of the ovule to produce a vigorous embryo and healthy seedling : that of a similar pollen grain in its own flower either does no positive harm, but has a feebler effect, or it may act like a poison. That of another pollen grain again may refuse to unite at all ; while that of a fungus hypha e.g. of Sclerotinia on Vaccinium may run down the style as does the pollen tube and produce death and destruction throughout the ovule. Or again, in Clover, we may have the hypha of a Botrytis with its .protoplasm unable to do more LIFE AND DEATH. 289 than penetrate into the cellulose walls and diffuse a poison into the adjacent cells, being utterly incapable of directly facing, or mingling with the living protoplasm of such cells, whereas the proto- plasm of another organism e.g. Rhizobium will penetrate directly into the cells, live in them for weeks or months without injury nay even with advantage to their life. And hundreds of similar cases can be selected. We may, therefore, conclude that Variation depends fundamentally on alterations in the structure or mode of building up and disin- tegration of the protoplasmic molecular unit, brought about either by direct modifying action of the inorganic environment nutrition, tem- perature, oxygen supply, light, etc., etc. or by the mingling with it of other protoplasm, the molecules of which since they have already a slightly different composition, configuration, mode of breaking down and building up, etc., affect its molecules by supplying them with altered nutri- tive atom-complexes, by competing with them for oxygen, etc., etc. Once these molecules are affected, we must assume that long sequences of other chemical and molecular changes will be also modified ; and although we have no conception of how these changes bring about changes in form, that they do so is only a conclusion of the same order as that which we hold regarding the much simpler changes concerned in the formation of crystals. That such variations may be of every degree as 290 DISEASE IN PLANTS. regards profundity, permanence, kind, etc., may well be imagined ; and there is nothing surprising in our being able to induce them more easily by the action of external factors in the readily accessible cell-protoplasm than in the less exposed nuclear-protoplasm ; because the latter is only accessible through the former, or through the agency of other nuclear protoplasm already modified. On these and similar phenomena depend the relative permanency and transmissibility of the variations. Our measure of the latter only begins when the effects referred to have become manifest in large masses of cells, because only then do they become appreciable to our senses. Further, variations thus induced may be of advantage to the continued life of the plant, or in all degrees disadvantageous or threatening to its existence. These latter variations are Disease, and if their interference with the normal rhyth- mical play of the building up and breaking down of the protoplasm molecules proceeds beyond certain limits, life ceases, and we have death supervening on disease. NOTES TO CHAPTER XXX. It appears probable that calcium is not always needed by living cells, and may not enter into the composition of protoplasm ; on the other hand traces of iron are perhaps necessary. The criticisms and summary of facts on which the hypo- thesis regarding protoplasm here adopted is based are developed at length in Kassowitz, Allgemeine Biologie, LIFE AND DEATH. 291 Wien, 1899, B. I. and II., where the collected literature may be found, and the reader introduced to the huge mass of controversial writings put forward since Darwin and asso- ciated with the names of Weismann and others. It will probably be noticed that I have employed the term molecular unit of protoplasm, and have not discussed the question of organised structure in the latter : this is because it seems clear to me that living protoplasm as such does not possess "organised structure" in the true sense of that term it is, rather, busy preparing and making "organised structure," and a molecular constitution would have to be ascribed to all "physiological units" of the nature of micellae, pangens, ids, etc., as truly as to the structural units of a starch-grain or cell- wall, or even of a crystal. In this connection, the student will find the necessary points of view put forward in Pfeffer, Physiology, pp. 37-83. INDEX. Absorption by roots, 49. Absorption of energy, 23. Absorption of light, 27. Absorption of water, 50. Abittilon, 183. A cants, 88. Accessory buds, 259. Acer, 214. Acid gases, 181, 191. Acids, 130, 136. Acquired characters, 283. Acrostalaginits, 238. Action of the environment, Adaptation, 176. Adapted races, 177. Adonis, 220. Adventitious buds, 224, 257, 260. jEcidiunii 88, 114, 116, 1 88, 189, 217, 223, 225, 247, 252. Aeration, 104. Aerobic organisms, 57. .Etiology, 89, too. Agaricus melleus, 115, 143, 234- Agents of disease, 113. Aglaospora, 223. Agriculture, 65. 271. 225, 187, 232, MS. Agricultural Chemistry, 2. Ajuga, 217. Albinism, 179, 182, 183, 186. Alder, 207, 219. Aleurone layer, 173. Algae, 215. Allinm, 258. Almond, 1 68. Alnus, 214. Aloe, 134, 161. Alpine plants, 250, 279. American blight, 164, 219. American vines, 155, 169, 172. Amides, 31. Amoeba, 144. Amount of energy stored, 25. Amygdalin, 173. Anabaena, 128. Anaerobic bacteria, 58, 237. Auagallis, 261. Analyses, 65. Analyses of waters, 58. Anemone, 247. Animals, 99, 108, 142, 207. Antettnaria, 232. Anthonoinos, 249. Anthrax, 144. Antiseptics, 162. 293 294 DISEASE IN PLANTS. Ants, 232. Aphis, 88, 109, 161, 165. 188, 213, 214, 232, 241, 253. Aphrophora, 233. Apogamy, 257, 261. Aporia Cralaegi, 187. Apospory, 257, 261. Apple, 170, 171, 187, 189, 192, 206, 217, 218, 219, 223, 226, 231, 233, 248, 249, 253. 254. Apricot, 1 88, 206. Apricots, 234. Area of root-surface, 37, 39. Arisarutn, 188. Aristolochia, 259. Aroids, 113. Arrest of growth, 246. Arsenic, 162. Artificial wounds, 194. Ascomycetes, 1 89,- 2 17, 269. Ascothyfa, 190. Ash, 182, 223, 225, 251. Asparagus, 180, 230, 251, 252. Aspergilltis, 231. Aspergillus niger, 58. Aspidiotus, 187. Assimilation, S, 21, 133, 271, 275, 277, 285, 286. Assimilates, 274. Atmosphere, I, 99. Atmospheric influences, 101. Atrophy, 246, 247, 266. Attractive substances, 136. Aucuba, 264. Autumnal colouring, 191. Autumnal fall, 93. Avalanches, 106. Bacteria, 102, 133, 143, 168, 173, 176, 182, 190, 200, 216, 219, 223, 227, 231, 236, 237. Bacteriosis, 227. Barberry, 176. Bark boring, 204, 205. Bark-beetles, 205. Barley, 176, 248. Barrenness, 246, 249. Bats, 244 Bean, 188, 190, 191, 268. Beech, 192, 222, 223, 225, 233, 240, 242, 254, 269. Beech Miner, 208. Bees, 142, 143, 164. Beet, 192, 216, 219, 230. Beet-rot, 230. Beetles, no, 143, 145, 205, 206, 207, 248, 254. Berkeley, 85. Bilberries, 116, 142, 217, 2i8 r 248. Biology of soil, 56, 102. Birch. 207, 218, 224. Birds, 1 08, 144, 164, 1 66. Birds'-eye Maple, 224. Black spots on leaves, 186, 189, 191. Bladders, 2 1 8. Blemish, 198. Blights, 86, 104, 179. Blisters, 230. Blue rays, 21. Bombyx, 187, 2 1 8. Bordeaux mixture, 162. Boring, 204. Botrytis, 131, 132, 136, 175, 230, 231, 243, 288. Boussingault, 5, 10. Bouvardia, 260. INDEX. 295 Bramble, 112. Branch stumps, 194, 199. Brand, 240. Breeding, 78. Briars, 113. Broom-rapes, 265. Browning, 122, 186, 235. Brown spots, 186, 189, 190, 191. Browsing, 244. Brnchiis, 248. Bruises, 194, 203. 240, 241. Bryony, 112. Bryophyllum, 260. Bud galls, 219. Bud variations, 92, 93. Bulb diseases, 227. Buried objects, 211, 269. Burning, 191. Burning-glass effect, 192. Burrows, 204, 205. Burrs, 222, 223, 224. Bursting of fruits, 227, 230. Butterflies, 145. Bye-products, 276. Cabbage, 253. Cabbage moth, 208. Caeonia, 252. Caesalpinia, 233. Calcium, 272. Calcium oxalate, 138. Calla, 183. Calliandra, 233. Callus, 119, 120, 124, 139, 140, 196, 197, 199, 201, 202, 210, 241, 260, 263, 269. Calyptospora, 116, 217. Cambium, 120, 196, 199, 222. Camellia, 187. Cancer, 127. Canker, 87, 222, 223, 241. Capnodium , 232. Capsella, 116, 175, 252. Carbohydrates, 16, 17, 20, 34, 122, 184, 272, 273, 277. Carbolic acid, 162. Carbon, 272. Carbon assimilation, 8, 10, 28, 106. Carbon-bisulphide, 163. Cardinal points, 288. Carrot, 164. Carpocapsa, 207. Cast branches, 123. Castor oil, 172. Caterpillars, 109, 164, 207, 208, 244, 254, 259. Cats, 164. Cattle, 108. Cauliflowers, 247, 250. Causes of disease, 89, 99, 108, 159, 278, 282. Cecidia, 212. Cecidomyia, 182, 213, 214, 218, 219, 254. Celery, 1 80, 230. Cell contents, 168. Cell-protoplasm, 279, 280, 290. Cellulose, 132, 277, 286. Celosia, 250. Centaurea, 1 88. Centhorhynchits, 219. Cephalairos, 188. Cephns. 248. Cercis, 259. Cereospora, 190. Cereals, 248. Change of conditions, 78. 296 DISEASE IN PLANTS. Charlock, 165. Checks to disease, 166. Chemical analysis, 32, 64, 103, 272. Chemical antiseptics, 1 59. Chemical energy, 29, 287. Chemotactic phenomena, 72, 130, 135, 137- Chermes, 153, 223. Cherry, 208, 209, 231, 234, 235, 247, 248. Chestnut, 190. Chlorine, 181. Chlorophyll, 19, 106, 122. Chlorophyll action, 184, 192. Chlorophyll corpuscles, 9, 18,22. Chlorosis, 122, 165, 179, 180, 181. Chrysanthemum, 243, 252. Chytridiaceae, 127, 136, 189,208. Cicada, 235. Cicatrix, 123. Cinchona, 168, 172, 173. Circulation of carbon, 62. Circulation of nitrogen, 62. Citrus, 1 68. Clasterosporium, 188, 209. Classification of diseases, 99, 101, 1 20. Claviceps, 232. Climate, i. Climbing plants, 112, 113,210. Clostridmm, 236, 237. Clothes, 142. Clover, 164, 187, 249, 252, 288. Cluster-cups, 188. Coal gas, 104, 182. Coccideae, 164, 232. Coccus, 223. Coffee leaf-disease, 114, 146, 1 66, 169, 242. Coleophora, 153, 206. Coleosporitim, 169. Coleus, 192, 220. Competition of fungi, 61. Complex interactions, 91, 99. Conifers, 125, 205, 223, 225, 234, 258. Constitution, 156, 283. Consumption, 248. Contact irritability, 125, 135. Contagium fluidum vivum, 183. Contortions, 252. Convallaria, 175. Convolvulus, 112. Copaifera, 234. Copper sulphate, 162, 165. Coppery leaves, 191. Cork, 119, 123, 194, 199. 216, 222. Cork wings, 217. Corky warts, 212. Corn, 248. Corrosion of marble, 46. Cossus, 206. Cost of epidemics, 146, 147. Cotton, 172. Crassula, 253. Creeping of mycelia, 142. Crepis, 252. Crimson spots, 189. Cross-breeding, 266. Cross-fertilisation, 69, 74, 77, 281. Cross-graining, 124. Crucifers, 219, 284. Cryptogams, 87, 108, in, 113. Cuckoo-spit, 233. INDEX. 297 Cucullate leaves, 253. Cucumber, 219. Cticurbitaria, 217, 243. Cultivation of pest and host plant, 168. Curculio, 248. Curling, 235, 246. Cuscuta, 134. Cuts, 119, 143, 194. Cuttings, 194, 198, 262, 263. Cyanide of potassium, 165. Cycads, 128. Cynips, no, 213, 219. Cystopns, 116, 136, 175, 187, 217, 247, 252. Cytases, 132. Cytisus A da mi, 264, 283. Daisy, 278. Damping off, 114, 144, 160, 229, 284. Dandelion, 247, 252. Daniel's researches, 283. Dark heat rays, 27. Darwin, 72, 125. Dasyscypha Willkommii, 152, 223. Death, 271, 272, 287, 290. De Bary, 85, 151. Deficiency of iron, 180. Defoliation, 109, 240, 244. Deformation, 132. Dematiuin, 135. Dcmatophora, 143, 145. Denitrification, 62. Derivation of Phytopathology, 8 5 . Destruction, 275. Development of root-hairs, 40. Dextrine, 173. Diagnosis, 85, 89. Diastases, 132. Diffusion, 53. Digestion, 133. Digraphis, 175. Dilophia, 188. Dionaea, 125. Dipsacns, 252. Diptera, 207. Dis-assimilation, 275, 277, 286. Discolorations, 179, 186, 192. Disease, 64, 91, 271, 272, 277, 287, 288, 290. Disease dodging, 168. Disease-fungi, 189. Disease of organs, 119. " Disease-proof" varieties, 168, 169, 171, 173, 177. I Disease-resisting varieties, 177. j Diseases of absorptive organs, 121. Diseases of assimilatory organs, 119. Diseases of bark, 120. Diseases of cambium, 120. Diseases of parenchyma, 1 20. Diseases of respiratory organs, 119, 121. Disintegration, 275. Distortions, 140, 246, 251, 252, 253- Dissemination of fungi, 142. Division, 127. Dodder, 113. Do Hum, 134. Dormant buds, 224, 225, 257, 259, 260. Double flowers, 247, 256. 2 9 S DISEASE IN PLANTS. Double ideals in selection, 168. Dracaena, 192. Drainage, 103. Drawing, 106, 180. Drip, 103. Drooping, 43, 179. Drops of water, 192. Dropsy, 228. Drought, 121, 183, 190, 191, 245, 248, 249. Dry-rot, 143, 237. Ducks, 144. Dutrochet, 7. Dwarfing, 246, 249. "Dying back," 190, 240, 242, 243, 244. Earwigs, 164, 207. Eau Celeste, 162. Edelfaule, 230. Eel worms, in, 248. Effects of environment, 262. Eggs of insects, 187. Elaborated sap, 94. Elm, 218, 224, 225, 233, 260. Empusa, 163. Endemic diseases, 153, 160, 1 66. Endive, 180. Endophytes, 130. Endophytic algae, 128. Endophytic fungi, 193. Energy in plants, 15, 25, 287. Engelmann, 20, 27. Entyloma, 187. Enzymes, 10, 130, 132, 136. Epichloe, 2 1 8. Epicormic shoots, 224, 257, 260. Epidemics, 108, 109, 113, 115, 142, 153, 160, 163, 166. Epiphytes, 113, 130, 135, 137. Epiphytic algae, 188. Epiphytic fungi, 161, 193, 232. Equisetnm, 113. Ergot, 131, 142, 144. Erincum, 88, 212, 214, 215. Erosions, 204, 207. Erysipheae, 135, 142, 161, 187,. 268. Essentials of fertilisation, 69. Estimates of loss, 146. Etiolation, 106, 179, 180, 229. Euphorbia, 116, 134, 247, 266. Excavations, 204. Excess of food, 229. Excess of minerals, 102. Excess of water, 100. Excessive growth, 246. Excessive nutrition, 250. Excrescences, 114, 212, 222. Excreta, 45, 130, 133. Exobasiditim, 128, 218. Exoascus, 1 1 6, 128, 188, 2o8 r 214, 218, 225, 247, 253. Expense of materials, 161. Experiments necessary, 1 68. Exposure of roots, 179, 184. External causes of disease, 99. Extinction of species, 91. Exudations, 227. Exudation under pressure, 5 r - Factors of an epidemic, 149, 165* Falling of fruit, 206. Falling leaves, 123. False chlorosis, 181. False etiolation, 180. Farfugium, 188. Fasciations, 230, 246, 251. INDEX. 299 Fats, 272, 286. Feeding, 14, 16. Fermentation, 58, 102, 130, 233. Ferns, 113, 247, 260, 261. Fertilisation, 71. Field-mice, 164. Figs, 113. Finger and toe, 114, 127, 163. Fire, 240. Flaming, 164. Flattened roots, 246, 252. Fleshiness, 228. Flies, 86, no, 142, 143, 145. Flux, 227, 231. Flying foxes, 244. Focussing of solar rays, 192. Foliage, 1 10. Fontaria, 134. Food, 1 8. Forest-fires, 241. Formic-aldehyde, 20. Foul products, 100. Foxy leaves, 191. Freezing, 121, 183. Frit fly, 182. Frost, 153, 160, 225, 229, 248, 249. Frost-beds, 243. Frost-blisters, 212, 218. Frost canker, 222. Frost-cracks, 204, 209, 242. Frost-patches, 240. Frost-ridge, 209. Futnago, 1 90, 232. Fumes, 104. Functions of roots, 43, 45. Functional depression, 96. Fungi, 89, 108, 143, 174, 189, 200, 205, 207, 208, 212, 216, 219, 223, 229, 231, 233, 238, 240, 241, 243, 248, 251, 255, 258, 265, 267, 283, 284, 288. Fungus attacks, 139. Fungus galls, 219. Fusarium, 143, 238. Fiisicladinm, 189. Fztsisporium, 237. Gagea, 258. Gall-apple, 21 8. Gall-flies, 219. Gall-insect, 139. Gall-like swellings, 128. Galls, 86, no, 120, 130, 138, 212, 214, 218, 255. Gangrene, 231. Garreya, 264. Gas, 1 60. Gases in soil, 104. Gastropacha, 225. Gelatine, 163. General death, 116. General disease, 119, 120. Germ-plasm, 267. Gesiieria, 260. Glechoma, 218. GloeosportuiH, 189, 190, 208. Gloxinia, 260. Goats, 164. Gooseberry, 217. Graft-hybrids, 262, 267, 27 1 , 283. Grafting, 78, 155. l6 9> 183, 250, 262, 271, 281. Grain-rust, 146. Grapes, 192, 230, 231. Grapholitha, 109, 207. Grass, in, 189, 190, 205, 2i8 r 233. 3 oo DISEASE IN PLANTS. Green fly, 161. Grew, 85. Greyish spots, 187. Growth, 271, 274, 275, 286. Grubs, no, 207. Gumming, 235. Gummosis, 227, 234, 235. Gymnosporangium, 114, 176, 223. Hail, 106, 240, 241. Hales, 85. Haltica, 209. Hardy varieties, 168, 170, 177. Haustoria, 134, 135, 136. Healing, 194, 196. Healing by cork, 123. Health, 272, 287. Health and disease, 91, 97, 287. Heliotropism, 126. Hemileia, 146, 169. Heredity, 72, 283. f/erpotrichia, 135, 190. Hessian Fly, 182. H eterodera, 219, 220. Hieracitim, 1 1 2. History of Phytopathology, 85. Holdfast of roots, 42. Hollyhock disease, 143. Holly, 217 Honey dew, 144, 227, 232, 233. Hops, 162, 187, 191, 232, 253. Hop-aphis, 146. Hop-disease, 166. Hop mildew, 161. //orniomyia, 219. Hornbeam, 224, 233, 242. Horse-radish, 260. Host, 284, 285. Hyacinth, 231, 261. Hyacinth disease, 143. Hybrids, 69, 156, 281. Hybridisation, 69, 75, 169, 266, 281. Hydrochloric acid, 181. Hydrogen, 272. Hymenomycetes, 206. Hypertrophy, 119, 127, 139, 213, 215, 247, 266. Hypochaeris, 112. ffypomyces, 237, Hyponomeuta, 254. Ice, 184, 209. Ichneumon-flies, 165. Icterus, 181. Illegitimate unions, 265. Immunity, 155, 156, 165, 168, 169. Impervious subsoil, 181. Inarching, 269. Increase in dry weight, 23. Indian agriculture, 172. Indian wheats, 168. Indispensability of elements, 278. Infection, 262, 265, 267. Ingredients of protoplasm, 272. Insect bites, 225. Insect diseases, 145, 146, 154, 189. Insect punctures, 88. Insects, 89, 98, 108, 109, 120, 138, 142, 153, 174, 187, 194, 2O3, 2O5, 2O6, 207, 2O8, 212, 223, 229, 241, 244, 2 4 8, 251, 254, 255, 258, 259, 269. Insolation, 180, 242. INDEX. 301 Intercellular endophytes, 136, 137- Intercellular mycelium, 128. Interference, 91. Internal causes of disease, 99, 101. Intracellular parasites, 127, 136- Intramolecular respiration, 277. Intumescences, 212, 215. Inulin, n, 17. Invertebrata, 108. Irritability, 125, 127. Irritation, 119, 139. Isaria, 163. Ivy, 113, 165. Japanese trees, 250. Jerusalem Artichoke, 264. Juncus, 219. Juniper, 114. Kidney bean, 192. Knauers, 22J. Knife wounds, 194, 195. Labour, 161. Lace- wings, 165. Lachnus, 223. Lady-birds, 164, 165. Lammas shoots, 257, 259. Larch, 168, 171. Larch disease, 115, 149, 152, 166, 171, 223, 241. Larvae, no. Lateral wounds, 132. Lawns, 112. Laying of wheat, 179, 180. Leaf-curl, 236, 253. Leaf-diseases, 1 14, 1 19, 120, 242. Leaf-galls, 217, 218. Leaf-miner, 86, 109, 204. Leaf perforations, 208. Leaf rolling, 214, 246, 254. Leaf-spots, 114, 190. Leguminoseae, 137, 219. Lemons, 235. Lenticels, 217. Lepicloptera, 187. Leptosphaeria, 249. Lichens, 137. Liebig, 4. Life, 271, 285, 287. Life and death, 271. Light, 27, 1.06. Lily disease, 143. Lime, 163, 215, 218, 232, 253; 254, 260, 269. Limes, 172. Limits of variation, 287. Ltnaria, 252. Liquid antiseptics, 160, 161,. 162. Living environment, 99, 108. Local action, 114. Local disease, 119, 121. Locusts, 109, 145, 163, 164. Longicorns, 205. Loranthus, 113, 245, 265. Losses due to epidemics, 142. Lowering of temperature, loo. Lucerne, 249. Lurking parasites, 142, 145. Lychnis, 232. Lyonetra, 206. Lysimachia, 217. Machine, plant compared to a, 79. Magnesium, 272. 302 DISEASE IN PLANTS. Maize, 116, 173, 219, 267. Majanihemum, 175. Malformations, 116, 130, 131, 246, 251. Mai nero, 190. Mallow, 252. Malpighi, 85. Mammals, 142. Man and plants, 108, 142, 143. Manna, 227, 235. Manna Ash, 235. Maple, 259. Maximum, 288. Maximum absorption, 19. Maximum assimilation, 19. Maximum temperature, 105. Mealy bug, 164. Melampsora, 176. Melon, 220. Messmates, 63. Metabolic products, 274. Metabolism, 23, 127, 271. Metabolites, 278. Metallic compounds, 162. Mice, 108, 163. Microbes, 227. Micro-organisms, 183. Mildew, 86, 164. Millardet, 169. Mineral salts, 101. Miniature trees, 250. Minimum, 288. Minimum temperature, 105. Misconceptions, 12. Mistletoe, 113, 265. Mites, 192, 214, 255. Mixed species, 166. Molecular structure of proto- plasm, 273, 274. Mongrel forms, 74. Monilia, 217, 231. Monstrosities, 246. Moraine plants, 250. Moths, no, 145, 206. Moulds, 230, 231, 237, 243. Mucor, 230, 231. Mulberry, 244. Mutilations, 252. Mycelial strands, 145. Mycelium, 188. Mycocecidia, 219. Mycorrhiza, 137. Myrtaceae, 258. Mytilaspis, 187. Natural checks, 159. Natural demise, 91, 93. Natural Grafts, 269. Natural Selection, 72, 99, 167, 286. Natural Wounds, 204. Nature of soil, 57. Necrosis, 240, 241, 243. Nectria, 145, 217, 223, 241, 243, 269. Nematodes, in, 134, 139, 219, 220. Nettle, 1 16, 252. Neurotits, 219. New formations, 255- Nitrate, 273. Nitrification, 62, 102. Nitrogen, 272. Nodosities, 219. Nodules on roots, 63, 137. Non-living environment, 99. Notommata, 140. Nuclear fusion, 267. INDEX. 303 Nuclear protoplasm, 271, 279, 280, 290. Nuclear substance, 71. Nucleo-plasm, 280. Nuts, 248. Oak, no, 188, 215, 218, 219, 223, 233. Oak leaf-roller, 254. Oat, 176. Occlusion, 200, 201, 222, 223. Odours, 144. Oedema, 228. * Olive, 223. Onion, 231. Onisfus, 182. Oospora, 216. Optimum temperature, 105, 288. Orange, 173, 187, 235, 247. Orange-coloured spots, 187. Orchard trees, 163. Orchestes, 206. Orchids, 113, 266. Organic acids, 50. Organisation, 89. Organised structure, 1 3. Organisms in soil, 60. Orobanche, 112. Osmosis, 26, 29, 46. Osmotic pressures, 18, 41, 52. Over-crowding, 104, in. Over-feeding, 102. Over-watering, 97. Oxalic acid, 134, 136. Oxidation, 124. Oxygen, 104, 272. Oxygen-respiration, 12, 64. Pallor, 179, 1 80. Palms, 192. Pangium, 134, 165. Parasites, 61, 113, 119, 130, 139, 174, '87, 230, 265, 269, 284. Parasitic algae, 188, 217, 219. Parasitic bacteria, 163. Parasitic diseases, 88, 119, 121. Parasitic epiphyte, 136. Parasitic fungi, 87, 97. Parasitism, 262, 264, 268, 271. Paris, 175. " Paris green," 162. Parti-coloured leaves, 191. Parti-coloured spots, 186. Pasture grasses, 69. Pathology, 121, 257. Pathology of cell, 119. Pathological conditions, 168,170, 246. Pea, 190, 191, 206, 208, 248, 268. Peach, 170, 253. Pear, 179, 187, 189, 191, 216, 218, 231, 240, 248, 249, 253, 257. Pedigree wheats, 69. Pelargonium, 198, 253. Peloria, 252. Penicillin in, 231. Peridermiinn Pint, 223, 234. Periola, 238. Permangate, 162. Perotwspora, 136, 160, 175, 187, 189, 208. Petasiies, iSS. Petroleum, 162. Pcziza, 115, 144, 152. Phanerogams, 108, in. 304 DISEASE IN PLANTS. Phellomyces, 238. Phoma, 217, 243. Phophylactic measures, 160. Phosphorus, 272. Photo-synthesis, II, 16. Phragmidium, 189. Phyllachora, 189. Phyllereus, 253. Phyllobium, 217. Phyllosiphon, 188. Phyllosticla, 188, 209. Physiology, I, 66, 85. Physiological diseases, 119, 121. Phytomyza, 206 Phytopathology, 85. Phytophthora, 115, 136, 144, 150, 151, 235, 236. Phytophysa, 219. Phytoptus, 189, 213, 214, 215, 218, 219, 253, 254. Pilea, 219. Pilobolus, 126, 140. Phylloxera, no, 145, 149, 154, 155, 163, 166, 172, 188, 219, 220, 268. Pines, 183, 223, 234, 251, 252. Pine apple, 258. Pith flecks, 204, 207. Plant as agent of disease, 99, 108. Plant, agricultural chemistry of, Plant and its food, 7. Plant and its surroundings, I . Plant, a machine, I, 15. Plant, central object of study, I. Plants, dying out of, 93. Plant, physiology, I. Plantago, 257. Plantain, 112, 257. Plasmodia, 163. Plasmodiophora, 114, 126, 127, 144, 163, 219, 284, 285. Plasmolysis, 47. Pleospora, 236. Pleotrachelus, 126, 140. Plum, 171, 189, 192, 209, 214, 206, 231, 235, 248, 249, 260. Poa, 258. Pocket-like galls, 155, 214, 218. Pocket-plums, 214. ^Pockets, 253. Poison, 102, 130, 136, 163, 216. Pollen grain, 288. Pollination, 248, 262, 265, 266, 271. Polydesmus, 236. Polygonatum, 175. Polygonum, 258. Poisonous gases, 181, 248. Polymorphism, 174. Polyporei, 142. Poly par us, 143, 206. Poly stigma, 189. Poplar, 1 88, 206, 215, 218, 254. Post and epidemics, 142. Potassium, 272. Potassium sulphite, 162. Potato, 162, 171, 194, 209, 216, 236, 237, 258. Potato-disease, 114,143,149,150, 166, 189, 207, 235. Powders, antiseptic, 159, 160, 161. Predisposition to disease, 98, 99, 105, 168, 169, 229, 262, 268, 277, 278, 282. Preventitious buds, 259, INDEX. 305 1'reventible diseases, 1 59. Prolepsis, 257, 259. Proliferations, 257, 258. Properties of soil, 57. Proteids, 132, 138, 272, 277, 286. Proteolytic enzymes, 132. Protomyces, 217. Protoplasmic molecules, 276, 278, 286. Protoplasm, 33, 41, 271, 272, 274, 276. Pruning, 105, 143, 194, 225, 250. Prussic acid, 163, 165, 173. Psylla, 253. Put:<:i?ita,88, 114, 169, 175, 176, 188, 189, 252, 247. Puckers, 214, 235, 246, 253. Puffing of spores, 142, 144. Punctures, 212. Pure culture, 166. Purple-black spots, 191. Pustules, 188, 190, 212, 217. Putrefaction, 234. Pyrethrutn, 161. Pyrus, 214. Pythium, 114, 119, 136, 144, 1 60, 230. Quassia, 161. Quinine, 173. Rabbits, 108, 142, 164, 194. Rain trees, 233. Rankness, 97, 227, 228. Rats, 108, 163. Rays of light, 18. Red light, 21. Red spider, 161, 187, 188, 192. Red spots, 1 88, 253. References in Bible, 85. Remedial measures, 89. Repellent substances, 136. Reproduction, 72, 281. Reserves, 274. Resin, 125. Resin-flux, 234. Resinosis, 227, 234. Resistance to disease, 155, 268. Resistant races, 172. Respiration, 17, 31, 130, 271, 275, 276, 285, 287. Reversions, 73. Rhinanthoideae, 265. Rhinanthzts, 112. Rhizobium, 289. Rhizoclonia, 238. Rhizomorph, 145. Rhododendron, 218. Rhubarb, 180, 230. Rhynchitis, 254. Rhytisma, 1 88. Ribbon grass, 183. Ribes, 214. Rice, 172. Rimpau's experiments, 69, 73, 77. Ringing, 194, 201, 202, 210. Ripened wood, 243. Robinia, 259. Rodents, 109. Roestelia, 217. Rolled leaves, 86. Root, 9, 35, 96, 1 20, 227, 270. Root-absorption, 181. Root-diseases, 119, 120. Root-excretions, 46. Root-fusions, 262. Root-galls, 221. 306 DISEASE IN PLANTS. Root-hairs, 34, 102, 163. Root-nodules, 212, 219. Root-parasites, 112, 265. Root-rot, 230. Roses, 232, 243, 257, 268, 278. Rosettes, 225. Rot, 97, 182, 227, 229, 231, 236. Rotation of crops, 69, 1 66. Rotifer, 140. Rot-organisms, 200. Rotting of wounds, 87. Rouen law, 85. Rushes, 114. Rust, 122, 142, 171, 172, 175, 191. Rye, 176, 248. Saccharomyces, 60. Sachs, 7, 36. Salvia, 214. San Jose scale, 187. Sand-blast action, 184. Sandy soils, 184. Saperda, 205. Saprophytes, 135, 137, 175,234, 243, 244. Scab, 189, 2 1 6. Scale, 187. Schinzia, 114. Schizoneura, 223. Scion, 183, 262, 264, 266, 282. Scleroderris, 223. Sclerotia, 143. Schwarz, 39. Sclerotinia, 142, 143, 144, 231, 248, 249, 288. Scolytidae, 205. Scorching, 240, 241. Scurf, 216. Sea-kale, 261. Secale, 76. Secretions, 130, 133, 173, 274. Sedges, 189. Seedless grapes, 247. Selandria, 208. Selection, 69, 74, 78, 169. Selective absorption, 53, 65. Self- fertilisation, 281. Semi-parasites, 112. Smecio, 188. Sensitive plant, 125. Septoria, 114, 187. Sewage waters, 59. Sexual act, 72. Shaded foliage, 1 1 3. Shanking, 246, 249. Shoots from old wood, 260. Shot holes, 204, 208, 209. Silver fir, 224. Silver leaf, 192. Sirex, 206. Skeleton leaves, 204, 207. Slime flux, 227, 233. Slime fungus, 219. Slugs, III, 164, 207, 269. Smut, 117, 143, 162, 190. Snails, III, 142, 207. Snow, 106. Soap, as insecticide, 161. Soil, l, 42, 99, 102, 142, 163. Soil-bacteria, 60. Soil-filtration, 59. Soil-organisms, 61, 143. Solar energy, 135. Somatoplasm, 267. Sooty moulds, 135, 190, 232. Sorbus, 207. Sorosporium, 2 1 6. INDEX. 307 Sour-rot, 231. Structure, 274. Sparrows, 164. Structure of protoplasm, 271. Specialised races, 168, 176. Structure of root-hairs, 40. Specific predisposition, 155. Struggle for existence, 105, 159, Spectrum, 19, 21, 26. 164, 165, 167, 286. Sphaerdla, 189. Study of causes, 85. Sphaerotheca, 187. Stumps, 194. Spermagonia, 144, 232. Subsoil, 57, 103. Sphaeroblasts, 222, 225. Substitutive selections, 286. Spicaria, 237. Suckers, 225, 260. Spiral grooving, 204, 2IO. Sugar, n, 17, 20, 173, 286. Spiral growth, 252. Sugar cane, 172. Spongospora, 216. Sugar cane disease, 166. Spontaneous variations, 78, 246, Sulphate, 273. 255. Sulphur, 161, 163, 272. Spores, 144. Sulphurous acid, 181. Sports, 93, 247. Sun-burn, 240, 241. Spots on leaves, 120, 186. Sun-cracks, 240, 242. Spraying, 159, 161, 162. Sundew, 232. Spreading of disease, 142. Sunflower, 256, 264. Squirrels, 108. Sun-spots, 192. Stag-head, 240, 244. Superstitions, 85. Starch, 9, 16, 17, 20, 23, 138, Surface energy, 26. 173- Surface roots, 112. Statistics of epidemics, 147. Sweet almond, 173. Steeping, 161. Symbiosis, 63, 130, 137, 219, Stem diseases, 120. 263, 265, 268, 285. Stereuin, 206. Symptoms of disease, 89, 122, Sterility of soil, 61. 179, 1 86. Stimulation, 119. Synckytrium, 127, 188, 217, 247. Stimuli, 126, 127, 139. Synthesis, 65. Stock, 262, 264, 266, 282. Syringa, 259. Stomata, 23. Syringing, 161, 164. Stool-shoots, 201, 225, 269. Stool stumps, 194, 201. Tamarisk, 235. Strangulations, 204, 209. Tannin, 138. Strawberry, 189, 257. Taphriiia, 218. Stripping, 194, 197. Tar, 164. Stroma, 217. Tea, 244. 308 DISEASE IX PLANTS. Teazel, 252. Teleutospore, 189, 191. Temperature, 99, 105. Tendencies to ill-health, 91. Tendrils, 125. Teratology, 246, 253, 254, 257. Tetraneura, 218. Tetranychus, 187, 192. Thawing, 183. Thelephora, 206. Therapeutics, 85, 89, 159. Thermotropism, 126. Thesium, 1 1 2. Thick-skinned organs, 168, 171. Thinning, 96, 105. Thistle, 247. Thrips, 88, 191, 208. Thyloses, 125. Tilia, 214. Timber diseases, 119, 120. Timiriazeff, 21. Tinea, 206. Tissue diseases, 119. Tobacco, 209. Tobacco powder, 161. Tomato, 171, 219, 230. Top-dry trees, 244. Topical remedies, 161. Tornicus, 205. Torsions, 246, 252. Tortrix, 254. Toxic agents, 130. Transformation of energy, 25, 28. Transformation of organs, 254, 255- Transmission of acquired char- acters, 264, 283, 290. Transplanting, 96. Transpiration, 181, 228. Trees, 109. Trichosphaeria, 135. Trilicum, 76. Tumescence, 227, 228. Tunnels, 206. Turgescence, 47, 228, 230. Turnip, 126, 162, 230. Twitch, 113. Tylenchus, 238, 248. Ulcer, 231. Unger, 85. Unsuitable soils, 101. Upheaval of seedlings, 179, 183. Uredineae, 114, 134, 136, 145, 169, 188, 189. Uredo, 88, 188, 191. Uredospores, 191. Uromyces, 116, 188, 191, 266. Urocystis, 220. Ustilagineae, 145, 190, 217, 248. Ustilago, 1 1 6, 117, 175, 190, 219, 255. Vacciniuiii, 128, 288. Variability, 174. Variation, 67, 72, 91, 92, 168, 174, 176, 246, 262, 263, 264, 271, 282, 286, 288, 289. Variegation, 179, 182, 183, 192. Varieties, 78, 247. Varieties of soil, 56. Vaucheria, 139, 140. Vegetable acids, 48. Vertebrata, 108. Verticillium, 145, 236. Viburnum, 214. INDEX. Vine, no, 149, 156, 162, 164, 169, 171, 189, 190, 191, 222, 248, 268. Vine disease, 143. Vivipary, 257, 258. Walnut, 190, 209, 253. Want of air, 100. Washing leaves, etc., 161. Wasp-flies, 165. Wasps, 145. Water, 272. Water and insects, 161. Water-culture, 65. Water in soil, 103. Waterlogging, 181. Weaving of fungi, 190. Webs, 190, 254. Weeding, 105. Weeds, 69, m, 113, 165, 229, 249. Weevils, 248. Wet feet, 181. Wheat, 169, IT i, 172, 176, 179, 180, 182, 183, 230, 248. Wheat rust, 86, 122, 146, 166, 169, 176. White spots, 1 86, 187. Willow, 206, 207, 219, 223, 233, 259- Willow beetle, 208. Wilting, 179, 181, 235, 249. Wind, 106, 142, 144, 153, 184, 209, 229. Wire- worms, 109, 181. Witches' brooms, 116, 222, 224. Wood, 124. Wood-ashes, 161. Woodbine, 112, 210. Wood-boring, 204, 205. Woodlice, 164. Wood-nodules, 225. Wood-wasps, 206. Woolly-aphis, 219, 223. Worms, 109, 142, 144, 194, 238. Wounds, 108, 139, 194, 204, 207, 213, 260, 263, 269. Wound-cork, 195. Wound-fever, 123. \Vound-fungi, 203, 204, 240. Wound-gum, 125. Wound- wood, 124. Wrens, 165. W'rinkling, 253. Xenia, 267. Xyloma, 88. Yeasts, 134, 172, 231, 233. Yellowing, 179, 181, 182, 184. Yellow leaves, 89. Yellow spots, 186, 187, 188, 253- Zoospores, 151. GLASGOW : 1'Kl.Nl By of i MACMILLAN AND CO.'S WORKS ON BOTANY. BY THE SAME AUTHOR. Crown 8vo. Price 6s. Timber and Some of its Diseases. H. 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