OHIVERSITY OF CALIFOBNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA POLLINATION OF THE BARTLETT PEAR BY WARREN P. TUFTS BULLETIN No. 307 May, 1919 UNIVERSITY OF CALIFORNIA PRESS BERKELEY 1919 Benjamin Ide Wheeler, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Director. Edward J. Wickson, Horticulture (Emeritus). Herbert J. Webber, Director Citrus Experiment Station; Plant Breeding. Hubert E. Van Norman, Vice-Director; Dairy Management. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Charles W. Woodworth, Entomology. Ralph E. Smith, Plant Pathology. J. Eliot Coit, Citrieulture. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Enology. Warren T. Clarke, Agricultural Extension. John S. Burd, Agricultural Chemistry. Charles B. Lipman, Soil Chemistry and Bacteriology. Clarence M. Haring, Veterinary Science and Bacteriology. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. James T. Barrett, Plant Pathology. Fritz W. Woll, Animal Nutrition. Walter Mulford, Forestry. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. Elwood Mead, Rural Institutions. J. B. Davidson, Agricultural Engineering. H. S. Reed, Plant Physiology. D. T. Mason, Forestry. C. L. Roadhouse, Dairy Industry. fFRANK Adams, Irrigation Investigations. J. C. Whitten, Pomology. F. L. Griffen, Agricultural Education. John E. Dougherty, Poultry Husbandry. S. S. Rogers, Olericulture. J. G. Moodey, Assistant to the Director. Mrs. D. L. Bunnell, Librarian. DIVISION OF POMOLOGY J. C. Whitten W. P. Tufts W. L. Howard *E. L. Overholser R. H. Taylor W. L. Sweet A. H. Hendrickson G. L. Philp M. N. Wood t In cooperation with Office of Public Roads and Rural Engineering, U. S. Department of Agriculture. * Exchange Professor from Cornell University, 1918-19. POLLINATION OF THE BARTLETT PEAR By WAEEEN P. TUFTS There is a growing interest on the part of pear growers in Cali- fornia concerning the question of whether the Bartlett, their chief variety, requires or is benefited by cross-pollination. The fact that Bartlett trees, planted without any regard to cross-pollination, have yielded profitable crops has led some to believe that the variety is self- fertile. On the other hand, these same growers have noticed when they have a stray tree of some other variety in their orchard that the surrounding Bartletts are more abundantly fruited than the remain- der of the orchard. This seems to indicate that the Bartlett, under valley conditions, is greatly benefited by cross-pollination. In the Sierra foothills thoughtful and observant growers have felt sure that the Bartlett, under their conditions, is always self -sterile, to a degree rendering cross-pollination imperative. Waite 1 discovered several years ago that the Kieffer, a leading variety of the eastern and middle states, was practically self-sterile. Since that time there has been much discussion throughout the country as to whether or not certain varieties besides the Kieffer could be benefited by cross-pollination and what varieties could be depended upon to serve as pollinators. Fletcher 2 states that : The results of hand-pollinating 8408 Kieffer pear blossoms and 9867 Bartlett pear blossoms in the years 1903, 1906, and 1907 indicate: 1. In West Virginia and Michigan, and probably in other parts of the east, unsatisfactory results may be expected from planting either Bartlett or Kieffer in large blocks so that cross-pollination by insects is not general. 2. Anjou, Lawrence, Duchess, and Kieffer are satisfactory varieties for plant- ing with Bartlett so far as pollination is concerned. Some years Kieffer does not blossom simultaneously with Bartlett but usually the blossoming seasons overlap sufficiently. Kraus 3 advises planting Clairgeau, Anjou, Howell, or Kieffer with Bartlett for the purpose of effecting cross-pollination. i Waite, M. B., Pollination of Pear Flowers, Bull. 5, Div. Veg. Path., U. S. Dept. Agr., 1894. 2 Fletcher, S. W., Eeport of Virginia Agr. Exp. Sta., 1909-1910. s Kraus, E. J., The Pollination Question, Ore. Agr. Exp. Sta., Cir. Bull. No. 20, 1912. 370 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Wisker 4 says that: California's pear profits would be greater if some other variety were planted with Bartlett for cross-pollination. Anjou, Bose, and Cornice are good pollinators and sell for higher prices than Bartlett. We have increased the fruitfulness of an old Bartlett orchard more than 200 per cent by inter -grafting other varieties. Weldon 5 believes that : The best results in growing Bartlett pears cannot be attained unless other varieties are planted with them in the orchard. Despite the evidence that this should be done, there are thousands of acres of Bartletts being planted in California each season with utter disregard of the benefits that might be derived by planting one or more other varieties for cross-pollination. In certain new pear- growiug sections, the writer has recently seen thousands of acres of Bartletts in blocks of five acres to more than one hundred acres each, with no other varieties near, except possibly an occasional tree of some favorite pear planted for home use. In some of these sections heavy winds and extremes of heat and cold occur, making more necessary the strictest attention to every detail that would tend to increase productivity. The fruit in general is good, and the trees everywhere have done well. So far as it has been possible to determine, they are self -fertile to a remarkable degree when the existing conditions are taken into consideration, but it is safe to predict that there will be years of short crops because of the absence of other pears, and the average production of these sections throughout a series of years will be far below what it would be with other varieties planted along with the Bartletts for cross-pollination purposes. There are older pear- growing sections of the state where large areas of Bartletts have been planted by themselves and where the same arguments against the practice may be used. Although practically all writers agree in advising the inter-planting of other varieties to pollinate the Bartlett, even in the valley locations in California, nevertheless the bulk of the acreage planted to Bartletts in the state contains no other variety. The writer outlined and per- formed certain experiments during the seasons of 1916, 1917, and 1918 designed to test the accuracy of the observations enumerated above. THE PROBLEMS OF BARTLETT PEAR POLLINATION In planning the work of which this bulletin is a report, the writer had in mind the differences of altitude, climate, soil, etc., existing in both foothill and valley locations in California and has attempted to solve the following problems for these various conditions. 1. Will the Bartlett pear be commercially profitable when planted in solid blocks, i.e., without pollinators? If found to be profitable 4 Wisker, A. L., Price List, Loma Eica Nurseries, 1915-1916. s Weldon, G. P., Pear Growing in California, California State Commission of Horticulture, p. 234, 1918. POLLINATION OF THE BARTLETT PEAR 371 under these conditions, this variety may be termed self -fertile;* if unprofitable, then it should be rated as self-sterile* 2. If the Bartlett will not, under all conditions, produce profitably in the absence of cross-pollination, what varieties would be best as pollinators? This is a question of inter- fertility* with the Bartlett. 3. Even if it should be found that by the inter-planting of other varieties with the Bartlett, the yield per tree can be increased, will the increase per acre make such interplanting commercially profitable ? 4. What variety should be planted with the Bartlett which, while being a good pollinator, will also produce fruit of high commercial value when grown under the same conditions as exist in so many of our present Bartlett orchards? Even though two varieties may pollinate each other, it is important that the following points should always be considered in selecting pollinators : (a) Commercial value of the pollinator. (&) Coincidence of bloom of the pollinator with that of the variety to be pollinated. (c) Succession of ripening of the varieties for convenience in har- vesting. (d) Amount of pollen produced by the pollinator. (e) Germinability of the pollen produced by the pollinator. ORGANIZATION OF THE WORK The methods employed for investigating the problems just outlined were those commonly in use in cross-pollination experiments. 6 Briefly stated, this part of the work consisted in the application by hand of the pollen desired, having first removed the flowers' own pollen- producing organs (the stamens). The introduction of foreign pollen through the agency of wind and insects was prevented by covering the band-pollinated blossoms with paper sacks. Accurate counts of the flowers were made and recorded. Later in the season the sacks were removed. The fruits resulting from these artificial pollinations * The writer prefers the terms, ' ' barren ' ' and ' ' fruitful, ' ' as explained by Kraus in the Journal of Heredity, Vol. 6, No. 12, pp. 549-557, rather than the inexact terms, ' ' sterile ' ' and ' ' fertile. ' ' The latter terms have been used in this paper, due to the fact that the general public is more or less familiar with these expressions as herein used. 6 The Division of Pomology has in preparation a bulletin dealing with the more technical phases of the whole pollination question. In this publication the tech- nique employed in the solution of various pollination problems will be discussed in detail. 372 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION were counted and the proper record made after the first and second drops and again at harvest. During the seasons of 1916 and 1.917 the experiments were conducted in the University Farm orchards at Davis where typical Sacramento Valley conditions prevail. In 1918, experiments were conducted in the Vaca Valley Ranch orchards of the Earl Fruit Company at Vacaville, and on the Loma Rica Ranch, Grass Valley, Nevada County. The trees in the University orchard are ten years old and in full bearing. They are planted in deep, rich valley soil and are in a thrifty growing condition. The trees on the Vaca Valley Ranch are in full bearing and in good condition, being planted in some of the best soil of the Vaca Valley. The trees on the Loma Rica Ranch are five years old and are beginning to bear quite well. This orchard is planted on typical red soil of the Sierra foothills at 3000 feet elevation. THE BLOOMING PERIOD OF CERTAIN PEAR VARIETIES Figure 1 (page 373) shows the average dates of the first, last and full bloom of eight leading varieties of pears for the years 1914 to 1918 inclusive (unless otherwise noted). Attention is called to the fact that the blooming period of the Bartlett is twice as long as that of any of the other varieties tested. A careful study of these data as well as of those presented by Weldon, 7 including the blooming dates of nearly sixty varieties, leads to the conclusion that with the possible exception of a few very early blooming sorts such as the Kieffer, Forelle, and Clairgeau the blooming period of the Bartlett overlaps very well with that of all likely pollinators. Even the very early blooming varieties just mentioned do not on the average reach the height of their bloom many days before the Bartlett and would in most years overlap sufficiently to make them available as pollinators. PRODUCTIVENESS AND VIABILITY OF POLLEN USED The pollen used in all experiments was in first-class condition and showed in all cases quite a satisfactory artificial germination test. Pollen of pears does not on the average give as high germination tests as that of some other fruits, particularly the almond, although a sufficient number of the pollen grains germinate to insure thorough pollination whenever the pollen is applied artificially. 7 Weldon, G. P., Pear Growing in California, California State Commission of Horticulture, p. 235, 1918. 4 M- ffq *| 1 h3 cr CD p cr P QTQ CD O o 3 P. o CO Hs rn o cr CD o <-h 3 P a a ^ CD P P H co CD « © o ^— ' <1 a CD 5 H a OS p ►tf CD H >-"• O Pi O Hs Hs O a H O N Ch O CD V! CD P H co CD it P i c r n> a) o n> J3 n> ^ n» rr C << o> r "D £t c/> XI -l © n« n» 33 3 C o r r o c r n« C_J« CJ1 ■fc- ^ CJ3J p rr s m c r r CP r o o 5 % 1 — 1 — 1 — * ■ — i — * N g" 1 1 * hi ■ - cr o 0" ^ ^ w * ©. » s - ^ ■D cr 3d D o n> ca O C GO S' O *T o i- o CD X3 rr I) s n> 374 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION In addition to the viability of the pollen produced by a certain variety, the orchardist in selecting a pollinator for his commercial orchard must also take into consideration whether the variety to be planted as a pollinator is a good pollen producer. Fortunately all the pear varieties under experiment have given every evidence of being abundant producers of pollen. Table I shows how well the pollen from different varieties grew in germination tests. Table 1. — Germinability of Pollen Percentage germination in a 12 per cent cane sugar solution, seasons of 1916-1918 1918 32 42 19 76 43 74 58 39 25 45 36 Variety Source 1916 1917 Angouleme University Farm 63 22 Anjou Grass Valley — .... Bartlett Grass Valley .... — Bartlett Vaca Valley — .... Bartlett University Farm 26 45 Clairgeau University Farm .... 26 Cornice Grass Valley .... .... Cornice University Farm .... .... Dana Hovey Grass Valley .... .... Easter University Farm .... 63 Howell University Farm — 21 Winter Nelis University Farm 9 41 Winter Nelis Average Grass Valley 33 36 44 RELATION OF NORMAL SET TO FINAL CROP Immediately after the petals fall each flower begins to form a fruit. During the following two or three weeks a certain percentage of these young fruits, for some reason or other, drop off and only a rather low percentage of the total number of original blossoms continue to develop. It is quite necessary that this drop take place, as the tree under average circumstances would never be able to carry so many fruits through to maturity. Just what factors determine which of these young fruits shall drop is not at present known. Later in the season there takes place still another falling of the young fruits known as the " June drop." The expression normal set is a more or less technical way of designating the percentage of fruit which the tree sets under normal conditions when left open to insect pollination. It is obvious that POLLINATION OF THE BARTLETT PEAR 375 the normal set may be determined at any time up to the end of the season by counting the fruits on a tree and comparing the number with the original bloom. In these experiments, approximately 2000 blossoms of each variety, well distributed over the trees, were counted each year and proper records kept to determine the normal set after the first drop, after the second drop, and at harvest time. The questions at once arise as to what percentage of normal set at harvest time constitutes a full crop, and what is meant by a " full crop." What constitutes a full crop of any fruit is perhaps largely a question of opinion, judgment, or guess. It varies with such factors as variety, soil, and moisture conditions. What would be considered a full crop under foothill conditions would not necessarily be the same in a valley location. Table 2 casts some light upon the question as it shows the normal set of the Bartlett pear for the years 1916 and 1917 as grown under the conditions existing in the University Farm orchards. The average yield of these trees has been reduced to an acreage basis. The trees under observation were planted during the spring of 1908, being set twenty-four feet apart by the square system which gives approximately seventy-five trees to the acre. Since plant- ing, these trees have received good commercial care. All figures are based on pears harvested. Table 2. — Comparison of Normal Set with Yields in Pounds per Acre University Farm, Davis, California, 1916-1917 1916 1917 Variety Percentage Pounds Percentage Pounds set per acre set per acre Bartlett 3.6 3510 8.2 3840 An examination of the figures presented in Table 2 shows that too much emphasis must not be placed on mere percentages. In 1916 the normal set was only 3.6 per cent as against 8.2 per cent in 1917, and yet comparatively little more fruit was harvested in 1917 than in 1916. This fact is explained by the heavier bloom occurring during the former season. These figures are presented to show the necessity of taking "normal set" counts each year in order to secure a correct basis for judging results of artificial pollination. BARTLETT PEAR POLLINATIONS, 1916-1917 During the seasons of 1916 and 1917 experiments on a rather small scale were conducted in the University Farm orchards, the main purpose being the determination of the question of self-fertility or 376 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION self-sterility of the Bartlett when grown under Sacramento Valley conditions. The results of this work are summarized in Table 3. Table 3. — Bartlett Pear Pollinations University Farm orchards, 1916-1917 - 1916 A 1917 A Bartlett crosses compared with normal set No. of blossoms counted No. of fruits set Per cent set April 29 No. of blossoms counted No. of fruits set Per cent set April 3 Bartlett Normal set 1116 42 3.6 1947 159 8.2 X Self 205 5 2.4 423 8 1.9 X Angouleme 73 8 10.9 380 26 6.9 X Clairgeau .... .... 201 6 3.0 X Easter — — 109 3 2.9 X Howell — — 105 1 1.0 X Winter Nelis 308 23 7.5 162 17 10.5 An examination of Tables 2 and 3 shows that a normal set of 8.2 per cent gave a crop of less than two tons to the acre (a low yield), but if no provision had been made in this orchard for cross-pollination, the yield would probably have been only about one-fourth as great. In certain years with a heavy bloom a normal set of 2 per cent may give a satisfactory crop, but during the bearing life of a pear orchard there are likely to be many years of light crops, if only one variety is planted. It will be noted from a study of the figures presented that self- pollinated flowers gave a distinctly lower set than did those crossed with pollen of other varieties. During both 1916 and 1917 the Bart- lett displayed distinct evidence of partial self-sterility. Of the varieties used as pollinators for the Bartlett, the Angouleme and Winter Nelis gave the best results both years. In 1917 the University orchards suffered a severe attack of pear blight and as a consequence many of the artificial pollinations were lost. SEASON OF 1918 Vaca Valley. — Table 4 gives the results of the experiments con- ducted in the orchards of the Vaca Valley Ranch of the Earl Fruit Company at Vacaville during the season of 1918. This orchard con- sists of a solid block of Bartletts, no provision having been made for cross-pollination. POLLINATION OF THE BARTLETT PEAR 377 It is unfortunate that the normal set count of the fruits matured could not be secured. The branches on which these records were made received the customary hand thinning early in the summer and there- fore later counts would not have been reliable. Table 4. — Pollination of the Baetlett, 1918 Vaca Valley Ranch, Vacaville Bartlett crosses compared with normal set No. of blossoms counted No. fruits apparently set May 10 Per cent apparently set May 10 No. fruits matured Per cent matured artlett Normal set 1276 108 8.4 .... X Self 424 79 18.6 41 9.7 X Cornice 278 90 32.3 73 23.3 X Easter 308 86 27.9 71 23.1 X Howell 238 63 26.4 47 19:8 X Winter Nelis 256 75 29.2 60 23.4 Those flowers which were self -pollinated matured a much smaller crop than did those which were crossed with pollen from other varie- ties. All four varieties used as pollinators for the Bartlett proved to be entirely satisfactory. Another very interesting point brought out in this table is the fact that the loss from the June drop of those fruits resulting from cross-pollination amounted to only 20 per cent, while with those self -pollinated the loss was 48 per cent. This would seem to indicate that one of the most important factors governing the June drop of pears is whether the fruit has been produced from self- or cross-pollination. Inasmuch as the resulting set in this orchard when the blossoms actually received their own pollen was more than double that of the normal set, it seems clear that the provision of some agency, prefer- ably honey bees, for the transfer of pollen from flower to flower would be highly profitable. Nevada County. — On account of the fact that the statement has been made repeatedly that the Bartlett requires cross-pollination in the foothill orchards of California, experiments were conducted on the Loma Rica Ranch, Grass Valley, in Nevada County. It is to be regretted that harvesting records could not be made in this orchard, but so many branches were lost from the ravages of blight that all later counts would have been of small value. The records here pre- sented were taken when the fruits had attained a good size, and there is no reason to believe that counts at harvesting time would in any 378 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION way have altered the final figures as given. Table 5 summarizes the results of these pollinations. Table 5. — Pollination of the Bartlett, 1918 Loma Eica Ranch, Grass Valley Bartlett crosses No. of compared with blossoms No. fruits normal set counted set Bartlett Normal set 2916 569 X Self 918 12 X Anjou 518 97 X Cornice 426 88 X Dana Hovey 459 71 X Easter 132 20 X Winter Nelis (pollen from Univ. Farm, Davis) 411 80 X Winter Nelis (pollen from Loma Rica Ranch) 418 87 * Fruits all small. Per cent set 19.5 1.3* 18.7 20.6 15.4 15.1 19.4 20.8 These results show that the Bartlett pear as grown in the Sierra foothills may be regarded as practically self -sterile. All fruits result- ing from self-pollination were small as compared with those from cross-pollinations. From the writer's experience in other years he feels that these fruits assuredly would" have been lost during the June drop. The normal set recorded for this orchard was very good and shows what may be accomplished by providing pollinating varieties and agencies for transferring the pollen from tree to tree. All five varie- ties tested proved entirely satisfactory as pollinators for the Bartlett. Winter Nelis pollen, not only from the trees on the Loma Rica Ranch, but also from trees in the University orchards at Davis, was used, but no material difference was noticeable between the two. Table 6 presents the results of an experiment designed to show from the average yield of two orchards the benefits to be derived from cross-pollination. The orchard of Mr. P. M. Beaser of Chicago Park, Nevada County, consists of a block of one thousand Bartlett pear trees, no provision having been made for cross-pollination. These trees are perhaps twenty-five years old. There is located on the Loma Rica Ranch, some eight miles away, a small block of old Bartlett pear trees of about the same age and vigor as those on the Beaser place. Many § 3 * HsQrq P • o i— i Hi O -t a> a> co O CO CD c+ CD M" y pu PS CD Q CD O Hi Hi H H- CD * 3 ?4 Pj &* P H5 5^ T3 5" o Pj (-•■ 3 O P Hi <*- CD 3 CD 3 g p p '"d H fcl P CD Ef. Pj erl" i«-i Pr 1 «i^' CD on? "^ o CO ^ CD CD o H 5: PP rn * r ^ r rn r ?5 P n> > r 3 "* re o r CO O I £ 2 ft £ ^ D §D O 15 H -H 2) ^k c it m O o X 1 1" ^X CT» C ^ s tt> fT r *< o rn r T X) 50 o o 3 O o 3 Q =TTf s i 3d Z5 ' © o 3 fl>. 3 o 8. 2o CO 5= Co = 33 3 3a o O I I 5 S C/) § ( O . o 00 s 380 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION of these trees have, however, during the past six or eight years been top-worked to other varieties. With the exception of the presence of pollinating varieties at Loma Rica, the two orchards just described are in all respects comparable. Both orchards bloomed profusely. Table 6. — Comparison of Yield on Bartlett Trees in Nevada County, with and without pollinating varieties, 1918 With pollinating varieties Without pollinating varieties (Loma Rica Ranch, Grass Valley) (Beaser Ranch, Chicago Park) No. of blossoms counted 3007 No. of fruits set May 17 449 Per cent set 14.9 Average yield per tree in packed boxes 2.19 r No. of blossoms counted 3170 No. of fruits set -Mny 17 192 Per cent set 6.0 > Average yield ner tree in packed boxes .77 The figures show that the trees provided with cross-pollination produced a crop almost three times as great as that produced by the trees which did not receive cross-pollination. It should also be noted that the favorable showing for the trees receiving cross-pollination was made in spite of the fact that this orchard lost much of its fruit as a result of the ravages of pear blight. RECOMMENDATIONS From a study of the experimental data presented and also as a result of the observation and experience of many orchardists of the state the conclusion is inevitable that all Bartlett orchards should be provided with facilities for cross-pollination. The question at once arises as to what variety shall be selected as a pollinator. For foothill locations where the bulk of the crop is offered for " green" shipment this question is probably not difficult to answer. In these places the Anjou, Bosc, Cornice, Howell, and Winter Nelis are all excellent pears, bringing prices equal to or better than the Bartlett for eastern shipment. These varieties may not per- haps give an equal tonnage with the Bartlett but all are fair producers. The Cornice should be avoided in windy locations, due to the fact that its skin is quite delicate and mars most easily. In valley locations the question of providing cross-pollination for the Bartlett is more difficult. The grower must concede that where the set can be increased 100 per cent or more by the planting of one pollinating tree to every eight Bartletts the returns per acre will still be greatly increased even though the fruit of the pollinating variety may have little or no commercial value. Although there has been no thorough attempt to test the adaptability of various varieties POLLINATION OF THE BARTLETT PEAR 381 to different locations, the writer feels that the information in hand indicates that either the Hardy or the Winter Nelis will prove, from all standpoints, to be the most efficient pollinator for the Bartlett when grown under valley conditions. Where the dried fruit is a primary product and where the fruit of the pollinator for the Bartlett must also be dried, special difficulties are presented. No experiments have as yet been conducted in this state to determine the drying qualities of the various varieties. In order to avoid the use of artificial evaporators, the pollinator must ripen its fruit at about the same time or shortly after the Bartlett. Practically all dried pears in California are sun-dried. Keeping specially in mind the time of ripening, the Howell and Hardy varie- ties seem to be the best pollinators for the Bartlett where the entire crop is to be dried. POLLINATING AGENCIES After having planted inter-fertile varieties the orchardist should, by all means, provide an agency for the transfer of the pollen from the trees of one variety to those of another. The common honey bee is by far the best carrier of pollen and it will pay the grower to keep bees even though he may not care to go into the honey business. Bees, however, are a very profitable side line for the orchardist, especially if alfalfa fields are available to work on after the blooming season of fruit has passed. About one hive of bees to an acre of bearing orchard should be provided. Preferably the hives should be scattered as widely as possible throughout the orchard during the blooming season. Experiment and experience have shown that little reliance can be placed on the efficacy of wind and of insects, other than the honey bee, in effecting the transfer of pollen from tree to tree or, in fact, from flower to flower. Eliminating from consideration all conditions which may influence the fruitfulness of an orchard except those occurring at blooming time, it may be said that the set is largely influenced by weather conditions at time of blooming. Cold weather, aside from killing the blossoms or lowering the vitality of the pollen, often prevents bees from working. The same is true if cloudy, wet and windy weather prevail. For their best work bees demand clear, warm and quiet days, and since the weather at the time of bloom is often quite un- settled, it is readily seen that the blooming period of the various pollinating varieties should overlap perhaps a week in order that there may be one or two days at least when the weather will be favor- able for insect pollination. 382 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION ARRANGEMENT OF THE ORCHARD FROM A POLLINATION STANDPOINT In planting an orchard it is desirable to have at least every sixth and preferably every fourth row of a pollinating variety. For con- venience in the management of the orchard, it is best to plant two rows of one kind, then two rows of the pollinating variety, and so on ; or, if it is desired to have more of one variety than another, four rows of the favorite variety, then two rows of the pollinator, and repeat. For one reason or another, it is often desirable to reduce the number of pollinating trees to the minimum. Under these circum- stances, one tree in twenty-five is perhaps sufficient, although at least one tree in eight is strongly recommended. It is seldom wise to graft- over a part of a tree to the pollinating variety as this tends towards confusion and added expense in harvesting. In planting one tree of the pollinator to seven or eight of the main variety, the pollinator should be placed as every third tree in every third row in such a way that the spaces in the pollination rows are broken thus, the "o" in each case representing a pollinator tree. If by chance a self-sterile variety has been planted in a solid block, the necessary pollinator may be introduced by grafting. Some relief may be obtained during the years while waiting for the trees grafted POLLINATION OF THE BARTLETT PEAR 383 over to pollinating varieties to come into bearing, by cutting off large limbs of pollinating varieties, placing the cut ends in buckets of water and distributing the same throughout the orchard during the blooming period. Such limbs will live for several days and continue to bloom, forming pollen for the bees to transfer to the self-sterile variety. SUMMARY OF BARTLETT PEAR POLLINATION 1. A 12 per cent cane sugar solution gave, during the seasons of 1916, 1917, and 1918, an entirely satisfactory artificial germination of pear pollen. 2. There is comparatively little variation in the amount of pollen produced by various pear varieties. Pollen production is always abundant. 3. The blooming period of most pears is comparatively short. 4. The eight varieties tested as pollinators for the Bartlett over- lap well in their blooming periods with the Bartlett. 5. The Bartlett has an exceptionally long period of bloom. 6. The Bartlett is to a limited degree self-sterile under valley conditions, as experienced at Davis and Vacaville. 7. The Bartlett is self-sterile under foothill conditions, as experi- enced in Nevada County. 8. In planting a commercial orchard of Bartlett pears, in either a valley or foothill location, the best plan seems to be to inter-plant another variety for cross-pollination purposes. 9. No cases of inter-sterility have been found to exist between pear varieties, and therefore any variety overlapping in blooming period with the Bartlett may be expected to be an efficient pollinator. 10. The experiments show that the Angouleme, Anjou, Clairgeau, Cornice, Dana Hovey, Easter, Howell, and "Winter Nelis will all pollinate the Bartlett successfully. 11. Pear fruits resulting from cross-pollination do not appear to exhibit the same tendency to fall at the June drop as do those result- ing from self-pollinations. 12. Pollinating agencies such as the honey bee are necessary to the set of a good crop of fruit. 13. One colony of honey bees should be provided for each acre of orchard. 14. Care should be taken in the arrangement of varieties in the orchard to facilitate cross-pollination and convenience in orchard management. 384 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION ACKNOWLEDGMENTS The writer wishes to express his appreciation of the help he has received in carrying out this work. Messrs. G. L. Philp and M. N. Wood, and Miss R. M. Amesbury, members of the Division of Pomology, have rendered invaluable assistance in the field and laboratory. Dr. W. L. Howard, also of this division, has given great service in suggestions and criticisms. The Earl Fruit Company, Mr. A. L. Wisker of the Loma Rica Ranch, and Mr. P. M. Beaser, of Chicago Park, have kindly allowed the use of their orchards for certain experiments and have at all times showed a kind and co-operative interest in the work. POLLINATION OF THE BARTLETT PEAR 385 Fig. 3. — A typical example of the normal set in the Vaca Valley Ranch orchards, 1918. Two pears matured from seventeen blossoms — a 11.7 per cent set. In years of normal bloom a 10 per cent set would assure a satisfactory crop. 386 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Fig. 4. — Two Bartlett pears set from eight blossoms pollinated with Bartlett pollen. A typical cluster of this variety when self -pollinated. Vaca Valley Ranch, 1918. POLLINATION OF THE BARTLETT PEAR Fig. 5. — Five Bartlett pears set from twelve blossoms pollinated with Cornice pollen. A typical cluster resulting from crossing Bartlett with Cornice. Vaca Valley Eanch, 1918. 388 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Fig. 6. — Four Bartlett pears set from seventeen blossoms pollinated with Easter pollen. A typical cluster resulting from crossing Bartlett with Easter. Vaca Valley Ranch, 1918. POLLINATION OF THE BARTLETT PEAR 389 Fig. 7. — Eleven Bartlett pears set from twenty-five blossoms pollinated with Howell pollen. A typical cluster resulting from crossing Bartlett with Howell. Vaca Valley Eanch, 1918. 390 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION Fig. 8. — Ten Bartlett pears set from twenty-five blossoms pollinated with Winter Nelis pollen. A typical cluster resulting from crossing Bartlett with Winter Nelis. Vaca Valley Ranch, 1918.