MAIN LIBRARY-AGRICULTURE AGRICULTURAL AND BIOLOGICAL PUBLICATIONS CHARLES V. PIPER, CONSULTING EDITOR BREEDING CROP PLANTS cMz Qraw-MlBock & 1m PUBLISHERS OF BOOKS FOP^ Coal Age v Electric Railway Journal Electrical World v Engineering News -Record American Machinist ^ Ingenieria Internacional Engineering 8 Mining Journal ^ Po we r Chemical 6 Metallurgical Engineering Electrical Merchandising oiiliiiiiiiiiiiiiiiiiiiHimillllllllniiiiiiiiiiiiiiiiniiiiiiiiiiiiMiiiiHiiniiiiii « 0 11 ll a ® i3 O 03 2 II B s BKEEDING CROP PLANTS BY HERBERT KENDALL HAYES PROFESSOR OF PLANT BREEDING, COLLEGE OF AGRICULTURE, UNIVERSITY OF MINNESOTA AND RALPH JOHN GARBER FORMERLY ASSISTANT PROFESSOR OF PLANT BREEDING, COLLEGE OF AGRICULTURE; UNIVERSITY OF MINNESOTA", NOW ASSOCIATE PROFESSOR AND HEAD OF, »' THE DEPARTMENT OF AGRONOMY, UNIVERSITY OF WEST VIRGINIA^ ' ' FIRST EDITION McGRAW-HILL BOOK COMPANY, INC. NEW YORK: 370 SEVENTH AVENUE LONDON: 6 & 8 BOUVERIE ST., E. C. 4 1921 S3 H3 COPYRIGHT, 1921, BY THE MCGRAW-HILL BOOK COMPANY, INC. -Y-ASRIGOL.TXI THE MAPI.E PRESS YORK: Z3o EDWARD MURRAY EAST 465031 PREFACE Since the early development of agriculture by primitive peoples, selection of seed for planting has been an important feature of agricultural practice. While many of our better varieties or strains of crop plants have originated as chance seedlings or from selections made by men who lacked a knowledge of the laws of heredity, there has been a growing appreciation in recent years of the value of training students for the occupa- tion of plant breeding. Studies in crop genetics carried on since 1900, as well as studies in field plot technic, have helped in a large measure to standardize methods of breeding. Information regarding the mode of inheritance of particular characters as well as a better knowledge of the wild relatives of our crop plants is con- stantly being obtained. The purpose of this book is to present fundamental principles of crop breeding and to summarize known facts regarding the mode of inheritance of many of the important characters of crop plants. Much of the material here presented has been used in courses in crop breeding which have been given in recent years at the College of Agriculture, University of Minnesota. Suggestions from others in relation to methods of treatment of various subjects have been of material value. Particular mention should be made of the helpful advice of Dr. M. J. Dorsey regarding the chapters on " Plant Genetics" and " Fruit Breeding;" of F. A. Krantz regarding the chapter on "Potato Breeding," and of John Bushnell and W. T. Tapley regarding the chapter on "Vegetable Breeding." We are also indebted to Miss Alice McFeely, Bulletin Editor, for many suggestions regarding presentation and for assistance in proofreading; to Mr. A. N. Wilcox for assistance in proof- reading; to Miss L. Mae Centerwall for help in obtaining a con- siderable number of publications from other libraries; and to Miss Alma Schweppe for checking the literature citations. Pre- vious summaries of certain phases of plant breeding methods were IX x PREFACE made available through the kindness of Professor Andrew Boss. The many helpful suggestions made by Dr. C. V. Piper, Consult- ing Editor of these publications have been of great value. Several illustrations have been supplied by investigators who have made intensive studies of particular crops; credit for these has been given in connection with the illustrations. Most of the other figures are from photographs by T. J. Horton, official photographer at University Farm, St. Paul. Figures on flower structure are from drawings made by G. D. George, illustrator. The papers of many investigators have been referred to in the text, as the advanced student will frequently desire to study the original publication. The possibilities of errors are very great in a text which reviews the studies of numerous investigators. The writers, therefore, earnestly invite the criticism of the readers. THE AUTHORS. UNIVERSITY OF MINNESOTA, June, 1921. CONTENTS CHAPTER I INTRODUCTION PAGE The founders of the art of plant breeding 2 The first demonstration of sex in plants 3 Further proof of plant sexuality 4 The studies of Koelreuter 5 Early studies of the cytology of fertilization 5 An answer to the question of hybrid fertilization 5 The great hybridist Gartner 6 Early English plant breeders 6 Other workers of this period 7 The relation of certain biologic principles to plant breeding 8 The doctrine of the constancy of species 9 Darwin's theory of natural selection 9 The stability of the germ plasm 10 DeVries' mutation theory 11 The pure-line theory 11 Mendel's law of heredity 12 Hybridization as a means of producing variations 13 The value of crop improvement in relation to a more efficient agriculture 14 CHAPTER II PLANT GENETICS Methods of studying inheritance of characters 16 The mode of sexual reproduction in flowering plants 17 The inheritance factors 20 Variability of characters 20 A cross in which the parents differ by a single factor 22 Inheritance of two independently inherited characters 23 Several factors necessary for the production of a character 25 Linkage of characters in inheritance 26 Inheritance of quantitative characters 27 Stability of inherited factors 32 CHAPTER III THE MODE OF REPRODUCTION IN RELATION TO BREEDING Natural crossing with self -fertilized plants 34 Wheat, 34; Barley, Oats, Tobacco, Flax, 36; Rice, 37; Cotton, Grain Sorghums, Peas and Beans, 38; Tomatoes, 39. The often cross-pollinated plants 39 Maize, Rye, 39; Alfalfa, 40; Grasses, 41. xi xii CONTENTS PAGE Effects of a cross in normally self -fertilized species 41 Effects of self-fertilization in normally cross-fertilized plants .... 45 Explanation of hybrid vigor 47 CHAPTER IV FIELD PLOT TECHNIC Soil heterogeneity ,. „ . .-".. . 51 Harris' method of estimating soil heterogeneity . ........ 51 Estimating soil heterogeneity by means of checks .....',.. 53 Use of checks in correcting yields. . . . . . . - . -.- ~. V . . • 53 Use of probable error in eliminating strains .- . r -. \. . . 57 The pairing method of securing a probable error . . . \ „ / .- . 57 Replication and its value • ." . 58 Size of plot '•.,'. . ". ..... . .\t , . 60 Shape of plot and border effect .....!*. 61 Competition as a factor in plot variability 62 Climatic variations 64 Summary of field plot technic 65 CHAPTER V CONTROLLING POLLINATION * Selfing plants artificially 67 Technic of crossing ....... 68 Crossing of smal' grains '. , 69 Crossing large-flowered legumes ..../. * . . 71 Depollination with water 73 Summary of technic of crossing 74 CHAPTER VI CLASSIFICATION AND INHERITANCE IN WHEAT Genetic classification .- . . . 75 Wheat species groups TV... 77 Poloni um crossed with other species 79 Some linkage results in wheat crosses '....?-... 79 Spike density ..;....;.... . . 81 Seed characters . . ... . . .' v . .. .*.".' .......... 81 Chaff characters . 84 Presence or absence of beards 85 Inheritance of disease resistance . ..•..-. 85 Inheritance of other characters 87 CHAPTER VII CLASSIFICATION AND INHERITANCE OF SMALL GRAINS OTHER THAN WHEAT Classification and inheritance in oats 89 Crosses betwen Avena fatua and A. sativa 90 Origin of cultivated varieties of A. sterilis 90 Differences in awn development 90 CONTENTS xiii PAGE Color of grain and straw I . ; 92 Hulled versus hull-less 93 Pubescence 94 Characters of base of lower grain 94 Open versus side panicle 95 Resistance to rust 95 Size characters 96 Linkage of characters 97 False wild oats 97 Classification and inheritance in barley 98 Species crosses 99 Simple Mendelian characters 102 Winter versus spring habit 103 Density of the spike 103 The barley awn in relation to yield 104 Some rye studies 106 Wheat-rye hybrids 106 Buckwheat 107 Breeding buckwheat 108 Rice 108 Inheritance of characters 108 CHAPTER VIII METHODS OF BREEDING SMALL GRAINS Method of keeping continuous records Ill New introductions 113 Select'on 114 Summary of methods of selection 115 Crossing 116 Technic of harvesting, thrashing, etc 117 CHAPTER IX SOME RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS Early investigators in selection of self -fertilized cereals 118 Selection within a pure line 120 Selection for the purpose of isolating a pure line 125 Wheat selections 128 Oat selections . . 129 Selections in other self -fertilized crops 131 CHAPTER X SOME RESULTS OF CROSSING AS A MEANS OF IMPROVING SELF-FERTILIZED CROPS The improvement of black oats at Svalof . . . . ... . -. „ . . . . . 132 A wheat cross made at Svalof ^ ...... 134 Wheat breeding at University Farm, Cambridge, England ...... 134 Farrer's wheat breeding in Australia .135 xiv CONTENTS PAGE Marquis wheat .- . . .s. . 136 Winter wheat breeding at the Minnesota Experiment Station . . . .137 Breeding beans resistant to Colletotrichum lindemuthianum 139 An improved strain of tobacco .....'.... 139 Summary 142 CHAPTER XI COWPEAS, SOYBEANS AND VELVET BEANS Cowpeas (Vigna sinensis) . ....... 143 Origin . . 143 Description and inheritance 143 Some results of selection and crossing 145 Soybeans (Soja max) ...'.- 146 Origin 146 Classification and inheritance 146 Breeding 148 Velvet bean (Stizolobium) 149 Origin 149 Important characters and inheritance 149 Mutations 150 Breeding 151 CHAPTER XII FLAX AND TOBACCO Flax 153 Species crosses 153 Inter-relation of factors for flower and seed colors 153 Inheritance of size characters 156 Wilt resistance in flax 157 Methods of breeding 158 Tobacco 159 The genus Nicotiana 159 Parthenogenesis . 160 Sterility • 160 Color characters «-.,......%... v . 161 Quantitative characters . . . . c. . .'.. .- ; . . . 162 Environment as a factor in tobacco breeding 165 Mutations in tobacco ."-.... 167 CHAPTER XIII COTTON AND SORGHUM Cotton . ; -i . . 173 Classification and inheritance . . . " 173 Mutations in cotton . .-..-. . . , . . . . . .«'..„• . . 177 Cotton breeding .;,... 177 CONTENTS xv PAGE Sorghum ......./..; » 178 Origin . * . . .-'..*•• 178 Classification and inheritance 178 Some results of selection 179 Methods of breeding sorghum . 179 CHAPTER XIV MAIZE BREEDING Origin and species 181 Inheritance of characters 183 Endosperm characters 183 Plant characters 187 Colors in plant organs 187 Podded condition 189 Auricle and ligule 189 Chlorophyll inheritance 190 Some seed and ear characters 192 Size characters 192 Chemica composition 193 Corn improvement by the trained plant breeder 196 Relation of ear characters to yield 197 Ear-to-row breeding 198 Home-grown seed 199 Relation between heterozygosis and vigor 200 Immediate effect of crdssing on size of seed 201 FI varietal crosses 202 Isolation of homozygous strains . 205 CHAPTER XV GRASSES, CLOVER AND ALFALFA Grasses 207 Breeding timothy . 210 Clovers 214 Red clover 214 Selection for disease resistant clover 215 Alfalfa 215 Grimm alfalfa and winter hardiness . ..'..' . . .216 CHAPTER XVI POTATO IMPROVEMENT Origin and species . . . , ... v . . 219 Inheritance ...".... 221 Production of new forms . . . i . 223 The difficulties of obtaining crossed seed 224 Improvement through seedling production 226 Clonal selection . . 228 xvi CONTENTS CHAPTER XVII BREEDING OF VEGETABLES PAGE Self -fertilized vegetables 234 Origin of vegetables 234 Peas 236 Some classification characters 236 Inheritance 236 Beans 241 Some classification characters 241 Inheritance 242 Tomato 245 Classification characters and inheritance 245 Peppers . 246 Classification characters and inheritance 246 Methods of breeding self-fertilized vegetables 247 Cross-fertilized vegetables 248 Radish; origin, inheritance and breeding 249 Beets; inheritance and breeding 250 Cultivated vegetables of the genus Brassica 251 Inheritance 251 Breeding . ., 252 Asparagus; rust resistance 253 Economic Cucurbitaceae 254 Introduction and classification 254 Immediate effect of pollination . 256 Cucumber 257 Muskmelon. . 257 Squashes and gourds 258 Watermelon 258 Breeding Cucurbitaceae 259 CHAPTER XVIII FRUIT BREEDING Origin and antiquity of some fruits .261 Some early studies of fruit improvement 264 Von Mons 264 Knight -...../. ; 264 American pomology 265 Some considerations of fruit breeding . . . . . 265 Overcoming soil heterogeneity 266 Self-sterility and heterozygosity 267 Inheritance of some characters 271 Apple.. ....,•'....'.- .">• * ;."-. 271 Raspberry ....'.., 272 Grape 272 Illustration of methods of breeding 273 Selection of bud sports 273 Controlled crosses ' 277 CONTENTS xvii CHAPTER XIX FARMERS' METHODS OF PRODUCING PURE SEEDS PAGE Determination of better varieties . ' 281 What is good seed?. ........ \ ..-..; 282 Adaptability. . . . v. 282 Yielding ability and quality 282 Purity 283 Hardiness 283 Strength of stalk 283 Disease escaping or resistance 283 Methods of seed production 284 Seed growers' methods for self-fertilized crops 284 Improved corn seed. ^ 287 Method of breeding corn for special breeders . ^. 288 Method of corn bleeding for average farmer ^ . . . 290 Potato seed (tubers) selection 290 Improvement by selection of such crops as alfalfa, clover and grasses 292 Seed registry or certification 293 DEFINITIONS 294 LITERATURE CITATIONS 299 INDEX. . 319 BREEDING CROP PLANTS CHAPTER I INTRODUCTION The origin and mode of development of nearly all of our principal cultivated crops is an obscure and much debated sub- ject. This is partly due to the fact that many crops have been grown for hundreds of years and often the same forms are culti- vated as were grown in early periods. It is very probable, for example, that the men of the old stone age, 50,000 years ago, had some sort of art of agriculture (Dettweiler, 1914). These con- clusions have been drawn from old engravings of this period which were made on cavern walls. Wheat and barley were cer- tainly grown in early times. A carving of the upper Paleolithic age in the Pyrenees mountains shows winter barley such as is now cultivated in that locality. Dettweiler writes very interestingly of the agriculture of the Lake Dwellers who lived during the period from 4,000 to 2,000 years B.C. He states that the Lake Dwellers of Switzerland cultivated the short-eared, six-rowed barley, Hordeum sanctum of the ancients; the dense-eared, six-rowed variety, H. hexastichon L., variety densum; two-rowed barley, H. distichon; small lake- dwelling wheat, Triticum vulgare antiquorum; true Binkel wheat, T. vulgare compactum; Egyptian or English wheat, T. turgidum, L.; an awnless thick-eared emmer, T. dicoccum, Schrank; one- grained wheat, T. monococcum, L.; meadow (common) millet, Panicum miliaceum, L.; club millet, P. italicum, L.; and a type of flax, Linum angustifolium, which still grows wild in Greece. An excavation was made in the village of Gleichberg, near Romhild in 1906. On an old fireplace, with remains of the oldest Bronze age, were found the following seeds: einkorn, spelt, binkel, and small lake-dwelling wheat, small lake-dwelling barley, vetch, peas, poppy, and possibly apple seeds. It is not the purpose to give the historical development of crops except to show that many were cultivated in very ancient 1 2 BREEDING CROP PLANTS times by primitive peoples who developed many varieties. As some of the varieties which were then grown are in existence today and are cultivated in some regions, a little idea of earlier work is obtained. Coming now more nearly to present times we may briefly consider the work of the Indians with maize. Piper speaks of the plan by which seeds of different colors were planted together in one hill with the thought that this method gave increased yields. It tended to keep the varieties in a heterozy- gous condition. During the last three or four years Squaw Flint from the Indian reservations in Minnesota has averaged as large a yield per acre at University Farm, St. Paul, as the more care- fully selected varieties. These facts should help to give the student of plant-breeding some idea of the great accomplishments in plant production in earlier times and to correct possible exaggeration of relative values of the results of recent work. Present-day breeding has achieved great results and will accomplish much more; the foundation, however, was laid many years ago. THE FOUNDERS OF THE ART OF PLANT BREEDING The relation between the science and the art of plant breeding is a very interesting subject. Through many years of trials, methods are improved; and a correct knowledge of the funda- mentals of the science often does not widely modify the actual practice involved. As a rule, scientific principles allow some short cuts in breeding methods and help to eliminate erroneous and useless practices. As will be constantly emphasized in this work, there is a close relation between the mode of reproduction and the methods of breeding a plant. A knowledge of sexuality was, therefore, almost a necessity before it was possible to develop the art of breeding. Sexual processes, while not thoroughly understood, were observed in animals three or four centuries B.C. by the Egyptians and Assyrians. Existence of fruit-bearing and sterile trees of the date palm was known to the people of Egypt and Mesopotamia in early times and records of artificial pollination as early as 700 years B.C. have been found (see Fig. 1). The Assyrians commonly referred to the date trees as male and female. The Greeks, however, to whom we look for early INTRODUCTION 3 scientific thought, failed to interpret this phenomenon. Theo- phrastus, for example, concludes that as other plants do not as a rule exhibit the same phenomenon, the date tree is not an example of real sexuality (Johnson, 1915). Little was actually known of plant sexual processes until comparatively recent times. The English physician Grew (1676) further developed the suggestion of Sir Thomas Millington that the stamens served as the male organs, by a hypothesis regarding the process of fertilization. The only means of demonstrating this phenomenon was by the experimental method. FIG. 1. — The date palm among the Assyrians. "Design from the palace of Sargon at Khorsabad (eighth century B.C.) showing that the male and female flowers of the date palm were clearly distinguished at that time. The worshiper in the middle is carrying a sprig of male or staminate flowers while the one at the right bears female or pistillate blossoms. The drawings should be compared with the photographs of actual flowers. The winged deity at the left, who is usually identified as the Palm God, holds in his hand a cone which is thought to typify the spathe of the male palm, and thus the principle of fertility in general." (After Johnson, 1915.) The First Demonstration of Sex in Plants. — Camerarius first made the experimental test by using isolated female plants of the mulberry, by emasculating the castor bean and by removing the stigmas from Indian corn. The results of these experiments were reported in a letter to Professor Valentin, of Giessen, written in 1694. The following statement, made by Camerarius and found in Ostwald's Klassiker, page 25, has been frequently quoted (John- son 1915.) 4 BREEDING CROP PLANTS "In the vegetable kingdom there is accomplished no reproduction by seeds, that most perfect gift of nature, and the usual means of perpetuating the species, unless the previously appearing apices of the flower have already prepared the plant therefor. It appears reasonable to attribute to these anthers a nobler name and the office of male sexual organs." Further Proof of Plant Sexuality. — The work of Camerarius was confirmed by several men. Thomas Fairchild, in 1719, produced a new variety of pinks by an artificial crossing; of two FIG. 2. — Male and female flowers of date palm about two times natural size. (Photograph taken by Swingle in Sahara Desert, 1X99.) varieties; and Bradley, two years earlier, found emasculated tulips set no seed. Miller, 1731, noted insects pollinating emas- culated tulips after first visiting untreated tulip flowers. Gover- nor Logan of Pennsylvania, in 1739, experimented with maize and observed that detasseled plants set no seed when isolated from untreated plants. He also removed the silks and found such treated plants were incapable of setting seed. Gleditsch INTRODUCTION 5 (1750) had a pistillate palm in Berlin which was 80 years old and had set no seed. He obtained a quantity of pollen from trees in Leipsic (then nine days' journey from Berlin) and after pollination seed was produced which germinated. The Studies of Koelreuter.1 — While these investigators and others confirmed the work of Camerarius, little advance was made in the art of breeding until Koelreuter (1761) made a careful study of artificial crosses and gave the first extended account. In tobacco crosses, for example, he found that the first generation was of intermediate habit and therefore showed the effect of the male parent. His work on the vigor of first generation crosses is of much interest. He believed the "oil" of the pollen grain after mixing with the stigmatic fluid penetrated the ovule. The belief of a union of male and female substances was a step in the right direction. The value of insects as carriers of pollen was also demonstrated. Early Studies in the Cytology of Fertilization. — Pollen tubes were first observed in 1823 by Amici who followed them to the micropyle of the ovule in 1830. Schleiden shortly afterward made numerous studies of the pollen tube and apparently thought the embryo developed in the embryo sac from the end of the pollen tube. This matter was not thoroughly cleared up until Strasburger (see Johnson, 1915) concluded, in 1884, that: "1. The fertilization process depends upon the copulation with the egg nucleus of the male nucleus which is brought into the egg. 2. The cytoplasm is not concerned in the process. 3. The sperm nucleus, like the egg nucleus, is a true cell nucleus. " An Answer to the Question of Hybrid Fertilization. — Al- though Koelreuter proved the fact of sexuality in plants it was not generally accepted, and early in the nineteenth century the Physical Section of the Royal Prussian Academy offered a prize for an answer to the question, "Does hybrid fertilization occur in the plant kingdom?" Among other results presented by Weigmann in answer to this question occurs the statement of the immediate effect of pollen in legumes. Weigmann made a study of 36 crosses using the following plants: onion, cabbage, pea, bean, lentil, pink, and tobacco. He observed the fact of variability due to crossing and thought gardeners should pay 1 For these facts the papers of other writers have been freely used. Those by ROBERTS (1919) have been especially helpful. 6 BREEDING CROP PLANTS more attention to the planting of their crops so that those of like kind did not grow so near each other that crossing through the aid of insects would take place. Sprengel, in & book published in 1793, showed the important role played by insects in pollination and studied the adaptations for crossing found in many flowers. He concluded that nature intended flowers should not be polli- nated by their own pollen. The Great Hybridist Gartner. — In extent and number of his experiments Gartner's work is very great. In 1835 he heard of the offer of a prize made by the Dutch Academy of Sciences at Haarlem regarding the place of hybridization in producing new varieties of economic and ornamental plants. Gartner's paper on this question, which received the prize, was published in extended form in 1849. He made thousands of crosses, involving nearly 700 species, and obtained about 250 hybrids. The work was so carefully controlled and checked that the fact of sex in plants was thoroughly proved. He made a classification of hybrids according to whether they resembled one or the other parent in all respects, whether they resembled one parent in one part of the plant and the other parent in some other characters, or whether there was an almost equal balance. In the last case in later generations, the inclina- tion toward the one or the other parent was supposed to be due to a slight overbalance of one or the other of the fertilizing materials. Gartner explains the appearance of the first hybrid generation as due to an inner force operating according to law. He, like Koelreuter and Weigmann, observed increased vigor in hybrids. He made experiments to determine the immediate effect of pollen with crosses between colorless and colored pericarp varieties of maize and in crosses between a brown-seeded Lychnis and one with a gray seed. As no change occurred, a law was developed to the effect that pollen does not immediately affect forms and external characters of seeds but influences the develop- ment of the resultant plant. He observed an immediate effect in some pea crosses and learned that the yellow cotyledon color dominated the green in the hybrid seeds. Early English Plant Breeders. — Knight, Goss, and Herbert, three English workers, did much to develop the art of breeding. Knight, who was a practical horticulturist, recognized the aid of artificial cross-pollination in producing new kinds. He INTRODUCTION 7 studied the question of the immediate effect of pollen. A variety of pea with a white seed-coat was fertilized with pollen of a gray-seeded variety. No immediate influence of pollen was obtained. However, when the resultant plant was pollinated by a white variety both gray- and white-seeded sorts were ob- tained in the next generation. William Herbert was a contempo- rary of Knight who learned of the work of Koelreuter some time after he had started his experiments. He opposed the idea that species crosses were necessarily sterile. Studies made by John Goss are considered of much interest as they showed results similar to those obtained later by Mendel. In 1820 flowers of Blue Prussian pea, which has bluish seeds, were pollinated with pollen of Dwarf Spanish. Three seeds were obtained which were yellowish- white like the male parent. Plants grown from the seeds produced some pods with all blue, some with all white, and some with both blue and white seeds in the same pods. When planted, the blue seeds bred true while the white seeds gave some segregates. No law, however, was developed. Other Workers of this Period. — At about this same period Sargeret, in France, was making studies with C ucurbitacece crosses. He observed the fact of dominance as the following crosses show. MUSKMELON (FEMALE) CANTALOUPE (MALE) FIRST GENERATION 1. Flesh, white Flesh, yellow Flesh, yellow 2. Seeds, white Seeds, yellow Seeds, white 3. Skin, smooth Skin, netted Skin, netted 4. Ribs, slightly evident Ribs, strongly Ribs, rather pronounced pronounced 5. Flavor, sugary and Flavor, sweet Flavor, acid very acid at same time He notes (Roberts, 1919) that: "The characters were not blended or intermediate at all, but were clearly and distinctly those of the one or the other parent." Naudin made quite careful studies and attempted to summar- ize, his results. He so nearly approached the law later laid down by Mendel that some- workers have spoken of it as the Naudin-Mendel law. He thought that if hybrids were self- fertilized they would return more or less rapidly to the parental types. Similar results were obtained if the hybrid was pollinated 8 BREEDING CROP PLANTS by one of its parents. He noted the uniformity of the first gen- eration and the production of many types in the second genera- tion some of which could not be told from the original parents. The results were explained by the segregation of specific sub- stances in the pollen and ovaries of the hybrid (Naudin, 1865). Wichura (1865) found reciprocal crosses gave like results and therefore concluded that the pollen and the egg have exactly the same share in the organism which results from fertilization. He studied species crosses in willows but did not deal with the individual characters of the species. Mendel's work, published in 1866, is now well known to all students of genetics and plant breeding. This early paper remained unnoticed until the rediscovery of the law in 1900 by each of three investigators, DeVries, Correns, and Tschermak. With the great advances made since that time rules can now be given which furnish a reliable guide for plant breeding operations. To quote from Pearl: "In the creation of new races by hybridization the plant breeder can and does take Mendelian principles as a direct and immediate guide. He has made Mendelism a working tool of his craft." THE RELATION OF CERTAIN BIOLOGIC PRINCIPLES TO PLANT BREEDING1 The art of plant breeding is closely related to those biologic principles which furnish the foundation for the science of breed- ing. For this reason there is a very close relation between the development of theories of evolution and scientific methods of breeding. The conception of evolution dates from the time of the Greek philosophers in the eighth century. This was the speculative period and evolutionary beliefs were not attained as a result of experimentation. Until the sciences of botany and zoology were built up it was impossible to do more than outline theories which appealed to the judgment of the writer. The modern inductive period is of comparatively recent times. Erasmus Darwin developed a theory of evolution which he did not think entirely adequate. Lamarck gave us the first well-rounded theory of evolution. It was based on the inherit- ance of acquired characters. By continued use an organ was 1 A bulletin by EAST (1907) and a book by SCOTT (1917) have helped materially and have been freely used. INTRODUCTION 9 strengthened and developed. Likewise, without use it was weak- ened. The supposed inheritance of these acquired characters was the basis of the production of the numerous species. The term species was first applied to animals and plants by John Ray (1628-1705) who used it to refer to a group of organ- isms with similar characteristics and which freely intercrossed. Many of the experiments of this period dealt with the question of species. The Doctrine of the Constancy of Species. — Linnaeus (1707- 1778) adopted a more strict definition although he was not always consistent in his use of the word. The doctrine adopted was that of the separate creation of fixed entities which were called species. Lamarck denied this theory and outlined his evolutionary hypothesis. Most naturalists of this period believed in the immutability of species. It is thought that the work of Lyell (1797-1875), an eminent geologist, had a marked effect on that of Charles Darwin, who was his intimate friend. Lyell insisted upon the continuity of the earth's history and the uniformity of agencies which wrought such profound changes upon the earth. This theory was in opposition to that of Cuvier, who believed that the earth's history was a series of times of destruction followed by periods of tran- quillity ("catastrophism"). After each such destructive period it was believed that new .creation took place. Darwin's Theory of Natural Selection. — The most influential worker in the history of development of the evolutionary con- ception was Charles Darwin. He and Alfred Russel Wallace independently developed a theory for the origin of species and united in presenting a preliminary paper in 1858. The publication of Darwin's " Origin of Species" in 1859 gradually brought about a belief in evolution. The work of Lyell had helped materially to develop a belief in the orderly progress of the world and assisted in preparing the way for the masterly presentation of Darwin. Darwin presented such a mass of evidence from widely different fields .that the entire thinking world was compelled to accept evolution as a fact. The evidence was grouped under such headings as organic relationship, com- parative anatomy, embryology, paleontology, and domestication. The fact of evolution is indisputable. The explanation is even yet not entirely satisfactory. Darwin's theory is founded upon a series of facts as follows: 10 BREEDING CROP PLANTS 1. Variability. — It is a matter of common observation that no two individuals are exactly alike. If sufficient individuals are examined the range of variation is found to be quite great. These variations are universally present. 2. A Struggle for Existence. — If all the progeny of some of the lower forms grew to maturity and each in turn produced as many progeny, the world would soon be overrun with a single form. There is competition also between different species and genera. 3. Natural Selection. — The conclusion would certainly seem reasonable that those forms would survive which possessed characters better adapted to a given environment and there- fore gave those particular forms advantage in the struggle for existence. 4. Heredity. — Variation produces the material for natural selection to work upon and heredity tends to perpetuate the variations. The mechanism of transmission of characters, the physio- logical cause of variations, and the inheritance of different- categories of variations were unanswered problems. Many criticisms were made of Darwin's work and many were considered by Darwin himself. Nearly all of these have a bearing upon plant breeding. In the improvement of crops, artificial selection takes the place of natural selection. The breeder is constantly faced with the question of the perpetuation of a variation. He also faces the question of whether the useful variation will per- petuate itself in crosses or will be lost. Darwin recognized two sorts of variations, the " fortuitous" or chance variations, i.e., those which are everywhere present and which cause every plant to be slightly different from other plants of the same species. These were considered to be of primary importance in evolution. While he recognized "definite" or discontinuous variations, the so-called mutations, these were not considered of primary importance. The Stability of the Germ Plasm. — Weissmann's theories are of much interest. He developed the idea of the continuity of the germ plasm and that external agencies could not modify 'inheri- tance without first affecting the germ cells. Plant breeders are not particularly interested in Weissmann's ingenious theories which were outlined to show that the inheritance of acquired characters was an impossibility. Apparently, in order that a new INTRODUCTION 11 character may be produced there must be a modification of the germ plasm. The real question, then, is what causes germinal change? In considering this question we must keep in mind the possibility that agencies which are of little importance from the standpoint of the plant breeder may be of profound importance in evolution. DeVries' Mutation Theory. — The more recent theory of evo- lution developed by DeVries attacks the question of the sort of variations which furnish the basis for evolution. DeVries gives only slight value to the small continouus variations and advances the hypothesis that large variations are of primary value. He believes in periods of mutation when from some un- known cause a species is producing many new forms, and other periods when stability of the species is the rule. DeVries recog- nized three sorts of mutations; (1) progressive, when an entirely new character appears ; (2) degressive, the appearance of a par- tially latent or hidden character; and (3) retrogressive, when an active character becomes latent. The cause, or causes, of these sudden changes was not known. Mutations are frequently not large but small. All sudden heritable changes which cannot be explained by the laws of segregation and recombination are called mutations. The Pure -line Theory. — The studies of Johannsen are of par- ticular value from the standpoint of the plant breeder. He worked with self -fertilized crops and found that while the progeny of a single self-fertilized plant varied quite widely, these varia- tions were not inherited. From single commercial varieties he found it possible to isolate numerous lines which in their means differed slightly from each other and which bred true. Johannsen considered a pure line to be the progeny of one or more self- fertilizations from a single homozygous ancestor. Selection within such a pure line was of no practical value. Numerous investigations with self -fertilized crops have been made and corrob- orate the results of Johannsen. Isolated cases of mutations in these pure lines have been reported, and while these are of much scientific interest they occur far too infrequently to be used as a basis for a system of breeding. Johannsen '& pure-line theory has been extended to cover clonal or asexual propagation in both plants and animals. At its proper place evidence will be given to show that in heterozygous organisms which are asexually propagated there sometimes occur 12 BREEDING CROP PLANTS bud sports or somatic mutations each of which may form the basis for a new race. Such bud sports in some plants apparently occur frequently enough to be of economic importance. Mendel's Law of Heredity. — Mendel's experiments, pub- lished in 1866, remained unnoticed until the facts were redis- covered in 1900 by De Vries, by Correns, and by Tschermak. This law furnished the starting point from which the modern study of genetics has developed. Many students will have taken a course in genetics before studying plant breeding. For such students it is sufficient here briefly to review Mendel 's law in its application to crop improvement. Mendel's law can best be understood in relation to cytology. It is well known that the chromosomes are the bearers of the herit- able factors. The number of chromosomes for each species is constant and the form and individuality is characteristic. Each chromosome is supposed to be composed of chromomeres and each chromomere may be the seat of a particular inherit- able factor. According to Morgan 's hypothesis, the factors are located in particular regions of the chromosome. The chromo- somes are considered to be in pairs and the two parts of each pair are in such a relation to each other that at reduction division, i.e., at the formation of gametes, the parts of each pair separate and the gamete contains only half as much chromatin as the somatic cell. The gamete then contains one member of each chromosome pair. Exceptions sometimes occur to the above rule when unusual cytologic divisions take place. A rather recent development of genetics is of primary impor- tance. At some time in preparation for reduction division there is a doubling of the spireme. Morgan supposes that at this time homologous parts of chromosome pairs lie next to each other. These spireme threads wind about each other and in some cases breaks occur. It is then supposed that the chromosomes may reunite in such a manner that a new chromosome is formed which contains parts of each of the homologous chromosomes that make up a pair. If factors are in particular loci this would allow for a different combination of factors in a chromosome containing parts of each chromosome pair. Most of the previous investigations show that many factors are inherited independently. This allows for numerous combina- tions when crosses are made. If there is a break, i.e., a cross- over or some other means by which factors which are usually INTRODUCTION 13 correlated may be recombined, a greater degree of segregation is possible than when factor correlation is absolute. In general we may say that the number of groups of correlated or partially linked factors is not greater than the number of chromosome pairs. Whether the above explanation is correct, partly so, or entirely wrong, it is a convenient theory with which to account for a large body of facts. It allows for classification of facts in such a way that correct breeding methods may be used. Mendel's law may then be summarized from the standpoint of the plant breeder as follows : 1. Plants breed true for certain characters when all factors necessary for the development of the character are in a homozy- gous condition. There is a relative stability of factors. Changes in factors or " mutations" are far too infrequent to furnish a basis for a system of breeding. 2. There is independent segregation of certain factors. 3. Partial coupling of certain determiners sometimes is found. The degree of linkage in transmission is quite constant. 4. Perfect coupling of certain factors occurs, i.e., constant association of characters in inheritance. As a possible exception to the usual behavior we may mention apparent segregation in the somatic cells of some hybrids. In some forms these changes apparently occur frequently enough to be of practical selective value. We may summarize Mendel's law in another way by saying that the first generation cross between stable forms may resemble one parent in one character, the other parent in another character or may be intermediate in the character in question. All mem- bers1 of FI are of uniform habit. Segregation occurs in F2 and " segregation of potential characters in the germ cells of hybrids arid their chance recombination" (East and Hayes, 1911) may be considered as a general law. In Fs and later generations some forms breed true, others segregate. Homozygous forms may be obtained which contain the de- sirable characters of both parents. Such forms are as stable as races which ha\e been bred by straight selection. Hybridization as a Means of Producing Variations. — A quite recent explanation for the cause of germinal variation and therefore the main cause of evolution is that of Lotsy (1916), 1 The meaning of FI, Ft, etc., and other genetic terms not explained in the text is given in the glossary. 14 BREEDING CROP PLANTS who gives to hybridization the major role in the production of variations. Some serious criticisms have been made of this hypothesis as an explanation of evolution. With the higher plants, however, natural crossing has doubtless played an im- portant evolutionary role. From the standpoint of the plant breeder crossing is of much importance and it is the only generally known means of producing variations of selection value that is available as a practical method. In cross-fertilized species crosses naturally occur followed by segregation, and recombination follows. Selection isolates desirable genotypes. THE VALUE OF CROP IMPROVEMENT IN RELATION TO A MORE EFFICIENT AGRICULTURE Maximum yields of crops can be obtained only when all factors relating to the various phases of crop production are favorable. The physical and chemical characteristics of the soil, correct time and rates of planting, and crop rotation must be considered. Recent studies have shown that there are marked differences in the effect of different crops upon those that follow them in the rotation. Of utmost importance is the necessity that the crop be adapted to the soil and climatic conditions in which it is to be grown and that profitable returns be obtained on the basis of the cost of production. After careful consideration of those factors which go to make up the home of the plant we turn our attention to the seed. The fact that there are remarkable differences in final yields from different varieties of the same crop is commonly known. We are as yet only on the threshold of the possibilities of crop improve- ment. Careful methods of seed inspection, registration, and treatment to control diseases are necessary to the greatest return from crop breeding. Education of the farmer will do much to overcome the evils of exploitation by the unscrupulous seed dealer or promoter who is anxious only to sell and make a profit on his seed. The business of growing carefully bred seeds is one that needs an appreciation of these and other factors in seed production. No great amount of special training is needed to carry on this work. To the careful worker who is willing to build up a reputa- tion by actual merit of his seeds, the business of seed production will prove a lucrative one. INTRODUCTION 15 The production of improved forms by breeding is a line of work which demands special training. This can be obtained only from a study of the underlying principles of genetics. Nearly all of our land grant colleges and experiment stations, as well as some private seed firms, are carrying on studies in plant breeding. Although these studies are yet in their infancy, results of much value are being obtained. By means of accurate field experi- ments carried on at research stations and with farmer codperators, the experiment stations and the federal Department of Agricul- ture are enabled to give accurate information regarding the better varieties to grow. In the past these studies have not always been made with a correct appreciation of the necessary technic. It is the purpose of this book to outline methods of breeding in relation to the underlying principles involved, and to present what are coming to be recognized as proper field methods of carrying on these studies. Because the subject is a compara- tively recent one, new methods of work are being constantly found. It is therefore necessary to present different viewpoints in order that the prospective breeder may learn why certain prac- tices are giving the better results. CHAPTER II PLANT GENETICS1 Since the rediscovery of Mendel's law in 1900 there has been an intensive study of the laws of inheritance through experimental breeding and other means. This has resulted in the develop- ment of a new biological science which is called Genetics. A knowledge of the principles of this science is a necessity if the student of crop breeding is to pursue his work in the most logical manner. The writers, therefore, believe that a study of genetics should precede plant breeding. There are, however, many people interested in crop improvement who have not had an opportunity to pursue an intensive study of genetics. For this reason it seems advisable to present genetic principles in some detail. Methods of Studying Inheritance of Characters. — The charac- ters of a plant are those qualities which serve to identify it. They are the means whereby one variety is differentiated from another. The production of a variety with only desirable characters is the main aim of the breeder. It is a commonly accepted fact among geneticists that Mendel's law may be used to explain the inheritance of nearly all plant and animal characters. The character is considered to be the end result of the interaction of certain inherited factors which are located in the germ cells; these factors under favorable environmental conditions cause the production of the character. Thus environ- ment and heredity both play important roles in character development. The laws of inheritance have been developed mainly by controlled crosses between parents of known in- heritance. By correlating the facts of character transmission from parent to offspring with known facts of cytology, an idea of the mechanism of heredity has been obtained. Before presenting a description of the factorial scheme which has been 1 In preparing this chapter other works on genetics have been freely used. BABCOCK and CLAUSEN (1918) and EAST and JONES (1919) have been par- ticularly helpful. 16 PLANT GENETICS 17 developed to explain Mendelian heredity, it will be necessary to give some of the main facts of reproduction in plants. The Mode of Sexual Reproduction in Flowering Plants. — Nearly all higher plants produce seeds as the result of the union of sexual cells or gametes. Each body cell which is capable of further division contains a nucleus in which the chromatin is located. This chromatin, which is composed of a definite number of parts or chromosomes, gains its name from the fact that it takes a dark stain with certain reagents when other parts of the Male Archesporial or megaspore mother cell eduction division Period Nucleus with of chromosomes divisio Embryo sac Archesporial, cell Synergids Egg cell or female gamete/ Embryo sac Polar or endosperm nuclei Antipodal cells of nutritional value 2-celled stage or diad 4 -celled stage or tetrad Pollen grain nucleus Generative nucleus FIG. 3. — Diagram to illustrate production of male and female gametes. A, In some forms as Lilium this megaspore mother cell functions directly as the embryo sac". Reduction division in Lilium occurs at the first nuclear division within the embryo sac. cell are unstained. In the soma or body of the plant the nucleus of each cell contains a definite number of chromosomes, half of which were obtained from the male sexual cell and half from the egg cell. Each new body cell results from the longitudinal division of the chromosomes of a preceding body cell. Thus all of the somatic cells of a plant contain the same number of chromosomes. Preparatory to the formation of the germ cells or gametes, the chromosomes assume a paired condition, one member of each 18 BREEDING CROP PLANTS pair being obtained from the male parent and the other from the female parent. At the formation of the sexual cells, or at reduc- tion division, one member of each pair of chromosomes passes to each daughter cell thus reducing the chromosome number to half that in the body cells. Following this reduction division there is an equating division whereby each chromosome is qualitatively FIG. 4. — Anther and pollen of the lily. A, Mature anther, showing the four locules, or chambers, containing pollen grains: the anther opens lengthwise on both sides along the lines of cells shown at a;; B, stages in the formation of pollen grains in a group of four (tetrad) within the pollen mother cell; C, mature pollen grain with early stages in the development of the male gametophyte; t, tube nucleus; g, generative nucleus. (After Bergen and Davis.) equally divided. This results in the formation of the male or female sexual cells or gametes as they are called (see Fig. 3). The male sexual cells are produced in the anthers and are carried in the pollen grains. A mature pollen grain contains two nuclei, a tube nucleus and a generative nucleus (see Fig. 4). After the pollen grain falls on the pistil the tube cell PLANT GENETICS 19 elongates, forming a pollen tube which passes down the style. This tube grows through the tissue of the pistil and reaches the embryo sac. The generative nucleus passes into the pollen tube and divides, forming two nuclei which are the male gametes. The pollen tube grows through the tissues of the pistil until it reaches the embryo sac, and the tip of the tube breaks after it penetrates the wall of the embryo sac. In fertilization one of FIG. 5. — Longitudinal section of the lower portion of the embryo sac of maize at the time of fertilization; pn, polar nuclei fusing; sn, sperm nucleus fusing with a polar nucleus, pn; e, egg; sn, sperm nucleus in the egg; pt, pollen tube; syn, synergid; v, vacuole. (After Miller.) these gametes of the pollen tube unites with the egg cell to form the embryo of the seed and the other unites with two polar nuclei to form the endosperm (see Fig. 5). If we represent the chromosome number of each body cell by 2x, each gamete would be represented by x, the embryo formed by the union of the generative cell with the egg cell would be 2x and the endosperm tissue 3x. 20 BREEDING CROP PLANTS The Inheritance Factors. — The inherited character is con- sidered to be the result of certain definite factors which are located in the chromosome. Moreover, a factor is considered to be located at a certain definite place in the chromosome. After the rediscovery of Mendel's law in 1900, numerous crosses were studied. In many cases the inheritance of each differential character in which the parents differed was easily explained by the hypothesis that one parent contained a genetic factor for the development of the character and that the other parent lacked this factor. This led to the erroneous conception that many characters were dependent on a single factor for their develop- ment. That this is not so may be easily seen if one considers that each character is a part of the physiological complex which goes to make up an organism. Thus, many genetic factors play a part in the development of the character. When a cross shows that two parents differ by only a single factor this does not mean that only a single factor is necessary for the development of a character. It does mean, however, that a single factor of in- heritance may cause a profound change in the expression of the character of an organism. Some crosses show that many factors play a part in the development of a single character. The present view is that a character is usually the result of the interaction of several factors. When a plant breeds true for a particular character each gamete produced contains all factors necessary for the development of the character. Before considering the results of certain crosses it. will be desirable to review briefly the subject of variation. Variability of Characters. — It is commonly recognized that no two plants or animals are exactly alike. These differences are called variations. Various means of classifying variations have been used. From the standpoint of the plant breeder variations are of two kinds: (1) non-heritable, (2) heritable. Non-heritable variations are those which are solely due to some difference or differences in the environmental conditions under which the plants develop, while heritable variations are due to some difference or differences in the hereditary characters of the organisms. Several illustrations may help to make clear what is meant by non-heritable variations. Baur (1914) cites races of Primula sinensis which under normal conditions breed constanlly true for red and white flowers respectively. If the red race is placed PLANT GENETICS 21 in partial shade in the greenhouse under temperatures of 30° to 35°C. only white flowers are produced. If those same plants are brought into another greenhouse with temperatures of 15° to 20°C. the flowers which then develop are the normal red color. It is pointed out that what this red primula inherits is not a red flower color but the ability to produce a certain flower color under certain conditions of the environment. Non-inherited variations have no value as a means of producing new varieties or strains. Such variations are, however, of importance to the breeder. For example, a small shriveled seed of wheat has the same inherited characters as a large, plump seed of the same pure line, nevertheless, the seedling produced by the shriveled seed may get an unfavorable start. Familiar examples of non- heritable variations are differences in height of plants, within a variety, which are dependent on differences in food supply, moisture, or sunlight. Inherited variations may be placed in two classes: (1) muta- tions, (2) new combinations. Mutations are due to a sudden change in the hereditary factors of an organism, or to the loss of a genetic factor. In some cases mutations result from abnormal chromosome behavior during the process of cell division. Before we can discuss profitably the reason why mutations occur it will be necessary to know much more about the nature of hereditary factors than we now do. Mutations are sometimes of much value to the breeder. Examples of mutations of economic importance will be found under a discussion of the breeding of various crops. When a desirable mutation occurs it can be utilized as a means of producing a new race. As there is no known means of artificially inducing mutations, the breeder can not depend on them as a means of producing improved varieties. New combinations result from crossing varieties which contain different hereditary factors. The first generation of a cross between homozygous parents which differ in a certain character may resemble either the one or the other parent or may be inter- mediated, but all FI plants will be of like habit. F2 plants, how- ever, are of different kinds, due to the segregation of hereditary factors in the germ cells of the FI plants. New combinations of factors may occur and thus new individuals may be produced which have some of the characters of the one parent combined with some characters of the other. In some cases characters 22 BREEDING CROP PLANTS which are not present in either parent, appear. These may result from the interaction of two or more factors all of which are necessary for the production of the character and part of which were contained in one parent and part in the other parent. These facts may be illustrated by the results of certain crosses. A Cross in Which the Parents Differ by a Single Factor. — Sweet corn when mature bears wrinkled seed while flint corn produces smooth seeds filled with starch grains. If sweet corn FIG. 6. — Inheritance of starchy and sweet endosperm in maize. A, Ear of sweet corn with wrinkled seeds; C, ear of flint corn with starchy seeds; B, immediate result of pollinating ear of starchy parent with pollen from sweet parent; D, produced by self-fertilizing an ear of an Fi plant of cross between sweet and starchy parent. Note the segregation into sweet and starchy seeds; E. An ear produced by planting wrinkled seeds of D; F, G, H, ears produced by planting starchy seeds of D. Note that one out of every three ears is pure for the starchy character. (After Babcock and Clausen.) is pollinated with pollen from a flint variety the resultant seed is starchy. There is an immediate effect due to double fertili- zation in which the endosperm results from the union of the polar nuclei with one of the gametes of the pollen grain. If the crossed seeds are planted and the resultant plants self-fertilized, the ears produced will contain starchy and sweet seeds in a 3 : 1 ratio. The facts may be presented by the use of the factor hypothesis. One of the chromosome pairs contains the factors for either the starchy or the sweet condition. Let S represent the sweet factor, F the starchy factor. In the following diagram only one of the chromosome pairs, which contains the starchy and sweet factors, will be shown. PLANT GENETICS 23 P| I F | Somatic cell of flint parent s | | S I Somatic cell of sweet parent Gametes of parents Ofstaicby appearance Will breed true seed of starch) habit . The factor foi starch development, F, produces a complele dominance over the sweet factor, S Male reproductive cells Female reproductive cells li Is! \\ F seeds wrinkled seeds Will segregate DIAGRAM 1 Will breed true Inheritance of Two Independently Inherited Characters. — Crosses between varieties which differ in two independently inherited characters may next be illustrated. The parental forms in the case of each differential character will be considered to differ in only a single inherited factor. PARENTS CHARACTERS GAMETES White Fife wheat Awnless spike, white seed AW Preston Bearded spike, red seed BR 24 BREEDING CROP PLANTS There is a dominance in FI of the red seed color (brownish red pigment in one of the bran layers) over the white and a partial dominance of the awnless over the bearded condition. The FI plants will therefore, have red seeds and a slight extension of the awns near the top of the spike. The inherited factors may be considered to be R for red seed, W for white seed, B for bearded, and A for awnless. W and R are considered to be located in homologous loci of one pair of chromosomes and B and A in homologous loci of another pair of chromosomes. The FI plants may then be considered as ABWR. The gametes of these FI plants may contain either A or B in combination with either W or R. The different combinations are supposed to occur in equal frequency. Wheat (Triticum vulgare) has eight pairs of chromosomes. The factors for bearded or awnless spike and for color of seed are independently inherited. Therefore they may be considered to be located in separate chromosome pairs. In the diagram only two chromosome pairs are shown. SOMT/C Ceu PAKEHTAL GAMETES F, ZYOOTC Ft GAMETES PAREHT }. dwn/ess spiff? White seed Bearded spike Red seed DIAGRAM 2 The F2 plants obtained by the self-fertilization of FI crosses will then be the result of all possible combinations of the gametes. The combination will be illustrated by the Punnet square. PLANT GENETICS 25 AR AW BR Female gametes AR AW BR BW AR AW BR BW AR AR AR AR AR AW BR BW AW AW AW AW AR AW BR BW BR BR BR BR AR AW BR BW BW BW BW BW BW Male gametes Collecting the various combinations we obtain: ^'2 PLANTS Ft BREEDING HABIT 1 AARR Awnless, red seed. Will breed true for awnless spike and red 2 ABRR Int. awns, red seed. Will segregate for spike character and breed true for red seed. 2 AARW Awnless, red seed. Will breed true for awnless spike and segregate for seed color. 4 ABRW Int. awns, red seed. Will segregate for both seed color and spike habit. 1 AAWW Awnless, white seed. Will breed true for awnless spike and white seed. 2 ABWW Int. awns, red seed. Will segregate for spike habit and breed true for white seed. 1 BBRR Bearded, red seed. Will breed true for bearded spike and red seed. 2 ABRR Int. awns, red seed. Will segregate for spike habit and breed true for red seed. 1 BBW W Bearded, white seed. Will breed true for bearded spike and white seed. Several Factors Necessary for the Production of a Character. — In many cases several factors are involved in the production of a single character. Thus the purple aleurone color found in Black Mexican sweet corn is dependent on the interaction of the factors R, C, A, and Pr (see Chapter XII, Maize Breeding). C and A are basic factors both of which must be present for the development of color. When R, C and A are present the color in the aleurone layer is red. Let us study a cross between Black Mexican which is homozygous for purple aleurone color and a white sweet which is homozygous for factors R and A but which lacks the factors C 26 BREEDING CROP PLANTS and Pr. The lack of the factor may be represented by a small letter. PARENTS APPEARANCE GAMETES Fi CROES Black Mexican Purple color PrRAC PrprRRAACc White Sweet White color prRAc As the FI seeds contain all factors necessary for the production of purple color in the aleurone layer, they will be purple. In later generations the factors R and A may be considered to be present in each gamete, as both parents were homozygous for these characters. The gametes of the Fl plants will, therefore, be PrRAC, prRAC, PrRAc, and prRAc. By the Punnet square method as illustrated in the previous topic, the student may determine the possible F2 combinations. These will be found to occur in the following proportions : COMBINATIONS APPEARANCE 1 PrPrRRAACC 2 PrprRRAACC 2 PrPrRRAACc 9 Purple aleurone 4 PrprRRAACc 1 prprRRAACC 2 prprRRAACc 1 PrPrRRAAcc 2 PrprRRAAcc 1 prprRRAAcc 3 Red aleurone 4 White aleurone Linkage of Characters in Inheritance.— Morgan and his co- workers have made an intensive study of the inheritance of characters in the fruit fly, Drosophila melanog aster. Over 300 inherited factors have been studied. If these inherited factors are located in the chromosomes and as there are only four pairs of chromosomes in the fruit fly, it would seem that certain fac- tors should be linked together in their transmission. That this is the case has been clearly proved by the result of many studies. What frequently happens is that factors which tend to be present in the same chromosome or gamete, sometimes change their linkage relations. That breaks do occur in the chromosomes seems evident from careful cytological studies. Preparatory to reduction division the two chromosomes which make up each each pair lie side by side. There is often a twisting of these chromosomes about each other and in some cases breaks occur and the parts are joined in a new relation which causes a modi- fication of the linkage relation. PLANT GENETICS 27 The diagram illustrates a change in linkage relations due to a cross-over. C and W are located in the same chromosome of one parent and c and w in homologous loci of a similar chromosome of the other parent. If there was perfect linkage the only gametes produced would be CW and cw. Owing to a cross-over, however, Cw and cW are also obtained although less frequently FIG. 7. — Diagrammatic representation of crossing-over and results. At the left, the two original chromosomes. In the middle, the twisted condition of the chromosomes in synapsis and their subsequent separation. At the right, the four types of chromosomes which result. (After Babcock and Clausen.} than the combinations CW and cw. The following outline expresses the result on a percentage basis: CW 38.7 percent.; cw 38.7 per cent. cW 11.3 per cent.; Cw 11.3 percent. non-cross-over gametes cross-over gametes Accepting the view that factors are located in particular places in the chromosome, the value of the cross-over hypothesis in explain- ing degrees of factor linkage becomes apparent. If certain com- binations of factors occur with less frequency than others, this means that the breeder must grow a much larger population in the segregating generations in order to obtain the combination desired than when the factors are independently inherited. Inheritance of Quantitative Characters. — Many of the impor- tant characters of economic plants are size or quantitative characters, such as height of plants, size of seed, or relative date of maturity. It was at first thought that these characters did not follow Mendel's law. The discovery that color characters were frequently due to the interaction of several inherited factors led to the explanation of the inheritance of size characters by similar means. Numerous controlled crosses have been studied. 28 BREEDING CROP PLANTS The general nature of the results in this field may be illustrated by a cross between barley varieties which differ in the average length of mternodes of the rachis (see Table I). In this cross between Hanna and Zeocriton, lax and dense varieties respectively, the F2 ranged from above the modal class of Hanna to the modal class of Zeocriton even though only 141 individuals were studied. The calculated coefficient of varia- bility for the F2 was three or four times greater than for the parental varieties. Several small F3 families were grown from Fz plants representing different densities. By examining the table one will note that some F3 lines bred comparatively true, FIG. 8. — Average spikes of the Zeocriton (left), Hanna (right) and four homo- zygous lines. Mean densities are as follows: Zeocriton, 1.9 mm.; Hanna X Zeocriton, 448-1, 2.3 mm.; 448-5, 2.9 mm.; 448-11-3, 3.7 mm.; 448-16, 4.3 mm.; Hanna, 4.6 mm. the ranges for density being no greater than for the parental lines and the coefficients of variability also being low. Other 7'!3 lines were as variable as the F2 generation while still others were more variable than the parents but less variable than the F2. Several Fs lines, which appeared homozygous, were tested in F4 and some of these on the basis of the more extensive test again, gave evidence of homozygosity. The general nature of the results is illustrated in Fig. 9. These results show that homo- zygous lines for density may be obtained in F3 and F4, and that in this cross homozygous lines were obtained which approached the densities of the parents as well as homozygous lines with PLANT GENETICS 29 >* OOOOOrHOOOOOOOOrHQOO ooooooooooo d N i-H i-l W i-l 1 cq,-iT-ii-irHT}O CO Th ^ tN ^ CO CO 1 *'8 00 Tj< OOCO OM< »OiO i-H CO •* i-H "5 O (N 00 OS CO rH CO rH r-l W I o-. t^ t^iO^rfl (NOO OOTH 0^ ^ CO b- CO Oi CO rH ; «•« ^ jH ^ , rHrH COrH^ o 9 s i-l COt-i t^ (N (N 2'2 IN --I 00 . i O5C35O5O5050i0503O5OSC35O5O50>O5O5a>a> O C3 O O3 C3 OS O Ct) Oi Oi O d 0 ; • O • • O £ S * "a * * "oi • • . • • • . ft . > S *-**--*-----oa> « ••§ -rn'r^^^J^^dUS CO || • 1 1 1 • "^ '• I rHrHrHCOCOCOCOCOCOOO c8 S ^jOpOOOOQOOpOOOOOOOOOOOOOO oooooboooboboboboooooo 30 BREEDING CROP PLANTS intermediate densities. The determination of just how many factors were involved could not be made without a more extensive test. The results can be explained on a genetic basis by the hypothesis that Zeocriton contains three independently in- herited factors for density and that Hanna lacks these factors. The added hypothesis may be made that each factor in a hetero- zygous condition gives half as great an effect as when homo- zygous. The factors may be considered to have a cumulative effect, two factors when present in a hornozygous condition / \ 1 / \ / \ / \ ^ h ocri 5,-Wl \ ton S^ 3 na \ "-*) IT.ib X ""•- ^ / y- *2- (Ufa x -5 w •S 40 •a *u $* » 20 i- £ o / ***>, \ / "-^ i ^ / \ I \ i ! 1 / / \ / ) \ A >^ \ f / 448 1 A-5\ ;\ 448- 1-3 -*- **" ^448- Ik x<. 1.4 1.61.82.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3-6 3-8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 la MM FIG. 9. — Diagrams showing the densities of parental forms and of the F generation in a cross between the Zeocriton and Hanna barleys (upper), of four pure lines (middle), and of several heterozygous lines (lower). (After Hayes and Harlan, 1920.) producing twice as great an effect as when a single factor is hornozygous. Other factors of a smaller value are also doubtless present which modify the expression of the main density factors. East and Jones have summarized the results of such controlled crosses and they find a number of general conditions fulfilled. "1. When pure or homozygous races are crossed, the FI populations are similar to the parental races in uniformity. This conclusion devolves from observations that if any particular factors AA and aa are homozygous in the parental races, they can only form Aa individuals in the FI generation. "2. If the parental races are pure, F2 populations are similar, no matter what FI individuals produce them, since all variability in the FI generation is the result of varying external conditions. PLANT GENETICS 31 "3. The variability of the F2 populations produced from such crosses should be much greater than that of the FI populations, and if a sufficient number of individuals are produced the grand-parental types should be recovered. The fulfillment of this condition comes about from the general laws of segregation of factors in F\ and their recombination in JFV "4. In certain cases F2 individuals should be produced showing a greater or a less extreme development of the character complex than either grandparent. This is merely the result of recombination of modifiers, as was explained above. "5. Individuals of different types from the F« generation should produce populations differing in type. The idea on which this state- ment is based is, of course, that all F2 individuals are not alike in their inherited constitution and therefore must breed differently. "6. Individuals either of the same or of different types chosen from the Fz generation should give Fz populations differing in the amount of their variability. This conclusion depends on the fact that some individuals in the F^ generation will be heterozygous for many factors and some heterozygous for only a few factors." From the standpoint of the student a hypothetical case may be given to show how the factor hypothesis may be used to explain the inheritance of quantitative characters. Given two barley varieties as follows: t Variety 1, average length of internode of rachis 2.0 mm. Variety 2, average length of internode of rachis 3.6 mm. Suppose these varieties differ by two separately inherited factors, A and B, each when homozygous causing a lengthening of the internode by 0.8 mm.; when heterozygous by 0.4 mm., Variety 1 aabb Gamete ab _, „ XT- • A n A A rt-o r* A n *! ZygOte AdBb Variety 2 A ABB Gamete AB Combinations in F2 would occur as follows: Fz PLANTS Fa BKEEDING NATURE 1 AABB Would breed true for length of internode of 3.6 mm. 2 AaBB Would segregate from 3.6 mm. to 2.8 mm. 2 AABb Would segregate from 3.6 mm. to 2.8 mm. 4 AaBb Would segregate as F2. 1 AAbb Would breed true for length of internode of 2.8 mm. 2 Aabb Would segregate from 2.8 to 2.0 mm. 1 aaBB Would breed true for length of internode of 2.8 mm. 2 aaBb Would segregate from 2.8 to 2.0 mm. 1 aabb Would breed true for length of internode of 2.0 mm. Probably few size characters are as simple in their inheritance as this illustration. However, the factor notation assists in gaining 32 BREEDING CROP PLANTS a conception of the mode of transmission of these size characters and there seems no good reason for believing that a different mechanism is involved than in the inheritance of color characters. Environmental conditions probably play a larger role in the modification of the appearance of size characters than for color characters. Stability of Inherited Factors. — That sudden changes in the appearance of a character are sometimes found is a well-known fact. The causes of these sudden changes are not so easily determined. Whether these are more logically explained as due to changes in certain inherited factors or due to a new recombina- tion of factors or by other causes is an unanswered question. The pure-line theory of Johannsen was a result of an experimental attack on the question of the stability of a character. A few sudden changes in characters have been observed. Nevertheless, plant characters of self-fertilized crops exhibit remarkable uniformity. Many of the inherited sudden changes which have been noted are most logically explained as the result of a natural cross. Others appear to be due to a sudden change in the hereditary factors of the organism or to the loss of a genetic factor. The view of factor stability which seems most helpful for the plant breeder has been clearly stated by East and Jones (1919). "For these and other reasons which might be given, could further space be devoted to the subject, we believe there should be no hesitation in identifying the hypothetical factor unit with the physical unit factor of the germ cells. Occasional changes in the constitution of these factors, changes which may have great or small effects on the characters of the organism, do occur; but their frequency is not such as to make necessary any change in our theory of the factor as a permanent entity. In this conception biology is on a par with chemistry, for the practical usefulness of the conception of stability in the atom is not affected by the •knowledge that the atoms of at least one element, radium, are breaking down rapidly enough to make measurement of the process possible." CHAPTER III THE MODE OF REPRODUCTION IN RELATION TO BREEDING General recognition of the stability of inherited factors has served to emphasize the importance of a knowledge of the mode of reproduction of crop plants. If the crop in question is nor- mally self-fertilized, and has been bred carefully, accidental crosses may cause serious mixtures in the variety and thus prohibit its sale as pedigreed seed. With naturally cross- fertilized plants, self-fertilization often has a detrimental effect. A knowledge of the mode of pollination of a crop is therefore an absolute necessity in outlining correct methods of breeding. As with other characters, environmental conditions play an important role. With crops which are adapted for insect pollination and yet which are self-fertile, the number and sort of insects found in the locality may greatly modify the amount of crossing which takes place. Variations in moisture conditions may determine the amount of cross-pollination. The age of the plant also is of importance. Aside from these there are often varietal differences in closely related forms. Plants may be placed in four groups according to their mode of reproduction. These groups, however, overlap because of prevailing conditions and inherent differences which the plants exhibit. Group 1. — Naturally self-pollinated: Wheat, oats, barley, peas, beans, flax, tobacco, tomatoes, cotton, sorghums.1 Group 2. — Often cross-pollinated: Maize, rye, sugar beets, root crops, grasses, alfalfa, cucurbits. Group 3. — Cross-pollination obligatory: (a) Self -sterile, red clover, sunflower, many fruits; (b) Dioecious plants, hops, hemp, asparagus, and date palm. 1 Some crops, such as sorghum and cotton, cross in the field frequently. As there is no sharp line of demarkation between cross- and self -pollinated plants and as sorghum and cotton should apparently be handled by the breeder in much the same manner as crops like barley, which is seldom naturally cross-pollinated, it seemed wiser to place sorghum and cotton in the self-fertilized group. 3 33 34 BREEDING CROP PLANT X Group 4. — Vegetatively propagated: Potatoes, sugar cane, many fruits. NATURAL CROSSING WITH SELF -FERTILIZED PLANTS Flower types are adapted for various degrees of self- or cross- fertilization. This in itself is a field in which much study might be made. The plant breeder, however, is chiefly interested in the final result. FIG* 10. — Natural hybrids in wheat. 1. From right to left: Spike of a pure variety produced from a cross of Turkey winter wheat and Wellman's Fife spring wheat. This is a bearded variety with smooth chaff. The progeny of a single plant of this variety gave 48 bearded, smooth chaffed plants and 2 plants with intermediate (tipped awns) and hairy chaff. 2. From right to left: Preston spring wheat; an F\ natural hybrid with intermediate awns and hairy chaff. The parental varieties from which these natural hybrids were obtained were grown alternately with Haynes Blue Stem the preceding year. Wheat.* — The individual florets of wheat and barley are much alike. The envelope of a floret of wheat, for example, consists of the flowering glume or lemma and an inner glume or palea. The sexual organs consist of a pistil with a two-branched, 1 POPE has reviewed much of the literature for cereal crops. See Journ. Amer. Soc. Agron., 8: 209-227. MODE OF REPRODUCTION IN RELATION TO BREEDING 35 fl feathery stigma and of three stamens with anthers, all of which are enclosed by the lemma and palea. Opposite the base of the palea are two tiny sac-like organs, lodicules. The increase in size of these organs due to water absorption causes the flower to open. This occurs when the stigma is receptive and at this time the elongation of the filaments causes the anthers to protrude from the glumes, when they promptly dehisce. The process of blooming is very rapid and seldom requires more than 20 minutes. Leighty and Hutcheson (1919) state that the opening of the glumes from beginning to completion may not require more than one minute, that the anthers may be ex- truded and emptied of their contents within two to three minutes and the glumes again become tightly closed at the end of 15 to 20 minutes. Kirchner (1886) states that about one-third of the pollen falls inside the flower. As the pollen is blown around the field by the wind it is easily seen that natural crossing may sometimes occur. Investigators differ in their be- liefs regarding natural crossing in small grains. De Vries (1906) says " wheat, barley and oats are self-fertile and do not mix in the field through cross-pollination . ' ' Biffin (1905) states that he has never observed a case of cross-pollination in wheat; while Fruwirth (1909) lists several German breeders who have given instances of natural crosses. Fruwirth says ties can be cultivated Nilsson-Ehle (1915), in Sweden, has found that some varieties show a much greater amount of natural crossing than others. Howard and others (19 10a), in India, carefully studied natural crossing in wheat for several years and recorded 231 natural FIG. 11. — Natural wheat-rye hybrids. Two spikes of parent wheat varieties are shown on the outside with hybrid spikes on the inside. (After Leighty.) " wheat varie- side by side for years without mixing." 36 BREEDING CROP PLANTS crosses. Smith (1912) reported eight natural hybrids in 96 rows of Turkey winter wheat and Saunders (1905) told of a natural hybrid which occurred at Ottawa. During the last three years at University Farm, St. Paul, at least 2 to 3 per cent, of natural crossing in wheat has occurred in the plant-breeding plots. Cutler (1919) mentions frequent natural crosses at Saskatoon, Canada. Barley. — Barley frequently is self-fertilized while the spike is in the sheath. In four-rowed barley the lateral rows overlap in such a way as to form a single row instead of two rows at each edge of the rachis, as in the normal six-rowed varieties. Fru- wirth (1909) observed natural crosses in four-rowed barleys and concluded there was practically no crossing in six-rowed forms. He records the observations of Rimpau, who noted only eight suspected natural crosses in barley after growing 40 varieties side by side for a period of eight years. Harlan, after several years' observation at University Farm, Minn, noted only two or three natural crosses. Barley probably, therefore, crosses much less frequently than does wheat. Oats. — The form of the individual flower of oats is very similar to that of wheat and barley. Tschermak (1901) reports four natural crosses observed by Rimpau, and Fruwirth (1909) records five or six crosses observed by Rimpau after cultivating 19 varieties side by side for eight years. A natural cross between a variety of Avena sterilis and A . nuda was noted by Pridham in 1916. These facts and numerous statements by breeders as to self-fertilization show that natural crossing occurs much less frequently in oats than in wheat. Tobacco. — In the tobacco plant the flowers are frequently visited by insects and some natural crossing doubtless takes place. As a rule only one variety of tobacco is grown in a locality. Howard and others (1910 b, c), in India, concluded that there is between 2 and 3 per cent, crossing in tobacco. They emphasize the necessity of producing artificially self-fertilized seed. In breeding experiments, artificially selfed seed is generally used and therefore few records regarding the degree of cross-pollination are available. As it is so easy artificially to self-fertilize tobacco and as each flower produces many seeds (98,910 seeds per plant, Jenkins, 1914) the amount of natural cross-pollination is of little breeding importance. Flax. — The flax flower, like the tobacco flower, is frequently MODE OF REPRODUCTION IN RELATION TO BREEDING 37 visited by insects which may cause natural crossing. Fruwirth (1909) states that crossing seldom takes place. Howard and others (1910a) have observed natural crossing under Indian conditions. Some idea of the frequency of natural crosses may be gained by a determination of the percentage of selected plants which breed true. Results of this nature have been presented by Howard and others (1919). NUMBER NUMBER YEAR PLANTS BREEDING SELECTED TRUE 1916 340 334 1917 233 232 1918 232 232 Only 0.9 per cent, of the progeny rows showed segregation. Rice. — In rice the inflorescence is a terminal panicle of perfect flowers. The one-flowered spikelet has a branched stigma and six stamens. The lodicules are strongly developed. Fruwirth (1909) observed the period of blooming in rice and found that 30 seconds elapsed from the time one flower began to open until it was fully open. Dehiscence of the anthers occurred about seven minutes later and the flower closed three hours after.wards. In rice self-pollination is the usual method, although oppor- tunities for crossing occur. Hector (1913) thinks crosses may occur at a distance of not more than 2 ft. by the agency of the wind. In lower Bengal 4 per cent, of crossing was estimated. Ikeno (1914) sowed alternate rows of blue- and white-seeded rice. Xenia occurs, blue being dominant, if the white-seeded variety is pollinated by the blue. Fifteen thousand kernels from 190 panicles were examined and no xenia was found. Thompstone (1915), in upper Burma, finds that pollination usually occurs before the glumes open; however, hybrids were frequently observed in fields of ordinary rice. Parnell and others (1918) observed the amount of natural crossing in pure green plants surrounded by others which possessed seed with a purple tip. A total progeny of nearly 15,000 plants grown from seed produced by the green plants were observed, more than 2,000 plants being studied in each of five different families. The percentage of crossing varied from 0.1 per cent, in one variety to 2.9 per cent, in another. Alkemine (1914) states that cross-pollination occurs if the anthers, on account of unfavorable environmental conditions, do not assume their natural position. This happens when the stig- 38 BREEDING CROP PLANTS mas protrude from the glumes and take a pendent position before anther dehiscence takes place. Cotton. — Probably cotton crosses to a greater extent than any of the other plants, except sorghums, listed as belonging to the naturally self-fertilized group. Because of the difference in ob- servations by investigators it would seem that varietal differences are one probable cause for the discrepancies. Leake (1911) observed 5 per cent, natural crossing in India. Figures given by Webber (1905) and Balls (1912) range from 5 to 13 per cent. Grain Sorghums. — Ball (1910) states: "All sorghums are adapted to open or wind pollination and most of them are probably adapted to self-fertilization. In adjacent rows of different varieties flowering on approximately the same date, as high as 50 per cent, of the seed produced by the leeward row was found to be cross-pollinated. It is probable that in a fairly uniform field of any given variety a similar percentage of natural crossing takes place." Graham (1916), in India, made a careful study of the amount of cross-fertilization in the Juar plant (Andropogon sorghumBrot.). Crossing was more frequent in the looser types of inflorescence than in the compact types. Single plant cultures were used for the study, which extended over a period of seven years. The percentage of crossing obtained by counting a given number of plants and noting those which were untrue to type gave 97 plants out of 1,577 (6 per cent.) in the loose headed type and only two plants out of 292 (0.6 per cent.) in the compact type of panicle. Preliminary studies were made by Karper and Conner (1919) of the amount of cross-pollination in plants of white milo which were found growing in a plot of yellow milo. The yellow and white varieties flowered simultaneously. Forty-one heads of white milo, which had been surrounded by yellow milo, were planted the following year. An average of 6 per cent, of natural crossing in plants so surrounded was noted. Peas and Beans. — Piper (1912) finds that natural crossing in the cowpea occurs but rarely in most localities. At Arlington Farm, in the experimental plots, instances of natural crossing have been observed. In some instances natural crossing occurs more frequently. Thus an Indiana farmer, who originally grew only eight varieties, found after several years that he had over 40 types. The new types, Piper concluded, were the result of MODE OF REPRODUCTION IN RELATION TO BREEDING 39 natural crosses. Similar crosses have been observed at the Michigan station. Harland (1919) has recorded a supposed case of a natural cross which occurred in one of his hybrid cowpea families. Natural hybrids of soybeans have been observed at the United States experimental farm in Virginia and also at the Kansas experiment station (Piper 1916). They were detected by the peculiar color of their seed. Varieties of soybeans were inter- planted at the Wisconsin station and the amount of natural crossing was determined by testing the progeny. More than 10,000 plants were tested and only a fraction of 1 per cent, of natural crossing was found (Russell and Morrison, 1919). Although horticultural peas and beans are largely self-polli- nated, cross-pollination does occasionally occur. Howard and others (1910a) give observations in India which indicate natural crosses both in garden and field peas. Tomatoes. — Jones (1916) planted alternate plants of dwarf and standard varieties of tomatoes 3 ft. apart in a field. Seed from the dwarfs was tested the following year. As standard habit is a dominant character, pollen from a standard plant fertilizing a dwarf would give a standard in F\. A total of 2,170 plants were grown from seed of dwarfs and 43 proved to be standards. This is practically 2 per cent. As there was nearly as great opportunity for dwarfs to be crossed with dwarf pollen it would seem that between 3 and 4 per cent, of crossing occurred in this experiment. THE OFTEN CROSS-POLLINATED PLANTS Maize. — Maize has been placed at the head of the often cross-pollinated group, as crossing is its normal form of repro- duction. Fruwirth (1909) found a setting of 24 per cent, in un- enclosed corn plants when far enough from other plants to prevent crossing. Knuth (1909), in similar experiments, found 16 per cent, selfing on the upper ear and 4 per cent, on the lower. Preliminary experiments have been made by planting corn with a recessive endosperm color in a field of a variety with a dominant endosperm character. Self-fertilization in these experiments was probably less than 5 per cent. (Waller, 1917, Hayes, 19186). Rye. — The flowers of rye are very similar to those of wheat and barley. According to Hildebrand the anthers project 40 BREEDING CROP PLANTS between the partly closed glumes until the bases protrude. They then tip over and dehisce, spilling part of the pollen outside the flower. Being lower than the stigma the pollen can not reach the stigma of the same flower. There is some evidence (Ulrich, 1902) (Fruwirth, 1909) which indicates that the rye flower is self-sterile, but that the spikelet is not necessarily so. Further studies are needed to clear up this point. Ulrich (1902) found significant differences between varieties and individuals of the same variety in the amount of self -sterility. The following table shows some of his results, obtained from artificial and natural pollination. Artificial pollination was obtained by covering the head with double paper bags. TABLE II. — SELF-STERILITY IN RYE Variety Artificial pollination Natural pollination, per cent. Individual spikes, per cent. Groups of more than 1 spike Same plant, per cent. Dif. plant, per cent. Petkuser 1.30 2.33 5.02 2.52 4.98 7.21 28.29 25.12 38.32 80 59 78 Probsteier Schlanstedter Heribert Nilsson (1916) isolated lines in Petkuser rye differing greatly in amount of self -sterility. Of 73 plant selections, 71 were practically self -sterile, one showed segregation, and one proved to be highly self -fertile. The rye flower is probably largely cross-pollinated and because of the heterozygous condi- tion, strains differing in fertility make up any particular variety. Alfalfa. — Piper and others (1914) working with alfalfa have found about the same percentages of seed set when a flower was self-pollinated as when it was crossed with pollen from flowers on the same plant. When cross-pollination was prac- ticed, approximately 50 per cent, more seed was obtained than from self-fertilization. They also found that pollen of Medicago falcata was as efficient in fertilizing M. saliva as pollen from other saliva plants. Waldron (1919), in North Dakota, planted together in equal numbers two species of Medicago, saliva and falcata. Seeds from each of the species were planted the following year and the number of hybrids noted. From M. falcata 42.7 per cent, of MODE OF REPRODUCTION IN RELATION TO BREEDING 41 hybrid plants were obtained and from the M. sativa seed about 7.5 per cent. A part of the difference in the results is doubtless due to the fact that the plants produce a smaller number of flowers and are procumbent to prostrate in habit. To find the amount of cross-pollination that normally occurs in alfalfa, one might average the above results and multiply the result by two. This gives in the neighborhood of 50 per cent, of natural crossing which is only indicative of the probable amount. Grasses. — Some studies with grasses have been reported by Frandsen (1917). Results obtained are given in the following table. Some sterility is indicated by comparing the results of self-fertilization with those of cross-fertilization and natural pollination. Poa fertilis and Bromus arvensis appear self-fertile. Considerable self-sterility is indicated in orchard grass, timothy, and fescue. TABLE III. — POLLINATION OF GRASSES Common name Scientific name Percentage seed setting Self: fertilizing Cross- fertilizing Free- flowering Orchard Tall meadow oat Fescue Dactylis glomerata Arrhenatherum elatius . Festuca pratensis Alopecurus pratensis.. . Lolium multiflorum. . . . Phleum pratensis 1.3-11.5 5.4-9.4 3.6-9.2 7.0-23.3 10.3 0.8- 8.5 59.7-66.8 66.6-80.0 4.3-75.8 47.9 17.8-54.0 29.0-69.5 50.0 51.0 35.2-47.7 73.2 79.8 91.3 70.4 77.2-89.2 Meadow foxtail. . Italian rye Timothy 52.0 63.5-65.5 80.4 Brome Poa fertilis Bromus arvensis EFFECTS OF A CROSS IN NORMALLY SELF -FERTILIZED SPECIES A cross between closely related varieties frequently exhibits a quite marked increase in vigor when compared with the parents. This is a manifestation of the same phenomenon as decrease in vigor which is commonly the result of self -fertilizing a naturally cross-fertilized species. With self -fertilized crops it is usually not possible to utilize this increased vigor because the cost of producing crossed seed is too great. Examples of FI crosses in tomato, tobacco, and wheat will be given. Table IV gives the comparative yields of first generation 42 BREEDING CROP PLANTS tobacco crosses and their parents. All crosses do not prove equally vigorous and a few give no increase as compared with the parental average. In general, however, the crosses show in- creased yields. As the tobacco flower produces many seeds, Houser (1912) believes the extra cost of production would not be prohibitive. Before this plan can be adopted commercially, extensive studies are needed to determine the value of particular Fi tobacco crosses. TABLE IV. — RELATION OF YIELD PER ACRE BETWEEN FIRST GENERATION HYBRID TOBACCO AND THE PARENT PLANTS Average yield of Average increase of hybrid Maximum increase of hybrid parents, Ib. over parents, Ib. over parents, Ib. 800- 900 260 485 901-1,000 212 464 1,001-1,100 185 354 1,101-1,200 153 315 1,201-1,300 153 285 1,301-1,400 159 239 over 1,400 156 189 Difference in yield of parents 1-100 197 485 101-200 131 181 201-300 189 260 301-400 97 360 401-500 164 215 over 500 175 465 The vigor of FI tomato crosses has received some study. The first extensive test was made by Wellington (1912) at the Geneva (New York) Station. A 3-year test was made under field con- ditions of a cross between Dwarf Aristocrat, a dwarf tomato, and Livingston Stone. Yields of the parents, the FI, and the F% generations are given. We are not so much interested at the present time in the com- mercial value of such crosses as in the development of the princi- ple involved. Wellington believes the above cross of sufficient value to more than pay for the cost of producing crossed seed. Similar results were obtained at the Connecticut Station in a cross between Stone and Dwarf Champion tomatoes. The MODE OF REPRODUCTION IN RELATION TO BREEDING 43 TABLE V. — YIELDS OF FRUIT IN THE FI AND F2 GENERATIONS OF A CROSS BETWEEN DWARF ARISTOCRAT AND LIVINGSTON STONE WITH THE PARENTS Data taken Year Dwart Aristocrat, Ib. Livingston Stone, Ib. *•», Ib. Fy, Ib. Ripe fruit per plant Ripe fruit per plant Ripe fruit per plant 1908 1909 1910 8.5 6.1 7.0 12.3 10.1 12.0 13.9 12.9 13.2 12.0 10.0 Average 7.2 11.5 13.3 11.0 Total fruit per plant Total fruit per plant Total fruit per plant Average 1908 1909 ,1910 14.8 9.7 14.8 13.1 20.9 17.7 24.7 21.1 25 3 20.0 27.7 24.3 20.0 25.1 22.6 experiment was carried on for four years (Hayes and Jones, 1916). The lowest increase in yield over the better parent was 11 per cent, and the highest 17 per cent. The cross averaged 15 per cent, more fruit by weight than the better parent. In average weight of fruit the cross exceeded the parental average by 8 per cent. It approached the fruit number of the Dwarf Champion parent and exceeded the average fruit number of the parents by 8 per cent. The cross also matured somewhat earlier than the early parent. A cross between the standard varieties, Lorillard and Best of All, was also studied. The parents produced about the same average size and weight of fruit and the cross about the same as the parents. A determination of the comparative vigor of F\ wheat crosses and their parents was made by Fred Griffee, a graduate student in plant breeding at the University of Minnesota. For this purpose pure lines of T. durum, T. dicoccum and T. compactum were crossed with pure line varieties of T. vulgare. Intervarietal crosses between pure lines of T. vulgare were also studied, as well as crosses between T. compactum with T. durum and T. dicoccum. A determination of the immediate effect of foreign pollen on size of seed was made. Parental plants were emasculated and then some of the spikes were artificially pollinated with pollen from other plants of the same pure line (incrossed seed) and in another series spikes were pollinated with pollen from another variety or species (crossed seed). Only those crosses were com- pared in which the average date of pollination was about the 44 BREEDING CROP PLANTS same for the incrossed and crossed seed. Results are presented in Table VI. .^, TABLE VI. — WEIGHT OF SEED OF INCROSSED PARENTS COMPARED WITH WEIGHT OF THE IMMEDIATE CROSS 9 Parent Cross Name of cross No. seeds Average weight seed, mg. No. seeds Average weight seed, mg. cross-female parent Marquis X Velvet Chaff Marquis X Penny 38 38 12.6±0.5 12 6 + 0 5 48 24 15.6±0.5 20 2 + 1 0 +3.0±0.7 4-7 6 + 1 1 Haynes Bluestem X Marquis Little Club X Marquis 49 39 17.2 + 0.8 10 . 1 ± 0 5 26 50 23.5±0.7 94 + 03 + 6.3±1.1 — 0 7 + 0 6 Emmer X Velvet Chaff Velvet Chaff X Mindum 44 104 26.4±0.8 19 9 + 0 6 24 23 27.1±1.3 15 9±0 6 +0.7±1.5 — 4 0 + 08 Emmer X Little Club 44 26.4±0.8 15 25.0±1.2 -1.4±1.4 All three crosses between varieties of T. vulgare gave increases over incrossed seed. These appear significant in relation to the computed probable errors. Of the crosses between wheat species only one gave a difference which appears at all significant. In the cross between Velvet Chaff and Mindum the incrossed seed seems somewhat heavier in the light of the probable error than the crossed seed. These results show an immediate effect of pollination on seed size in crosses between varieties of T. vulgare. The emasculation and artificial pollination causes a reduction in seed size as compared with normally produced seed. In- crossed, normally produced seed and crossed seed were grown in the greenhouse under controlled conditions and the comparative vigor of parents and crosses was determined. As there were no significant correlations between size of seed planted (even when incrossed seed was compared with normal seed) and resultant plant vigor, the differences between the parents and crosses may be explained on the basis of inheritance. Average yield of plants in grams of seed will be used as a measure of vigor (see Table VII). The crosses between varieties of T. vulgare and the crosses between T. vulgare and T. compactum gave in every case slightly greater yields per plant than the average of the parents. On the other hand, F\ crosses between durum or emmer varieties and varieties of common or club wheats were all significantly lower in yield than the parents. The low yields of these species MODE OF REPRODUCTION IN RELATION TO BREEDING 45 TABLE VII. — AVERAGE YIELD PER PLANT OF Fi WHEAT CROSSES AND THEIR PARENTS Aver- Crc ss Name of one parent No. of indi- viduals Yield, grains Name of other parent indi- viduals Yield, grams age weight parents, No. of indi- Yield, grams viduals Marquis 15 1.9 Penny 36 2.4 2.2 18 2.7 Marquis 15 .9 Bobs 59 3.0 2.5 65 3.3 Velvet Chaff 38 .5 Penny 36 2.4 2.0 28 2.5 Velvet Chaff 38 .5 Bobs 59 3.0 2.3 92 2.9 Penny 36 .4 Bobs 59 3.0 2.7 23 2.8 Haynes Bluestem . . 47 .4 Marquis 15 1.9 2.2 18 2.5 Marquis 15 .9 Little Club . . 46 2.2 2.1 45 2.3 Velvet Chaff 38 .5 Little Club . . 46 2.2 1.9 37 2.5 Average 30 1.9 45 2.5 2.2 41 2.7 Little Club 46 2.2 Emmer 48 1.1 1.7 9 0.3 Little Club 46 2.2 Mindum .... 49 2.1 2.2 1 1.0 Marquis 15 1.9 Mindum .... 49 2.1 2.0 13 0.3 Velvet Chaff 38 1.5 Mindum .... 49 2.1 1.8 8 1.1 Velvet Chaff 38 1.5 Emmer 48 1.1 1.3 23 0.5 Marquis 15 1.9 Emmer 48 1.1 1.5 18 0.6 Average 33 1.9 49 1.6 1.8 12 0.6 crosses are due in a large measure to sterility for there was an appreciably smaller setting of seeds in the crosses than in their parents. Crosses between distinct species of self-fertilized plants have been carefully studied in the tobacco genus, Nicotiana. Results obtained may be summed up as follows (East and Hayes, 1912): "(a) plants so different that they will not cross; (6) crosses that pro- duce seed that contain no proper embryo ; (c) crosses that produce seed with embryo, but which go no further than the resting stage of the seed; (d) crosses less vigorous than either parent; (e) crosses more vigorous than the average of the parents; and (/) crosses more vigorous than either parent." Apparently in wide crosses the normal physiological processes are interfered with. The statement is frequently made that this is due to lack of compatibility between the parents. The specific physiological cause is not yet known. EFFECTS OF SELF-FERTILIZATION IN NORMALLY CROSS- FERTILIZED PLANTS This subject will be studied in relation to the specific outline for breeding some normally cross-fertilized plants, such as maize and rye. A few data will be presented in 46 BREEDING CROP PLANTS order to illustrate the general results. The theoretical explana- tion is given, as an appreciation of these phenomena is essential in obtaining a correct plant breeding perspective. The most extensive studies made have been those with maize. As this crop is almost entirely cross-pollinated under natural field conditions it is an admirable one to contrast with self- fertilized plants. Table VIII presents differences in yield and height obtained at the Connecticut Station with four self-fer- tilized strains of Learning Dent. These strains were grown only in small plots, therefore differences are only indicative of the general results which may be expected. Crosses between in- dividual plants within a strain that had been selfed six or seven years, were not appreciably more vigorous than the progeny of self -fertilized seed. These strains also differ in other characters, such as shape of ear, width of leaf, and color in various organs. One strain of Learning Dent No. 1-12 was self-fertilized for about seven years. It produced well-developed tassels but few ears and was eventually lost. TABLE VIII. — THE EFFECT OF INBREEDING ON THE YIELD AND HEIGHT OF MAIZE Year grown No. of genera- tions selfed 0 The four strains 1-6-1-3, etc. 1-7-1-1, etc. 1-7-1-2, etc. 1-9-1-2, etc. Yield, bu. per acre Height, in. Yield, bu. per acre Height, in. Yield, bu. per acre Height, in. Yield, bu. per acre Height, in. 1916 74.7 117.3 74.7 ; 117.3 74.7 117.3 74.7 117.3 1905 0 88.0 88.0 88.0 88.0 1906 1 59.1 60.9 60.9 42.3 1907 1907 1908 2 95.2 59.3 59.3 51.7 1908 1908 1909 3 57.9 46.0 59.7 35.4 1910 4 80.0 63.2 68.1 47.7 1911 5 27.7 86.7 25.4 81.1 41.3 90.5 26.0 76.5 1912 6 1913 38.9 1914 1913 7 41.8 39.4 45.4 85.0 1915 1914 8 78.8 96.0 47.2 83.5 58.5 88.0 21.6 1916 1915 9 25.5 24.8 30.6 78.7 1917 1916 10 32.8 97.7 32.7 84.9 19.2 86.9 31.8 82.4 1917 11 46.2 103.7 42.3 78.6 37.6 83.8 i MODE OF REPRODUCTION IN RELATION TO BREEDING 47 From these and other results (Jones, 1918) it is apparent that selfing in maize produces: 1. Strains which can not be perpetuated. 2. Strains which can be perpetuated only with difficulty. 3. Strains which exhibit normal development but vary in amount of growth attained. EXPLANATION OF HYBRID VIGOR1 The studies of the early hybridizers, Koelreuter, Gartner, Knight, and others, gave results which can be summed up in a single sentence as follows (East and Hayes, 1912) : " Crosses between varieties or between species often give hybrids with a greater vegetative vigor than is possessed by either parent." Darwin made extended and careful studies of the effects of cross- and self-fertilization in plants. He conclusively proved that in general there is an advantage in cross-fertilization. While he noted some self-fertilized families he believed these would eventually perish. Lacking as he did a knowledge of Mendelian phenomena it was impossible for Darwin to develop as logical an explanation of these results as we now have. Darwin thought the results could best be explained by the nature of the sexual elements rather than in the act of crossing. Several explanations of hybrid vigor have been advanced since the rediscovery of Mendel's law. In all cases hete'rozygosis has received a major place in the explanation. The results of these studies have been summed up as follows (East and Hayes, 1912): "1. The decrease in vigor due to inbreeding naturally cross-fertilized species and the increase in vigor due to crossing naturally self-fertilized species are manifestations of one phenomenon. This phenomenon is heterozygosis. Crossing produces heterozygosis in all characters by which the parent plants differ. Inbreeding tends to produce homozy- gosis automatically. " 2. The phenomenon exists and is in fact widespread in the vegetable kingdom. "3. Inbreeding is not injurious in itself, but weak types kept in 1 A recent monograph by EAST and JONES (1919) presents in a clear and concise way the effects of inbreeding and cross-breeding in the light of modern theories of genetics. This publication has been used very freely in this section. 48 BREEDING CROP PLANTS existence in a cross-fertilized species through heterozygosis may be isolated by its means. Weak types appear in self-fertilized species, but are eliminated because they must stand or fall by their own merits." Biologists commonly believe that internal or external agencies do occasionally modify the germ plasm. It is also commonly accepted that somatic modifications do not impress themselves upon the germ plasm. From the facts of segregation as explained by the Mendelian law and the acceptance of the theory of factor stability, we may next consider what may be expected in self- pollinating a naturally cross-fertilized plant, such as corn, or what will result in later generations after making a cross in naturally self-fertilized plants. Several slightly different formulae have been advanced to show the theoretical expectation. The simplest formula for the per- centage of homozygous types in any generation following a cross /2n— l\m between different forms is f ) . In this formula n is the number of segregating generations which has elapsed since the cross was made and m is the number of separately inherited allelomor- phic pairs of factors involved. In self-fertilized organisms this would not absolutely hold unless all the progeny of each genotype were equally productive numerically. In artificially self-fertilizing naturally cross-pollinated plants, such as corn, it is theoretically possible to select a completely heterozygous individual in each generation for self-fertilization and thus obtain no reduction in heterozygosis. Jones (1919) has worked out theoretical curves for 1, 5, 10, and 15 allelo- morphic pairs of factors for from one to 10 generations of self- fertilization following a cross. Some facts regarding the effects of self-fertilization in genera- tions following a cross are apparent from a consideration of this figure. When only a single allelomorphic pair is concerned, the first generation of selfing reduces the percentage of heterozygous individuals by half. When a number of factor pairs are con- cerned reduction of the percentage of heterozygous individuals is comparatively slow for the first few years of selfing. At the end of 10 years the percentage of heterozygotes is veiy low whether the Initial cross was heterozygous for 15 allelomorphic pairs or for a single allelomorphic pair. From the above discussion it is apparent that after several years of self-fertilization following a MODE OF REPRODUCTION IN RELATION TO BREEDING 49 cross between different varieties a large percentage of the plants are homozygous and will breed true for their characters if self- fertilization is continued. The number of different biotypes which can be isolated from a cross depends upon the number of allelomorphic pairs of factors involved and their linkage relations. Formerly the heterozygous condition was believed to carry with it an increased developmental stimulus. It was also believed that this stimulus was greater when the mate to an allelomorphic 100* Percentage of Heterozygous Individuals in each Selfed Generation when the Number of Allelomorphs Concerned Are: 1,5,10,15. 456 Segregating Generations 10 FIG. 12. — The percentage of heterozygous individuals and the percentage of heterozygous allelomorphic pairs in the whole population in each generation of self-fertilization. (After Jones.) pair was lacking than when both were present. The physiolog- ical cause of this growth stimulus was not known although it was recognized that "the greater the degree of heterozygosis the greater is the vigor of the resulting plant" (East and Hayes, 1912). A considerable number of studies showed that the rapidity and amount of cell division was increased. A Mendelian explanation of this growth stimulus which is so frequently found in crosses, has been advanced. Jones (1918) has explained the vigor of F\ which has been called heterosis on the 4 50 BREEDING CROP PLANTS basis of dominance and linkage. In comparing crosses with their parents it is quite common to find that the F\ generation has a higher value for nearly every growth character than has the aver- age of the parents. Modern geneticists recognize that each character is due to the interaction of many inherited factors. If each growth factor gives as great an effect when heterozygous as when homozygous or proves partially dominant when heterozy- gous, it would be easy to explain heterosis by the actual physio- logicial growth development which is a part of the normal expression of a particular inherited factor. This explanation was formerly advanced to account for heterosis but was con- sidered unreliable, as it was difficult to account for the almost universal decrease in vigor when such plants as maize were selfed. This can be explained by the facts of linkage, as it is possible to have a greater number of different growth factors present in a heterozygous than in a homozygous individual. The explanation has much in its favor. CHAPTER IV FIELD PLOT TECHNIC In carrying out crop-breeding studies the number of varieties and strains has been greatly multiplied. Vilmorin's isolation principle, whereby the value of any selection is determined by the breeding nature of the progeny, has been universally adopted. The field is then the plant-breeder's laboratory and the question of correct field technic is of the utmost importance. The difficulties, of making all conditions of similar nature for a large number of strains or varieties which must be tested, are very numerous. The method used must be such that the per- formance will be a correct indication of the comparative value of the strains being tested when grown under farming con- ditions. The purpose of the present chapter is to discuss field plot technic for such disturbing factors as soil heterogeneity and climatic conditions. SOIL HETEROGENEITY The field selected for the comparative trials should be repre- sentative of the soil and climatic conditions under which the crop will be grown. The land must then be cropped in such a manner that it is kept in a uniform state of good productivity. In order to do this, it is necessary to observe some one of the standard rotations. It is a good practice to have one or more bulk crops rotated with the breeding plots in order to keep the land uniform. If only one area of land is available there is then no choice and the investigator must see that this field is treated in the best possible way. If more than one field is available, it is possible to deter- mine which is more nearly uniform by a correlation of contiguously grouped plots as outlined by Harris (1915). Harris' Method of Estimating Soil Heterogeneity. — By Harris' method the coefficient of correlation is used as an index of soil uniformity . This statistical constant measures the degree of correlation between contiguous plots grouped in a certain way. If the variation in yield from plot to plot is simply due to random sampling, there will be no correspondence between contiguously grouped units. On the other hand, if the field is "patchy" 51 52 BREEDING CROP PLANTS certain contiguous units tend to yield high while others show a tendency in the opposite direction. Under these conditions a high correlation coefficient results. If variability due to random sampling only is entering, the correspondence between some contiguous plots will be counterbalanced by the lack of corre- spondence between others, providing that the number of ultimate units is sufficiently large to permit an expression of the law of average. It is obvious that in the application of Harris' method the field must receive the same treatment (seed, cultivation, fertilizer, etc.). The division of the field into the desired units may be made at any time before the crop is harvested, but preferably before or soon after planting in order to minimize possible injury to the growing crop. A simple illustration will make the calculation of the correla- tion coefficient clear, although a much larger number of units should be used in an actual study of the reliability of a field for plot work. Suppose a certain field is divided into 16 units and these units are in turn arranged in groups. Let p\, p2, Pz, etc., represent the ultimate units and CPl, CPV etc., represent the groups. By assigning values for yield in bushels per acre to the ultimate units, one may make the calculation necessary to apply the formula. The value of any particular group is the sum of the ultimate units in it. DIAGRAM ILLUSTRATING HARRIS' METHOD (2) (2) (4) (6) Pi C P2 Pi Ps C P4 P2 (3) (3) (6) (4) P5 Pe P^ Ps (3) (3) (5) (5) P9 C PIO P3 Pii C Pl2 P4 (5) (5) (4) (4) Pis Pu Pii Pl6 p = Average yield of all ultimate units = n = Number of units in each group = n = Number of groups = = Sum of squares of the yields assigned for ultimate units = 4 4 4 280 S(CP2) = Sum of squares of the group yields = 1,080 = (1.2247)2 = 1.4999 The numbers enclosed in parentheses represent assumed values (bushels per acre). Now according to the formula FIELD PLOT TECHNIC 53 Where rPlP2 is the constant sought, S is indicative of summation, CP the calculated values for the groups, pl9 p2, etc., the as- signed values for the ultimate units, m the number of groups, n the number of units in each group, p the average value of all the ultimate units and If <• +j « » < & 1 c << r- « r^ W) fcfl "c K fl C fl ft 0 1—1 r-l T-i tic ^ 4) <0 0 .2 .2 1 ! 1 O CQ O2 Segregatio NT CHARA( • X I o CO 3 white to 3 short to -2 ^ § ° 4) ^ & £ CO OS iNTITATIVE Complex s Complex s Segregatio Improvem 2 H H PL. £ -2 .5 & £ .1 0) s i 1 S * S 1> W PH g c 1 03 j "°* 1 g 1 Is li III II 3 Susceptibility •«l! £2 * &• GO **5 Black Green or intei Red Fi almost j dominant \ Fi intermedia Intermediate 4 • . , . • , '. '. ? ' • bib t : : S :^ 1 : £ '• "3 5 $ ! U i • i* ill: Contrasted charact< Character of endosperm Red t>s. white seed Red vs. white seed Red 0s. colorless awn Palea brown vs. colorless Palea colorless vs. yellow Stigma purple rs. colorless . . . Grains not readily falling vs. \ Susceptibility to disease causi sphceria Cattanei vs. immuni White vs. red glume Long vs. short glume Awned vs. awnless glume . . . . Black vs. reddish brown awn Green vs. golden color of inne Red vs. green leaf sheath High vs. low stature Long vs. short panicle Thick PS. thin stem Amount of tillering Time of appearance of first ps Compact vs. loose grain arran Broad vs. narrow leaf Time of flowering Quality of grain and yield . . . 110 BREEDING CROP PLANTS Time of culm formation was carefully studied by Hoshino (1915), who crossed an early with a late variety. The parents averaged 83.8 and 113.2 days, respectively, from time of planting to jointing, the parental average being 98.5 days, while the FI gave an average of 94 days from planting to jointing. The F% generation equalled the combined range of the parents. Some forms bred true to the parental types in F3. One form which segregated in F3 was much less variable than the F2. This line could be explained by the presence of a single heterozygous factor for time of shooting. The author suggests that three multiple factors will explain the results. Kock (1917) crossed Karang Serang, an early maturing good quality rice, with Skrivimankotti, a variety of high yielding ability. Results were not easily explained on a factor basis. After seven years some hybrids showed considerable uniformity. Improvements in quality and quantity of yield were obtained as shown by a comparison of the parents and the better of these hybrid lines. These facts show that correct methods of breeding rice are similar to those of the other small grains. CHAPTER VIII METHODS OF BREEDING SMALL GRAINS The progeny test is now recognized as the best means of determining the comparative productivity of varieties and strains. Vilmorin's isolation principle was first used in the United States in 1897 by Hopkins, of Illinois, for corn breeding, and in 1890 by Hays, of Minnesota, for small grains. Studies in field-plot technic and in crop genetics have led to standard methods of breeding self -fertilized crops. One of the important steps for the breeder is to obtain a broad knowledge of the crop plant with which he is to work. This consists of a knowledge of the home of the plant, its wild and cultivated relatives, the existing varieties and their important economic characters. It is also necessary to learn the needs of the crop for the locality in which the breeder is to work. The importance of this knowledge can not be over-emphasized. After obtaining a fundamental knowledge of the crop, the work in crop improvement naturally falls under three heads: (1) In- troduction, (2) Selection, (3) Crossing. Before taking these up, attention will be given to a system for recording plant pedigrees. Method of Keeping Continuous Records. — There are numerous methods of keeping records and as a rule each investigator will modify some general scheme to fit his own particular needs. It is also recognized that a plan which might prove satisfactory for an experiment station investigator who works only in one particular region might not be at all desirable for a federal worker who has charge of crop investigations over a wide area. The Minnesota plan has proved quite satisfactory, although it is recognized that other methods of equal simplicity and value have been developed by other workers. It is given only as suggestive of the necessity of accurate records and as one means of attaining that end. When a new introduction is first brought to Minnesota it is given a Minnesota accession number and the history, source, and other data are entered in the number book for that crop. If the new introduction is a pedigreed form from a nearby state and seems promising it is placed at once in the 111 112 BREEDING CROP PLANTS variety test. If its value is unknown it is handled in the plant- breeding nursery. The three groups, introductions, selections, and crosses, are given nursery class and stock numbers for means of identification. The year of the first test in Minnesota is also carried (except in the case of crosses where the year that the cross was made is used) , together with a series number from 1 to as many forms as are handled in the class for the year and crop concerned. The following classes are used with the sup- position that the forms were first tested in the nursery in 1920: Class 1-20-1, 1-20-2, etc Selections. Class II-20-1, II-20-2, etc Crosses. Class III-20-1, III-20-2, etc New Introductions. Supposing for example 20 new wheat introductions were grown, these would be classed as III-20-1 to 111-20-20. All individual plant selections are placed in class I if they are made from com- mercial varieties or new introductions. The year that they are first placed in the nursery is also carried, as well as the series number. These class and series numbers are not changed as long as the form is continued in the nursery trial. Crosses are not given a series number until the strain gives evidence that it is homozygous. For the first few years the method of numbering used by the United States Department of Agriculture is followed. Thus a cross made between 1-14-1 and 1-14-20 is labeled at the time of crossing 1-14-1 X 1-14-20. The female parent is written first. On growing this cross in FI a convenient number or letter is used. Later generations for the letter method would appear as A for F\, A-l for Fz, A-l-1 to A-l-200 if 200 plant selections were grown in F3. As soon as a cross is purified, that is, when particular selections appear homozygous, they are placed in the rod-row test and given a series number; thus the cross made in 1918 would be labeled as follows: First year, Class 11-18, A Second year, 11-18, A-l Third year, 11-18, A-l-1 to A-l-200 Suppose A-l-10 and A-l-50 appear homozygous and look promising, they would be placed in the rod-row test and receive series numbers as II-18-1 and II-18-2. Bank figuring books have been found to be quite satisfactory METHODS OF BREEDING SMALL GRAINS 113 for the yearly field notes, a separate book being used for each crop. The following illustrates the method of keeping records for the year 1922. 1921 HEIGHT, DATE OTHER NAME N.S.N. SOUHCE IN. HEADING FIELD NOTES Turkey X Odessa. . II-18-1 A-l-W After obtaining yield and taking notes on grain characters, the yearly results are drawn off on 8J^ by 11 paper, summarized, and filed for reference and further study. Only general notes are taken, such as date heading, date mature, height in inches, per cent, lodged, degree lodged, per cent, and kind of destructive diseases, botanical characters, grain color, plumpness and quality, weight per bushel, and yield. New Introductions. — By means of new introductions the breeder is enabled to obtain varieties or strains which have been produced by other breeders, or native varieties from the original home of the crop. There is no value in attempting to produce a variety which is adapted to a particular condition if the quali- ties desired are to be found in some variety already grown in another locality or country. The United States Department of Agriculture has a trained corps of workers who are constantly introducing new plant sorts from foreign countries. At the present time the Office of Cereal Investigations of the Bureau of Plant Industry acts as a medium for the introduction of new varieties of small grains. Through cooperation with this office, promising new introductions are being tested in localities to which they seem adapted. In small grains no conclusion can be drawn from the first- year test of a new introduction obtained from a widely different climate. Often the seed does not give a high percentage of germination or for some other reason the results secured are not even indicative of the value of the introduction. The first year the different introductions may well be grown in short rows. The following year a rod-row of each new introduction may be grown as a part of the regular crop breeding row trials, and yield and other characters determined. Those which are at all promis- ing by this test may then be placed in the regular row trials and handled in the same manner as pure-line strains. After two or three years those introductions which give results of promise will be used as a basis for individual plant selection, providing the introduction was not already a pure-line. 114 BREEDING CROP PLANTS Selection. — The plant-selection method is used for the purpose of isolating the best possible pedigreed strain of a commercial variety. If the variety is of considerable value a large number of individuals (500 to 1,000) may be selected. Often a smaller number is all that the breeder can afford to test. The number chosen will depend on the productive capacity of the commercial variety or new introduction which is used as the basis of selection. Plant selections are grown in short rows the first year, the same number of seeds being placed in each row. Two general methods have been rather widely adopted for the initial head-selection plot. In either method the same number of seeds is placed in each row. The difference lies in the spacing of the seeds. Some prefer to place the seeds approximately the same distance apart in the row and at sufficient distance (2% to 3 inches) that the plants can be separately observed. Others scatter the seeds in short rows, placing them so close together that individual plants cannot be differentiated at maturity. The latter method more nearly approximates the rod-row plan and needs less room. In either case the rows are usually a foot apart. The field, after being carefully harrowed, is raked by hand, if necessary. It is then marked out by the use of a sled marker, from 7 to 12 rows being marked at a time. The rows are opened with a wheel hoe and covered either with it or a rake or a hand drag with numerous iron teeth. Those selections which by field inspection seem to be of inferior vigor, to have weak straw or other undesirable characters, are eliminated before harvesting. A few others are discarded on the basis of yield, although the experimental error in a yield comparison of this kind is much too large to justify rejection. The following year each selection may be grown, if sufficient seed is available, in three systematically distributed 18-foot rows, 1 foot being removed from each end of every row before harvesting. According to Love and Craig (1918a), J. B. Norton, of the United States Department of Agriculture, was the first to put the rod-row method into general use. By varying the length of the row and obtaining the yield in grams it is possible to con- vert yields into bushels per acre by multiplying by a simple conversion factor. If the length of oat rows harvested is 15 ft. and the yield is obtained in grams, the yield per acre in bushels may be obtained by multiplying by 0.2. For wheat and barley, METHODS OF BREEDING SMALL GRAINS 115 if the rows harvested are 16 and 20 ft. long, respectively, the conversion factor will be 0.1. The rod-rows are about twice as far apart as the rows made by a field grain drill. As from one and one-half to two times as much seed is planted per nursery row as under field planting, the rate of seeding per acre does not differ materially in the two methods. These row trials have been shown to give results similar to those from field tests, although the average yield of the crop is not the same (Montgomery, 1913; Love and Craig, 1918a). As has been previously noted, 'there are two general methods of work, i. e., the use of single- and three-row plots. Three-row plots in which the central row only is used to secure yield are de- sirable as they help to control mixtures at planting and harvest- ing time, overcome competition between nearby varieties and help in obtaining more dependable data on lodging. They require more land and the cost is somewhat greater for planting and cultivating. In sections where soil heterogeneity is very great it is possible that the use of single-row plots and numerous repli- cations may be somewhat better than three-row plots and fewer replications. On land that is well suited for field plot work the use of three-row plots and three replications is advised. After a strain has been grown for three years it may well be removed from the row-yield trial and either increased if it shows promise or discarded if it appears to be of no value. At Cornell new sorts are introduced to the farmers for trial directly from the rod-rows. In many cases the new sort is finally tried in variety plots planted by the usual field-plot method. This gives an expression of yield under normal methods of planting and favor- able field conditions. Summary of Methods of Selection. — 1. Determination of the varieties which possess economic possibilities. These may be commercial varieties or new introductions. 2. Head selection of these promising varieties. 3. Test of head selections in plant-rows. The very undesirable strains are eliminated in the field by inspection. A few may be discarded on the basis of yield or seed characters. 4. Yield determinations of the selections, using three plots of a single row each, systematically replicated, if seed is available. 5. Continuation of the row test. When land is well suited it is believed that four systematically distributed plots of three rows each will give reliable results. Possibly the arrangement 116 BREEDING CROP PLANTS of selections of like nature together, the use of single rows and more replications, may be desirable under certain condi- tions. 6. Computation of a probable error for the method of test. The use of this probable error as a means of determining signifi- cant differences. 7. Increase of the better selections and either a trial by careful farmers or a further test in field variety plots followed by distri- bution of the better strains. If placed in field variety plots, borders should be removed and each variety tested in repli- cated plots. Probable errors should be obtained and used as in the row trials. From five to eight years' time is needed before the new selection is introduced to the farmer. Crossing. — The improvement of commercial varieties of self fertilized small grains by the head or plant method of selection is a very easy process, although several years are required to do the work. The production of new forms by crossing is not so simple. A standard plan of attack has been developed which is the application of the Mendelian method. The first step is the initial cross. Promiscuous crossing is not advised, but each cross should be the result of a determination of parents which most nearly approach the ideals in mind. By recombination of characters there is the possibility of obtaining a sort which is more desirable. The FI generation is grown so that each plant has space for individual development. A knowledge of the inheritance of characters allows those plants which are not crosses to be elimin- ated in FI. The F2 generation plots should be as large as can be studied and each plant grown with enough free space that it may be examined. Numerous selections of plants which have de- sirable field and seed characters should be made. Each of these Fz plants selected should be grown in an individual progeny plot in F3 and individual plant notes taken. Selection of desirable plants should be continued in later generations. When plots show apparently uniform progeny of a desirable sort, the strain should be included in the rod-row tests and compared with standard varieties. Knowledge of the results of continued self-fertilization in generations following a cross, shows the reliability of another method which was first used at Svalof, Sweden (Babcock and METHODS OF BREEDING SMALL GRAINS 117 Clausen, 1918) and is now being tried by other investigators. It consists of growing a bulk plot of the cross for several generations. At the end of from six to ten years, head selections may be made with the knowledge that a large part of these selections will breed true. The adoption of this plan will in a large measure do away with the technic of studying individual plants in a heterozygous population. It is desirable for those workers who would like to use crossing methods but who do not have time for individual plant studies. It is not so rapid as the Mendelian method. Technic of Harvesting, Thrashing, Etc. — Slight variations in methods are used by different workers. At Cornell rows of like kind are taken to the thrashing shed and hung head down until thrashed. At the Minnesota Station the straw is cut near the base, the bundles tied with the stake, label near the bottom, and the heads wrapped with a cheese-cloth covering. Bundles of the same selection are then tied upright to a stake and later taken to the thrashing shed when needed. The row trials at the sub- stations are harvested by cutting off the heads. These are then put into cloth sacks and shipped to the Central Station. Several machines which can be cleaned easily have been devised for thrashing. The chief requisites of a machine to be used for experimental purposes are that it be easily cleaned and that so far as possible there be no ledges or ridges upon which seeds may lodge. The alternate thrashing of different nursery crops is a desirable procedure. Each of the plots of one strain of wheat may be thrashed separately in rotation and then a strain of oats may be thrashed in the same way. At the Minnesota Experiment Station winter wheat is thrashed alternately with barley and spring wheat with oats. This plan helps materially to reduce the roguing of accidental mixtures from the plots. Various machines have been made to assist in individual head and plant thrashing. A machine constructed by H. W. Teeter, of the Department of Plant Breeding at Cornell (Love and Craig, 1918a), is very satisfactory. As no screen or fan is used, all seeds are saved. After thrashing, the seed is passed through a wind blast. This machine is so arranged that mixtures may be avoided. CHAPTER IX SOME RESULTS OF SELECTION WITH SELF- FERTILIZED CROPS In its broadest sense, selection is really at the basis of all animal or plant improvement by breeding. Evidence accumulated by early plant breeders indicated to them that selection of the most desirable plants for seed was highly profitable, irrespective of whether the plants were naturally cross-fertilized or self-fertil- ized. Darwin believed that the mean type of any population could be changed by a plus or minus selection. It was left for Johannsen (1903) to point out the true significance of selection within a naturally self -fertilized crop. Before discussing Johannsen 's pure-line concept and its rela- tion to the improvement of self-fertilized crops by selection, a brief survey of early work on improvement of naturally self- fertilized cereals is desirable. EARLY INVESTIGATORS IN SELECTION OF SELF-FERTILIZED CEREALS John Le Couteur and Patrick Shirreff were first to use the prog- eny test in making selections. The former did considerable work with wheat. In the early part of the nineteenth century he grew what he supposed to be a uniform variety. Professor La Gasca, of the University of Madrid, upon inspecting Le Cou- teur 's wheat in the field pointed out no less than 23 distinct forms. This observation led the latter to make a collection of 150 varieties. Le Couteur simply took it for granted that the progeny of any one individual would breed true. Patrick Shirreff, another breeder of cereals, who lived in the middle of the nineteenth century, worked along somewhat different lines. He searched for the exceptional plant to start a new variety, and discovered seven such varieties. Frederic F. Hallett also followed rigid selection of individual plants in his wheat breeding. Furthermore, he proceeded on the theory that the selection of the best spike on the plant and the 118 RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS 119 best seed on the spike would yield correspondingly the best plant. Le Couteur and Shirreff placed all the emphasis on the original plant selection, while Hallett believed he could improve the prog- eny of an individual plant by further selection. Needless to say, Hallett made no progress after the initial selection. A number of his improved varieties were introduced and widely grown. Louis Leveque de Vilmorin formulated a breeding principle as a result of a series of experiments performed by himself and his father which was published in monograph form (1852). These early studies were carried on with vegetables and the con- clusion was reached that the only way to determine the breeding value of a plant was to grow and examine its progeny. Much study was made by the younger Vilmorin with the sugar beet. This is not a self-fertilized plant, but the principles learned have a direct bearing on selection with self -fertilized crops. In the first few years the problem of determining the sugar content of mother beets without injury to the roots received particular attention. Weighing a small ingot of silver in the juice extracted from a small piece of root was found to be an accurate method of determining density and thus sugar content. Roots of similar sugar content were then used as mother plants and their breeding nature determined. Some gave progeny with high sugar content without pronounced variability; other mother plants gave varia- able progeny some of which were high in sugar content and others much lower, while some mother beets produced progeny of such inferior sugar content that all were immediately discarded. Later the sugar content was determined by means of polarized light (Babcock and Clausen, 1918). As an example of his results may be mentioned a strain of beets which, after three years' selection, gave juice with an average density of 1.087 while unselected seed grown in the same field gave an average density of only 1.042. Andre Leveque de Vilmorin produced a desirable cultivated form of carrot by three years of selection from wild forms. Louis de Vilmorin also made a collection of wheats and other grains from all parts of the world. After 50 years of selection within isolated lines of wheat, no notable change was observed (Hagedoorn, A. L. and A. C., 1914). Willet M. Hays, formerly of the Minnesota Experiment Station, was the first in America to adopt the "Vilmorin method" for small grains. In 1891 he introduced what is known as the centgener method of grain breeding (Hays and Boss, 1899). 120 BREEDING CROP PLANTS Briefly, it consisted of growing and harvesting a 100-plant plot from each plant. Selection was continued the following year. The selections of most promise were increased and given exten- sive trials by farmers. By this method new forms of superior value were discovered. The pure-line method of breeding self-fertilized crops was independently discovered and later adopted (1891) by the Svalof experiment station in Sweden. The director of the station, H. Nilsson, was led to its adoption by the accidental discovery that only those plots planted with seed coming from a single plant exhibited uniformity (Newman, 1912). DeVries (1907) says: 11 To this accidental circumstance, combined with the exact scientific method of keeping extensive records, the discovery of the cause of the diversity of the cultures was due. For precisely those cultures which were derived from one ear only were found to be pure and uniform, all others offering to the eye a more or less motley assemblage of forms." The fact that many of the agricultural varieties grown in Sweden at the present time are the result of this method of breed- ing is sufficient evidence of its success. In addition to individual plant selection, the older mass selection is' sometimes used with self -fertilized crops. Mass selection is the selection of a group of individuals which seem to embody the desired characters. No attempt is made to grow the offspring of the different individuals separately and hence a pure-line study is impossible. In spite of this fact, mass selection sometimes has a place in correct breeding. For example, it may be advantageous to let nature eliminate non-hardy forms of a winter wheat variety before beginning a study of individual plant progenies. SELECTION WITHIN A PURE LINE Early in the twentieth century Johannsen (1903, 1913) began his famous experiments with beans and barley which resulted in the discovery of facts which led to the development of the pure- line theory. Johannsen found that selection within a pure line was futile. Table XIX is typical of what he obtained by selection within each of 19 different pure lines of beans. Since Johannsen announced his pure-line concept, several investigators working with other crops and other characters have verified his conclusions. RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS 121 TABLE XIX. — SELECTION EFFECT DURING Six GENERATIONS IN LINE I OF PRINCESS BEANS Harvest Total num- Mean weight of mother beans of the select strains Differ- ence, Mean weight of progeny seeds of select strains Difference, years ber of B A beans A-minus B -plus A-minus B -plus 1902 145 60 70 10 63.15 + 1.02 64.85 + 0.76 + 1.70±1.27 1903 252 55 80 25 75. 19 ±1.01 70.88±0.89 -4.31±1.35 1904 711 50 87 37 54.59±0.44 56.68±0.36 + 2.09±0.57 1905 654 43 73 30 63.55 + 0.56 63.64 + 0.41 +0.09±0.69 1906 384 46 84 38 74.38 + 0.81 73.00±0.72 -1.38 ±1.08 1907 379 56 81 25 69.07 + 0.79 67.66 + 0.75 -1.41 + 1.09 Fruwirth (1917) made selections within a pure line of each of the following: lentil (Lens esculenla), vetch (Vicia saliva), snap bean (Phaseolus vulgaris), field pea (Pisum arvense), and white mustard (Sinapis alba), but failed to change significantly the mean of the character subjected to selection. In other words, the genotype was not altered. Fruwirth also conducted experiments within pure lines of oats. He selected for number and length of hairs on the lower grain in addition to selecting for percentage of two-grained spikelets per plant. The work was carried on from 1906 to 1915 without effecting permanent altera- tion in the hereditary complex. Table XX taken from Fruwirth, illustrates a typical case. TABLE XX. — SELECTION FOR PERCENTAGE OF BRISTLING IN OATS Minus selection Plus selection vr _ Per cent, of bristling of Per cent of bristling of Progeny Progeny Mean S.D. Mean S.D. 1907 i 5.11+0.682 1.68±0.482 1908 2.5 5.47 + 1.37 4.32+0.97 4.8 4.05+0.882 2.78+0.622 1909 0 4.70 + 1.03 3.24+0.72 9.2 4.75±0.99 3.12±0.69 1910 0.67 2.94+2.36 11.80 + 1.66 10.0 8.46 + 1.61 8.06 + 1.14 1911 0 0.14±0.07 0.33+0.05 21.9 8. 88 ±1.47 5.50 + 1.04 1912 0 0.93+0.22 0.85+0.12 18.7 1.02+0.35 1.65±0.25 1913 0 1.20±0.33 1.67±0.24 5.1 3.58±0.98 4.18±0.69 1914 0 0.1/4+0.09 0.44+0.06 13.0 0.64+0.25 1.25±0.17 1915 0 2.65+0.46 2.32+0.33 5.7 3.14+0.55 i2. 74+0. 39 i i ! 1 Practically no bristles. 2 Mean error. 122 BREEDING CROP PLANTS In the above table mean error is used instead of probable error (mean error X 0.6745 = probable error). The means, both in the minus and in the plus direction, show no effect of continuous selection. In 1914 Hutcheson published the results of 13 years of con- tinuous selection in wheat carried on at the Minnesota Station. Here again no significant effects of selection are found. Table XXI presents a comparison of the yields for the first five-year period with those of the last five-year period. TABLE XXI. — COMPARISON OF AVERAGE YIELD PER PLANT IN GRAMS OF FIRST FIVE-YEAR PERIOD WITH THOSE OF LAST FIVE-YEAR PERIOD IN CONTINUOUS SELECTION OF WHEAT Variety First five-year period Last five-year period Hedgrow 2.67 2.34 Russian 1.99 2.18 Speltz 2.51 2.40 Kainoiiska, ' 2.01 1.97 Polish 1 1.54 1.61 Polish 2 1.62 1.31 Average . . . 2.06 1.97 In the tobacco breeding work of the Connecticut Experiment Station (Hayes, 1913b) the inheritance of number of leaves was TABLE XXII. — NUMBER OF LEAVES OF SUMATRA, 403; BROADLEAF, 401; HAVANA, 402; AND CUBAN, 405 » Progeny Number Year grown Leaves of parent Range of varia- Total Average c.v. tion 403 1910 24-31 150 28.2±0.08 5.27±0.21 403-1 1911 29 23-31 125 26.5 + 0.11 6.64 + 0.28 403-1-2 1912 29 21-32 151 26.2±0.12 8.28 + 0.32 401 1910 17-22 150 19.2+0.05 5.00±0.19 401-1 1911 20 16-22 108 19.1+0.08 6.54+0.30 401-1-1 1912 22 17-23 145 19.9±0.07 6.03+0.24 405 1910 16-25 150 19.9+0.08 7.53+0.28 405-1 1911 21 18-23 124 20.6+0.07 5.29±0.23 405-1-1 1912 23 17-25 150 20. 9 ±0.07 6.17±0.24 402 1910 17-24 150 19.8 + 0.07 6.98 + 0.27 402-1 1911 20 16-25 143 20.3+0.10 8.87 + 0.35 402-1-1 1912 20 17-22 150 19.4 + 0.05 4.59±0.18 RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS 123 studied. • The parental forms were grown with the hybrids for comparison. Although tobacco is naturally self-fertilized, the plants were bagged to insure self-fertilization. The behavior of the parental forms selected in a plus direction is shown. It is obvious from the data presented that tobacco, like other self- fertilized crops, does not respond to selection within a pure line; at least not to a degree which would encourage the plant breeder to use this method of seeking improvement. (See Table XXII.) Love and Craig (1918b) recently reported on the effect of selection for height of plant within a pure line of oats. No evi- dence of selective effect was obtained, as is shown in Table XXIII. TABLE XXIII. — SELECTION FOR HEIGHT WITHIN A PURE LINE OF OATS Average height of parents Average height of offspring Year selected, in cm. produced, in cm. Tall line Short line Tall line Short line 1913 1914 1915 1916 Average. . 85.8 86.9 94.9 97.1 58.8 60.4 67.8 74.9 74.2 82.6 89.4 95.9 75.7 82.9 88.8 94.5 91.2 65.5 85.5 85.5 An average difference of 25.7 cm. in height of plant between the parent forms chosen, failed to change the genotype. One of the old mooted questions among investigators of field crops was the relation between the weight of seed planted and the resultant yield. Earlier workers adhered to the belief that the selection of large seed would give increased yield. In a pure line of a self-fertilized crop, heavier seeds possess larger endosperms a'nd consequently contain more stored food material for the young plantlet than the smaller seeds. It seems that it would be pos- sible to have the environment during the germination period such that the larger seeds would have an advantage over the smaller ones. The important fact to bear in mind, however, is that all seeds of the same pure line have the same inheritance. Some work has been done (Arny and Garber, 1918) on the relation between size of seed planted and resultant yield in Marquis wheat. The seeds were individually spaced 4 in. apart. The relation between the weight of the seed in milligrams, and the resultant yield in decigrams was expressed by means of a 124 BREEDING CROP PLANTS correlation coefficient. The coefficients for the years 1914, 1915, 1916, and 1917 were 0.143 ± O.C38, 0.088 ± 0.028, 0.445 ± 0.020, and 0.478 ± 0.024, respectively. In this investigation each plant was given the same space for individual development. The results show that under these conditions relatively large amounts of stored plant food in the germinating seed may or may not give the resultant plants an advantage, depending on environmental influences other than the amount of endosperm. Several investigators have attacked this problem from a prac- tical viewpoint. Seeds were separated into light, medium, and heavy by means of a fanning-mill. The productivity of the plants coming from the various classes of seed was compared under field conditions. Some investigators procured a slightly greater yield from plants produced by heavy seed than from those coming from light seed. Others obtained no such difference. Plants from medium or ungraded seed in almost all cases proved as productive as those from heavy seed. The work carried on at the Ohio Station may be taken as a typical example of, these investigations. Table XXIV presents the average results (Williams and Welton, 1911) of an experiment with weight of seed wheat over a period of seven years. The grades are first, second, and third, represent- ing heavy, medium, and light seed, respectively. Two methods of seeding were practiced, namely, a uniform rate by weight and a varied seeding to obtain approximately an equal number of plants on equal areas. TABLE XXIV. — THE RELATION OF WEIGHT OF GRAIN TO YIELD IN WHEAT Seven-year Average Results Grade Seed used, Av. wt. per bu., Ib. Bushels per acre Crop Harvested av. wt. per bu., Ib. Uniform seeding Varied seeding Average of both series First 61.6 59.8 57.7 31.3 31.4 31.3 31.3 30.9 30.7 31.3 31.2 31.0 59.4 59.0 59.0 Second Third In the case of oats (Williams and Welton, 1913) a greater difference was obtained between light and heavy seed, but the un- screened seed yielded only a little less than the large seed. Table XXV presents the average data of a four-year period. RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS 125 TABLE XXV. — THE RELATION OF WEIGHT OF GRAIN TO YIELD IN OATS Four- Year Average Results Grade Seed used Bushels per acre Crop harvested, av. wt. per bu., Ib. Av. wt. per bu., Ib. No. per ounce Uniform seeding Varied seeding Average of both series Light .... 27.5 30.7 27.3 1,052 1,684 1,286 59.0 58.0 58.4 59.0 55.3 58.0 59.0 56.7 58.2 28.6 28.4 27.8 Heavy Unscreened. . . A current popular belief is that plants from large or heavy seeds yield more than plants from light or small seeds. The data col- lected by various investigators do not substantiate this view. As a matter of fact, from a practical viewpoint it would be difficult to demonstrate any increase in yield as the result of the use of a fanning mill. The fanning mill, however, is very useful in removing weed seeds or diseased light grains. SELECTION FOR THE PURPOSE OF ISOLATING PURE LINES The determination of the better selections requires at least five years. Accordingly, there have been consistent attempts to find some character or characters which were so closely associated with yield or other economic qualities that they were of actual selection value. If such could be found it would be possible to use them as checks on the yield results. Manifestly they would be of especial value in the early period of head selection, for the results from short rows planted from individual heads are not very accurate indications by which to discard selections. In this connection DeVries (1907) states that " correlation between botanical marks and breeding qualities are to be con- sidered as reliable guides in the work of selection." As an illus- tration of such correlations, the belief that there is an association between two-grained spikelets of oats and yield may be mentioned. Some of the early data collected at Svalof indicated that such was the case. After fifteen years further study, five or six of the best yielding oat varieties were examined. Some were three- grained types and others were two-grained types. Newman (1912) in summarizing these results concludes that "there seems, there- fore, to be no definite relationship between the yield of a given strain and the number of kernels per spikelet by which it is char- acterized." The relationship between other characters was 126 BREEDING CROP PLANTS likewise studied, such as early maturity and high yield; short- haired rachilla and high brewing qualities in barley; weight of 1,000 grains in wheat, oats, and barley and yield; stooling with yield and quality; size of spike or panicle and yield. In some cases there seemed to be a relation between yield or quality and some particular character, but when sufficient numbers were studied no consistent association between any one morphological character and yield was found. Much investigational study has been made on this subject by others and similar conclusions have been reached. At the Minne- sota Station correlations between yield and the following char- acters in wheat have been sought; stooling, height of plant, size of seed, date heading, and date of maturity. In some seasons the early varieties were the better yielders and in other seasons the later varieties. Stooling was obtained from plots in which plants had room for individual development, and the correlation of stooling and yield was computed for two years for wheat, oats, and barley. Yield was obtained from the replicated rod-row test. The results showed no association between stooling and yielding ability. Quite consistent association between weight of 1,000 plump seed and yield of wheat as determined by the rod-row test was obtained as is here shown. TABLE XXVI. — CORRELATIONS BETWEEN WEIGHT OF 1,000 PLUMP SEED or T. vulgare AND YIELD Number of selections or varieties in the population Class and year Correlation coefficient 70 Spring, 1914 0.431+0.066 70 Spring, 1915 0.519±0.059 35 Spring, 1917 0.580 + 0.076 63 Spring, 1918 0.109 + 0.084 54 Winter, 1916 0.356 + 0.080 83 Winter, 1917 0.436+0.060 Fairly consistent results of this nature would seem to show that weight of seed was associated with high yield in wheat. Mont- gomery (1912) isolated more than a thousand pure lines of Turkey winter wheat at the Nebraska Station and found both large- and small-seeded strains among the higher yielders. Simi- lar results have been obtained at Svalof . A study of the correlation between lodging and morphological RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS 127 characters of the stems of cereals has been carried out at the Minnesota Station (Garber and Olson, 1919). Number of fibro- vascular bundles, area of sclerenchyma cells in the cortex and bundle and other characters were studied in relation to lodging. Stiffness and thickness of wall of the sclerenchyma seemed to be associated in oats but no such relation was found in wheat and barley. No other instance of a close association between any one of the characters studied and lodging was obtained. Some correlations are of value in selection or in obtaining accurate data. Thus, if one desires to classify a number of selections according to comparative maturity, reliable results may often be obtained by taking such notes as date of awn emer- gence in barley and date of heading in wheat and oats. In years favorable for normal development, a high correlation between date of heading and maturity has been obtained. In unfavorable years, date of heading is a more reliable indication of the inherited differences between strains in relation to their normal period of maturity than a note taken at maturity. In general, it seems safe to conclude that no one character is closely enough associated with yield to be of selection value in picking out the highest yielding strain. It is possible, how- ever, in many crops to weed out the very undesirable plants by inspection. The yield test must then be used to determine the better pure lines. This seems reasonable when we realize that yield is the final result of many growth characters. A strain which excels in all characters, such as stooling, disease resistance, size of seed, size of head, fertility, etc., naturally will be a high yielder. As so many characters — of which the above are only a few of the more easily seen — are essential to high yield, no single botanical character is of great selection value. This has led to the present method which is summarized as follows by Newman (1912): "Thus instead of basing the isolation of superior individuals purely upon botanical or morphological characters as was formerly the case, the principle has become to select a large number of individuals without special regard to such characters." The value of these individuals is determined by the study of yield continued over several years. Numerous experiments have proved the value of this method. In this connection it is of interest to point out progress that has already been made with self -fertilized crops. 128 BREEDING CROP PLANTS WHEAT SELECTIONS A new winter wheat, Kanred (Jardine, 1917), discovered at the Kansas Experiment Station as a result of testing out 554 head selections made from Crimean (No. 1,435 of the Office of Cereal Investigations, United States Department of Agriculture) is a rather striking example of what may be accomplished by this method of work. As an average of six years' tests, Kanred yielded 4.6 and 5.2 bu. more than Turkey and Kharkov respectively. These varieties gave best results under Kansas conditions until Kanred was found. Table XXVII shows a comparison in yield between commer- cial varieties and selections made from them (Love and Craig, 1918a) at the Cornell Station. TABLE XXVII. — THREE-YEAR AVERAGE YIELD PER ACRE OF WINTER WHEAT VARIETIES AND SELECTIONS MADE FROM THEM Varietal selections Three-year i average yield i Gain, bu. per acre, bu. Klondyke 28 2 Klondyke 126-26 30.4 2.2 Klondyke 126-44 Fulcaster 31.3 26 0 3.1 Fulcaster 123-23 27.9 1.9 Fulcaster 123-32 (beardless) 30.2 4.2 Red Wave 27 7 Red Wave 128-47. . 31.1 3.4 Red Rock winter wheat, which is highly satisfactory in Michi- gan, comes from a red seed picked out of a white wheat (Ply- mouth Rock) (Spragg and Clark, 1916). Here we have an example of selecting and increasing an individual obviously differ- ent from the type in which it occurred. The red seed may have been due to one of several causes, admixture, natural crossing, or a mutation. Whatever the cause, selection immediately isolated a wheat which was different in appearance and which proved valuable. On a percentage basis, the average yield of Plymouth Rock at the Michigan Experiment Station during the period 1912-1915 is 73.4. The yield of Red Rock for the same period is taken as 100. Besides yield and quality other characters of economic impor- RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS 129 tance may be improved by selection. The illustration below, taken from Williams (1916), shows clearly what has been accom- plished at the Ohio Station in the way of isolating a strain with stiff straw. The three pure lines shown are selections from the commercial variety Fultz. FIG. 26. — Variation in stiffness of straw in pure line selections of Fultz wheat. (After C. G. Williams.) OAT SELECTIONS The Maine Experiment Station has made somewhat extensive studies of pure lines in oats (Surface and Zinn, 1916). The yields of commercial varieties were compared with that of their respective pure-line selections. In Table XXVIII are given a part of the data reported in Maine Agricultural Bulletin 250. The average of the seven pure lines of Banner for the entire period of the test is 81 bu. per acre, while that of the commercial Banner is 79.7 bu. The average difference for the period of the test between commercial Irish Victor and the four pure lines is nearly 6 bu. per acre. 9 130 BREEDING CROP PLANTS TABLE XXVIII. — YIELDS OF COMMERCIAL VARIETIES AND PURE-LINE SELECTIONS Bushels per acre Variety 1913 1914 1915 Mean Banner (commercial) 62 7 94 5 81 8 79.7 Banner, Maine 355 (p.l.) 71 0 105 3 83 6 86 6 Banner, Maine 281 (p.l.) Banner, Maine 351 (p.l.) Banner, Maine 230 (p.l.) Banner, Maine 307 (pi) 73.1 70.0 69.4 66 9 97.0 98.2 93.8 95 8 81.2 75.5 76.8 77 0 83.8 81.2 80.0 79 9 Banner, Maine 286 (p.l.) 70.9 87.1 75.7 77.9 Banner, Maine 357 (p.l.) Irish Victor (commercial) Irish Victor, Maine 340 (p.l.) Irish Victor, Maine 337 (p.l.) Irish Victor, Maine 336 (p.l.) Irish Victor Maine 346 (pi.) 70.0 67.0 74.1 58.4 75.3 71.9 83.1 82.4 95.8 103.9 89.2 89.5 79.1 76.2 83.6 79.2 75.1 77.0 77.4 75.2 84.5 80.5 79.9 79.5 Kiesselbach and Ratcliff (1917) have reported in Bulletin 160 of the Nebraska Experiment Station the yields of numerous pure lines of Kherson together with the yield of the commercial variety for a four-year period. TABLE XXIX. — YIELD TEST OF KHERSON OAT STRAINS GROWN IN FIELD PLATS. 1913 TO 1916 Strain Yield in bushels per acre No. 1913 1914 1915 1916 Average Original 44.4 58.9 29.9 - _ 83.0 54.1 21 58.0 71.7 32.4 85.4 61.9 23 61.0 67.3 27.7 85.9 60.5 15 51.8 50.4 26.5 77.3 51.5 25 62.1 64.2 . 30.7 83.9 60.2 6 64.1 63.2 24.4 81.6 58.3 33 58.5 63.7 35.7 86.1 61.0 27 50.8 67.1 31.7 81.9 57.9 38 62.1 .64.9 31.9 81.1 60.0 35 33.9 53.0 22.8 76.7 46.6 4 61.0 67.8 30.5 80.3 59.9 5 65.2 33.3 83.3 19 50'. 1 21.4 74.8 RESULTS OF SELECTION WITH SELF-FERTILIZED CROPS 131 As shown by the last column of the table, only two of the thirteen pure lines gave lower average yields than the commer- cial variety for the four-year period. They are Nos. 15 and 35. Among oat selections (Anonymous, 1919) which have proved their practical value may be mentioned Iowa 103, Iowa 105, and lowar, all of which are pure-line selections from Kherson. These selections were made by Burnett at the Iowa experiment station. SELECTIONS IN OTHER SELF -FERTILIZED CROPS An exhaustive account of the work that has been done in isolating and testing pure lines of self-fertilized crops would alone make a large volume. In this somewhat brief treatment only a few typical examples are chosen. The Iron cowpea (Orton, 1911), which is resistant to wilt, is one of the notable examples of what has been accomplished by the introduction of a promising variety. The isolation of this form alone has produced thousands of dollars for the farmer. M. A. C. Robust bean (Spragg, 1919), which is a selection out of the ordinary navy bean, has proved to be very much superior in yield to the commercial variety. At the Svalof Experiment Station (Newman, 1912), in Sweden, progress has been made in isolating pure lines of barley which possess superior brewing qualities. These few examples show the value of selection as a means of crop improvement. The effect of selection is to isolate the more desirable types from the commercial variety. After this has been accomplished, crossing may be resorted to as a method of obtaining a variety which combines the desirable characters of several strains. CHAPTER X SOME RESULTS OF CROSSING AS A MEANS OF IMPROVING SELF-FERTILIZED CROPS In the preceding chapters it was shown that the selection and increase of a homozygous individual plant isolated a pure line. No one of these pure lines contains, as a rule, all the characters desired. What usually happens is that one pure line excels in one character, while another is superior with regard to some other character. The only way in which the desirable characters belonging to different strains can be com- bined is by crossing and then selecting the desired segregate. To attain success in this field, it is important to use as parents those forms which most nearly approach the combination of characters desired. The old idea of indiscriminate crossiiig in order to procure superior economic characters, such as yield, has been largely abandoned, which is reasonable from our knowl- edge of what selection accomplishes and of Mendel's law of in- heritance in crosses. Love (1914) compared the yield of oat selections with hybrids which were the result of more or less indiscriminate crosses made by J. B. Norton. The average yield of the hybrids was but little higher than the average yield of the selections. It is probable that the comparison would have shown a greater difference if the parents had been chosen on the basis of their performance records. Two forms may be crossed because each possesses to the greatest degree the character sought, with the hope of obtaining transgressive segregation; or a cross may be made to combine different characters. The Improvement of Black Oats at Svalof.— Nilsson-Ehle (1917) has reported experiments carried on from 1901 to 1917 for the purpose of improving the black oats grown in Sweden. The native oats formerly grown had weak straw and lodged badly. Black Tartarian oats was introduced to overcome this difficulty. Little by little this form mixed with the native oats and probably naturally crossed to some extent. The resultant complex (Svart Tartarish Plymhafre) was especially suitable for selection and the 132 IMPROVING SELF-FERTILIZING CROPS 133 isolation of desirable forms. This was done at the Svalof Station. The selections, Klock 1 and Stormogul, maturing early and late respectively, were obtained. Both possess stiff straw and Storm- ogul has good yielding ability. The improvement of the latter character was sought by crossing with higher yielding light colored forms. The following diagram indicates the method followed. The varieties and strains were purified before the crosses were made. Svart Tartarish Plymhafre Milton (Probsteier type) I Klock I (1901) X Guldregn (1903) I Stormogul (1901) X Klock II (1909) Klock III (1917) Klock II is the result of crossing a good-yielding black oat of stiff straw (Klock I) with a high-yielding yellow oat (Guldregn). The offspring has the stiff straw of Klock I and the high-yielding ability of Guldregn. One selection in the cross of Klock II with Stormogul gave a strain, Klock III, which has the early maturity of Klock II, a somewhat higher yielding ability than Stormogul, as well as non-lodging ability, which last character both parents possessed. In Table XXX the yields of three of the strains are shown. TABLE XXX. — RESULTS OF COMPARATIVE YIELD TRIALS OP THE VARIETIES KLOCK II, STORMOGUL, AND A SEGREGATE KLOCK III OF A CROSS BETWEEN STORMOGUL X KLOCK II AS OBTAINED AT SVALOF FROM 1912 TO 1916 Yield pe r hectare Relative Grain 1912, kg. 1913, kg. 1914, kg. 1915, kg. 1916, kg. Average, kg. Klock II = 100 Klock III Stormogul Klock II . . 3,780 3,860 3 730 4,170 4,160 3 870 2,560 2,700 2360 3,010 3,030 2 280 4,580 4,160 4 230 3,620 3,582 3 284 109.9 108.7 100 0 Straw Klock III 5,060 4530 2 470 3 825 7 850 4 747 100 3 Stormogul .... Klock II 5,810 5 260 5,330 4 470 2,850 2 310 4,550 4 300 7,630 7 330 5,234 4 734 110.6 100 0 134 BREEDING CROP PLANTS A Wheat Cross Made at Svalof.— The highest yielding winter wheat grown at the Svalof Station, reported by Newman (1912), was a cross, Extra Squarehead II, No. 0290. This wheat is one of the offspring of Old Extra Squarehead X Grenadier II. It combines the winter-hardiness and rust resistance of the former with the stiff straw and high yield of the latter. As an average of four years' trial at Svalof and Alnarp, 1his wheat has yielded 18 per cent, more than Old Extra Squarehead and 8 per cent, more than Grenadier II, which was next. No variety of winter wheat has proved so generally popular among the farmers of southern Sweden as Extra Squarehead II. It may be of interest to point out that preceding the cross, hundreds of selections out of Grenadier II were examined in search of a pure line with the combination of rust resistance and high yield. Wheat Breeding at University Farm, Cambridge, England. — Most of the wheat varieties grown in England are very susceptible to yellow rust (Puccinia glumarum). Biffen (1917) set himself the task of breeding a high-yielding, resistant form. He crossed American Club, which is very resistant to this parasite, with several susceptible varieties in order to study the mode of in- heritance and develop a standard technic of operations. In all crosses the Fz generations showed monohybrid segregation with resistance behaving as the recessive. The resistant individuals were rather clear-cut, although they sometimes exhibited uredinia. The susceptible plants showed a wide range of variation. No recognizable morphological character has been found correlated with resistance. The constancy of resistance in wheats of hybrid origin has also been studied by Biffen. For the purpose he used a resistant strain produced from a cross between American Club, a resistant variety, and Michigan Bronze, which is one of the forms most susceptible to -yellow rust. During eight years of observation the hybrid variety proved just as resistant as the American Club. A resistant variety of Russian origin, found among some Gurka wheats, which was not adapted to local conditions, was crossed with Square Head's Master, the variety most commonly grown in England. Among the resistant offspring is one that gives considerable promise. Comparative trials of this wheat (Little Joss) over a period of seven years show it to yield about 4 bu. per acre more than the best of the English and French wheats. The explanation for this would seem to be that Little Joss in- IMPROVING SELF-FERTILIZING CROPS 135 herited the yielding capacity of Square Head's Master as well as the resistance of the Russian wheat parent. Farrer's Wheat Breeding in Australia. — Probably no one has made more wheat crosses that have proved valuable than William Farrer of Australia (Sutton, 1910). Most of his work was done without the application of a knowledge of the Mendelian princi- ples. He, however, made crosses for definite purposes and in reality followed the Mendelian mode of work without recognizing the law involved. Farrer strongly featured composite crossing, i.e., the crossing of parents which were themselves of hybrid origin. Federation, a variety very popular in southern Australia, was produced in this manner. As a typical example of Farrer's method, the history of Federation will be given somewhat in detail. This variety was the outcome of a deliberate attempt to produce a wheat especially suited to gathering with a stripper, a harvester used in Australia. Federation is early maturing, stiff-st rawed, erect, and of somewhat short growth. Despite its rather unattractive appearance, it is one of the highest yielding wheats for the section in which it is grown. The upright habit makes it easy to harvest. Furthermore, the grains are held tight enough to prevent shattering but not tight enough to interfere with the operation of the stripper. Federation resulted from a cross between the varieties Purple Straw and Yan- dilla. The parentage is indicated in the following diagrammatic scheme: Improved Fife X Etawah Purple Straw X Yandilla Federation The history of the origin of Bunyip, another Farrer production, is indicated as follows: Improved Fife X Purple Straw Blount's Lambrigg X Hornblende I An unnamed cross-bred X King's Jubilee Rymer X Maffra Bunyip 136 BREEDING CROP PLANTS Among other varieties produced by crossing which are of economic importance may be mentioned Comback, Cedar, Firbank, Bobs, Florence and Cleveland. Farrer's method of breeding seems to have been based on inducing maximum variation through composite crossing and then subjecting the progeny to selection. He was a keen observer and possessed ability to pick out forms which proved of economic value. This emphasizes the need of a knowledge of the charac- ters of a crop with which the breeder is to work, which is as essential as a knowledge of laws of breeding. FIG. 27. — A section of the winter wheat plant breeding nursery in the spring of 1918. The three rows at the right are Minhardi, a very winter-hardy wheat produced from a cross of Odessa with Turkey. In right center are three rows of Turkey, Minn. 1487. Marquis Wheat. — If the spring wheat known as Marquis (Saunders, 1912) were the only one of economic importance which had been produced by artificial crossing, the practice would be justified. The early history of this wheat is somewhat obscure. It is one of the descendants of a cross between an early ripening wheat from India, Hard Red Calcutta 9 and Red Fife cf . The cross was made by A. P. Saunders, probably at the experimental farm at Agassiz, Canada, in 1892. The crossed seed or its progeny was transferred to the Ottawa Experimental Farm. In 1903 Chas. E. Saunders took charge of the cereal breeding at this place and immediately initiated a series of selections from the IMPROVING SELF-FERTILIZING CROPS 137 progeny of cross-bred wheats. The progeny from the cross made by A. P. Saunders was found to differ strikingly in gluten content of seeds. The laborious practice of chewing a small sample of each pure line was made the basis of selections . One of the high-gluten selections isolated from this mixture of types was named Marquis. It was first grown as a pure form in 1904 and the bread-making tests made in 1907 fully established its bread-making qualities. In addition to this character it is early ripening, thus often escaping rust, has stiff straw, high yielding ability, distinctive appearance of seed, and remarkably wide adaptations. These qualities have made it popular among the farmers in the spring wheat belt. FIG. 28. — Minhardi, Minn. 1505. Grown in 1918. This variety is very winter-hardy. Winter Wheat Breeding at the Minnesota Agricultural Experi- ment Station. — One of the most urgent needs in order to bring about the successful production of winter wheat in Minnesota is a strain which will withstand the severe winters. This ideal has more or less been the goal of breeding operations from the first. Yield and quality also have been given considerable attention. Before attempting crossing, varieties were obtained from all over the world. Odessa, an awnless, red-chaffed variety of Russian origin, has proved most winter-hardy , although 138 BREEDING CROP PLANTS some of the more recent Turkey selections are nearly as hardy. In the large number of crosses that have been studied since 1902 there is an outstanding fact worthy of emphasis from a plant breeding viewpoint. Of the different crosses made, none proved as winter-hardy as the Odessa-Turkey combination, although numerous crosses between other winter wheats were studied. This shows the necessity of studying carefully prospective paren- tal material to determine what should be used. When Odessa was used it furnished an hereditary complex capable of with- standing severe winters (Hayes and Garber, 1919). FIG. 29.— Turkey, Minn. 529. Grown in 1918. very badly. This variety winter-killed At University Farm and at Waseca one of the Odessa-Turkey crosses, Minhardi, (Minnesota No. 1,505) has proved more winter-hardy than the Odessa parent. This cross also possesses very high yielding ability but the quality of seed is somewhat inferior. Its ability to yield is probably inherited from the Turkey, which yielded high in favorable seasons. Minturki (Minnesota No. 1,507) is a bearded wheat obtained from a cross of Odessa with Turkey. It is somewhat less winter-hardy than Minhardi but it excels in quality and yielding ability. Table XXXI presents data on some of the more promising forms of winter wheat for Minnesota conditions. IMPROVING SELF-FERTILIZING CROPS 139 TABLE XXXI. — AVERAGE YIELDS AND AVERAGE WINTER INJURY OF THE BETTER WINTER WHEATS GROWN AT UNIVERSITY FARM AND AT THE WASECA SUBSTATION Variety or N.S.N. Minne- sota acces- Minne- sota University farm Waseca, 1918 Average winter cross sion No. No. 1916, bu. 1917, bu. 1918, bu. bu. injury, per cent. Turkey 1-03-213 829 829 24.7 30.0 7.7 40.1 51 Odessa 1-01-3 558 1,471 32.9 36.1 24.8 30.6 38 Turkey X Odessa 11-02-195 829 X 558 1,505 37.0 43.1 40.9 35.3 29 Turkey X Odessa 11-02-280 829 X 642 1,507 38.7 47.5 20.9 32.5 42 Turkey 1-03-68 529 1,487 27.5 39.0 5.3 20.2 73 Turkey 1-03-120 1,488 1,488 33.4 40.9 36.5 36.4 40 The pure-line parentage of the Turkey-Odessa crosses is not known, although the parents are believed in all cases to have originated from a single plant. At the time the crosses were made, in 1902, there was no pedigreed Turkey available with the winter- hardy ability of Minnesota 1488. One of the recent crosses made is between the best Turkey selection, Minnesota 1488, and Min- hardi. Both parents are winter-hardy and are good yielders; the Turkey likewise produces good quality seed. This shows what is believed to be the correct procedure in plant breeding. Breeding Beans Resistant to Colletotrichum Lindemuthia- num. — Extensive tests of the reaction between physiological strains of anthracnose and host plants were made at the Cornell Station. Four groups of beans were obtained; (1) Resistant to both strains, (2) resistant to strain A and susceptible to strain F, (3) susceptible to A and resistant to F, (4) susceptible to both F and A. Wells' Red Kidney was practically immune to strain A and highly resistant to strain F, while Michigan Robust carried resistance to the F strain only. The latter is a white navy bean of superior yielding ability as was pointed out in the preceding chapter. McRostie (1919) crossed these two varieties to obtain a bean which possessed in addition to the characteristics of Robust, resistance to strain A of anthracnose. Segregation occurred for resistance to strain A on a simple Mendelian 3: 1 ratio with susceptibility recessive. In the second and third generations, a white navy bean homozygous for resistance to both physiological strains of anthracnose was obtained. An Improved Strain of Tobacco. — Connecticut Havana tobacco introduced among Wisconsin farmers gave satisfactory results as 140 BREEDING CROP PLANTS to quality but the yield was low. Johnson (1919), of the Wiscon- sin Agricultural Experiment Station, attempted to overcome this objection by breeding. In 1909 a pure-line study revealed the fact that there were no less than three distinct morphological types present in the particular variety, grown at the experiment FIG. 30. — Tobacco No. 27. A pure line strain with a high leaf number and a low breadth index of leaf. (After Johnson, 1919.) station, which was introduced from Connecticut. Selections No. 26 and "No. 27 differed distinctly from the normal or prevailing type. Form 26 carried fewer leaves but of larger size than the normal, while form 27 possessed more leaves which were some- what smaller in size than the normal. A cross between 269 and 27 cf was made in 1910 with the hope of combining the desirable features of the two forms. The success of the cross is indicated in the following data taken from Johnson. IMPROVING SELF-FERTILIZING CROPS 141 TABLE XXXII. — SUMMARIZED DATA OP MOST SIGNIFICANT CHARACTERS OP CONNECTICUT HAVANA No. 38 TOGETHER WITH PARENT AND NORMAL STRAINS. AVERAGE OP EIGHT YEARS Strain • Leaf No. Average of to] bottom 3, middle, and leaves Breadth index of Length, in. Width, in. leaf No 26 14 2 20 0 11.3 56 5 No. 27 18.0 18.0 9.6 53.6 No. 38. = 26 X 27 No. 33 16.9 15.5 19.1 18.2 10.6 9.8 55.8 53.8 FIG. 31. — Tobacco No. 26. A pure line strain with a low leaf number and a high breadth index of leaf. Note the method of insuring self-fertilization by covering the terminal inflorescence with a manila paper bag. (After Johnson, 1919.) No. 33 is a desirable strain of the normal Connecticut Havana type produced by continued selection and inbreeding. Breadth 142 BREEDING CROP PLANTS index is obtained by dividing the average leaf breadth by the average length and multiplying by 100. The table shows that by crossing, a form, No. 38, was obtained which combined some- what the desirable features of the parents (Nos. 26 and 27) and is superior in both number and size of leaves to the better pure-line obtained by selection (No. 33). As an indication of the commercial reception of this new form, it was esti- mated that at least 10,000 acres of No. 38 were grown in Wisconsin in 1919 cut of a total of about 40,000 acres. Here we have an example of crossing two closely-related forms and obtaining from the resul- tant progeny a strain of more commercial value than either parent. The illustrations bring out more clearly some of the features of the parents and progeny. Summary. — In this chapter concrete evidence FIG. 32.-Tobacco No. 38. This strain °f the Value °f crossing as was produced by crossing No. 26, which ex- a means of producing im- cels in leaf breadth, with No. 27, which is nrftvpj VJ,riWiV QJ D. ft T3 r2 « d rf S tits? „ 'a'ft'ft '"So ,2 ,2 ^ C 95 M w « .2 .2 2 .2 02 02 02 w ^ -/ / -/ If ll 1*1 ap est ft ft 73 T) T3 T3 « ^ ft ft -a -a » CJ ^ II 88 varietie Adapted Average O O O O t^ rH 00 CO rH TjH t» O S rH ft c sf < -I s S § 2 g i i £ iiini i ii S s .2 r? W W £ « S 2 S S If if r; ^22 •8 -8 'S ft o o Q 204 BREEDING CROP PLANTS general rule can be given and the only sure means of determining the value of a cross is by the experimental test. Results have shown that FI crosses between good yielding varieties which differ from each other in several characters frequently yield con- siderably more than either parent and more than pay for the trouble of producing crossed seed. Thus the tests made in Con- FIG. 47. — Fi cross of Minn, self-fertilized strains No. 1 X No. 4. necticut (Jones et al, 1919) and those carried out in Minnesota (Hayes and Olson, 1919) showed thatFi crosses between selected eight-rowed flints and dents very frequently exceeded either parent in yielding ability. For each growth character in which the parent varieties differ there is usually an intermediate condi- tion in FI. There is a tendency for a partial dominance and the first generation often exceeds the average of the parents in most MAIZE BREEDING 205 of its characters. Fi crosses are of value from the standpoint of earliness. Thus a cross, studied at Minnesota in 1919, between Squaw flint and Minnesota No. 13, approached the dent parent in height of plant and the flint parent in earliness and exceeded both in yield. Such a cross would be of much value as a silage or husking variety under northern conditions. The production of crossed seed is not very difficult. The va- rieties to be crossed may be planted in alternate rows and the tassels removed from one variety before any of the pollen has matured. Seed produced by the detasseled variety is known as first generation crossed seed. If the varieties to be crossed differ in maturity they should be planted at different times so that both bloom at about the same date. Isolation of Homozygous Strains.— Shull (1908, 1909) first suggested the utilization of crosses between self-fertilized strains as a means of increasing yield in corn. Such crosses often give very high yields. The chief objections to this method are that self-fertilized strains are usually of very low yielding capabilities and that the seeds from self ed lines are usually much smaller than from normally pollinated corn. Even though crosses between self -fertilized lines yielded very vigorously, the method has not seemed commercially desirable. Low yields of seed per acre would increase the cost of seed. Under unfavorable conditions the food supply of the seed might not give the young F± plant a vigorous start. Jones (1918) has made a suggestion which removes some of these objections. After isolating selfed strains, tests are made to determine which four biotypes are most desir- able as parents. Suppose these are numbered 1, 2, 3, and 4 respectively. Numbers 1 and 2 are crossed, also 3 and 4, by detasseling all of one biotype in each group. Seed from the plants of each detasseled biotype is then planted in alternate rows in an isolated plot and all of one combination, as 3 X 4, detasseled. Seed from these detasseled rows is used for com- mercial planting. This method seems worthy of more extensive trial. Such a cross was compared at the Connecticut station with the best dent variety obtained from a varietal survey followed by a vari- ety test. The highest yielding dent variety gave a yield of 92 bu. while the cross under similar conditions yielded 112 bu. Every investigator who has produced self-fertilized strains of corn has been impressed by the large number of undesirable 206 BREEDING CROP PLANTS abnormalities which are isolated. These abnormalities through ordinary seed selection are not eliminated from the commercial variety. Self -fertilized strains, however, stand or fall upon their FIG. 48. — Average yields of 4 self-fertilized corn strains above; F\ crosses in the center; the double cross below. (After Jones.) own merits. Through self-fertilization the unfavorable strains may be eliminated. Crossing of the more desirable strains followed by selection seems a logical method for synthetically producing improved maize varieties. CHAPTER XV GRASSES, CLOVER, AND ALFALFA The importance of hay crops in the world's agriculture makes desirable their consideration from the standpoint of improve- ment by breeding. Grasses, clover, and alfalfa differ strikingly in (see Chapter III) amount of seed set when artificially self- pollinated. Red clover (Trifolium pralense) is practically self- sterile; white clover (Trifolium repens) sets -few seeds when protected from insect pollination; timothy (Phleum pratense) under a bag produces few seeds; and brome grass (Bromus arvensis) under the same conditions sets seed abundantly. Although common alfalfa (Medicago saliva) and yellow alfalfa (M. falcala) cross freely, seed of either may be produced by selfing. Enough examples have been cited to show that there are not only differences in the modes of pollination in the three mentioned classes of hay crops but also differences within each class. Carefully controlled experiments with grasses to determine the percentage of naturally crossed and naturally self- fertilized seed are very limited. To what extent decrease in vigor will result from artificial self-pollination is also an un- answered question. When self-sterility is not a limiting factor, the methods of breeding all these crops are essentially alike. The ease with which some of them may be clonally reproduced has led to slight modifications in breeding technic. In the following brief discussion, the aim has been to choose a few examples rather than to enter into an exhaustive treatment of the entire field. GRASSES Timothy ranks far ahead of the other grasses in importance. Some of the other hay grasses which may be mentioned are orchard grass (Dactylis glomerala), tall oat-grass (Arrhenatherum elalius), and brome grass (Bromus inermis). These three grasses are adapted to certain conditions better than is timothy. Some important pasture grasses are Kentucky bluegrass (Poa pralen- 207 208 BREEDING CROP PLANTS sis), Canada bluegrass (Poa compressa), arid redtop (Agrostis palustris) . The variability (see Fig. 49) of each of the different species of grasses presents a wealth of material for breeding purposes. FIG. 49. — Individual timothy plants grown under like conditions. The upper plants are undesirable, one having weak stems and the other lacking vigor. The lower plants are more desirable. They differ in density of plant and number of culms. (Courtesy of Myers.) Moreover, the fact that many of them may be conveniently prop- agated as clones facilitates a study of the value of individual FIG. 50. — Flowers of timothy. 1. Spike. 2. Floret — a, anther; b, filament; c, branched stigma; d, style; e, ovary; /, outer glume. 3. Outer glume. 4. a, feathery stigma; 6, style; c, ovary. 5. Spikelet showing a, palea; b, floral glume. (After Seal after Trinius and Scribner.) Size: 1, $in; 2, 80n; 3, 4, 5, greatly enlarged. ' "• GRASSES, CLOVER, AND ALFALFA 209 a FIG. 50. 210 BREEDING CROP PLANTS plant selections. The hereditary constancy of forms so isolated may be tested by selfing or by adopting methods which insure close breeding. Breeding Timothy. — The United States Department of Agri- culture has carried on extensive experiments in timothy breeding at New London and North Ridgeville, Ohio, but unfortunately the work has not been published. As a result of breeding, two improved varieties have been widely distributed through the Ohio Experiment Station. The Cornell and Svalof Experiment Stations have done considerable timothy breeding. Webber et al (1912) published a detailed report of the experi- ments as carried on at Cornell. Samples of timothy seed were procured from various sources in the United States, Canada and other countries. This seed produced an abundance of different forms from which selections were made. Individual plants were selected on the basis of the following characters; 1. High-yielding ability. 2. Height. 3. Broad and thick plants, which stool abundantly. 4. Many and dense culms. 5. Erect, non-lodging plants. 6. Many large leaves. 7. Leaves extending well toward the top of the plant. 8. Leaves remaining green until plant is nearly ready to harvest. 9. Rust resistance. 10.' Spikes of medium size, setting seed freely. The ultimate aim was to produce a high-yielding variety. A selected plant was dug up and vegetatively propagated by separating bulblets from it. The bulblets were set out in rows (16 to 24 per row) and allowed plenty of space for individual development. Self-fertilized seed from these various clones was planted in sterilized soil and the seedlings were transplanted in rows as above. By a comparison of these rows and the re- spective clones from which they came it was found whether they were breeding true for the characters desired. When sufficient seed was available, plots were sown broadcast and yields obtained. As soon as a form appeared valuable and bred comparatively true, it was isolated and increased. According to Webber self-fertilized seed may be produced by placing several spikes of the same plant, just before blooming, under a paper bag. At University Farm Minn., only a few seeds GRASSES, CLOVER, AND ALFALFA 211 were obtained by this method. Another method is sometimes used in clonally propagated rows. Each row is surrounded by a fence about 10 ft. high made of finely woven cloth. This method does not prevent some cross-pollination but it does bring about a high degree of inbreeding. A tall growing crop, such as rye, surrounding isolated plots prevents pollination with un- desirable strains or varieties. As would be expected in dealing with a heterozygous crop, the self-fertilized seed of the various isolated clones produced plants which showed considerable difference in their inheritance. Some of the clones bred fairly true when reproduced by selfed seed, others did not. Table L, taken from Webber et al (1912), illustrates the transmission of yielding ability in some clones. TABLE L. — TRANSMISSION OF YIELD IN TIMOTHY BY CLONAL AND SEED PROPAGATION Number of original plant Plat No. Average yield per plant of mother by clonal propagation (ounces) Plat No. Average yield per plant of progeny by self- fertilized seed propaga- tion (ounces) LIGHT-YIELDING PLANTS 12.07 1,797 1.005 3,216 2.121 9.03 1,713 1.830 3,109 3.364 104.30 1,794 1.982 3,213 4.071 191.19 1,785 2.283 3,142 3.143 811.02 1,728 2.542 3,166 1.925 128.19 1,799 2.462 3,217 0.966 211.31 1,792 2.806 3,211 1.905 212.36 1,653 2.811 3,143 4.140 8.04 3,011 2.941 1,959 3.714 107.30 3,033 3.158 1,960 1.182 HEAVY-YIELDING PLANT 271.26 1,660 13.521 3,152 11.455 887 . 10 1,620 13.783 1,905 7.600 875.30 ,752 13.811 3,182 7.915 224.15 ,619 14.133 1,904 9.000 860.30 ,744 14.517 1,934 7.636 820.27 ,740 15.587 3,206 10.844 860.25 ,743 15.970 1,931 9.428 889 . 31 3,189 16 . 000 3,190 9.043 245.28 1,796 16.308 3,215 9.457 37.31 1,630 20.274 3,122 7.636 212 BREEDING CROP PLANTS The practical results which have been attained by this method of breeding are brought out in Table LI, also taken from Webber etal TABLE LI. — SUMMARY, SHOWING YIELD OF FIELD-DRY HAY Yield in pounds per acre 1910 1911 Average yield of 17 new varieties. Average yield of 7 checks Actual average increase 7,451 6,600 851 7,153 4,091 3,062 FIG. 51. — View of vegetatively propagated row plots of timothy. Each plot is propagated from a single, original plant. Note that the two central plots are comparatively late in maturity; also note differences between these two strains, one having erect culms and heads, the other having somewhat spreading culms and long loose heads. (Courtesy of Piper.) The season of 1911 was particularly unfavorable for the growth of timothy. The new varieties gave a greater increase that year than in the preceding and more favorable one. Webber et al attribute this difference partly to the rust resistance of the new strains. The method of breeding timothy at Syalof as reported by Witte (1919) is not essentially different from that practiced at GRASSES, CLOVER, AND ALFALFA 213 Cornell. Individual plant selections are vegetatively propa- gated in plots isolated as much as possible. Seeds produced by the better clones are planted in varietal plots for comparison. The best commercial varieties are also grown for comparison. When a new variety proves superior and has practical uniformity, it is increased and distributed on a large scale. A comparison of ordinary timothy and two improved forms distributed by the Svalof Station is shown in Table LII (Witte, 1919). TABLE LII. — YIELD OF DIFFERENT VARIETIES OF TIMOTHY IN TRIALS AT SVALOF, 1909-1918 Variety Kilograms of green fodder per hectare First year's lay Second year's lay Total Yield per cent, compared to ordinary Swedish timothy Svalof s Gloria .... Svalof 's Primus. . . . Ordinary Swedish. . 14.48 13.46 11.57 11.03 10.21 9.59 25.51 23.67 21.16 120.6 111.9 100.0 Timothy, like many other grasses, is susceptible to a rust (Puccinia graminis). It has already been mentioned that in making selections at the Cornell Station some attention was given to resistance to this fungus. Eleven of the better Cornell selections have been tested for rust resistance (Hayes and Stakman, 1919). The relation of other characters to resistance was also studied. The rust classes are; 1, no rust; 2, slight in- fection; 3, moderate infection: and 4, heavily rusted. Average erectness is taken with 1 as a basis of an erect plant and 10 a procumbent one. Table LIII presents the data. From the table it is apparent that the Cornell selections possess a high degree of resistance. Relatively few plants are found in rust classes 3 and 4. The Minnesota selections show the reverse condition, i.e., most of the plants are found in classes 3 and 4. These facts show that a variety of rust-resistant timothy may be isolated. Timothy breeding may be briefly summarized as follows: 1. Individual plants propagated vegetatively in rows. Bulb- lets are placed far enough apart in the row to give ample room for individual development. 2. The clones produced in 1 are closely inbred or seed is saved from vegetatively multiplied plants in isolated plots. By 214 BREEDING CROP PLANTS TABLE LIII. — RUST RESISTANCE IN TIMOTHY IN RELATION TO OTHER CHARACTERS AS SHOWN BY VARIOUS DATA Variety Rust classes a iB in Erectness mean •3* S ;*"§ %"•* Average height, cm. Average numbers of stools 1 2 3 4 Cornell 1,611 80 11 l 0 1.1 1.0 2.3 11.9 88 122 Cornell 1,620 77 26 3 2 1.3 1.0 2.2 18.3 88 138 Cornell 1,630 79 15 9 2 .3 0.9 3.1 12.3 85 141 Cornell 1,635 61 14 9 3 .5 0.8 2.9 11.4 85 109 Cornell 1,671 56 13 13 5 .6 0.8 2.9 11.6 89 123 Cornell 1,676 87 10 3 6 .3 0.9 2.9 13.1 87 116 Cornell 1 687 86 12 9 6 .4 0.9 3.7 11 .3 88 131 Cornell 1,715 90 11 6 3 .3 0.8 3.0 10.3 86 98 Cornell 1,743 100 3 12 7 .4 1.0 5.6 10.9 84 134 Cornell 1,777 36 0 4 0 .2 0.9 5.7 11.5 83 142 Cornell 3,230 32 5 5 0 .4 0.9 3.1 12.9 86 117 U. 8. Dept. Sel. 1 ... 2 12 70 40 3.2 0.6 2.6 13.0 91 64 U. S. Dept. Sel. 2 ... 4 4 19 13 3.0 0.8 3.3 10.9 87 90 U. S. Dept. Sel. 3 ... 0 7 15 9 3.1 0.8 2.8 13.3 92 72 L. L. May Sel. 1 .... 2 3 24 15 3.2 0.6 2.3 12.4 86 70 L. L. May Sel. 2.... 8 5 25 6 2.7 0.6 3.1 10.1 80 92 Griggs Bros. Sel. 1 . . 1 1 15 5 3.1 0.7 3.0 12.8 86 91 planting the seed so produced clones are tested for transmission of the desired characters and also for uniformity. 3. When sufficient seed is available, plots are sown broadcast and tests for yield are obtained under ordinary field conditions. 4. A selection which has shown performance ability is in- creased in isolated plots and distributed to the farmers. CLOVERS The importance of clovers as forage crops and their role in soil improvement make them of great economic value. Tri- folium pratense, or ordinary red clover, is by far the most widely grown. Alsike clover (T. hybridum), because it may be grown in more acid soil than the other clovers, is favored in certain localities. Some of the other clovers are white (T. repens), crimson (T. incarnatum) , and the sweet clovers (Melilotus alba and M. officinalis). All of these species are biennial or peren- nial except T. incarnatum, which is an annual. Red Clover. — It has been demonstrated several times that the species T. pratense will set practically no seed when protected from the visit of insects, particularly bumblebees. However, this is not the only factor which influences fertilization. West- gate et al (1915) found that moist soil and atmospheric condi- GRASSES, CLOVER, AND ALFALFA 215 tions induced the formation of a large percentage of infertile ovules. All the cells remained sporophytic, no reduction taking place with the formation of an embryo sac. As much as 100 per cent, ovule infertility was found in the first clover crop. The rate of pollen tube growth was shown to be much slower in self- than in cross-pollinated plants. It is probable that pollen- tube growth is too slow to effect fertilization when the plant is selfed. The pollen of red clover is easily burst by an excess supply of moisture. Martin (1913) demonstrated that good artificial germination of pollen could be obtained on membranes which were just moist enough properly to regulate the supply of water to the pollen. He suggests that the stigma of red clover performs the same function as the membranes. The above facts necessitate a method of breeding which is essentially a restricted form of mass selection. Before starting selection it is desirable to make comparisons of the varieties prod- uced by other breeders and of commercial seed from different sources to obtain the best form for further breeding operations. A seed plot may then be used, in which each plant is spaced so that its characters may be determined. Undesirable plants should be removed before pollination. By repeating this process, forms with the desired characteristics and with practical uni- formity may be isolated. Selection for Disease -Resistant Clover. — Clover anthracnose (Colletotrichum trifolii), causes serious injury to red clover in certain regions. Bain and Essary (1906) issued a preliminary report on isolating an anthracnose resistant red clover. Healthy plants in a badly infested field were located late in the season after most plants had been killed by the disease. The seeds of the chosen plants were planted separately in alternate rows with ordinary commercial seed. Measures were taken to insure the infection of every seedling with anthracnose. By June 1 the commercial plants began to show symptoms of the disease and by the middle of September not more than 5 per cent, of them were living, while 95 per cent, of the selections were healthy and making a fair average growth. Some of the latter showed small lesions, but growth was not seriously injured. ALFALFA Alfalfa is one of the oldest, if not the oldest, plant cultivated for its forage only (Piper, 1916). Most of the cultivated forms 216 BREEDING CROP PLANTS belong to the species Medicago saliva. The only closely related species of economic value is M. falcata, sometimes called sickle alfalfa or yellow-flowered alfalfa. The two species cross readily, as Waldron (1919) has shown (for pollination studies on alfalfa see Chapter III. Piper et al (1914) found that alfalfa set more seed when cross-pollinated than when selfed, although the selfed set considerable seed. It also was demonstrated that automatic tripping with consequent self-pollination may occur under certain conditions. Grimm Alfalfa and Winter Hardiness. — Westgate (1910) and later Brand (1911) suggest that the origin of Grimm alfalfa is probably the result of natural crossing between cultivated alfalfa, M. saliva, and wild plants of the yellow-flowered sickle lucern, M. falcata, found especially in Germany, Austria, Roumania, and certain regions of Italy. The seed from which the Grimm variety eventually resulted was brought to Carver County, Minnesota, by a German immigrant farmer, Wendelin Grimm, in 1857. Here for 50 years the original variety was subjected to the severe Minnesota winters and as a result the non-hardy types were eliminated. At the present time Grimm alfalfa is probably the hardiest variety grown. Waldron (1912) reported the result of testing for winter hardi- ness sixty-eight different strains of alfalfa assembled from various parts of the world. The trial was made at Dickinson, N. Dak. during the severe winter of 1908-09. The two strains of Grimm alfalfa included in the experiment proved to be the hardiest. On an average, less than 5 per cent, of the Grimm plants were killed and only one other strain showed less than 10 per cent, killed. Disregarding twelve strains which were destroyed completely, the average percentage killed for the other strains, considered as a unit, was 77.5. To bring out the fact that differences between strains in their respective reactions to cold are genetic, Waldron computed FIG. 52. — Structure of alfalfa flowers. 1. Branch showing flowers in position. 2. Single flower showing — a, standard; 6, sexual column in contact with standard; c, keel; dt wings. 3. Seed pod. 4. Flower parts in position — a, undeveloped pod; b, ovary; c, filament; d, anther. . 5. Same with all anthers removed except one to show stigma. 6. Anther. Size: 1, about %n; 2, about 2n; 3, about }^n; 4, 5, 6, greatly enlarged. GRASSES, CLOVER, AND ALFALFA 217 6 FIG. 52. 218 BREEDING CROP PLANTS correlation coefficients. Two nurseries had been planted on succeeding years with the same strains taken from the same original lot of seed. The percentage of killing of the various strains in one nursery during the winter of 1908-09 was correlated with similar data collected from the other nursery after the winter of 1910-11. A correlation coefficient of +0.62 ± 0.06 was obtained. Some of the surviving plants of the different alfalfa strains were selfed and the seeds so obtained were planted separately in FIG. 53. — Comparative hardiness of Grimm and common alfalfas. The two rows in the center are from Grimm seed. At either side are rows grown from southern grown common seed. 1916 season. (Photo loaned by Arny.) a third nursery. Percentage of winter killing of these strains was taken and the. correlation coefficient between the percentage of winter killing of the parental stock and that of the new strains was determined. The correlation coefficient obtained was -f-0.46 ± 0.07. The mean winter killing (expressed in per- centage) of the parental stock was 27.43 ± 1.75 as compared with 6.43 ± 0.66 for the strains coming from selfed seed. In other words, progress has been made toward isolating hardy biotypes. CHAPTER XVI POTATO IMPROVEMENT Potatoes have been generally introduced into cultivation since the discovery of America, and are now a crop of major im- portance in many countries. The large number of varieties is an illustration of the rapid development in domestic plants of varie- ties which are suited to special soil and climatic conditions. As potatoes are reproduced commercially by tubers, they furnish an excellent illustration of the way in which vegetative reproduction modifies breeding methods. Origin and Species. — There are from five to 100 species of tuber-bearing potatoes according to the number of forms which are recognized as separate species (East, 19086: Wight, 1916). Whether the cultivated potato arose from a single wild species or from several is a debatable question. The preponderance of opinion is that there is only a single wild species, Solanum tubero-, sum L., which deserves to be considered as the stem form from which all cultivated varieties arose. Wight (1916), after care- fully examining herbarium material, previous records, and wild species, makes the following statements: "Every reported occurrence of wild S. tuberosum that I have been able to trace to a specimen, either living or preserved in the herbarium, has proved to be a different species. I have not found in any of the principal European collections a single specimen of Solanum tuberosum collected in an undoubted wild state." Berthault (1911) cites Heckel, Planchon. and Labergerie as examples of recent workers who believe that other wild species gave cultivated S. tuberosum forms by mutation; Planchon be- living that the original form was S. commersonii; Heckel that S. maglia through mutation produced cultivated potatoes; while Labergerie believed both of these species gave cultivated forms through mutation. Berthault attempted to answer the question by growing seeds and tubers of both these species and also by growing seed of several cultivated varieties. Progeny of seed or tubers of S. maglia and S. commersonii gave no forms which 219 220 BREEDING CROP PLANTS approached in calyx or corolla characters the conditions found in S. tuberosum cultivated varieties. Progeny of seed of cultivated varieties showed Mendelian segregation, but no characters were obtained which had not been observed in ancient cultivated varieties. Wittmack (1909), after a careful botanical study of species, reached the conclusion that S. tuberosum was the stem species from which all cultivated potatoes arose. The evidence presented by De Candolle (1886) seems sufficient to prove that the potato was wild in Chile and in a form which is very similar to that of our cultivated plants. Heckel (1912) reports a study of changes under cultivation of Solatium tubero- sum forms collected in the wild in Bolivia and Peru by M. Verne. The wild plants were 0.25 meter in height, bore blue flowers and deep green foliage and tubers about the size of a hazel nut each produced at the end of a long stolon. These tubers were planted at Marseilles in a garden heavily fertilized with manure. Little change was observed in flower and fruit characters but there were pronounced changes in the subterranean parts. The yellowish tubers, each borne at the end of a much shortened stolon, con- tained a much greater amount of starch than wild tubers, while the characteristic bitter taste of the wild tubers disappeared. Much more profound changes occurred under cultivation with tubers of S. maglia (Heckel, 1909). There seems to be no good reason for speaking of all these tuber changes as mutations. It seems more in line with modern genetic usage to consider them as the normal expressions of the inherited factors under the new conditions of environment which occur under cultivation. The cultivated potato was first introduced into Spain and Portugal by the Spaniards during the first half of the sixteenth century.1 Clusius described and illustrated the potato from plants sent him in 1588 by the governor of Mons. The published description was made in Clusius' "Rariorum Plantarum Histor- ia" which appeared in 1601. The original plant obtained by Clusius bore two tubers and a fruit ball. This variety bore red- dish tubers and light purple flowers. The spread from this in- troduction was probably next into Italy and from there early in the seventeenth century to Austria, then to Germany, from Ger- many to Switzerland and then to France. Drake, after a West India piratical trip, took back the Roanoke 1 EAST, 19086. POTATO IMPROVEMENT 221 colony to England, including Thomas Herriott. Probably pota- toes were part of the stores obtained in the West Indies by Drake and these Herriott introduced into Ireland about 1586. This was the second introduction into Europe, the Spaniards deserving the credit for the first introduction. It is not known from what source the English colonists of Virginia and Carolina first obtained the potato, but it is generally believed to have been from Spanish or other travelers. Gerard described and illustrated the potato in his "Herbal" in 1597. The variety he described possessed light brown to yellowish tubers and violet to almost white flowers. Inheritance.1 — The transmission of potato characters through the seed is in conformity with Mendelian principles. Vegetative propagation allows the breeder to perpetuate any desirable geno- type even though heterozygous, which is the usual condition in the potato plant. While, in general, self-fertilization of a com- mercial variety gives rise to seedlings which vary a great deal, it is comparatively easy to obtain homozygosity for some characters. Tuber shape and size are important characters which are used as one means of varietal classification. Tuber shape has been found to depend essentially on the presence or absence of a single factor for length. According to this hypothesis a tuber may be homozygous long, homozygous round, or heterozygous long. Heterozygous long is the most variable of the three conditions. In one experiment two varieties with round tubers when selfed produced nothing but round tubers, while twelve varieties with oval tubers, when selfed, produced long, oval, and round tubered progeny. Nilsson (1912-13) found one variety of potato that did not breed true for round tubers. Long tubers were dominant to round in Fruwirth's (1912) experiments. Depth of eye is a character of considerable economic impor- tance. In general, shallow eyes were found to be dominant over deep eyes. Several factors, in addition to a chromogen body, have been recognized in tuber coloration. Red potatoes contain two genes, R, a reddening factor, and D, a developer of pigment. Purple and black tubers have, in addition to R and D, another factor, 1 The following discussion is based on inheritance studies made by SALAMAN (1909-11, 1910-11, 1911, 1912-13) and EAST (19106) except where otherwise noted. 222 BREEDING CROP PLANTS P. Segregating ratios were in accordance with the above fac- torial hypotheses. Wilson (1916) obtained only white tubers from selfed white-tubered varieties. Similar results have been obtained by other plant breeders which show that white is a recessive character. A certain amount of coloring in the young sprouts or shoots, stems, and sometimes in the leaf petioles was found associated with the presence of color in the tubers. With regard to flower color, three white-flowered varieties, selfed, produced only white flowers; and three out of four colored varie- ties, when selfed, produced both colored and white forms. Color is, therefore, dominant to its absence. Inheritance of this character may be explained by assuming the presence of a chro- mogen body and modifying factors. Heliotrope flowers are due to the chromogen body plus a reddening factor; purple flowers are produced by the addition of a purpling factor; white flowers may be due to the absence of one or more of these factors. Fruwirth (1912) found red tubers dominant over white, yellow flesh over white, and lilac-colored flowers over white. It was also found that different gradations of color were inherited. Nilsson (1912-13) found a complicated flower color inheritance. A variety with violet-blue flowers gave, on selfing, progeny with red, violet-blue, near-red, purple, dark and light blue, and white flowers. A variety with light blue flowers, on selfing, yielded progeny showing simple monohybrid segregation with white recessive. Evidence that several factors were operating in the inheritance of tuber flesh color was also obtained. Some of the varieties with yellow flesh (tubers) bred true when selfed, others segregated as dihybrids with white recessive. The inheritance of habit of growth was also studied. Plants may be upright, bushy, or procumbent. Bushy plants are heterozygous for habit of growth and many of them exhibit a dis- tinct tendency to become procumbent. Homozygous forms of upright and sprawling plants may be isolated easily. Period of maturity is used as a means of varietal classification. It is prob- ably inherited in the same manner as with other crops. Sterility of the anthers has been found to be a dominant character. At first Salaman believed that its inheritance was due to a single differential factor but later evidence indicated a more complex manner of transmission. Plants producing pale helio- trope flowers were found to be heterozygous for pollen sterility. MacDougal (1917) crossed the wild potato of Arizona, S. POTATO IMPROVEMENT 223 fendleri, which grows at a high altitude and endures extremes of climate, with a domestic variety. The wild form produces small tubers. In the Fz generation forms appeared which were identical with the wild parent together with many intermediate types. Most of the observed variations in cultivated varieties have occurred in the tubers, although the English ash-leaf varieties are examples of a variation in leaf shape (East, 19076). Production of New Forms. — For the purpose of differentiating between two important phases of potato improvement, Stuart (1915) has referred to "selection" as the "isolation and asexual propagation of desirable strains or types" while "breeding" is used only for sexual reproduction. With certain crops, such as the potato, this terminology is distinctive. Such a restricted usage of the word "selection" seems undesirable from the plant- breeding standpoint. The same idea can be obtained by the use of "clonal selection" to refer to the asexual propagation of de- sirable strains or types. Systematic plant breeding with the idea of combining the desirable characteristics of two parental varieties can be carried out only after the breeder has familiarized himself with the characters of particular varieties and of their wild relatives. Thus, with the potato as with other crops the breeder should first determine the ideal toward which he will work. Parental varieties should than be selected because of some desirable characters. By recombination of the favorable characters of both parents, improvement may be obtained. Gilbert (1917) has listed certain characters of the potato which are universally de- sired. Some of these are: 1. High yield. 2. Good quality. 3. Disease-resisting capabilities. 4. Good keeping quality. 5. Good color of flesh and skin. 6. Skin of desirable texture. 7. Tubers of good shape. 8. Shallow eyes relatively few in number. 9. Upright, vigorous plants. 10. No tendency to make second growth. The desirability of most of these characters is self-evident. The chief difficulties in the way of developing a standardized method of attack arise from: 224 BREEDING CROP PLANTS 1. The heterozygous condition of most varieties. 2. The difficulties of obtaining crossed seed. The heterozygous condition need not be further emphasized. Conditions are much the same as in the fruit crops. The Difficulties of Obtaining Crossed Seed. — The technic of making a cross is very simple. According to East (1908a), "The flowers close slightly about dusk and open in the morning between five and six o'clock. The pollen appears to be in the best condition for use on the second day of blooming." Stuart (1915) collects flowers to be used as the male parent in small sacks. After the pistil is removed from these flowers the anthers are tapped sharply with a pair of forceps, the pollen is collected FIG. 54. — Emasculated and unemasculated potato blossoms. (After Stuart.) on the thumb nail and then applied to the pistil of the emascu- lated flower. The flowers are receptive two to four days after emasculation. East ( 1908a) stated the belief that the potato is usually self-fertilized. He also observed the fact that insects were seldom seen to visit the flower. Salaman (1910-11) believes it unnecessary to cover the flower before or after pollination. Stuart, however, used 1-lb. bags and found that if a certain amount of foliage was included in the bag the use of bags did not cause a lowering of the number of seeds set. An average of between one and two hundred seeds was obtained from each suc- cessful cross by Stuart. The chief difficulty is that many varieties do not bloom very POTATO IMPROVEMENT 225 freely, although the general belief is that all varieties may bloom under certain conditions of environment. East (1908a) classified varieties as follows: "1. Varieties whose buds drop off without opening. "2. Varieties in which a few flowers open, but which immediately faU. "3. Varieties whose flowers persist several days, but which rarely produce viable pollen. "4. Varieties which under most conditions always produce viable pollen." In 487 out of 721 varieties under observation the buds fell off before the flowers opened. Stuart, however, obtained a much higher percentage of varieties which produced flowers in which the blossoms opened before the buds fell. These results are given to emphasize the fact that conditions widely influence seed production. The lack of fertile or healthy pollen in many varieties prohibits their use as parents. The relation between the percentage of healthy pollen and fruit production was determined by East (1908a) for a considerable number of crosses (see Table LIV). TABLE LIV. — RELATION BETWEEN PERCENTAGE OF VIABLE POLLEN AND FRUIT PRODUCTION Viability Fruit production None Difficult Medium difficult Medium Good 0- 25 26- 50 51- 75 76-100 per per per per cent, cent, cent, cent. healthy 20 5 5 2 I 3 3 6 2 3 healthy healthy healthy Somewhat similar results were obtained showing a positive correlation between fruit production and the percentage of multi- nucleate pollen grains. Such grains may be determined under the microscope by their slight protuberances. Germination tests in seven per cent, sugar solution showed that a pollen tube grew from each protuberance in a multinucleate grain. These results obtained by East have been corroborated by the studies of Stuart. In some cases, however, seed production is not difficult to obtain as the data from Stuart show (Table LV). 15 226 BREEDING CROP PLANTS TABLE LV. — RESULTS OP POTATO CROSSES MADE ON THE POTOMAC FLATS, WASHINGTON, D. C. IN 1910 Parentage of cross Date emascu- lated Date pollin- ated Number of flowers crossed Number of seed balls formed Num- ber of seeds Percent- age of seeds germi- nated Irish Cobbler X Irish Seedling . July 28 July 30 6 6 964 80.0 Irish Cobbler X Irish Seedling . July 28 July 30 7 5 984 83.5 Eureka X Keeper July 28 July 30 7 5 1,154 78.3 Improvement Through Seedling Production. — Probably no statement could be more illuminating than Stuart's discussion regarding early studies of potato improvement in the United States. The facts here related are taken from Stuart's pub- lication. During the period from 1840 to 1847 the wide occurrence of potato blight f ocussed the attention of potato growers upon the need of more resistant varieties. Rev. C. E. Goodrich, of Utica, N. Y., believed this susceptibility to diseases was a result of long-continued asexual propagation. Through the agency of the American consul at Panama, South American varieties were introduced. Goodrich grew seedlings from Rough Purple Chili, one of the introduced varieties, and obtained a new variety which he named Garnet Chili. This new variety was introduced into the trade in 1857. Between 1849 and 1856 Goodrich raised a total of 8,400 seedlings. These experiments had considerable effect on the work of other breeders. In 1861 Albert Bresee, of Hubbardton, Vt., grew a naturally fertilized seed ball produced by Garnet Chili. One of the seedlings produced was distributed under the name of Early Rose. The most careful breeder of this period was C. C. Pringle, of Charlotte, Vt. He selected varieties for crossing because of desirable characters. A variety by the name of Snowflake was one of the best known of his productions. Pringle, in the early seventies, contracted to produce potato seed for $1,000 a pound. Numerous varieties, probably resulting from naturally polli- nated seed, were introduced by E. S. Brownell, of Essex Center, Vt. Among the better known of these were Brownell's Best, Beauty, Eureka, and Winner. Among other well-known varie- ties which were introduced about this time was the early maturing variety, Early Ohio. Alfred Reese produced this variety, which PO TA TO IMPRO YEMEN T 227 was introduced in 1875, from a seedling of Early Rose. One of the first plant productions of note of that celebrated breeder, Luther Burbank, was obtained by growing seedlings of a potato ball which he found on an Early Rose vine in his mother's garden at Lancaster, Mass. Of 23 seedlings grown, one was of much promise. This was introduced by Gregory in 1872 as Burbank's Seedling. From naturally fertilized seed of Garnet Chili, FIG. 55. — An extra-promising first-year seedling. Crop of 1910. 24 tubers. (After Stuart.) E. L. Coy of West Hebron, N. Y., obtained a variety that was introduced in 1878 as Beauty of Hebron. These early experi- ments which produced some varieties that are still grown illus- trate the marked effect which the introduction of a single variety may have on the production of new forms. Some of the varieties which resulted from the introduction and breeding experiments of Rev. C. E. Goodrich are here listed : TABLE LVI. — PEDIGREES OF SOME POTATO VARIETIES Breeder Variety used for seed Seedlings named Rev. C. E. Goodrich . . . Albert Bresee Rough Purple Chili Garnet Chili Garnet Chili Early Rose Alfred Reese Luther Burbank E. L. Coy Early Rose Early Rose Garnet Chili Early Ohio Burbank Beauty of Hebron 228 BREEDING CROP PLANTS These early studies illustrate the general mode of production of new potato varieties. Certain methods are of value in giving the seedlings a good start. In the latitude of Washington, Stuart recommends sowing the seed in the greenhouse early in March and transplanting the seedlings from 3-in. pots into the field in May. The plants are placed in rows 3 ft. apart and spaced at a distance of 2 ft. apart in the row. Results indicate that seedlings producing tubers of irregular shape or those with deep red or purple skin may well be discarded after the first year's trial. After another year's study those strains with undesirable characters such as low yielding ability, undesirable shape, deep eyes, unusual susceptibility to fungous diseases and FIG. 56. — An unpromising first-year seedling. Crop of 1910. Note large number of small, irregular shaped tubers. (After Stuart.) the straggling or weak vine growth, should be discarded and the few more promising types given a wide test to determine their adaptability and value under different conditions. Clonal Selection. — The subject of bud mutations in potatoes is a somewhat difficult one, for there are numerous reported cases of such sudden changes. Many of the experiments were not performed with sufficient care to furnish acceptable evidence, although numerous apparently authentic cases of color changes have been reported. As an illustration of carefully controlled experiments those carried on by East (1910a) may be cited. In these studies each variety worked with was started from a single hill. During the course of the study, five permanent changes POTATO IMPROVEMENT 229 from pink to white tubers, two permanent changes from long to round tubers, and four instances of changes from shallow to deep eyes were observed. On the basis of the modes of inheritance of these characters, the hypothesis was made that the changes resulted from the loss of dominant factors. Experiments in selection for high nitrogen content gave negative results. The statement was made, "it is true that all of the asexual variations have been losses of characters, while in sexual reproduction the formation of new characters occurs." This certainly substan- tiates the belief that the production of improved varieties of potatoes through bud mutation is not a promising method of attack. East quotes A. W. Button, who states: "I have no hesitation in affirming that there is no potato in commerce in England, and I might say in Europe, which owes its origin as a dis- tinct potato to bud variation in any form whatever." If this statement is true, it seems fair to conclude that there has been a somewhat loose usage of the term "bud mutation" in its application to raising the standard of a variety by any of the well-known methods such as tuber unit or hill selection (see Chapter XVIII). Accumulated evidence certainly points to the belief that the chief value of such work rests on the prob- able elimination of degenerate strains. Evidence from Canada presented by Macoun (1918) is particularly illuminating. Four varieties, Early Rose, State of Maine, Empire State, and Dela- ware, were grown in Canada at the Experimental Farm at Ottawa from 1890 to 1909 inclusive. The better tubers were selected from each year's crop and used to plant the following crop. Results are presented in Table LVII. TABLE LVII. — AVERAGE YIELD OF POTATOES OVER THE FIRST FOUR AND LAST FOUR YEARS OF A 16-YEAR PERIOD AND SUBSEQUENT YEARLY YIELDS OVER A FOUR-YEAR PERIOD Year Variety 1890-1893, bu. 1902-1905, bu. 1906, bu. 1907, bu. 1908, bu. 1909, bu. Early Rose 257 317 150 128 69 18 State of Maine.. . . Empire State Delaware 325 301 296 361 338 352 132 132 103 174 117 114 97 117 156 62 62 53 Average 295 342 129 133 110 49 230 BREEDING CROP PLANTS For the 16-year period from 1890 to 1905, inclusive, the varieties were kept in a high state of productivity "due, no doubt, to careful selection and good cultivation each year. " In 1906, however, there was a marked falling off in yield due to the unfavorable season. In the early part of the season there was sufficient rain but at about the time of the last cultivation, hot dry weather set in and continued throughout the season. During July there was also a severe attack by aphis. The vines, therefore, presented a stunted appearance and dried up early in the fall, the yield of tubers being very low. In 1907 and 1908 the seasons were also very unfavorable. The best tubers were again planted in 1909 and although the tubers used for planting presented a very favorable appearance, the yields were very low. A com- parison was made in 1909 of tubers grown continuously at the Central Experiment Farm and newly imported tubers grown under more favorable conditions. The yielding ability of the imported tubers exceeded that of the Central Farm tubers by as high as 500 per cent, in some cases. The plant breeder is naturally interested in the subject of whether these are instances of bud variations due to unfavorable environment. If so, they should be permanent changes. If, on the other hand, they are non-heritable variations, this does not affect the practical importance of tuber selection as a means of obtaining high yields. Macoun (1918) has furnished evidence which helps to clarify our ideas on this question. From time to time tubers were sent from Ottawa to the branch stations, on the prairies, where potatoes usually grow very vigorously. In 1916 the following question was asked: "You will, no doubt, remember that potatoes sent you from Ottawa are usually weak growers when you receive them. I would be glad if you would inform me for how many seasons that weak growth con- tinues, or do they make a strong growth the next year, the same as the ones you have been growing for several years?" Answers made by the superintendents of these prairie farms showed that the first year's crop from tubers sent from the Cen- tral Farm was very small. From one to three years elapsed before varieties introduced from the Central Experimental Farm yielded as well on the prairies as those varieties which had been continually grown on the prairies. Much of the so-called "running out" or degeneracy in pota- POTATO IMPROVEMENT 231 toes has been traced to certain plant diseases (Stewart, 1916; Orton, 1914) which have been variously named as leaf roll, mosaic, and curly dwarf. Quanjer (1920) has presented evidence to show that these three diseases may be stages of the same disease, which is transmissible from plant to plant. Similar results have been obtained at University Farm.1 The disease is called " mosaic dwarf" by Krantz and Bisby in unpublished investigations. That rejuvenation of a variety is possible through its introduction and growth under a more favorable environment is illustrated by studies which have been carried on FIG. 57. — Progeny of single tubers as grown at University Farm, 1918. Some tubers give vigorous progeny, others produce only small, weak, degenerate plants. (Courtesy of Krantz.) cooperatively between the Division of Horticulture of the Min- nesota Central Station and the sub-stations. Yields for Min- nesota No. 2 at University Farm for 1914, 1915 and 1916, respectively, were 196, 169 and 22 bu. This shows the rapid reduction in yield which is obtained by the continued use of tubers saved at University Farm. Tubers of Minnesota No. 2 saved from the 1916 University Farm plot gave a yield cf 170 bu. at Duluth in 1917. Tubers from this Duluth plot yielded 300 bu. at Grand Rapids in 1918. Whether a badly diseased 1 Data on running out and on field experiments in Minnesota were furnished by F. A. KRANTZ of the Division of Horticulture, Minnesota Experiment Station. 232 BREEDING CROP PLANTS variety can be rejuvenated by planting under a favorable environ- ment tubers from diseased plants is as yet an unanswered question. The rapid deterioration of varieties when University Farm tubers are used for their propagation is believed to result from these transmissible diseases which are now called " mosaic dwarf." Degeneracy can apparently be prevented by covering that part of the field in which tubers are to be saved for the next year's planting by a cheese-cloth cover. The following data with the variety Green Mountain seem sufficient authority for this statement: TABLE LVIII. — DEGENERACY PREVENTED BY USING TUBERS OF VINES WHICH WERE COVERED WITH A CHEESE CLOTH COVER Source of timber used Year grown Yield per acre, bu. Probable error in per cent. Grown in open at University Farm, 1917. . . . Grown under cheese-cloth cover at University Farm, 1917 1918 1918 172 223 4.6 5 4 Newly introduced stock 1918 205 1 7 Grown in open at University Farm, 1917 and 1918 1919 1.5 Grown under cheese-cloth cover at University Farm 1917 and 1918 1919 285 4 8 Newly introduced stock 1919 272 1 5 All seed stock was obtained from the same grower at the North Central Experiment Station, Grand Rapids, Minn. Whether all degeneracy is due to such transmissible diseases is as yet un- answered. Possibly unfavorable cultural conditions may also affect the development of the tuber so that the yield of the fol- lowing year's crop may be modified. Another explanation of degeneracy has been commonly mentioned. This is the hypothesis that continued asexual propagation causes senility or degeneracy. Perhaps this question may be answered for the potato by the consideration of a fact reported by Heribert Nilsson (1913). In a report of yields of 67 varieties, as tested in Sweden, he emphasized the fact that a variety "Hvit Jamtlandspotatis" which has been cultivated more than 100 years proved to be the highest yielder. This is given as a refutation of the theory of senility. It has not been the intention in this discussion to lead the POTATO IMPROVEMENT 233 practical breeder to discard "clonal selection" as one means of obtaining high yields, for it is a recognized fact that seed plot methods are of much practical importance. The results, how- ever, are probably not due to the isolation of bud mutations but rather to the use of tubers which have developed normally and which furnish the right conditions to give the resultant plants a favorable start. May not the conditions be much the same as with any vegetatively propagated plant. Bonnier, for example, found that about three years are required before a low- FIG. 58. — Tubers produced under such a cheesecloth cover have given good yields during the seasons 1918 and 1919 while tubers from uncovered vines produced very inferior yields. University Farm, St. Paul, Minnesota. (Cour- tesy of Krantz.) land dandelion transported to alpine conditions fully expresses the characters of a dandelion plant which had been grown under these conditions for many years. On returning the same plant to the lowlands about the same number of years elapsed before the. plant had again fully attained the lowland habit. This is probably not a germinal change but the normal expression of the plant under a particular environment. With the clonally propagated potato there is a cumulative response to unfavorable conditions. Such conditions modify the plant's development and therefore influence the development of the following year's crop. There seems no reason for believing that an actual ger- minal mutation has occurred. CHAPTER XVII BREEDING OF VEGETABLES SELF-FERTILIZED VEGETABLES The long periods of cultivation and the various environments to which many of our vegetables have been subjected, have served to increase the number of varieties. Most of the vegetable varieties have been produced by commercial seed firms or by seed growers. An examination of any seed catalog shows numer- ous new forms which are being constantly introduced into cultivation. There has been a marked tendency among seeds- men to give new trade names to old standard varieties. This has led to a great deal of confusion in nomenclature and much difficulty has been experienced in varietal identification. There is need of a more scientific test of varieties prior to introduction and of a standardization of varieties. Considerable progress has been made in classification of some vegetables. More information is needed regarding the mode of pollination and inheritance of special characters before methods of breeding can be intelligently applied. In this chapter the origin of both cross- and self- fertilized vegetables is briefly summarized. The mode of inheritance of special characters of the self-fertilized vegetable species, pea, bean, tomato, and pepper are given, together with a brief discussion of methods of breeding. Origin of Vegetables.1 — The ancient Greeks and Romans were familiar with some of our garden vegetables; on the other hand, many are of more recent origin and new varieties are being constantly introduced. The discovery of America introduced to civilization such important vegetables as the Irish potato, 1 For a complete history of the origin of vegetables see DE CANDOLLE, A., Origin of Cultivated Plants, Kegan Paul, Trench & Co., London, Second Edition; 468 pages, 1886; HENSLOW, G., The Origin and History of Our Garden Vegetables and Their Dietetic Values, in Jour. Roy. Hort. Soc., vols. 36 and 37, 1910-11 and 1911-12; STURTEVANT, E. L., History of Garden Vegetables, in Am. Nat., vols. 23, 24, and 25, 1889, 1890, and 1891. 234 BREEDING OF VEGETABLES 235 sweet corn, tomato, bean, sweet potato, pumpkin, squash, and pepper. Nearly all the other cultivated vegetables of temperate climates are indigenous to Europe or Asia. Sweet corn, which is one of the most highly prized foods grown in America, is probably of recent origin. In Bailey's (1900) Cyclopedia of American Agriculture, Volume 2, page 402, the following statement occurs: "The first sweet corn cultivated in America was derived from the Susquehanna Indians in 1779 by Captain Richard Begnall, who accom- panied General Sullivan on his expedition to subdue the Six Nations." How long Zea mays saccharata had been under cultivation is TABLE L1X. — ORIGIN AND ANTIQUITY OP SOME VEGETABLES Vegetable Botanical name Probable origin Years culti- vated Asparagus Asparagus officinalis Europe, west temperate Asia. ' B Bean, lima Phaseolus lunatus Brazil. E Bean, common .... P. vulgaris S. Am. found in Peruvian tombs. Ed) Beet Beta vulgaris Canaries, Mediterranean basin, B Tops as food western temperate Asia. Roots as food A result of cultivation. B Cabbage Brassica oleracea Europe A Carrot Daucus carota Europe, west temperate Asia (?). B Celery j Apium graveolens Temperate and southern Europe, B : » northern Africa, western Asia. Corn, sweet Zea mays var. saccharata Cucumber Cucumis sativus India. A Lettuce Lactuca sativa Southern Europe, northern Africa, B western Asia. Muskmelon Cucumis melo India, Beluchistan, Guinea C Onion Allium cepa .... Persia, Afghanistan, Beluchistan, A Palestine (?) Parsnip Pastinaca sativa Central and southern Europe. C Pea, garden Pisum sativum From the south of the Caucasus to B Persia (?), northern India (?). Pepper Capsicum annuum Brazil (?) E Pumpkin Cucurbita pepo Temperate North America. E Radish Raphanus sativus Temperate Asia. B Salsify Tragopogon porrifolium Southeast of Europe, Algeria. C (?) Spinach Spinacia oleracea Persia (?). , C Sweet potato Convolvulus batatas Tropical America (where?). D Tomato Lycopersicum esculentum Peru. E Turnip Brassica rapa Europe, western Siberia (?) A Watermelon Citrullus vulgaris Tropical Africa. A A = Species cultivated more than 4,000 years. B = Species cultivated more than 2,000 years. C = Species cultivated less than 2,000 years. American species: D = Cultivation ancient in America. E = Cultivation before discovery of America, but not showing signs of great antiquity. 236 BREEDING CROP PLANTS not known, but there is considerable evidence to substantiate the belief that at least the main types of corn, Zea mays indentata and Zea mays indurata, were cultivated a long time before the discovery of America. Table LIX taken from De Candolle (1886) presents a summary of the origin of some common vegetables. PEAS Some Classification Characters. — Considerable historical in- terest attaches' to the pea because of the fact that in studying the inheritance of certain characters in this plant Mendel dis- covered his now famous principles. Garden peas (Pisum sati- vum) are of two kinds, shelling and edible-pod. In the former, seeds only are used as food, while in the latter both pods and seeds may be so utilized. By far the greater part of the garden peas grown belong to the shelling group. Commercial varieties of garden peas are classified on the basis of habit of growth — climb- ing, half-dwarf, and dwarf; and length of time to mature — early, medium, and late. Peas of the early varieties may be round or wrinkled. Most of the medium and late maturing varieties belong to the sugar peas, which have wrinkled seeds when mature. Size of pod is another important classification charac- ter. Ripened pods may be inflated or somewhat constricted. Inheritance. — In a reciprocal cross of the varieties Autocrat and Bountiful, it has been suggested (Keeble and Pellew, 1910) that the inheritance of the character tallness involved two factor differences, one for length of internode and one for thickness of stem. In certain crosses White (1918) finds the inheritance of stature still more complicated. Tall varieties (over 4.5 ft.) are divided into three groups and half-dwarfs are separated into two groups. The factorial scheme suggested is as follows : FIG. 59. — Flower structure of pea. 1. A single flower — a, petals of calyx; 6, side view of corolla. 2. Front view of fully open flower — a, petal of calyx; b, standard; c, whig; d, keel. 3. The sexual organs removed from the bud. (Adapted from Muller.) a, Filament; b, anther; c, style; d, stigma hairs. 4. 5. Anthers. 6. Cross section ovary. 9. Longitudinal section ovary. Size: 1, %n; 2, Y§n\ 3, greatly enlarged; 4, 5, lOOn; 6, greatly enlarged; 7, 8n; 8, 40n; 9, 40n. BREEDING OF VEGETABLES 237 d gives purple pods. With Pi gives purple pods. Black-eyed seed-coat pattern. Round, smooth seeds with simple, oval starch grains, low water content and with excellent powers of germination under unfavorable weather conditions. Pods with seeds separated or free. Tall, robust plants, large number of internodes. Leaves with tendrils. Dark self-colored purple seed-coat. With P gives parchmented. smooth pods. With (B1) gives glaucous foliage, pods. The presence and absence of these thirty-five factors are gene- tically responsible for seventy or more differential characters. As is noted in the table, there is a modifying effect of one factor upon another in certain cases. Studies have also shown that certain environmental conditions may modify a particular inheritance in such a way that the true genetic nature can not be determined by inspection. This is an instance which should help to impress upon the student the necessity of the controlled breeding test as a No. Factor j A 2 B ' 3 (Bl) 4 (Bt) 5 C(A] 6 D 7 E[A] 8 (Ef) 9 F 10 (Fa) 11 (Fn) 12 (Gc)[A] 13 G 14 (Gp) 15 H 16 I 17 J 18 Li[A] 19 Lz 20 (Le) 21 (Lf) 22 M 23 N 24 O 25 P 26 Pi 27 P2 28 (PI) 29 R 30 S 31 T 32 (Tl) 33 U 34 V 35 W BREEDING OF VEGETABLES 241 means of determining the genetic nature of any particular variety or strain. Factors A, C, E, (Gc) and L\ appear absolutely coupled and may, therefore, be considered to be a single factor with several separate expressions. This factor gives salmon-pink or rose color to the flower, and to the leaf axil, and to the stem in the presence of D; purple dotting on seed-coats in the presence of F and B, with reddish dots when B is absent and F is present; yellowish green to grayish brown seed-coat color, brown hilum; indent peas in the presence of L2. The results of examining many thousand F2 generation progeny indicate that factors A, B, (Fa), /, (Le), G, and R are indepen- dently inherited. Four groups of linked factors were found. These, according to the factorial notation used by White, are: GROUP PARTIALLY LINKED RATIO OF NON-CROSSOVERS TO CROSSOVERS 1 (Bl)S 8:1 2 A(Lf) 7:1 3 R(Tl) 63:1 4 GO Undecided BEANS Some Classification Characters. — The species1 of garden beans most commonly grown are Phaseolus vulgaris and P. lunatus. The former is divided, from the standpoint of use as food, into snap and shell beans, although there is some overlapping in these groups. Shell beans are sometimes used as snap beans and vice versa. Time required to mature, habit of growth, whether climb- ing or bush, and size of plant are characters always described by commercial seedsmen. Length of bearing period is also an im- portant character. Commercial growers sometimes desire varie- ties which may be harvested in a few pickings but for the home and general gardener, a variety with a longer bearing period is usually preferred. Size and shape of pod, number of seeds per pod in the case of snap beans, quality and color of the pod, are used in classification; with snap beans, stringless, fleshy, fine-grained pods are most desirable. The ease with which dry 1 For a discussion of the classification of garden beans and a description of varieties see TRACY, W. W., American Varieties of Garden Beans, U. S. D. A., B. P. I. Bull. 109, 173 pages, 1907; JARVIS, C. D., American Varieties of Beans, Cornell Agr. Exp. Sta. Bull 260: 149-255: 1908. 16 242 BREEDING CROP PLANTS shell beans may be thrashed is of economic importance. In this group, color, size, and shape of seeds are usually included in varietal descriptions. Both productivity and disease resistance may differ strikingly in different varieties of beans. Inheritance. — Seed-coat color has been shown by Shaw and Norton (1918) to involve several factor differences. The work was carried on with twenty-one varieties including more than 40,000 plants. Crosses between mottled and self-colored varie- ties yielded mottled beans in FI and showed 3 : 1 ratios in F<^. Mottled X white varieties gave mottled in FI, and in Fz the ratio of 9 mottled to 3 self-colored to 4 white usually resulted. It was demonstrated that pigment patterns and pigment colors were controlled by distinct factors. All plants with white or eyed beans bore white flowers while plants with mottled or self-colored beans usually bore pink flowers. The inheritance of stature in beans, as in peas, is in some crosses dependent on a single factor difference while in other crosses several factor differences are involved. Emerson (1916) has explained the result of crossing a tall pole (indeterminate growth) bean and a short bush (determinate growth) bean or a short pole bean and a tall bush bean, by a three-factor hypothesis. The following values to be added to an initial value of three inter- nodes were assigned to the factors : Factor A either homozygous or heterozygous added 10 internodes approximately, while fac- tors B and C each added two internodes when homozygous and one when heterozygous. Results were explained factorially as follows: Parent 1 AABBCC = 17 internodes or AAbbcc = 13 internodes Parent 2 aabbcc = 3 internodes or aaBBCC = 7 internodes FI AaBbCc = 15 internodes or AaBbCc = 15 internodes Many new forms would naturally be produced in F%. Tschermak (1916) has brought together and summarized the FIG. 60. — Flower structure of bean. 1. Small branch showing — a, developing pod; b, c, flowers in different stages of development. 2. Front view of fully opened flower — a, calyx; b, wing; c, standard; d, keel. 3. Enlarged keel. 4. Keel with outer part broken away to show — 6, style; c, anther; d, undevel- oped pod; e, ovary. 5. 6. Longitudinal and cross section of pod. 7. Enlarged stigma showing— a, stigma hairs. 8. Anther. Size: 1, n; 2, about 2w; 3 to 8, greatly enlarged. BREEDING OF VEGETABLES 243 FIG. 60. 244 BREEDING CHOP PLANTS data on the inheritance of economic characters in the garden bean. Table LXII is made up from his summary. TABLE LXII. — INHERITANCE IN THE BEAN Contrasted characters Fi condition Fz behavior Colored X white (flowers) Colored 3:1 Green X yellow (unripe pods) Green 3:1 Non-constricted X constricted (pods) . Non-constricted 3:1 Round X flat (pods) Round 3:1 " Non-stringi ness " X "stringy" Intermediate or approach- Stringy pods recessive (pods). ing non-stringiness (1 out of 4). Blunt X sharp (pod ends) Approaches blunt Approaches 3:1. Broad X narrow (pods) Approaches broad Segregation irregular. Long X short (pods) Approaches long Segregation irregular. Cylindrical X spherical (seeds) .... Approaches cylindrical Segregation irregular with spherical seeds constant. Cylindrical X kidney-shaped (seeds) . . Approaches cylindrical Segregation irregular with kidney-shaped seeds constant. Yellow X green (cotyledons) Yellow (apparent on crossed 3 : 1 (segregation ap- seed) parent on Fi plants). The inheritance of resistance to various diseases is extremely important. One of the most injurious diseases of the bean is anthracnose (Colletotrichum lindemuthi anum) . Barrus (1918), as the result of an extensive study, was able to place beans in four groups with respect to susceptibility or resistance to this disease. Over two hundred varieties of beans commonly grown, besides many others, were tested. A considerable number of plants belonging to closely related genera were also examined. The cultures of anthracnose used for inoculating the varieties were obtained from widely separated geographical areas. By study- ing the reaction of the various cultures to each bean variety, two strains of anthracnose, alpha and beta, were discovered. With respect to their reaction to these two anthracnose strains, varie- ties of beans were placed in four groups: (ab) Varieties susceptible to both strain alpha and strain beta. (aB) Varieties susceptible to strain alpha but resistant to strain beta. (Ab) Varieties susceptible to strain beta but resistant to strain alpha. (AB) Varieties showing some resistance to both strains. The most resistant variety of the last group is Wells Red Kidney. Results of crosses between varieties whose anthracnose BREEDING OF VEGETABLES 245 reactions are known indicate (McRostie, 1919; Burkholder, 1918) that resistance to either the alpha or beta strain is inherited as a simple dominant, involving but a single factor difference. It seems, therefore, very easy to produce resistant varieties to both strains by crossing and selection and thus to combine de- sirable economic characters and anthracnose resistance. McRostie (1921) has recently published an interesting paper on further studies of disease resistance in common beans. The more extensive results obtained bear out the earlier views on the mode of inheritance of resistance to bean anthraxnose. The studies carried out show that bean mosaic susceptibility is in- herited. In FI there was a partial dominance of susceptibility over resistance and in Fz a segregation which indicated a two factor hypothesis. In crosses between susceptible and resistant varieties in relation to the dry root rot, caused by the fungus, Fusarium martii phaseoli Burk., there was a dominance in F\ of susceptibility and a segregation in F2 that appeared to be on a 9 : 7 basis. In nearly all cases resistant F2 plants bred true to this character in Fs. Results of this nature show the great practical importance of the application of Mendelian principles to breeding for disease resistance. It seems very likely that a large part of our serious plant diseases will be controlled even- tually by the production of disease resistant varieties. TOMATO Classification Characters and Inheritance. — The tomato be- longs to the genus Lycopersicum of which there are several cultivated species. Tomatoes are classified on the basis of vine habit, either standard or dwarf, leaf type, period of maturity, size and color of fruits, and other characters. As a result of breeding experiments, many different combinations of characters have been made. Price and Drinkard (1908) were among the first investigators to report on the simple Mendelian behavior of certain tomato characters. Table LXIII, taken from similar ones compiled by Tschermak (1916) and Jones (1917), presents a brief summary of inheritance in the tomato. Fruit shape is dependent on several factors according to Crane (1915) and Groth (1912, 1915). Some of the foliage characters are also somewhat complicated in their inheritance (Groth, 1911). The inheritance of each of the other characters listed in the table is dependent on single factor differences. 246 BREEDING CROP PLANTS TABLE LXIII. — INHERITANCE OF CHARACTERS IN THE TOMATO Characters Dominant Recessive Fruit shape .... Spherical Pear-shaped Fruit shape Loculation of ovary Endocarp color Epicarp color Fruit surface Leaf margin Roundish conical Two-loculed Red Yellow Smooth Serrate (normal or fine Roundish compressed Many-loculed Yellow Colorless Pubescent Entire (potato or coarse Leaf type leaf) Pintpinellifolium type leaf) Esculentum type Leaf color Green Yellow Inflorescence type Vine habit and leaf surface Height of plant Simple Standard, smooth Tall or normal Compound Dwarf, rugose The Fz segregation ratio is 3:1. Jones (1917) has pointed out that the data of Hedrick and Booth (1907) and Price and Drink- ard (1908) show linkage relations between the factors for vine habit and fruit shape and also between those for leaf color and loculation of ovary. Heterosis in the F\ generation of certain tomato crosses and its commercial possibilities for increased production have been pointed out (Wellington, 1912; Hayes and Jones, 1916). Groth, of the New Jersey State College Experiment Station, made a study of size inheritance in the tomato fruit. The re- sults are explained by what the author (1914) terms " Golden mean." If (a) and (6) represent the respective magnitudes or volumes of size characters of the parents, the FI is represented by \/ab rather than (a + 6)/2. This hypothesis was put forward as non-Mendelian and in explanation of results in size inheritance frequently attributed to multiple factors. Emerson (19146) has shown that the hypothesis is essentially based on multiple factors. PEPPERS Classification Characters and Inheritance. — Garden peppers which are commonly grown for pickles or for condiments belong to the species Capsicum annuum. From the standpoint of their utilization as food, peppers may be divided roughly into two groups — hot and mild, depending on flavor. Mild peppers are BREEDING OF VEGETABLES 247 frequently used green for slicing or stuffing, whereas hot peppers more often serve as a condiment in spice mixtures. Number of days to mature is usually given by seeds-men in describing varieties. Color, size, shape, and uniformity of fruit are other important commercial characters. A limited number of inheritance studies with this vegetable have been made. Webber (1911) and Ikeno (1913) report the behavior of certain characters in the second generation after a cross. Below is given a tabular summary of a part of the results obtained. TABLE LXIV. — INHERITANCE IN THE PEPPER Contrasted characters Fi condition Fz behavior Violet X white (flower).. . . 3:1 Violet (considerable variation in amount of violet coloring) Violet flower associated with violet coloring in leaf-stem and ripe fruit. White flower associated with green leaf and stem except for a dark spot near attachment of petiole. Umbel X non-umbel (in- Non-umbel Red Less hairy than hairy parent Pungent florescence) Red X orange (ripe fruit).. Pubescence X non-pubes- cence (stems and leaves) Pungent X sweet (fruit) . . . 3:1 3:1 15 pubescent:! non-pubescent Approx. 3:1 In the inheritance of size of leaf, Webber obtained results which clearly indicated that several factor differences were involved and a like result was obtained by both Webber and Ikeno with regard to size of fruit. The character of the peduncle, whether erect or recurved, was found by Ikeno to be dependent on a single factor difference, erect being the recessive condition when the fruit had ripened. During the flowering stage and early development of the fruit the heterozygous individuals for this character-pair showed dominance for the erect peduncle. Methods of Breeding Self -Fertilized Vegetables. — The vege- tables discussed above together with others which are naturally self-fertilized or which will produce ample viable seed when self ed may be considered as a single group from a breeding stand- point. The method of breeding this group is identical with that already outlined for naturally self-fertilized crops and hence 248 BREEDING CROP PLANTS need not be repeated here. Yield, quality, and disease resistance are the three most important economic characters. To bring about a desirable combination of these characters, both selection and hybridization have been practiced. Selection has been used by Edgerton (1918), of the Louisiana Agricultural Experiment Station, to isolate tomatoes resistant to wilt (Fusarium lycopersici) . The improved technic followed is worthy of consideration. Seeds of a particular variety were planted in soil which had been sterilized previously and then inoculated with a pure culture of the wilt-producing organisms. When seedlings showed wilt infection they were pulled and dis- carded. Only plants which showed resistance were transplanted to the field. Tomatoes had grown continuously for eight or ten years on this field and it was known to be heavily infected with the wilt fungus. The use of this method permits a smaller acreage and insures the contact of each plant with the wilt organism. A selection made from a row of Acme grown in 1909 named " Louisiana Wilt-Resistant" was extremely wilt resistant but possessed other characters which made it undesirable for Louisiana conditions. Selections from the progeny of crosses between this form and Earliana showed considerable promise. Durst (1918) reported the result of five years' selection for resistance to Fusarium of tomatoes. Varieties were found to differ a great deal in their resistance and unfortunately the most resistant ones produced poor fruit. After five years, some of the better strains stood up in soil which proved fatal to the original varieties. In addition to disease resistance, the selections also showed good yielding ability. Of seventy-four lots grown one year the highest fourteen yields were produced by selected strains. Whether selection alone or hybridization and selection together are to be used as a means of improving a crop is dependent upon the nature of the material. If the character combination is not already present, the only practical means of bringing it about is crossing followed by selection. Cross-Fertilized Vegetables Crops have previously been classified as belonging to four groups according to their mode of reproduction. Cross-fertilized vegetables may be roughly divided into three main divisions; BREEDING OF VEGETABLES 249 1. Those which are normally cross-pollinated but which set seed freely on selfing and show no evidence of sterility. 2. Those which are wholly or partially self-sterile. 3. Those which are cross-fertilized owing to the dioecious condition. Much more study of the mode of pollination of vegetables is necessary before it is possible accurately to classify vegetables according to their mode of reproduction. The crops here considered have been purposely chosen as illustrations of breeding results within these three groups. RADISH Origin, Inheritance, and Breeding. — The cultivated radish, Raphanus sativus, was grown by the ancient Greeks and Romans. There has been considerable discussion as to its origin. Some writers have thought that the cultivated form with its fleshy root arose directly from R. raphanistrum. This belief was ap- parently substantiated by experiments in which the wild form was grown under cultivation and after several years cultivated radishes were obtained. Riolle (1914) tested this hypothesis by a controlled experiment. The wild form was grown under culti- vation and self-fertilized. Three years of selection failed to produce roots which resembled the fleshy roots of R. sativus. On the other hand, when the wild and cultivated forms were both grown on the same plot and seed was saved from the wild form, it was found to be an easy matter, after three years' selection, to obtain roots which resembled the fleshy roots of R. sativus. These results were believed to be due to natural crossing of the wild and cultivated forms. This hypothesis was tested by making an artificial cross. Segregation for root condition oc- curred in FZ. This led Riolle to conclude that former experiments in which cultivated radishes were obtained from the wild through selection were best explained through natural crossing. R. sativus roots contain sugar while wild roots contain ho sugar. FI crosses contain less sugar than the cultivated forms. The presence of starch in the root of the wild radish, particularly in the bark, is a character which separates it from the cultivated varieties. This proved a dominant in crosses. Cultivated radishes show various color intensities. Color is apparently inherited in much the same manner as in other crops. Individual 250 BREEDING CROP PLANTS radish plants were grown under cover by Riolle and self-fertilized seed was produced in abundance. This led Riolle to suggest that homozygous strains be first produced. These would then furnish material for accurate inheritance studies as well as be of much value for economic breeding purposes. On the other hand, Stout (1920) has stated that there is considerable self -sterility in the cultivated radish. Up to the present, mass selection has been most frequently used as a means of breeding radishes (Tschermak, 1916). BEETS Inheritance and Breeding. — Both garden beets and sugar beets belong to the species Beta vulgaris. Kajanus (1913) made a study of the inheritance of root forms in mangels and sugar beets. In general, the F\ roots were intermediate between the parental forms. Sugar beet crosses in which wedge-shaped forms were involved proved to be exceptions. Wedge-shape was completely dominant over walnut-form and also over long, somewhat slender roots (post-shape). The other beet shapes studied were oval and round. Most of the ratios obtained in Fz could be satisfactorily explained on the basis of four factors — two involving length of root and two concerned with form. A marked increase in the sugar content of the sugar beet was produced by Vilmorin through the application of the progeny test method (see page 119). There is some difference of opinion regarding the ease of producing self -fertilized seed. Shaw (1915) demonstrated that the sugar beet, isolated (two miles from any other beet plants), will set some seed. To what extent self- sterility is a factor is unknown. The production of homozygous forms through self-fertilization would seem worth trying as a means of obtaining homozygous material for breeding studies. This method seems a logical procedure for all vegetables which are naturally cross-fertilized but which also set seed freely under conditions of self-fertilization. Mass selection is often used in breeding beets. Only those roots which come up to an adopted standard are stored over winter and set out the following spring to become the seed- producing plants. Carrots and parsnips, when bred by mass selection, are handled in a similar manner. Although varieties of any one of the crops, beets, carrots, or parsnips, freely inter- cross, there is no crossing between the three different kinds of BREEDING OF VEGETABLES 251 vegetables (Malte and Macoun, 1915). This fact may be utilized in making planting plans. CULTIVATED VEGETABLES OF THE GENUS BRASSICA Cabbage and several other vegetables such as cauliflower, brussels sprouts, kohl-rabi, and rutabagas, belong to the genus Brassica. Few inheritance studies have been made with this group of vegetables. Cabbage has received more attention from a breeding standpoint than the others. Inheritance. — The evidence so far accumulated indicates that cabbage belongs to the cross-fertilization obligatory group. Price (1911-1912) and Jones and Oilman (1915) were not able to produce self-fertilized seed under a bag. Tschermak (1916) maintains that many of the kinds of vegetables belonging to the cabbage group freely intercross when in close proximity at blooming time. The above facts are fundamental and show the method of breeding which must be used. They may also aid in explaining some unusual inheritance results. Price crossed varieties of crinkled-leaf and smooth-leaf cabbage, obtaining dominance for crinkled leaf in FI with no segregation of this character in F2 ,i.e., all plants (419) had crinkled leaves. With respect to size, shape, and solidity of heads, color of foliage, and length and thickness of stem, considerable more variability was obtained in F2 than in FI. In a cross between a crinkled- leaf cabbage and a cauliflower, the thick, leathery leaf of the latter was dominant in FI and was the only apparent leaf char- acteristic in P2. Head cabbage crossed with headless produced nothing but headed forms both in the FI and F2 generations. As to type of head, the cabbage or leafy form was found to be dominant over the type of head of the cauliflower. In F2 the cabbage head form was maintained without apparent segrega- tion. Crosses between cabbage and brussels sprouts gave FI and F2 generations identical with respect to habit of growth, i.e., all were determinate. Axillary buds were more common in the hybrids than in ordinary cabbage. The thick stem of kohl- rabi was found to be dominant in a kohl-rabi-cabbage cross and a limited number of F2 individuals showed no segregation of this character. Button (1908) crossed reciprocally kohl-rabi and Drumhead cabbage, obtaining, in F^ 3 non-kohl-rabi plants to 1 resembling 252 BREEDING CROP PLANTS kohl-rabi. The parental forms did not appear in the F2 genera- tion. Drumhead cabbage crossed with Thousand-headed kale produced 204 plants in F2. Of these, 176 resembled a dwarf type of Thousand-headed kale with leaves broader than usual and fewer branches; 26 resembled cabbage; and two plants were much like brussels sprouts. The difficulty of a study of inheritance in the Brassica genus arises from the heterozygous condition of many forms and the self-sterile condition. Before the results are accepted as ex- amples of non Mendelian behavior, a criticial study in which all facts are considered should be made. In cabbage there is appar- ently a complicated inheritance. The above results are satis- factorily explained on a multiple-factor hypothesis. In crossing heterozygous forms, the FI generation may be as variable as the F2. In the inheritance of any particular character, the number of factor differences may be so large as to make the appearance of parental forms improbable in a small Fz generation. Breeding. — The breeding of cabbage resistant to yellows (Fusa- rium conglutinansWollenw.) at the Wisconsin Experiment Station (Jones and Oilman, 1915) is of great economic importance. Less than a decade ago, truck farmers in certain sections of Wis- consin were so discouraged from the ravages of yellows that they were about to abandon cabbage growing. The method of pro- ducing resistant cabbage strains may be briefly summarized. It had been noticed that there were usually a few plants which escaped the disease in a field where nearly all plants were badly infected with the organism. These apparently resistant plants were selected on the basis of type. After storing over winter, all that were of the same general type were planted together and were far enough removed from any other similar planting to insure against contamination by foreign pollen. Selfed seed was not obtained but most plants not bagged set seed abundantly. Some plants were eliminated because of low seed production. Progeny of the retained plants were grown sepa- rately and their resistance to yellows was tested. In this way several strains of cabbage highly resistant to yellows have been produced. Further studies have been reported and numerous resistant varieties have been produced (Jones et al, 1920). The writers emphasize the fact that resistance is not absolute and that environmental factors influence very markedly the develop- ment of the disease. They state, however, that: BREEDING OF VEGETABLES 253 "By following the proper methods any skillful cabbage grower who has Fusarium sick soil may either undertake with reason- able confidence to develop a resistant strain of his own, or having secured one of these resistant strains he can maintain its resistance and produce his own seed." ASPARAGUS Asparagus (Asparagus officinalis) is dioecious in habit of flow- ering altho hermaphrodite plants have been discovered (Norton, 1911-1912). With this vegetable, cross-pollination is usually necessary for seed production. Rust-Resistant Asparagus. — The fungus, Puccinia asparagi, has occasioned a great deal of alarm among commercial asparagus growers, particularly those of the eastern United States. This rust differs from that occurring on the small grains in that all stages of the rust occur on the asparagus plant. At the invita- tion of Massachusetts growers, the United States Department of Agriculture in cooperation with the Massachusetts Agricultural Experiment Station undertook to produce a resistant variety. Norton (1911-1912, 1913) has reported on this investigation. Because of the dioecious habit of asparagus it was necessary to select two kinds of plants — male and female. Selections were based on rust resistance, i.e., only plants which showed a high degree of resistance were chosen. In 1909 the first test of the transmission of relative rust resistance was made. Twelve lots saved from as many plants showing various degrees of rust re- sistance were planted in duplicate in short rows. After the young shoots appeared they were dusted several times with fresh uredospores. Later in the season observations were made on the degree of infection. The results are given in Table LXV (Norton, 1913). Table LXV shows clearly that rust resistance is inherited. Various artificial crosses were made between forms showing rust resistance. The progeny of some of these crosses proved highly resistant and in some cases were more resistant than the parents. By this method several strains of asparagus with a high degree of resistance have been produced. In the production of a new form a male plant obtained in 1910 from a lot of New American of un- known origin proved of marked ability in transmitting vigor and rust resistance to the progeny. The female plants known as Mary and' Martha were selected from the variety Reading 254 BREEDING CROP PLANTS TABLE LXV. — TRANSMISSION OF RUST RESISTANCE IN ASPARAGUS Row Source of seed Type of plant Rank of seedlings in resistance First lot Second lot Average 1 2 3 4 5 6 7 8 9 10 11 12 A 1-6 A3-61 A4-7 ,4 4-17 A7-5 A7-15 47-25 524-27 524-28 Old field Old field Frank Wheeler old bed Badly rusted, near rusty bed 7 6 3 10 4 2 5 11 9 12 8' 1 9 5 7 8 3 4 2 10 11 12 6 1 8.0 5.5 5.0 9.0 3.5 3.0 3.5 10.5 10.0 12.0 7.0 1.0 Very resistant female Resistance good .... Resistance fair Resistance good Resistance good Resistance good . . . Very rusty Very rusty Rusty Resistant Best resistant female Giant. Two or three other females have been selected and the crossed seed obtained from these selected plants has been distrib- uted under the name Washington asparagus (Norton, 1919). Some of these strains are now being offered for sale by commercial seedsmen. Norton suggests the following method for breeding asparagus; After two mated plants have proved their value by the progeny test, they should be dug up and propagated by crown division. These clones are isolated together and retained exclusively as breeding stock. Isolation may be accomplished by a fine-meshed cage to prevent the entrance of bees or by planting at a safe distance from other beds of asparagus. Producing seed in a greenhouse by hand pollination has also been found successful. ECONOMIC CUCURBITACE^ Introduction and Classification. — The family Cucurbitacece is of considerable historical interest. Sageret (1826) and Naudin (1856, 1859a, 18596), two pre-Mendelian workers, made extensive hybridization studies with some species belonging to this family. Naudin made a species classification on the basis of genetic behavior which is accepted at the present time. All the forms which cross readily were placed in the same species group. BREEDING OF VEGETABLES 255 Cucumis sativus — Cucumber Cucumis melo — Muskmelon, cantaloupe Cucurbita pepo — Pumpkin, gourd, summer squash, and varieties of winter squash. Peduncle hard and ridged. Cucurbita maxima — Large field squash and winter squash. Peduncle soft and fleshy. Cucurbita moschata — Squash. Little grown in United States. Peduncle much enlarged where attached to fruit. Citrullus vulgaris — Watermelon, citron. FIG. 61. — Structure of flowers of squash. 1. Female flower — a, corolla; b, calyx; c, fruit. 2. Male flower. 3. Male flower with calyx and corolla removed. 4. Female flower with calyx and corolla removed showing — a, stigma; 6, style; c, point of attachment of calyx and corolla; d, undeveloped fruit. 5. 6. Longitudinal and cross sections of fruit. Size: 1, 2, Y±n; 3, 4, >£n. Cummings (1904) experienced no great difficulty in crossing Golden Custard ( 9) with Crookneck (cf), varieties of squashes belonging to C. pepo. The reciprocal cross proved difficult, only five out of 284 pollinations producing fruit with viable seed. A histological examination revealed the fact that the male generative 256 BREEDING CROP PLANTS nucleus of Custard penetrated the ovary of Crookneck and took up a position which, in many cases, was in close proximity to the egg cell but for some reason fusion did not occur in most cases. Bailey (1890), as the result of many artificial pollinations, concludes "that the field pumpkins and the summer and fall types of bush squashes (C. pepo) do not cross with the running squashes of the Hubbard, Marblehead, Boston Marrow, turban, and mammoth types (C. maxima)." In the Cyclopedia of American Horticulture, Bailey (1900) states that C. moschata and C. pepo may be crossed artificially but it is doubtful if they cross naturally. Cucurbitacece in general are monoecious and largely dependent on insects for pollination. Immediate Effect of Pollination. — There is a popular belief widely disseminated that pumpkins and watermelons should not be grown in close proximity to one another because of the immediate effect of cross-pollination. A similar belief exists with regard to cucumbers and muskmelons. Evidence accumu- lated by various plant breeders shows that this idea is not founded on fact. The work of Bailey at Cornell and Pammel at Iowa may be cited. The former (1890) was unable to find any immedi- ate effect of cross-pollination between varieties of C. pepo and likewise between varieties of C. maxima. Bailey not only was unable to demonstrate any immediate effect of pollen in varieties which could be crossed but he was even unable to produce crosses between cucumbers and muskmelons. Ninety-seven flowers of several varieties of melons were pollinated with different varieties of cucumbers. Not a single fruit set. Twenty-five reciprocal pollinations were also made. One fruit developed but produced no seed. The setting of parthenocarpic fruit without fertilization is not an infrequent occurrence in cucumbers. Pammel (1892), in an intermingled planting of varieties of each of the following species, Citruttus vulgaris, Cucumis melo, Cucurbita maxima, Cucumis sativus, and Cucurbita pepo provided excellent facilities for inter-specific pollinations. Neither the watermelons nor the muskmelons showed contamination. Some hand pollinations be- tween species were made, but no cross-fertilization was obtained. The variability in flavor of commercial varieties of melons is undoubtedly partly responsible for the erroneous belief that they may be contaminated by other species of cucurbits grow- ing in close proximity. At the Connecticut Station an extensive varietal test was made. Most of the varieties were of very inferior quality even though they were exposed only to BREEDING OF VEGETABLES 257 rnuskmelon pollen. Even if crossing occurred, there is no con- clusive evidence that xenia would result. CUCUMBER Wellington, (1913) studied the inheritance of the following char- acters: color, size, number of spines, smooth or rough skin, and obtained ratios indicating monohybrid segregation. Smooth skin and small spines, few in number, appear to be linked. Heterosis shown by increased number or size of fruit, has been observed in the F\ of certain cucumber crosses (Hayes and Jones, 1916). The Fi of a cross (Reeves, 1918) between American type (20 per cent parthenocarpic) and English type (normally parthenocarpic) showed 20 per cent parthenocarpy. MUSKMELON Lumsden (1914), of the New Hampshire Agricultural Experi- ment Station, has made rather extensive studies of inheritance in the muskmelon. The following tabular statement gives a summary of his work: TABLE LXV1. — INHERITANCE IN THE MUSKMELON IN A CROSS BETWEEN THE VARIETIES SUTTON'S SUPERLATIVE AND DELICES DE LA TABLE Characters No. of Fi plants „ No. of Fz plants Fi ratio Color of skin . . . . Yellow * green Yellow X green 1 Yellow 79 2 76:1 Form of fruit . . Round • elliptical Round X elliptical 1 Round 79 2.76:1 Ribbing 5-45 46-100 per cent. per cent, ribbing • ribbing Ribbed X non-ribbed. . . 1 Ribbed 79 1:1.82 Netting 5-45 46-100 per cent. per cent. Netting • netting Netted X smooth 1 Netted 79 1 : 1 . 63 •Size of seed Large X small 1 Large 79 Large : small 2.95:1 Size of fruit Large * small Large X small 1 Large 79 2.59:1 17 258 BREEDING CROP PLANTS These data show that all the characters studied segregated in the second generation. There is some indication that the contrasted characters in color of skin, size of seed, size of fruit, and form of fruit are each separated by a single main factor difference. In a cross between varieties producing round and elliptical fruits respectively the F\ fruit was recorded as round, while the Fz gave a ratio of 2.76 round to 1 elliptical. The other two characters, netting and ribbing, indicate more complex in- heritance. Delices de la table (cf) has deep ribbing and no netting; Sutton's Superlative ( 9 ) has no ribbing and pronounced netting. The F2 generation showed a variation of from 5 to 100 per cent, with respect to each character. SQUASHES AND GOURDS Emerson (1910), while at the Nebraska Experiment Station, made a study of size inheritance in a cross between Yellow Crookneck and White Scallop summer squashes. He found that length of neck and diameter of bowl were intermediate between the parents in FI. The second generation showed a complete series of dimensions and shapes from one parent to the other. The same investigator crossed Striped Spoon gourd with Filipino Horned gourd. Results similar to those of the squash cross were obtained. WATERMELON One of the most serious handicaps to the production of water- melons in the Southern States is the presence of wilt, due to an organism, Fusarium niveum. "Citron" or "stock melon," so- called locally, is a non-edible variety of Citrullus vulgaris resistant to wilt. Orton (1911) conceived the idea of crossing this form with edible forms. He hybridized Eden, a good quality melon, with citron. The FI was very vigorous and of intermediate type. Between three and four thousand Fz plants were grown and ten fruits selected on the basis of resistance and quality. After selecting the resultant progeny for several years the variety Conqueror was isolated. It is disease resistant, has a tough rind, and does not sunburn easily. The flesh is juicy and of good quality, although not equal to the finest. These studies were made in South Carolina. It was found that Conqueror BREEDING OF VEGETABLES 259 retained its resistance when grown in Iowa but seemed to lose it when grown in Oregon, on the Pacific Coast. No very satisfac- tory explanation has been offered for this phenomenon. It is possible that a similar condition exists with flax wilt. FIG. 62. — A strain of Hubbard squash isolated by self-fertilization which is comparatively uniform for the production of large fruits of uniform shape. Minnesota Exp. Sta. (Courtesy of Bushnell,) Flax strains resistant to wilt seem to lose their resistance when grown for a few years in wilt-free soil. Breeding Cucurbitaceae. — Each botanical species of this family in most cases constitutes a freelv inter-crossing group of FIG. 63. — A small fruited strain isolated from a commercial variety of Hubbard squash by self-fertilization. Minnesota Exp. Sta. ( Courtesy of Bushnell.) varieties. The monoecious character of the plant encourages cross-fertilization. In spite of these facts the authors believe that in some cases progress may be made by breeding methods 260 BREEDING CROP PLANTS recommended for self-fertilized crops or more specifically for crops which yield ample seed on selfing. When such a plan is adopted for naturally cross-fertilized crops it becomes necessary to insure selfing by artificial means. By reducing ordinary varieties to pure lines, a much more exhaustive study of the material at hand may be made, and on the basis of this study desirable combina- tions affected by hybridization or pure lines of commerical value may be isolated. The method which is adopted after the isola- tion of homozygous lines through self-fertilization will depend on the degree in which vigor is lost as a result of selfing. That homozygous lines may be isolated in squashes is a demonstrated fact, the result of three years' study as carried on by John Bushnell, of the Minnesota Station. Some lines which are comparatively uniform appear vigorous while others are less vigorous. Types for markedly different characters which are relatively uniform have been isolated. CHAPTER XVIII FRUIT BREEDING The improvement of fruit crops offers an interesting field of study for the trained investigator. Many fruits are in a complex heterozygous condition. For this reason and because fruits are propagated by asexual methods Mendel's law does not have here the same value as for the breeder of self -fertilized crops. There are also many fruit crops which are totally self -sterile so that cross-pollination, either natural or artificial, is essential to the production of fruit. Unlike an annual crop the individual fruit tree often takes many years to grow before fruiting. For these reasons methods of handling are often of much greater importance than methods of breeding. It is, therefore, of utmost importance that the student first make an intensive study of the botanical relatives, methods of culture, varieties, and environmental necessities of the crop before undertaking breeding operations. ORIGIN AND ANTIQUITY OF SOME FRUITS1 Wild fruits without doubt played an important role in the food supply of primitive man. As the art of agriculture came to be developed because of the necessity of obtaining enough food to supply the increasing human population, the fruit crops were gradually introduced into cultivation. Some of our most prized fruits, as the apple, grape, and plum, have been cultivated since earliest times; while others, as the strawberry, black rasp- berry, and blackberry, have been brought under cultivation since America was discovered. The wild species from which our fruits have been developed may still be found today. Wild plums may be found in nearly every state of the United States, while in central and northern Asia the wild relatives of apples, pears, apricots, cherries, and plums are of frequent occurrence. The wild crabs are found in abundance, in both the Eastern and the Western Hemispheres. As the cultivated European 1 A paper by WHITE (1916) has been used very freely in this discussion. 261 262 BREEDING CROP PLANTS varieties gave good results when introduced into the United States, the breeding of apples has not been seriously undertaken until comparatively recent times. The cultivated varieties are very numerous. Our pears were developed from two very dif- ferent wild species, Pyrus communis, the wild pear of western Asia and Europe and the hard, gritty sand pear of northern China. P. communis is the source of our eating pears, such as the Bartlett, while inter-species crosses furnished our cooking and winter pears. Peaches were first developed in China. When one compares the little hard, bitter wild peach of China and our cultivated varieties the results of early breeding are strikingly illustrated. There are three groups which are commonly accepted as the ancestral forms of our cultivated plums: (1) The thorny wild European species which produces dark purple fruits about the size of a pea. These are the source of our prune varieties. (2) North American native wild plums which have a very juicy flesh without much meat. Several species are recognized (Wight, 1915). (3) A Chinese-Japanese wild species. Many of the cultivated varieties of plums are largely of hybrid origin. There are over 120 wild species of cherries which are native to Asia and from 200 to 1,500 wild species of raspberries and black- berries. The variation in type of the wild red raspberries of New England is a good illustration of a wide diversity of forms. Some of these are probably results of crosses with escaped culti- vated varieties. Natural hybridization certainly played a large part in the evolution of such fruits and the selection of promis- ing wild seedlings furnished the major part of our cultivated varieties. Fletcher (1916) has described 1879 varieties of strawberries which originated in North America and 26 European varieties which have attained prominence in this country. The straw- berry is largely a hybrid product of four or more species. The citrus fruits are all of Asiatic origin. Present cultivated varieties have for the most part been produced during the last 100 years. The grapefruit industry of the United States has been developed in the last 25 years. This fruit, which is a native of islands lying to the south of Asia, was introduced into the West Indies early in the eighteenth century and more recently from the West Indies into Florida. Table LXVI1, which is part of a table published by White (1916), is a summary statement of the source FRUIT BREEDING 263 and the length of time under cultivation of some of our most highly prized fruit crops. TABLE LXVII. — ORIGIN, PROBABLE LENGTH OF TIME OF CULTIVATION, AND COMMENTS ON SOME CULTIVATED FRUITS (AFTER WHITE, 1916) Name Date Origin Remarks Apple Apricot Blackberry • A A F E. Europe, W. Asia Central Asia, China United States Very different type common to China. Wild species variable. Wild species verv variable. Blueberry . . F E. and N. North America. Four species, often confused Cranberry Currant, red Cherry, sour Cherry, sweet Grape, Old World . . Grape, New World . . Gooseberry F C B B A F C E. and N. North America Northern Hemisphere Asia Minor, S.E. Europe (?) S. Europe, E. Asia West temperate Asia North America N. Europe, N. Africa, W. with huckleberry. Cultivated for about 100 years. White and yellow varieties are forms. California and Old World grape. Many probably hybrids. Old and New World species dis- Grapefruit Lemon B B Asia, United States Malayan and Pacific Islands east of Java India tinct. Largely cultivated in United States. Orange, sweet Peach C A India China Numerous hybrids with other species. Hundreds of varieties. Pear A Temperate Europe and Asia, Two species and hybrids be- Plum Raspberry, red Raspberry, black . . . Strawberry A C F F N. China S. Europe, W. Asia, N. America. N. Europe, Asia, N. America Middle North America Temperate N. America, tween them. Much hybridized group. Varieties and hybrids of two species. \t least three species involved Pacific Coast of N. and 8. America, Europe A, cultivated for more than 4,000 years. B, cultivated for more than 2,000 years. C, cultivated for less than 2,000 years in the Old World. F, cultivated since the discovery of America. Often only very recently. The mode of origin of some of the better United States fruit varieties has been compiled by Dorsey (1916) from the New York Agricultural Experiment Station fruit monographs. A summary statement is presented in Table LXVII I. These data show that nearly 85 per cent, of the commercial fruit varieties of apple, cherry, plum, and grape have been obtained by selecting promising chance seedlings, that one parent was known for a little more than 10 per cent, of the varieties described, 264 BREEDING CROP PLANTS TABLE LXVIII. — ORIGIN OF VARIETIES OF APPLE, CHERRY, PLUM, PEACH AND GRAPE Fruit Both parents known One parent known Neither parent known Origin as bud sports Total Apple 3 39 588 4 634 Cherry 20 61 1,064 n 1,145 Grape . ... 74 57 72 0 203 Plum 49 108 524 1 682 Peach 2 13 69 1 85 Total 148 278 2,317 6 2,749 while over 5 per cent, of the commercial varieties originated from crosses in which both parents were known. Only six out of 2,749 varieties are known to have originated as bud sports. SOME EARLY STUDIES IN FRUIT IMPROVEMENT1 The preceding discussion gives some idea of the great number of varieties of our fruit crops. While many of these are from chance seedlings, a considerable percentage resulted from definite attempts to produce improved forms. Von Mons. — One of the earliest horticulturists was a Belgian by the name of Von Mons, who was born in 1765 and died in 1842. He was a chemist but followed horticulture as an avoca- tion. His studies were carried out for the purpose of proving the truth of certain philosophical theories. While he did not succeed in substantiating the theories, his work was of considerable value to horticultural science and practice. His most important studies were with pears. In 1823 there were 80,000 seedlings in his nur- sery. About this time he issued a catalog in which 1,050 pears were described by name or number. Of these, 405 varieties were of his own production. His practice was to sow, select, and resow, and without doubt a part of his great accomplishments was a direct result of cumulative selection. Knight. — Thomas Andrew Knight has already been mentioned as a man who contributed much to the art of plant breeding. He was born in England in 1759 and died in 1838. A part of his work was carried on with such fruit crops as apples, pears, and peaches. 1 For an account of the evolution of American fruits the reader is re- ferred to BAILEY, 1898; MUNSON, 1906. FRUIT BREEDING 265 He emphasized the value of crossing as a means of producing improved forms for he believed this method was more rapid than Von Mons' selection practice. American Pomology. — Throughout the nineteenth century American pomologists made great progress in the improvement of fruits. While many American named varieties occurred as chance seedlings, others were the result of careful breeding. The strawberry and grape are examples of fruits in which many of the varieties are a result of controlled breeding. Selection and crossing both played important parts in the improvement of varieties. Hovey was one of the best known of the early straw- berry breeders who worked during the first half of the nine- teenth century. The production of improved American varieties of grapes well illustrates a common method of the production of new fruits. Old World grapes did not succeed in the greater part of the United States, as European varieties proved very susceptible to diseases, particularly mildew. The production of American varieties from native wild species gave us many of the cultivated types. Some of the best of the early varieties arose as chance seedlings. Con- cord was thus discovered by Ephraim W. Bull and introduced about 1853. It has been frequently used as a parent for the production of the improved forms. Some improved forms have resulted from crosses between native and European varieties, Delaware being generally thought to have been so produced. With the plum, as with the grape, the native American species have furnished the source from which a large part of the American varieties have been produced (Wight, 1915). Several wild species have been used and frequently the varieties which have proved best adapted to a given locality have been produced from the wild form which is native to the same region. SOME CONSIDERATIONS OF FRUIT BREEDING Fundamental laws of heredity furnish the same foundation for a development of correct breeding technic in the fruits as with other crops. There are, however, some factors which, modify breeding methods. For example, a single tree takes up con- siderable field space and thus has a greater value than a single plant of wheat or corn. In comparing varieties and clonal lines the question of soil heterogeneity must be considered for this is 266 BREEDING CROP PLANTS probably a frequent cause for the variation in yield from different trees of the same variety when grown in the same orchard. Self- sterility, which is so prevalent among fruit crops, often prevents the production of homozygous material ; while the use of heterozy- gous material does not allow the breeder to make systematic crosses with a knowledge of the genetic constitution of the parents. In spite of these difficulties which the fruit breeder must face, there has been a consistent attempt to use fundamental breeding principles and at present methods are becoming somewhat standardized. The advantage which comes to the breeder from the fact that an improved variety may be propagated asexually and need not be reduced to a homozygous condition, tends to offset other difficulties Some of the more general problems will be here illustrated. Overcoming Soil Heterogeneity. — Batchelor and Reed (1918) have made an interesting study of variability in orchard plots. They used orange, lemon, walnut, and apple trees in the investi- gation. From 224 to 1,000 trees of each of the different fruits were studied and the coefficient of variability for yield of single trees determined. The coefficient of variability of the clonal varieties ranged from 29.72 to 41.23 per cent. Thirty-five per cent, might be considered a fair average. Multiplying this by 0.6745 gives 23.6, the probable error in percentage of the mean. The effect on the coefficient of variability of increasing the number of trees in a plot was studied ; a comparison of plots con- taining 1, 2, 4, 8, 16 and 24 trees being made. Table LXIX gives an average of tests with oranges, lemons, apples, and walnuts. The results are based on a study of more than 2,000 individual trees. TABLE LXIX. — EFFECT OF INCREASING THE NUMBER OF TREES PER PLOT Number | of trees \ per plot '; Average coefficient of variability Average reduction of coefficient of variability by increasing number of adjacent trees per plot Increase from Average reduction 1 37.7810.52 2 30.89 + 0.55 1 to 2 6.89±0.76 4 26.76±0.62 2 to 4 4.13±0.83 8 24.27 + 0.77 4 to 8 2.49 + 0.99 16 22. 58 ±1.01 8 to 16 1.69 + 1.27 24 19.74 + 1.08 16 to 24 2.84 + 1.48 FRUIT BREEDING 267 From these results the conclusion is reached that eight trees is about the correct number which should be used in a plot. The question of replication, i.e., the systematic distribution of plots over the field, is taken up. Results computed for four- and eight-tree units are given for oranges, apples, walnuts, and lemons. Table LXX gives an average of data from these crops. TABLE LXX. — EFFECT OF REPLICATION IN FOUR- AND EIGHT-PLOT UNITS Four trees in a unit Eight trees in a unit Number of systematically replicated plots 26.76±0.60 24.27 + 0.77 1 15.12±0.47 12.84±0.56 2 13.58±0.53 11.27±0.63 3 9.29 + 0.40 9.54±0.57 4 8. 40 ±0.40 8 49 + 0 49 7.95 + 0.49 5 6 The conclusion seems warranted that four systematically replicated plots greatly reduces the error which arises from soil heterogeneity. The data also show that four systematically distributed plots of four trees each are somewhat more reliable than two plots of eight trees each. As was presented in Chapter IV, Harris has given a reliable means of estimating soil heterogeneity by the correlation between the neighboring plots of a field. The test was applied to an orange grove which' appeared to have uniform soil conditions. The correlation between the yield of eight-tree plots as ultimate units and grouped combinations of four such adjacent plots was found to be: r= +0.533 + 0.085. This showed a pro- nounced heterogeneity in the soil of this orchard. However, the correlation computed between the yield of an eight-tree ultimate unit and the yield of the combination of four such systematically distributed units was not much larger than the probable error. These facts show the unreliability of yields of single trees as a criterion of productivity, that eight-tree plots give much more reliable results, and that plot replication is of as much value in studies of fruit-yield as of farm crops. Where quality is a major criterion, single trees give fairly reliable information. Self -sterility and Heterozygosity. — One of the chief difficul- ties of systematizing methods of work is due to the heterozyogus 268 BREEDING CROP PLANTS condition of most fruit material. A commercial variety may be extremely valuable and yet be heterozygous for many characters. On the other hand, the commercial variety may be homozygous for a large part of its characters. It seems reasonable to con- clude that the more nearly homozygous the parental variety, other things being equal, the greater value it would have as a parent. The ability of impressing its characteristics upon the larger part of its offspring has been called prepotency by animal breeders. Such prepotency is genetically explained by the supposition that the prepotent parent is homozygous for certain dominant factors for the characters under observation. Hedrick and Wellington (1912) showed that some crosses between apple varieties pro- duced a considerable percentage of individuals with small fruits. Thus the cross between Rails and Northern Spy gave great variability in size of apples, while the cross between Sutton and Northern Spy gave progeny in which no trees were obtained which produced small fruit. One of the great difficulties is that it takes several years to learn the varieties which when crossed will give certain desired combination. Another difficulty which must be considered is that many varieties of fruits are self-sterile. This is of utmost importance in commercial fruit production for it is necessary to interplant such a variety with some variety which produces an abundance of pollen which is capable of fertilizing the variety in question and TABLE LXXI. — INCREASE IN WEIGHT OF SEED AND FRUIT DUE TO CROSS- POLLINATION Pollination Average weight of seed, grams. Average weight of fruit, grams. Newtown X self 0 05 73 Newtown X Bellflower 0 40 104 Newtown X Spitzenberg 0 66 147 Newtown X Jonathan 0 65 162 Newtown X Grimes Golden 0 60 173 Spitzenberg X self 0 13 100 Spitzenberg X Newtown 0 65 126 Spitzenberg X Arkansas Blk 0 68 128 Spitzenberg X Jonathan 0 70 144 Spitzenberg X Baldwin 0.71 157 FRUIT BREEDING 269 which blooms at about the same period. The self-sterile habit likewise prohibits the reduction of the material to a homozygous condition. Frequently self-fertile varieties give great increases in weight of seed and fruits as a result of cross-pollination. There- fore, pollinators, varieties which have proved desirable as pollen parents, are often of considerable commercial value in increasing yield in the case of self-fertile varieties. Table LXXI gives two typical cases taken from the work of Lewis and Vincent (1909) with the apple. The large increases in weight of seed as a result, of crossing are TABLE LXXII. — CONDENSED STATEMENT OF NUMBER OF SELF-FERTILE, SELF-STERILE, AND PARTIALLY SELF-FERTILE VARIETIES OF FRUIT CROPS Number Number Number | Fruit self- self- partially Authority Remarks fertile sterile self-sterile Grape 7 13 5 Dorsey, 1914 Self-fertile and partially self- I after Beach fertile have upright stamens (1898, 1899) and pollen with germ pore. Self-sterile varieties have re- flexed stamens and pollen with no germ pore. Grape | . .-"• Beach, 1902 Pollen of self-sterile varieties I can not fertilize other self- sterile varieties. i Plum All cultivated varieties of Dorsey, 1919 Results given by Dorsey are native American species ex- from studies of Waugh cept New Ulm and Robinson (1896, 1897, 1898, 1899, are self-sterile. 1900, 1901) Goff (1894, 1901), and Waite (1905). Plum 18 16 5 Sutton, 1918 All self-sterile varieties set fruit when pollinated with any other variety with few exceptions. Cherries . . . 3 17 2 Sutton, 1918 Apples .... 8 16 10 Sutton, 1918 Apples ... 28 59 Lewis and Vin- From 50 to 200 pollinations cent, 1909 were made for each variety. If no seed set, variety is « classed as self -sterile. All varieties with some seed setting are classed as self- fertile, although some are partially self-sterile. Pears j Bartlett and Kieffer pears are Fletcher, 1911 self-sterile. 270 BREEDING CROP PLANT* very noticeable. Increases in size of fruit are also of much importance. For the commercial grower or the fruit breeder, it is essential to know which varieties are self-sterile. In order to illustrate the conditions generally found regarding sterility, a compilation of some results is presented in Table LXXII. Citations to literature are given so that the reader may go to the original sources when he desires to know what category any particular variety belongs to. The causes of sterility have been determined in some cases. In the strawberry it is due to at least two causes (Valleau, 1918) : 1. The dioecious condition. 2. The production of aborted pollen grains or microspores in otherwise normal anthers. In the grape, Dorsey (1914) has found sterility to be asso- ciated with both hybridity and the dioecious condition. The varieties which produce reflexed stamens seldom produce fertile pollen. Dorsey states that : " Sterility has been found to be due to the pollen rather than in the pistil. Sterile pollen in the grape results from degeneration processes in the generative nucleus or arrested development previous to mitosis in the microspore nucleus." Pollen abortion occurs both in pure and hybrid forms but is not considered a cause of lack of fertility as abundant pollen is pro- duced in the grape. In the plum, pollen abortion is not as a rule the cause of self- sterility. The outstanding features as given by Dorsey (1919) are: "(a) A constancy of expression of self-sterility even in P. domestica in which about one-half of the varieties are self-f ertile ; (b) the occur- rence of cross-sterility; and (c) the slow growth of pollen tubes under the condition of self- and cross-sterility." This type of sterility is comparable with that in the tobacco crosses previously discussed, where sterility resulted from slow pollen tube growth. In this case the pollen tube growing from the pollen grain into the tissues of the style never reaches the embryo sac. The self-sterile condition is believed by Dorsey to be a dominant character in the plum and to be inherited, segrega- tion into sterile and fertile forms occurring at reduction division. FRUIT BREEDING 271 Knight (1917) has made a study of self-sterility in the apple and the conclusions reached show the manifold causes which must be considered in a study of the problem. For this reason the conclusions are here given verbatim. "1. Self -sterility in Rome Beauty is not due to sterility of the pollen as has been shown to be the case in certain varieties of grapes. "2. Sensitiveness of pollen to over-abundant moisture supply is not involved here as a factor, as has been shown by Jost for the pollen of many grasses, barley especially; and by J. N. Martin for the pollen of red clover. The pollen of Rome Beauty and many other varieties germinated in distilled water. "3. Rome Beauty stigmatic fluid extracts offer no inhibition to the germination and growth of Rome Beauty pollen. "4. Rome Beauty stigmas offer no particular mechanical obstruction to the penetration of Rome Beauty pollen tubes. "5. Self -sterility of Rome Beauty is not due to inability of its own pollen tubes to grow deep enough to reach the egg. This has been sug- gested as the cause of self-sterility in certain pear and apple varieties by the work of Osterwalder. "6. From present indications one important factor in self -sterility of Rome Beauty is the relatively slow rate of growth of Rome Beauty tubes in Rome Beauty stylar tissue. Doubtless other factors will be found upon further examination." Inheritance of Some Characters. — The mode of inheritance of most fruit characters has as yet not been determined. There are, however, numerous experiments under way for the purpose of learning how individual characters behave in crosses. The lack of information in this field is due to the heterozygous condition of many fruit varieties and to the fact that with many fruit crops so long a period elapses between the time of sowing the seed and the production of fruit. Apple. — Inheritance in the apple is well illustrated by a study made at the Geneva Station by Hedrick and Wellington (1912). Crosses were made in 1898 and 1899 and 148 seedlings were grown. In 1912, 106 of the seedlings had come into bearing. These 106 seedlings resulted from 11 crosses. The first genera- tion naturally does not furnish very reliable data as a means of deciding the mode of inheritance of individual characters. Three types of skin color were studied, red, yellow, and inter- mediates. The conclusion was reached that Ben Davis and Jonathan were both pure for red color of skin, as crosses between 272 BREEDING CROP PLANTS these varieties gave seedlings which produced fruit with a red skin. Other crosses led to the belief that yellow is recessive and that a cross between red and yellow is intermediate in skin color. Sweetness was believed to be a recessive character to acidity with the indication that the F\ was intermediate. Raspberry. — Bailey (1898) believed that the purple rasp- berry, Rubus neglectus, was a natural hybrid between the black and red varieties. This was definitely proved at the Geneva Station by a cross between Smith No. 1, a black raspberry, and Lonboro, a red seedling, which gave 209 purple raspberries (Wellington, 1913, Anthony and Hedrick, 1916). The same Smith No. 1 crossed with June, a red raspberry, gave 50 purples and 46 blacks. Selfed seedlings of Columbian, a purple variety, gave 31 purple, 7 red wine, 2 reddish, 1 yellow, and 1 black. The mode of inheritance of colors can not be determined, although it seems that several of the black varieties are heterozygous for color and that several factors for color are present. The presence of bloom on the canes proved to be a partially dominant charac- ter over the absence of bloom. The number of spines on canes showed segregation in selfed seedlings of Columbian. Yellow raspberries could be told in the seedling stage from the black and purple by the absence of red tinge on the leaves. The production of promising varieties from crosses between the red and black varieties was especially mentioned. Grape. — The Geneva Experiment Station, in New York, (Hedrick and Anthony 1915) likewise furnished the greater part of our data on inheritance of characters in the grape. Table LXXIII gives the results of crosses for skin color. TABLE LXXIII. — INHERITANCE OF SKIN COLOR IN GRAPES Color of parental types Color of seedlings Black Purple to dark red Medium ! to light White red White X white 6 43 49 44 3 52 13 45 13 1 40 166 8 42 54 50 12 32 Light red X light red 8 38 407 5 41 100 Dark red X dark red Black X black White X dark red White X black .... Black X dark red FRUIT BREEDING 273 The chief conclusions which may be reached from these results are that nearly all varieties are heterozygous for color and that white is a pure recessive. In studies of inheritance of quality there is a proof of the value of selecting as parents the types which excel for the character being worked with. Table LXXIV gives some of the results of crosses in which quality was studied. TABLE LXXIV. — INHERITANCE OF QUALITY IN THE GRAPE Parental types Total Percentage of good or better Parents good or of higher quality 682 27 Good X fair or poor 56 11 Medium X medium 213 " 10 Poor X poor 51 4 Nearly all grapes of high quality at the New York station contain some V. vinifera blood. This is easily understood when one remembers the long period of breeding of the European varieties and that American varieties were only recently obtained from the wild. Inheritance of size of grape berry and ripening period showed the value of selecting as parents varieties which excel in the character which the breeder wishes to obtain. Illustrations of Methods of Breeding. — Methods of breeding fall naturally under three main heads; 1. Selection of bud sports. 2. Seedling selection. 3. Controlled crosses. As has been already mentioned many of our varieties have resulted from chance seedlings, others from seedlings in which only one parent was known. A review of the subject leads to the conclusion that the improvement of fruits by the use of self- fertilized seed is a less desirable method than by the use of crossed seed. When selfed seed can be produced the progeny are as a rule less vigorous than those obtained from crossed seed. As these subjects have been touched upon in some detail under other headings, seedling selection will not be discussed further. Selection of Bud Sports. — It is now a commonly accepted fact that mutations or sudden changes in the germinal material do occasionally occur. Likewise, in asexually propagated species 18 274 BREEDING CROP PLANTS bud sports have been found, and in some cases these have been used as the foundation of improved races. To justify a method of breeding founded upon their utilization, such bud sports must occur frequently enough to pay for the trouble of making a systematic search for them. A review of the experimental evidence is of considerable interest, for this is the only means we have of deciding whether the selection of particular trees or branches for propagating pur- poses is a reliable means of producing new varieties. Of the four apple bud sports mentioned in Table LXVIII the chief changes were in the color of the fruits. In the Isabella grape several sports were obtained which produced black grapes of larger size than Isabella, and which excelled in sweetness (Powell, 1898 cited from Dorsey, 1916). Dorsey (1916) records two large-fruited variations in the Concord grape which arose as bud sports. Instances of bud variations in ornamental horticultural plants are quite common. As an example of their frequency, the work of Stout (1915) will be briefly discussed. Extensive asexual or clonal selections were made in Coleus and numerous color changes were isolated as well as changes in leaf shape. The same varia- tions were obtained through bud sports as by seed reproduction. Some clonal lines sported much less frequently than others. The work on citrus fruits (Shamel and others, 1918) which has been carried on in California, has drawn the attention of many horticulturists and plant breeders to the subject of bud sports and their place in correct fruit-breeding methods. Valencia oranges were originally introduced from three sources, but all have proved of similar type and are now called Valencia. From this variety 12 important strains originating as bud sports have been isolated. As a rule, single off-type branches produce fruits showing charac- ters which are different from the fruits borne on the remainder of the tree. Many of these sports are of highly undesirable type. The Washington navel orange was introduced from Brazil in 1870 by the Department of Agriculture at Washington. Thir- teen distinct strains have been isolated through bud selection. Thompson, one of these strains, has proved a very desirable type. Likewise, bud sports have occurred in the grapefruit which was introduced in California from Florida in 1890. The Marsh is the best of six strains which were obtained by selecting bud sports. Similarly bud sports have occurred in lemon orchards. Shamel (1919) records an occurrence of a sporting branch in a FRUIT BREEDING 275 French prune tree which was first observed in 1904. Several grafts from this branch were placed in bearing trees. These grafts reproduced the characters of the sporting branch. In 1914, trees in alternate rows of an orchard were top-worked by the use of buds from the new strain and compared with buds from the normal French prune variety. The top-worked trees from the bud sport bore larger fruit than those from the normal prune. The fruits were also more evenly distributed over the tree than in the original French prune variety. The above are some of the more striking instances of the pro- duction of new varieties through the isolation of bud sports. Crandall (1918) has made an extensive test in Illinois of the value of bud selection in apples as a means of improving the variety. Two distinct lines of study have been followed. 1. The value for propagating purposes of buds selected in different ways. The experiments included a comparison of large versus small buds, of buds from different parts of the tree and from different locations on the shoot. 2. Selection of trees because of special merit. Comparison of seedlings produced from large and small apples produced by these selected trees. A considerable number of varieties was used for the first study and a total of 5,400 buds were selected. A careful measure- ment was then made of the yearly growth of wood from the buds which had been previously selected. Growth curves were made and on the basis of these results the conclusion was reached that all buds from healthy shoots were of equal value for propagation purposes. The characters of seedlings grown from seeds of large and small fruits borne on trees of special merit were carefully studied. Seeds from large fruits produced seedlings which were somewhat more resistant to adverse conditions than seedlings grown from small fruits. The hypothesis that this may be explained by the fact that large fruits and large seeds frequently occur from crosses, seems reasonable in the light of the work of Lewis and Vincent previously cited. Stewart (1912) has discussed the value of cion selection in tree- fruit improvement. Individual apple tree data over a period of from ten to fourteen years were presented. Under apparently the same conditions some trees were consistently higher yielders than others. A review of considerable experimental evidence led 276 BREEDING CROP PLANT* Stewart to conclude that there was more evidence in favor of purity of the clone than in favor of the value of clonal selection as a means of producing higher-yielding strains. Similar con- clusions were reached from an experiment carried on by Tyson brothers, in New York, with the York Imperial apple. Two trees were selected which bore unusually similar fruits and these were used for propagation. More than 8,000 trees were planted in the new orchard. Examination of trees of this orchard when they came into bearing showed them to be not superior to the usual York Imperial apple (Dorsey, 1917). The cited cases show the present status of the problem of selec- tion of bud sports as a means of improvement of fruit crops. The studies with the citrus genus appear to justify the belief that degenerate or inferior bud sports are of frequent occurrence. This leads to a conclusion that only those limbs which produce normally healthy fruit should be used for propagation purposes. Even among the citrus fruits there is as yet no very conclusive proof that the selection of cions from high-yielding trees will accomplish more than to prevent possible "running out" of the variety. The evidence from apples would seem to justify the belief that bud sports are very infrequent. The breeder, then, can well afford to make careful observations with the hope of discovering bud sports. If apparently desirable sports are found, these may then be used for propagation. In such crops as citrus fruits and with such plants as Coleus, bud sports are of frequent occurrence. There is, then, some evi- dence for the belief that sports occur more frequently in hetero- zygous than in homozygous material. As Stout (1915) obtained the same changes through asexual selection as by the use of self- fertilized seed, it seems reasonable to suppose that some sort of segregation and recombination occurs in somatic tissue. No cytological evidence has been given to account for such a supposi- tion. With heterozygous material the loss of a single dominant factor would be immediately apparent in the soma. This is one reason why bud sports occur more frequently in heterozygous forms (East and Jones, 1919). Nabours (1919) has shown that similar cross-overs occur in parthenogenetic reproduction in the grouse locust as in those forms which are produced by the recom- bination of gametes containing the haploid number of chromo- somes. If the usual sort of cross-overs occurred in homozygous material, there would be no change in the homologous parts of FRUIT BREEDING 277 chromosome pairs. In heterozygous material, however, new combinations of factors would be produced which might cause changes in the external appearance of the organism. No cyto- logical basis for such cross-overs has been demonstrated. Controlled Crosses. — One of the earliest controlled experi- ments in the breeding of fruits by crossing was started by Swingle, in 1893, in Florida. This was an attempt to produce hardier types by the use of wild citrus species. The hardy Chinese species, Citrus trifoliata, was used as one of the parents. In 1897, 212 crosses were made between this species and orange varieties. The three fruits that were produced gave thirteen hybrids, which were so different from existing varieties of citrous fruits that they were called "Citranges." Other crosses between citrous species were made. One of the promising combinations was a cross between the West India lime and the kumquat orange. This orange is one of the hardiest of the evergreen citrous trees while the lime is very tender. Further experiments are under way and other promising wild relatives of the citrous fruits have been obtained. Crosses of this nature are producing fruit varie- ties which are successful in regions where citrous fruits could not be grown formerly. The work shows the necessity of a thorough botanical knowledge of the wild relatives of the crop which it is hoped to improve by breeding. A somewhat similar method of work with the hope of producing hardy apples for the Canadian Northwest was started by William Saunders in Canada in 1888. The wild Siberian crab, Pyrus baccata, which proved hardy on the prairies and withstood temperatures of 50° below zero, was used as the female parent and crossed with commercial apple varieties. Macoun (1915) states that the fruit of Pyrus baccata averages % in- in diameter and is quite astringent. The fruits obtained from some of the more promising ol the crosses were not so large as desired, although some compared very favorably in size with ordinary crabs. They were of good flavor and proved hardier than any varieties of apples and crabs that had been tested up to that time. Several are here listed. Jewel, P. baccata X Yellow Transparent. Size 1.4 by 1.3 in. Columbia, P. baccata X Broad Green. Size 1.8 by 1.6 in. Charles, P. baccata X Tetofsky. Size 1.6 by 1.5 in. Recrosses between the best of these and apple varieties were 278 BREEDING CROP PLANTS made and 407 trees were grown. Some varieties were obtained with larger fruits but these as yet have not been thoroughly tested for hardiness. Pears have been frequently tried in the Dakotas but have failed for two causes (Hansen, 1915): (1) Lack of hardiness; (2) susceptibility to blight. The Chinese sandpear, Pyrus FIG. 64. — Wolf, a hardy variety of plums which lacks quality of fruit. (Photo loaned by Dorsey.) sinensis Lindley, obtained from Dr. Sargent, of the Arnold Arboretum, proved perfectly hardy and resistant to blight. Various crosses between this species and cultivated pears be- longing to Pyrus communis have been made. Preliminary tests have shown that some of the seedlings were blight resistant and hardy. These results indicate that the problem of producing FIG. 65. — Burbank, a plum of high quality produced by Luther Burbank. It lacks hardiness when grown in Minnesota. (Photo loaned by Dorsey.) pears for the Northwest may eventually be solved. In a some- what analogous manner, Hansen (1911) has produced new plum varieties by crossing the native sand cherry with Japanese plums. This has resulted in a "happy combination of hardiness, rapid growth and early bearing of tree, with large size and choice quality of fruit." FRUIT BREEDING 279 It will be of interest here to present briefly an instance from the fruit-breeding work at the Minnesota Station in which desirable new plum hybrids were obtained when the tender parent, Burbank (P. triflora) was crossed with Wolf which is a hardy variety of P. americana mollis. The percentage of hy- brids killed during winter dormancy is taken as a basis for clas- sification. It will ,be seen that some of these hybrids, as No. 8 or No. 9, are hardy in the bud like the staminate parent Wolf. The two which have been named Red Wing and Tonka, are inter- mediate in hardiness but of excellent fruit characteristics. FIG. 66. — Tonka, Burbank X Wolf, No. 21. Has high quality and is nearly as hardy as the hardy variety of Wolf. (Photo loaned by Dorsey.} TABLE LXXV. — SHOWING THE PERCENTAGE OF BUDS KILLED IN AN FI PROGENY WHEN ONE OF THE PARENTS is HARDY AND THE OTHER TENDER1 Parent Percentage of buds killed 1916-17 Parent Percentage of buds killed 1917-18 Burbank 100 Hybrid No. 9 0 Wolf 0 10 50 Hybrid No 1 50 11 1 2 35 12 (Red Wing) 10 3 5 14 . . 25 4 5 15 5 5 10 16 5 6 10 17 ... 0 7 0 20 25 8 . .. 0 21 (Tonka) 25 These few instances have been given as indicative of the methods of work which are being used by some of the most pro- gressive fruit breeders. Some general conclusions regarding methods of work may be here given. 1 Data furnished by M. J. Dorsey. 280 BREEDING CROP PLANTS 1. A knowledge of the botanical relationship and wild relatives of the fruit are necessary if greatest progress in improvement is to be obtained. 2. Some varieties and species transmit their characters to a much greater degree than do other varieties. A knowledge of the more prepotent varieties materially aids in planning a cross. 3. Varieties selected as parents should contain in the highest degree possible the character or characters desired in the progeny. 4. The larger the numbers of progeny grown, the greater the chances of obtaining the combination desired. 5. Most fruit crosses give variable progeny in FI. Numerous crosses should, therefore, be made. 6. Information regarding the mode of inheritance of particular characters will assist in selection of varieties to be used as parents. CHAPTER XIX FARMERS' METHODS OF PRODUCING PURE SEEDS The production of new varieties of farm crops is a specialized line of work and should be undertaken as a rule only by men who have had special training in crop breeding. • The expense and time necessary for this kind of experimental work are too great for the individual farmer. The aim of the farmer or seed grower should be to maintain the improved form and not to allow contamination through crossing with inferior stock, admixture, plant diseases, etc. A method of producing seed which will stand this test and at the same time meet with the approval of the farmer must be simple, effective, and inexpensive. A nation or state cannot afford to maintain an experimental laboratory only to have the products of that laboratory deteriorate because of subsequent treatment. The maintenance of pure, improved varieties as well as their discovery by selection or synthesis by crossing is an essential factor in economic food production. Be- fore taking up in detail methods of producing pedigreed seed by farmers, a few observations regarding seeds in general will be made. DETERMINATION OF BETTER VARIETIES Certain general facts regarding varieties should be understood. The breeder and grower must recognize that no one variety is best adapted to a particular locality or for all seasons. In some seasons an early oat gives the best yield. Owing to slight seasonal variations, a later variety may excel in yield. Thus, a single season's test is not reliable as a means of determining the better sort to grow. For this reason carefully conducted tests are carried on each crop season. By means of these the experiment stations are in a position to determine and advise as to the better varieties. The final decision as to which variety to grow must of course be made by the farmers and based on their act- ual field experiences. 281 282 BREEDING CROP PLANTS WHAT IS GOOD SEED? There are certain characters of farm crops which must be considered if the grower wishes to produce good seed. Good seed of any farm crop must belong to a variety that is superior in the following respects : 1. Adaptability to the locality and soil. 2. Yielding ability. 3. Purity to type for small grains or self-pollinated crops, and comparative purity for corn and other cross-pollinated crops. 4. Quality for the particular characters for which the crop is grown. 5. Hardiness. 6. Erectness or ability to withstand lodging. 7. Disease escaping or resistance to disease. The seed of the particular variety itself must be superior in the following: 1. Germinating ability. 2. Good color, plumpness and weight. 3. Uniformity. 4. Freedom from diseases transmitted by seed. 5. Freedom from any other damage. 6. Freedom from obnoxious weeds. 7. Freedom from mixture with other varieties. Adaptability. — We have already indicated that no one variety always excels in yield or quality. All that the experiment stations can do is to determine the few better varieties and in this way assist the farmer to decide which to grow. There are decided advantages in limiting the number of varie- ties. It is of considerable value for one locality to produce large quantities of a particular variety. Several reasons are apparent, chief of which are: (1) The buyer can obtain a large amount of seed of that particular variety. (2) The production of only a few varieties or a single variety is of material help in keeping purity of type, as there is not so much opportunity for (a) mix- tures in thrashing, growing, etc., or (b) cross-fertilization between varieties, which causes variability of seed and plant characters and, therefore, loss of purity of type. Yielding Ability and Quality. — Variety tests carried on under experimentally controlled conditions are the best means of deter- mining comparative yield and to some extent comparative quality FARMERS METHODS OF PRODUCING PURE 8EEDH 283 of different strains. Many farmers sustain annual losses, which are not small, due to using seed of an over-exploited variety which has not proved its worth in competitive tests. With many crops, quality is of prime importance and must receive some consideration if a No. 1 grade product is to be obtained. Purity. — For crops like wheat, oats, and barley, which are self-fertilized, uniformity is the rule, providing the grower is willing to pay some attention to eliminating accidental mix- tures. For cross-fertilized crops, of which corn is a good example, purity of type is of less importance, although certain general standards of purity are desirable. Hardiness. — Hardiness is a feature of adaptability but it de- serves especial mention. Ability of annual crops like rye and wheat to withstand winter-killing as well as winter hardiness for perennial crops such as alfalfa is of high importance and is generally given much consideration by experimenters before recommending a particular variety. Strength of Stalk. — Ability to stand up, which obviates injury from lodging, is of much importance in grain and hay crops. In small grains early lodging often causes shriveled seeds. The difficulty of harvesting is greatly increased when the crop is flat. Disease Escaping or Resistance. — Some varieties are much freer from disease than others. There are various factors, but the chief ones may be considered under disease escaping and disease resistance. Disease escaping may be due to early maturity, as in the case of Marquis wheat, which often escapes stem rust epi- demics when late varieties such as Bluestem are seriousty injured. Disease resistance is the condition which obtains when the organism gains entrance to the plant yet causes no appreciable injury. There is, for example, a distinct tendency for durum wheat to be resistant to stem rust; some durum strains being much more resistant than others. The above are some of the important agronomic or horticultural characters which separate one variety from another. By a knowl- edge of these the grower is enabled to obtain the best available strain for his conditions. Seed of this selected variety must then be saved in such a manner that it will have germinating ability, i.e., will grow vigorously. In order to do this the seed must be mature and well developed and free from transmissible diseases. Freedom from obnoxious weed seeds is also an important con- sideration. 284 BREEDING CROP PLANTS METHODS OF SEED PRODUCTION After obtaining the better variety for the locality, the seed grower has the problem of keeping this variety in the same high state of production and if possible to improve it. The purpose of this chapter is to outline methods for the various crops which may be used by the seed grower or by the average farmer. Farm crops may be placed in four groups according to their modes of reproduction. There is a close relation between this characteristic and the farmer's methods of seed production. The four groups mentioned are as follows: Group 1. — Generally self-fertilized: Barley, wheat, oats, peas, beans, flax, tobacco. / Group 2. — Often cross-pollinated: Corn, rye, most grasses, root crops. Group 3. — Cross-pollination obligatory: Red clover, sunflower. Group 4. — Vegetatively propagated : Potatoes, sugar cane, sweet potatoes. Among farm crops, the production of seed generally depends on a union of the male reproductive cell, contained in the pollen grain, with the female reproductive cell — the egg cell. The pollen grains of corn are produced in the tassel and each thread of silk leads to an ovary which contains the egg cell. In order to produce seed, the male reproductive cell must pass down through the silk and unite with the female cell. This process is called fertilization. If pollen and silk are borne by the same plant the process is self-fertilization, and if by different plants, cross-fer- tilization. As the egg cell and the pollen grain of self-fertilized plants are, as a rule, alike in their inherited characteristics, the progeny of a single self-fertilized plant, such as barley, wheat, or oats, have the same inheritance. There is, of course, considerable variation in all characters, owing to environmental effect, but all evidence shows that these differences are not truly inherited. Occasional crosses occur in self-fertilized crops which cause inher- itable variability. Mass selection serves to eliminate these off types. SEED GROWERS METHODS FOR SELF-FERTILIZED PLANTS For self-fertilized plants the grower can, as a rule, obtain a pedigreed strain which is nearly adapted to his conditions. The only thing that he can do with this variety is to save seed in such a way that mixtures of other strains or occasional crosses are eliminated, together with obnoxious weed seeds and diseases. FARMERS' METHODS OF PRODUCING PURE SEEDS 285 The strain in question can be kept in a pure condition for its characters, and if it is not entirely pure at the outset a correct method of seed selection will tend to purify it and thus to increase its value. The work for self-fertilized crops is very simple as compared with the production of improved seed of cross-fertilized crops or the production of highly bred livestock. For self-fertilized crops the method outlined is essentially that which is compulsory for the production of registered seed by the Canadian Seed Growers' Association. The steps are given here with the understanding that the grower has already obtained the best available variety for his soil and climatic conditions. The chief points are as follows: 1. The use of a yearly hand-selected seed plot of at least K acre in size, in a good state of cultivation, free from weeds, under a proper rotation, and sown at the regular rate of seeding. 2. The hand selection from this plot of enough seed of uniform character, thoroughly mature and free from disease, to plant the following year's seed plot. This selection may be accomplished before the plot is harvested or from the shock before thrashing. 3. The selected heads, panicles, or pods should be thrashed by hand and the seed carefully stored. 4. The removal of all impurities, weed seeds or mixtures of other varieties, from the seed plot before it is harvested. Purity of seed is important. 5. The bulk crop on the seed plot should be allowed to mature thoroughly, should be harvested carefully, and used the following year to sow as much of the bulk field as possible. According to plans adopted by the Canadian Seed Growers' Association, seed may be registered which is not more than three generations away from the hand-selected seed plot. Such seed is inspected in the field and after being thrashed, and must conform to certain standards of purity and freedom from diseases. The seed plot method is of particular interest to farmers for grain crops — barley, wheat, and oats. It could be used to advantage for flax, beans, and possibly peas, although in the case of peas the selection of seed would be somewhat more difficult. For these crops there seems to be no good reason why the seed plot could not be a part of the main field, although the grower must not forget that the seed plot needs some extra attention if the work is to be worth while. 286 BREEDING CROP PLANTS The seed plot method is here outlined by means of a diagram. DIAGRAM OF FARMER'S METHOD OF MAINTAINING THE PURITY OF SELF FERTILIZED CROPS 1st year Field Hand selected seed plot M A) H 25-30 Ibs. . ^ and selected seed plot Seed of a variety recommended for the locality or •which has been grown successfully in the locality 25-30 Ibs. of typical heads Impurities removed before harvesting selected by hand selected by hand \ \ Canadians register seeds as 1st, 2nd or 3rd generation seed according to the source of the seed and the number of generations away from the H. S. P. (Hand Selected Seed Plot.) For the tobacco crop there is no necessity of a seed plot. The grower should select good-type plants in the field and save these for seed production. The best growers insure the produc- tion of self-fertilized seed by covering the inflorescence before any of the flowers open, with a 12-lb. manila paper bag. It is necessary to remove the bag from time to time to shake out the dead parts of the corolla so that the seed will not become damaged. Ten or twelve plants handled in this manner furnish sufficient seed for a large acreage. If the farmer is troubled with flax wilt he can easily over- come this difficulty by seed selection. All that is necessary is to select from a plot on which the wilt disease is causing con- siderable loss those plants which appear to be free from the dis- ease. Experiments carried on by Bolley at the North Dakota Station which have been recently corroborated (Stakman et al, 1919), have shown that a wilt-resistant type can be produced by three years of continuous selection. Methods of producing wilt resistant seed are presented here in diagrammatical form: — FARMERS' METHODS OF PRODUCING PURE SEEDS 287 DIAGRAM OF METHOD OP CONTROLLING FLAX WILT BY SELECTION Wilt resistant 1st year Seed Plot Select enough resistant plants 2d year Seed Plot Wilt sick Wilt sick soil seed soil by hand for seed plot of following year If wilt resistant seed is not available produce it by selecting plants which are resistant under wilt conditions; three years of continuous selection will accomplish this. Field IMPROVED CORN SEED The determination of the better variety of corn to grow is not difficult. The farmer can obtain reliable advice from the local county agent or by consulting the nearest experiment station. The introduction of new varieties of corn from other states before they have been tested for the climatic conditions in question is a very undesirable practice and as a rule a cause of much annual loss to the corn grower. The problem with corn is somewhat different from that with the self-fertilized crops. Corn is cross- fertilized, therefore constant inherited variability is the rule. When a variety is introduced from another locality it undergoes a process of selection which may markedly change its characters. Selection in a pedigreed line of wheat, on the contrary, does not change its characters and serves only to keep the variety in the same state of purity by artificially removing any possible mix- tures which may occur. This brief discussion will probably serve to show that seed selection on the farm is a very impor- tant practice for the corn grower, unless there is a local grower of high grade seed. The corn seed grower faces another difficulty which the small- grain seed producer does not have to consider. With small grains — barley, oats, and wheat — purity for all characters is the general rule. This has led the corn breeder also to attempt to obtain purity of type. Carefully controlled investigations have served to show a possible fallacy in this practice. The 288 BREEDING CROP PLANTS report of a recent study at the Minnesota Station (Olson, Bull, and Hayes, 1918), which contains experimental evidence together with a review of other experiments in relation to score card characters and yield, show no correlations between individual characters 'such as trueness to the ideal score card ear type and subsequent yield of these ears. Artificial self-fertilization in corn isolates homozygous types which are less vigorous than normally cross-pollinated plants. All other evidence seems to show that too close a purity of type corn tends to a reduction in vigor. The grower whose method of selection is based upon ear type is certainly obtaining no gain in yield of shelled corn per acre. The detrimental results of too close selection to type may not be very apparent and may be more than counterbalanced by the extra attention from a cultural standpoint, for an interest in ideal ear types certainly stimulates the farmer to produce better corn. It is not, however an increase due to better breeding but to better cultural practice. The present purpose is to outline methods of seed selection. As there is no apparent relation between score card characters for type of ear planted (within a particular variety) and resultant yield, even though such selection may be constantly practiced, we may pay little attention to those characters as far as our breeding plan goes. The grower should, of course, produce corn of one variety which is pure, judged by easily evident characters, such as color of seed and cob. Abnormalities, such as very large butts, badly flattened cobs, or very irregularly rowed ears, should not be used as foundation stock. Aside from these there is no need of paying much attention to type. Ability of a variety to mature under the conditions, is very important and needs much attention. Two methods of work are outlined here, either of which may be of considerable value in increasing yield. METHOD OF BREEDING CORN FOR SPECIAL BREEDERS Nearly all discussions of corn breeding are based on the ear-to- row method. Such a method takes considerable time and can be carried out only by the breeder or occasional seed specialist. The ear-to row test is commonly understood. It consists of growing the seed of a certain number of ears in individual rows and determining the better yielding ones. Each ear saved is FARMERS' METHODS OF PRODUCING PURE SEEDS 289 then a basis of further selection. Complicated methods have been used for the introduction of new blood and to keep up the vigor of the strain. The method here outlined is an attempt to simplify this practice and at the same time obtain as good results as can be obtained by the more detailed procedures. It is based on experimental studies carried on at the Nebraska Station (Montgomery, 1909). The details are as follows: 1. Select from 100 to 200 ears of the variety to be grown. If possible, select these ears in the field from those stalks which if in a perfect stand will give a good yield. 2. Make an ear-to-row test of these selected ears, saving half of the seed from each ear planted. From this ear-to-row test the 25 best ears may be determined. 3. Mix the remnants of the 25 highest yielding ears and plant the following year in a seed plot. Select all ears obtained which are fairly desirable, eliminating only the very undesirable types. 4. Use the selected seed for planting as much of the corn acreage as possible. DIAGRAM OF PROCEDURE FOR SPECIAL CORN BREEDER 1st Year 2nd Year 3rd Year 4th to 3th Year Use remnants of 25 best yielding ears Save seed in fall Seed Plot of at least from per- fect stand hills and 1 acre V vigorous \ stalks. Repeat ear-to-row test at the end of the 8th year and proceed as before. Field U> 290 BREEDING CROP PLANTS 5. Give special attention to a part of the field so that a uniform stand may be obtained. Select enough seed from this part of the field for the entire acreage. Select seed for the following year's seed plot in the fall before a killing frost, from perfect stand hills and from those stalks which appear free from disease and which under competition show ability to produce one or more good ears. Throw away only the ears of very undesirable type. 6. Continue the method outlined under 5 for a period of four or five years and then use again the ear-to-row method as outlined under 1 and 2. METHOD OF CORN BREEDING FOR AVERAGE FARMER The average corn grower does not have time or facilities for accurate ear-to-row work. The method here outlined is very simple, yet is probably nearly as good for the average corn variety as the more complicated one previously given. 1. (a) Give special attention to a part of the field, or use a seed corn plot. (6) Plant and cultivate carefully, using the hill method, and grow four stalks per hill. (c) Each fall before frost select enough seed for the follow- ing year's seed plot from stalks which give a good yield and which grow in four-stalk hills. (d) Discard only the very undesirable ears and store each selected ear in a careful manner. (e) Test all seed used for germination. 2. Save all good seed produced by the yearly seed plot to plant the general field. 3. Continue 1 and 2 each crop season. POTATO SEED (TUBERS) SELECTION All localities are not equally good for producing potato tubers for planting, therefore it will be better for some farmers to buy tubers from a different locality. At University Farm, experi- ments of the Division of Horticulture show that tubers should be produced at some other locality if high yields are to be ob- tained. For the farmer, however, who lives in a locality where FARMERS' METHODS OF PRODUCING PURE SEEDS 291 desirable tubers for planting are produced, there are some methods which are of help to him in saving tubers. Ordinarily the plants grown from tubers of a single plant are alike except for the occasional changes which occur in the inherited characters of the plant itself. Mixture in commercial tubers is one common cause of lack of purity of type. The selec- tion of tubers, therefore, gives the grower an opportunity to im- prove his variety and also insures a constant supply of tubers free from diseases. This freedom from disease is a very important point (Tolaas and Bisby, 1919). The first step of the grower is to obtain the best available variety, true to type and free from disease. After obtaining such a variety, one of the following plans may be followed. Both are alike for the first year's work. DIAGRAMMATICAL ILLUSTRATION OF TUBER SEED PLOT SELECTION OF THE POTATO 4M year 4th year 1st year 2d year 3d year 1st year 2 d year 3d year Dig 100 bills by hand Field Method I Bulk seed plot Best Bulk seed plot / Bulk, seed plot bills / Etc. Method 11 H ill- to -row seed plot Best Hill- to- row seed plot strains 292 BREEDING CROP PLANTS First Year. — (a) Remove from the part of the field used for saving tubers all plants which show evidences of diseases. This should be done during the growing season. (b) At harvest time dig at least 100 hills by hand, keeping each hill separate. (c) Use tubers from a number of the better hills for the stock plot the following year. Second Year. — Method I. — Plant all good tubers from previous year's selection of best hills in a bulk seed plot. Enough tubers should be used to plant about Y± acre. This requires approxi- mately 5 bushels, which allows some tubers to be discarded. Method II. — This is the hill-to-row method. In order to compare the productive capacity of each selected hill it is desir- able to have each row the same length and planted from the same total weight of potatoes. All of the progeny of some hills will be discarded this second year. Those that give a good yield and are desirable in other ways may be further tested. Third Year. Method I. — Continue the stock plot by the same means as used in Method I for the second year's work, and use all good tubers produced each year in this seed plot for field planting. This work may be continued each succeeding season by the same plan. Method II. — Make a further test of the best selections as determined by the second year's test, growing much longer rows, thus obtaining more reliable results. All tubers free from disease, of the best yielding strain or strains, may be used to increase the stock the following year. The essential features of these two methods are presented on page 291 in diagrammatical form. Method II probably is somewhat better if all details of the test are carefully performed. For the average farmer, Method I is less cumbersome and if constantly practiced would probably give about as good a result as Method II. IMPROVEMENT BY SELECTION OF SUCH CROPS AS ALFALFA, CLOVER, AND GRASSES Obtain, if possible, a variety which is especially adapted to the conditions. Breeding work should aim at producing a variety which excels in resistance to winter injury and to plant diseases and is also a high producer of hay and seed. FARMERS' METHODS OF PRODUCING PURE SEEDS 293 The following is an outline of the possible steps : 1. Obtain 3 or 4 pounds of the best available seed. 2. Plant in a seed plot isolated as far as possible from other crops of a like kind. Plant seed in rows 3 ft. apart and plant two or three seeds in each hill, spacing the hills two ft. apart in the row. 3. Remove all but a single plant from each hill when the plants are well started. 4. Keep the plot free from weeds. 5. Discard all weak plants from time to time as they become apparent. 6. Save seed of all desirable plants and increase. The improvement of the class of crops here mentioned is somewhat more difficult than with small grains, corn, and pota- toes, and should be undertaken only by the few seed producers who are willing to take the necessary trouble to carry out care- fully the details as outlined. Controlled experiments at some of the state experiment stations and in Europe have shown that much gain can be obtained by such selection. SEED REGISTRY OR CERTIFICATION The outlined seed plot methods are based upon fundamental breeding principles. In order to protect the seed grower who follows such a practice, some system of seed certification is advisable. Various methods have been developed by crop improvement associations. The details of procedure are those which are based upon fundamentally sound business practice. Seed that is eligible for registration must conform to certain standards of purity and freedom from plant pests. The seed-plot methods, if carefully followed, insure the production of seed of a certain standard grade. Certification or registration shows that the seed has been approved by the trained seed inspector. DEFINITIONS1 Acquired Character. — A modification of bodily structure, function, or habit which is impressed on the organism in the course of individual life. Aieurone. — The outermost layer of the endosperm in cereals, when it is rich in gluten. Allelomorph. — One of a pair of contrasted characters which are alternative to each other in Mendelian inheritance. Often used, but with doubtful propriety, as a synonym for gene, factor, or determiner. Allelomorphism. — A relation between two characters, such that the determiners of both do not enter the same gamete but are separated into sister gametes. Alternative Inheritance. — A distribution of contrasting parental or ancestral characters among offspring or descendants, such that the individ- uals exhibit one or other of the characters in question, combinations or blends of these characters being absent or exceptional. Anthesis. — The period or act of flowering. Awn. — A bristle-shaped elongated appendage or extension, to a glume, akene, anther, etc. Barbed. — Furnished with rigid points or short bristles, usually reflexed. Biotype. — A group of individuals all of which have the same genotype. Bran.— The coat of the caryopsis, consisting of pericarp and seed-coat united. Caryopsis. — A one-seeded dry fruit with the thin pericarp adherent to the seed, as in most grasses. Centgener. — Originally used by W. M. Hays, at the Minnesota Station, to refer to a 100-plant plot in which each seed was planted a certain distance from each other seed. Chaff. — The floral parts of cereals, generally separated from the grain in thrashing or winnowing. Chimera. — An association of tissues of different parental origin and genetic constitution in the same part of a plant. Chromosome hypothesis. — The hypothesis advanced by Morgan in which factors are arranged in the chromosomes. Class. — In genetics a group that includes variates of similar magnitude. Clone. — A group of individuals produced from a single original individual by some process of asexual reproduction, such as division, budding, slipping, grafting, parthenogenesis (when unaccompanied by a reduction of the chromosomes), etc. Coefficient of Variability. — A relative index of variation obtained by expressing the standard deviation in percentage of the mean. Coupling. — Such a relation between the genes of two unit-characters that they have a more or less marked tendency to be included in the same gamete when the individual is heterozygous for both of the genes in question. 1 Many of the genetic definitions are taken from Shull (1915), Babcock and Clausen (1918) or others. Ball and Piper's (1916) papers on termi- nology have been used for agronomical terms. 294 DEFINITIONS 295 Cross. — Synonymous with hybrid. Cross-fertilization. — The union of the egg cell of an individual with the sperm cell of a different individual whether the organisms belong to the same or different genotypes. Cross-over. — A separation into different gametes, of determiners that are usually coupled, and the. association of determiners in the same gamete which are generally in different gametes. Detassel. — To remove the tassel, as in maize. Cryptomere. — A factor or gene whose presence can not be inferred from an inspection of the individual, but whose existence can be demonstrated by means of suitable crosses. Determiner. — Synonymous with gene or with factor as applied in genetics. Dominance.— In Mendelian hybrids the capacity of a character which is derived from only one of the two generating gametes to develop to an extent nearly or quite equal to that exhibited by an individual which has derived the same character from both of the generating gametes. In the absence of dominance the given character of the hybrid usually presents a "blend" or intermediate condition between the two parents, but may present new features not found in either parent. Dominant. — (1) A character which exhibits dominance, i.e., that one of two contrasted parental characters which appears in the individuals of the first hybrid generation to the exclusion of the alternative "recessive" character. (2) An individual possessing a dominant character in contrast to those individuals which lack that character which are called "recessives." Ear. — A large, dense or heavy spike or spikelike inflorescence as the ear of maize. Popularly applied also to the spike-like panicle of such grasses as wheat, barley, timothy and rye. Emasculation. — The act of removing the anthers from a flower. Endosperm. — The substance which surrounds the embryo in many seeds, as the starchy part of a kernel of wheat or corn. Factor. — An independently inheritable element of the genotype whose presence makes possible a specific reaction or the development of a particular unit-character of the organism which possesses that genotype; a gene or determiner. Floret. — A small flower, especially one of an inflorescence, as in grasses and Compositse. Fi, F2, F?, etc. — 1st, 2nd, and 3rd, etc. generations following a cross. Gamete. — A reproductive cell containing x number of chromosomes. Gene. — Synonymous with determiner or factor. Genotype. — The fundamental hereditary constitution or sum of all the genes of an organism. Glabrous. — Smooth, especially without hairs. Glume. — One of the two empty chaffy bracts at the base of each spikelet in grasses. Grain. — Cereal seeds in bulk. Group. — In genetics a broad general term for a complex of other categories and not for a complex of any particular category. Head. — A dense, short cluster of sessile or nearly sessile flowers on a very short axis or receptacle, as in red clover or sunflower. 296 BREEDING CROP PLANTS Heredity.— The distribution of genotypic elements of ancestors among the descendants; the resemblance of an organism to its parents and other ancestors with respect to genotypic constitution. Heterozygosity. — The condition of an organism due to the fact that it is a heterozygote; the state of being heterozygous; the extent to which an indivi- dual is heterozygous. Heterozygote. — A zygotic individual in which any given genetic factor has been derived from only one of the two generating gametes. Both eggs and sperms produced by such an individual are typically of two kinds, half of them containing the gene in question, the rest lacking this gene; conse- quently the offspring of heterozygotes usually consist of a diversity of individuals, some of which possess the corresponding character while others lack it. Heterozygous. — The state or condition found in a heterozygote. Heterosis. — The increased growth stimulus often exhibited in the Fj generation of a cross. Homozygosis. — The state of being homozygous; the extent to which an individual is homozygous. Homozygote. — An individual in which any given genetic factor is doubly present, due usually to the fact that the two gametes which gave rise to this individual were alike with respect to the determiner in question. Such an individual, having been formed by the union of like gametes, in turn gener- ally produces gametes of only one kind with respect to the given character, thus giving rise to offspring which are, in this regard, like the parents; in other words, homozygotes usually "breed true." A" positive "homozy- gote with respect to any character contains a pair of determiners for that character, while a "negative" homozygote lacks this pair of determiners. Homozygous. — The state or condition found in a homozygote. Hybrids. — The progeny of a cross-fertilization of parents belonging to different genotypes. Hull. — A term applied to include the lemma and palea when they remain attached to the caryopsis after thrashing. Hypostasis. — That relation of a gene in which its usual reaction fails to appear because of the masking or inhibitory effect of another gene; con- trasted with "epistasis." Inflorescence. — The flowering part of a plant. Keel. — A central ridge resembling the keel of a boat, as in the glumes of some grasses, etc. ; also the inferior petal in the legume flowers. Kernel. — Matured body of an ovule; seed minus its coats. Lethal. — A genetic condition causing death. Linkage. — The type of inheritance in which the factors tend to remain together in the general process of segregation. Lodicule. — A minute scale at the base of the ovary opposite the palea in grasses, usually two in number, and probably representing the reduced perianth. Mean. — The arithmetical average. Mode. — The class of greatest frequency. Mendelize. — To follow Mendel's law of inheritance. Multiple Allelomorphs. — Three or more characters which are so related that they are mutually allelomorphic in inheritance. DEFINITIONS 297 Mutant. — An individual possessing a genotypic character differing from that of its parent or those of its parents, and not derived from them by a normal process of segregation. Mutate. — To undergo a change in genotypic character independently of normal segregation. Ovule. — Female sex cell with its immediate surrounding parts. Ovum. — Egg cell. PI, Po, etc. — The 1st, 2nd, etc. generation of the parents. Palea. — The upper of the two bracts immediately enclosing each floret in grasses. Panicle. — A compound inflorescence with pediceled flowers usually loose and irregular, as in oats, rye, proso, etc. Pedicel. — A stalk on which an individual blossom is borne. Peduncle. — The primary stalk supporting either an inflorescence or a solitary flower. In grasses the uppermost internode of the culm. Pericarp. — The matured wall of the ovary. Phenotype. — The apparent type of an individual or group of individuals, i.e. the sum of the externally obvious characteristics which an individual possesses, or which a group of individuals possess in common; contrasted with genotype. Presence and Absence Hypothesis. — The hypothesis that any simple Mendelian difference between individuals , results solely from the presence of a factor in the genotype of the one individual, which is absent from that of the other. Presence and absence of unit-differences as a convenient method of describing the results of genetic experiments should be carefully distin- guished from the presence and absence hypothesis. The method is purely objective and entirely free from hypothetical implications. Probable Error. — A measure of accuracy for results obtained by statistical methods. The chances are even that the true value lies within the limits marked by the probable error. Probable Error of a Single Determination. — S. D. X ±0.6745. Probable Error of a Difference. — The square root of the sum of the squares of probable errors of the two results, or the probable error of a single determination multiplied by the \/2. Pubescent. — Hairy in a general sense; in special use, covered with short, soft hairs. Pure Line. — A group of individuals derived solely by one or more self- fertilizations from a common homozygous ancestor. Sometimes erroneously applied to groups of individuals believed to be genotypically homogene- ous (a homozygous biotype or a clone) without regard to the method of reproduction. Recombination. — Union of parental factors in individuals of the second or later generations after a cross. Reduction Division. — That in which homologous chromosomes separate preparatory to formation of gametes. Repulsion. — Such a relation between two genetic factors that both are not, as a rule, included in the same gamete, referring especially to cases in which the factors in question give rise to obviously different characteristics; also called "spurious allelomorphism." 298 BREEDING CROP PLANTS Replication.— Systematic repetition. Used in field work to designate the systematic distribution of plots of each strain or variety to overcome soil heterogeneity. Two replications means the use of three plots systemati- cally distributed. Roguing. — The act of removing undesirable individuals from a varietal mixture in the field by hand selection. Seed. — -The mature ovule, consisting of the kernel and its proper coat. Self-fertilization. — The union of the egg cell of one individual with the sperm cell of the same individual. Self -sterility. — That condition in which the male gametes of an organism are incapable of fertilizing the female gametes of the same individual. Segregate. — With reference to Mendelian unit-characters, to become separated through the independent distribution of the genetic factors before or at the time of the formation of the gametes. Sex-linked Inheritance. — The association of the determiner for any unit- character with a sex-determiner, in such a manner that the two determiners are either generally included in the same gamete, or that they are generally included in different gametes. Somatic Segregation. — Segregation during somatic division. Species. — A group of varieties or a single variety which in botanical characters and genetic relationship can be differentiated from another group or variety belonging to the same genus or to other genera. Spikelet. — A small or secondary spike, especially in the inflorescence of grasses. Spike. — A simple inflorescence with the flowers sessile or nearly so on a more or less elongated common axis or rachis. Sperm or Sperm Cell. — Male sex-cell. Standard Deviation. — An absolute measurement of variation in terms of the mean. The square root of the sum of the deviations squared divided by the number of variates. Sterility. — Inability to reproduce; when male and female gametes, through incompatibility or some other cause, are incapable of mating or fertilization. Strain. — A group within a variety which constantly differs in genetic factors or a single genetic factor difference from other strains of the same variety. Tassel. — Used to designate the staminate inflorescence of maize. Unit-character. — In Mendelian inheritance, a character or alternative difference of any kind, which is either present or absent, as a whole, in each individual, and which is capable of becoming associated in new combinations with other unit-characters. Variate. — -A single magnitude determination of a character. Variety. — A group of strains or a single strain which by its structural or functional characters can be differentiated from another variety. Variety Group. — A complex of varieties which resemble each other more than varieties belonging to a different group. Of lower grade than species. Xenia. — The apparent immediate effect of pollen. It results from double fertilization. Zygote. — The body formed by the union of two gametes and containing 2x number of chromosomes. LITERATURE CITATIONS AARONSOHN, A., 1910. Agricultural and botanical explorations in Palestine. U. S. Dept. Agr., Bur. Plant Indust., Bull. 180, 52 pp. ALLARD, H. A., 1919a. Some studies in blossom color inheritance in tobacco, with special reference to N. sylvestris and N. tabacum. Am. Nat., 63 : 79-84. 19196. Gigantism in Nicotiana tabacum and its alternative inheritance. Am Nat., 53 : 218-233. ALKEMINE, M., 1914. tlber das Bliihen des Reises und einige sich daran anknupf ende Erscheinungen. Zeitschr. f iir Pflanzenziicht. , 2 : 339-375. ANONYMOUS, 1919. New Iowa oats and the man who found them. Wallace's Farmer, 44 : 771. ANTHONY, R. D., and HEDRICK, U. P., 1916. Some notes on the breeding of raspberries. New York (Geneva) Agr. Exp. Sta., Bull. 417: 75-88. ARNY, A. C., and HAYES, H. K., 1918. Experiments in field technic in plot tests. Jour. Agr. Res., 15: 251-262. ARNY A. C. and GARBER, R. J., 1918. Variation and correlation in wheat, with special reference to weight of seeds planted. Jour. Agr. Res., 14 : 359-392. BABCOCK E. B. and CLAUSEN, R. E., 1918. Genetics in relation to agricul- ture. McGraw-Hill Book Company, New York, 675 pp. BACKHOUSE, W. O., 1916-17. Note on the inheritance of "crossability." Jour. Genetics, 6 : 91-94. 1918. The inheritance of glume length in T. polonicum. A case of zygotic inhibition. Jour. Genetics, 7 : 125-135. BAILEY, L. H., 1898. "Evolution of Our Native Fruits." The Macmillan Co., London, 472 pp. 1890. Experiences in crossing cucurbits. Ann. Rept. Cornell Agr. Exp. Sta., 3 : 180-187. 1900. Cyclopedia of American horticulture. The Macmillan Co., New York, I, 410 pp. 1909. Cyclopedia of American agriculture. Vol. 2. Crops. The Macmillan Co., New York, 699 pp. BAIN, M., and ESSARY, S. H., 1906. Selection for disease-resistant clover. A preliminary report. Tenn. Agr. Exp. Sta., Bull. 75, 10 pp. BALL, C. R., 1910. The history and distribution of sorghum. U. S. Dept. Agr. Bur. Plant Indust., Bull. 175, 63 pp. 1910. The breeding of grain sorghums. Amer. Breeders' Mag., 1: 283-293. 1911. The importance and improvement of the grain sorghums. U. S. Dept. Agr., Bur. Plant Indust., Bull. 203, 42 pp. BALL, C. R. and Piper, C. P., 1916. Contributions to agronomic terminology. Jour. Amer. Soc. Agron., 8: 1-9, 197-204, 228-237, 310-315. 299 300 BREEDING CROP PLANTS BALLS, W. L., 1908. Mendelian studies of Egyptian cotton. Jour, of Agr. Sci., 2 : 346-379. 1911 (1913). The inheritance of measurable characters in hybrids between reputed species of cotton. Int. Conf . de Genetique, 4 : 429-440. 1912. The cotton plant in Egypt, studies in physiology and gene- tics. The Macmillan Co., London, 202 pp. BARBER, C. W., 1914. Note on the influence of shape and size of plots in tests of varieties of grain. In Maine Agr. Exp. Sta., Bull. 226: 76-84. BARRUS, M. F., 1918. Varietal susceptibility of beans to strains of Colleto- trichum lindemuthlanum (Sacc. & Magn.) B. & C. Phytopath., 8:589-614. BATCHELOR, L. D., and REED, H. S., 1918. Relation of the variability of yields of fruit trees to the accuracy of field trials. Jour. Agr. Res., 12: 245-283. BAUR, E., 1914. Einfiihrung in die experimentelle Vererbungslehre. Geb ruder Borntrseger. Berlin (2nd edition). 401 pp. BEACH, S. A., 1898. Self-fertility of the grape. New York State Exp. Sta., Bull. 157: 397-441. 1899. Fertilizing self-sterile grapes. New York State Exp. 6ta., Bull. 169: 331-371. 1902. Investigations concerning the self-fertility of the grape. New York State Exp. Sta., Bull. 223: 269-290. BEAL, W. J., 1876-1882. Reports Michigan Board of Agriculture 1876, 1877, 1881 and 1882. BELLING, J., 1912o. Third generation of the cross between velvet and Lyon beans. Rpt. of Florida Agr. Exp. Sta. for 1912: 115-127. 19126. Crossing Corn. Florida Agr. Exp. Sta. Press Bull. 97. 1913. Report of Assistant Botanist. Rpt. of Florida Agr. Exp. Sta. for 1913 : 104-130. 1914o. The mode of inheritance of semi-sterility in the offspring of certain hybrid plants. Zeitschr. fur Ind. Abstamm. u. Vererb , 12 : 303-342. 19146. Inheritance of pod pubescence and partial sterility in Stizolob- ium crosses. Rpt. of Florida Agr. Exp. Sta. for 1914 : 31-55. 1915«. Inheritance of mottling of the seed-coat. Rpt. of Florida Agr. Exp. Sta. for 1915: 111-127. 19156. Inheritance of length of pod in certain crosses. Jour. Agr. Res., 6:405-420. BERTHAULT, P., 1911 (1913). Note Preliminaire sur 1'origine specifique de la Pomme de Terre. Int. Conf. de Genetique, 4 : 377-380. BIFFEN, R. H., 1905. Mendel's law of inheritance and wheat breeding. Jour. Agr. Sci., 1: 4-48. 1907a. Studies in the inheritance of disease resistance. Jour. Agr. Sci., 2:109-128. 19076. The hybridization of barleys. Jour. Agr. Sci., 2 : 183-206. 1912. Studies in the inheritance of disease resistance. II. Jour. Agr. Sci., 4:421-429. 1916. The suppression of characters on crossing. Jour. Genetics, 6 : 225-228. LITERATURE CITATIONS 301 1917. Systematized plant breeding. In Seward's "Science and the Nation," pp. 146-175. University Press, Cambridge. BOLLEY, H. L., 1901. Flax wilt and flax sicksoil. North Dakota Agr. Exp. Sta., Bull. 50: 27-57. 1903. Flax and flax seed selection. North Dakota Agr. Exp. Sta., Bull. 55: 171-198. 1909. Some results and observations noted in breeding cereals in a specially prepared disease garden. Proc. Amer. Breeders' Assoc., 5: 177-182. BRAND, C. J., 1911. Grimm alfalfa and its utilization in the Northwest. U. S. Dept. Agr., Bur. Plant Indust., Bull. 209, 66 pp. BURKHOLDER, W. H., 1918. The production of an anthracnose-resistant white marrow bean. Phytopath. 8 : 353-359. CAPORN, A. ST. CLAIR, 1918. An account of an experiment to determine the heredity of early and late ripening in an oat cross. Jour. Genetics, 7 : 247 - 257. CARLETON, M. A., 1916. The small grains. The Macmillan Co., New York, 699 pp. CARRIER, LYMAN, 1919. A reason for the contradictory results in corn experiments. Jour. Amer. Soc. Agron., 11: 106-113. COE, H. S., 1918. Origin of the Georgia and Alabama varieties of velvet bean. Jour. Amer. Soc. Agron., 10: 175-179. COLLINS, G. N., 1909. The importance of broad breeding in corn. U. S. Dept. Agr., Bur. Plant Indust., Bull. 141: 31-44. 1910a. The value of first-generation hybrids in corn. U. S. Dept. Agr., Bur. Plant Indust., Bull. 191, 45 pp. 19106. Increased yields of corn from hybrid seed. Yearbook of U. S. Dept. of Agr. for 1910 : 319-328. 1912. The origin of maize. Jour. Washington Acad. of Sci., 2 : 520-530. 1917. Hybrids of Zea ramosa and Zea tunicata. Jour. Agr. Res., 9 : 383-395. COLLINS, G. N. and KEMPTON, J. H., 1911. Inheritance of waxy endo- sperm in hybrids of Chinese maize. Int. Conf . de Genetique, 4 : 347-357. 1913. Effects of cross-pollination on the size of seed in maize. In Misc. Papers, U. S. Dept. Agr., Bur. Plant Indust., Cir. 124. 1914. Inheritance of endosperm texture in sweet X waxy hybrids of maize. Amer. Nat., 48 : 584-594. COOK, O. F. 1913. Heredity and cotton breeding. U. S. Dept. Agr., Bur. Plant Indust., Bull. 256, 113 pp. 1915. Brachysm, a hereditary deformity of cotton and other plants. Jour. Agr. Res., 3 : 387-400. CORRENS, C., 1899. Untersuchungen iiber die Xenien bei Zea Mays. Ber. d. Deutsch. Bot. Ges., 17: 410-417. 1901. Bastarde zwischen Maisrassen mit besonderer Beriicksichtigung der Xenien. Bibliotheca Botanica, 53 : 1-161. CRANE, M. B., 1915. Heredity of types of inflorescence and fruits in tomato. Jour. Genetics, 5: 1-11. CRANDALL, C. S., 1918. Apple-bud selection, apple seedlings from selected trees. Illinois Agr. Exp. Sta., Bull. 211: 181-264. 302 BREEDING CROP PLANTS CUMMINGS, M. B., 1904. Fertilization problems: A study of reciprocal crosses. Maine Agr. Exp. Sta. Bull., 104: 81-99. CUTLER, G. H., 1919. A dwarf wheat. Jour. Amer. Soc. Agron., 11: 76-78. DARWIN, CHARLES, 1877. The effects of cross- and self-fertilization in the vegetable kingdom. Appleton & Co., London. DE CANDOLLE, ALPHONSE, 1886. Origin of cultivated plants. Kegan Paul, Trench & Co., London, 468 pp. DETTWEILER, 1914. Aryan agriculture. Jour. Heredity, 6: 473-481. DE VRIES, HUGO, 1899. Sur la fecondation hybride de 1'albumen. Comptes Rend, des Seances de 1' Academe des Seances, 129 : 973-975. 1906. Species and varieties: their origin by mutation. Open Court Pub. Co., Chicago. 1907. Plant breeding. Open Court Pub. Co., Chicago, 360 pp. DILLMAN, A. 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Experiments in wheat breeding. U. S. Dept. Agr., Bur. Plant Indust.,Bull. 269, 61 pp. MORROW, G. E., and GARDNER, F. D., 1893. Field experiments with corn. Illinois Agr. Exp. Sta., Bull. 25: 173-203. 1894. Experiments with corn. Illinois Agr. Exp. Sta., Bull. 31: 359- 360. MUNSON, W. M., 1906. Plant breeding in relation to American pomology. Maine Agr. Exp. Sta., Bull. 132: 149-176. NABOURS, ROBERT K., 1919. Parthenogenesis and crossing-over in the grouse locust Apotettix. Am. Nat. 63 : 131-142. NAUDIN, CH., 1856. Nouvelles recherches sur les caracters specifiques et les varietes des plantes du genre Cucubita. Ann. d. Sci. Nat. Fourth series, 6 : 5-73. 1859a. Especes et des varietes du genre Cucumis. Ann. d. Sci. Nat. Fourth series, 11 : 1-87. 18596 Revue des Cucurbitacees. Ann. d. Sci. Nat. Fourth series, 12:79-164. 1865. Nouvelles recherches sur' 1'hybridite dans les vegetaux. Nouvelles archives du' Museum d' Histoire Naturelle de Paris I. 25-176. NAWASCHIN, S., 1898. Resultate einer Revision der Befruchtungsvorgange bei Lilium martagon und Fritillaria tenella. Bull. Acad. Imp. Sci. St. Petersbourg. s. 5, t. 9, No. 4: 377-382. NEALE, S. T., 1901. Pedigreed sorghum as a source of cane sugar. Dela- ware Agr. Exp. Sta., Bull. 51, 24 pp. NEWMAN, L. H., 1912. Plant breeding in Scandinavia. Ottawa, Canada, 193 pp. 312 BREEDING CROP PLANTS NILSSON-EHLE, H., 1908. Einige Ergebnisse von Kreuzungen bei Hafer und Weizen. Botan. Notiser, Lund., pp. 257-298. 1909. Kreuzungsimtersuchungen an Hafer und Weisen. Lunds. Univ. Arsskr. N. F. Afd. 2, Bd. 5 Rr. 2pp. 122. 191 la. Ueber Falle spontanen Wegf aliens eines Hemmungsfaktors beim Hafer Zeitzchr. fiir Induk. Abstamm. u.Vererb., 5: 1-37. 19116. Kreuzungsuntersuchungen an Hafer und Weizen. Lunds. Univ. Arsskr. N. F. Afd.2, Bd. 7, Nr. 6, pp. 3-84. 1911c. Mendelisme et Acclimatation. Int. Conf . de Genetique, 4 : 1- 22. 1912. Zur Kenntnis der Erblichskeitsverhaltnisse der Eigenschaft Winterfestigkeit beim Weizen. 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PEARL R., and MINER, J. R., 1914. A table for estimating the probable significance of statistical constants. Maine Agr. Exp. Sta., Bull. 226: 85-88. PIPER, C. V., 1912. Agricultural varieties of the cowpea and immediately related species. U. S. Dept. Agr., Bur. Plant Indust., Bull. 229: 1-160. 1916. Forage plants and their culture. The Macmillan Co., New York, 618 pp. PIPER, C. V. EVANS, M. W., McKEE, R., and MORSE, W. J., 191£. Alfalfa seed production; pollination studies. U. S. Dept. Agr., Bull. 75, 32 pp. PIPER, C. V. and MORSE, W. J., 1910. The soybean: History, varieties and field studies. U. S. Dept. Agr., Bur. Plant Indust., Bull. 197, 84 pp. POWELL, G. H., 1898. Plant breeding. Am. Gard., 16: 466-467, 514-515. PRICE, H. L., and DRINKARD, A. W., JR., 1908. Inheritance in tomato hy- brids. Virginia Agr. Exp. Sta., Bull. 177: 18-53. 1911-12. Inheritance in cabbage hybrids. Ann. Rept. Virginia Polytec. Inst. Agr. Exp. Sta. for 1911 and 1912 : 240-257. PRIDHAM, J. T., 1916. Oat breeding experiments. Agr. Gaz. N. S. Wales, 27:457-461. QUANJER, H. M., 1920. The mosaic disease of the Solanacese, its relation to the phloem-necrosis and its effect upon potato culture. Phytopath., 10 : 35-47. REEVES, F. S., 1918. Report of Hort. Exp. Sta., Vineland, Ontario, for 1916-17:39-41. RIOLLE, Y. T., 1914. Recherches morphologiques et biologiques sur les Radies cultives. Ann. Sci. Agron., 31st year: 346-550. ROBERTS, HERBERT F., 1919. The founders of the art of breeding. Jour. Heredity, 10: 99-106, 147-152, 229-239, 257-275. 1919. The contribution of Carl Friedrich von Gartner to the history of plant hybridization. Am. Nat., 53 : 431-445. RUMKER, K. VON, 1913. Ueber Roggenzuchtung. Btr. zur Pflanzen- ziicht., 3 : 8-28. RUMKER, K. VON, and LEIDNER, R., 1914. Ein Beitrag zur Frage der Inzucht bei Roggen. Zeitschr. fur Pflanzenziicht., 2 : 427-444. 314 BREEDING CROP PLANTS RUSSELL, H. L., and MORRISON, F. B., 1919. Service to Wisconsin. Wis. Agr. Exp. Sta., Bull. 302, 35 pp. SAGERET, A. 1826. Considerations sur la production des hy brides, des vari- antes et des varietes en general, et sur celles de la famille des Cucur- bitacees en particulier. . Ann. d. Sci. Nat., 8: 294-314. SALAMAN, R. N., 1910—11. The inheritance of color and other characters in the potato. Jour. Genetics, 1 : 7-46. 1912-13. A lecture on the hereditary characters in the potato. Jour. Roy. Hort. Soc., 38: 34-39. 1909-11. Male sterility in potatoes, a dominant Mendelian character; with remarks on the shape of the pollen in wild and domestic varieties. Jour. Linnean Soc., 39: 301-312. 1911 (1913). Studies in potato breeding. Int. Conf. de Genetique, 4 : 373-375. SALMON, CECIL, 1914. Sterile florets in wheat and other cereals. Jour. Am. Soc. Agron., 6: 24-30. SAUNDERS, C. E., 1905. A natural hybrid in wheat. Proc. Amer. Breeders' Assoc., 1: 137-138. 1912. Marquis wheat. Exp. Farms' Rept., Ottawa, Canada, pp. 118-120. SCOTT, W. B., 1917. The theory of evolution. 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J., 1916. Red Rock wheat. Michigan Agr. Exp. Sta. Cir. 31, 7 pp. STAKMAN, E. C., HAYES, H. K., AAMODT, OLAF S., and LEACH, J. G., 1919. Controlling flax wilt by seed selection. Jour. Am. Soc. Agron., 11:291-298. STAKMAN, E. C., LEVINE, M. N., and LEACH, J. G., 1919. New biologic forms of Puccinia graminis. Jour. Agr. Res., 16: 103—105. STEWART, F. C., 1916. Observations on some degenerate strains of potatoes. New York (Geneva) Agr. Exp. Sta., Bull. 422: 319-357. STEWART, J. B., 1908. The production of cigar-wrapper tobacco under shade in the Connecticut Valley. U. S. Dept. Agr., Bur. Plant Indust., Bull. 138, 31 pp. STEWART, J. P., 1912. Factors influencing yield, color, size, and growth in apples. Ann. Rept. Pennsylvania State College for 1910-11 : 401-492. STOUT, A. B., 1915. The establishment of varieties in Coleus by the selec- tion of somatic variations. Carnegie Inst. of Wash., Pub. 218: 1-80. 1920. Further exper mental studies on self-incompatibility in herma- phrodite plants. 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INDEX Aaronsohn, wild wheat, 78 Alfalfa, flower structure, 217 Grimm variety, 216 origin of, 215 pollination in, 40, 207 selection in, 293 winter resistance, 218 Alkemine, rice pollination, 37 Allard, tobacco, inheritance in, 161 tobacco mutations, 167 Apples, self -sterility in, 269, 271 inheritance in, 271 Arny and Garber, weight of seed and plant vigor, 123 Arny and Hayes, border effect in plot tests, 61 Asparagus, pollination in, 253 rust resistance, 253 B Babcock and Clausen, crossing over, 27 homozygosis following a cross, 117 selection in sugar beets, 119 Backhouse, wheat species crosses, 79 wheat-rye crosses, 106 Bailey, crosses in cucurbits, 256 evolution in fruits, 264 inheritance in raspberries, 272 sweet corn, origin, 235 Bain and Essary, clover anthrac- nose, 215 Ball, pollination, grain sorghums, 38 Balls, cotton, natural crosses, 38, 173 cotton, inheritance, 175 Barley, awn in relation to yield, 104 classification, 98 Barley, independent Mendelian in- heritance, 102 inheritance of spike density, 28, 103 pollination in, 36 species crosses, 99 winter vs. spring habit, 103 xenia in, 103 Barrus, disease resistance in bean, 244 Beach, self-sterility in grape, 269 Beal, varietal crosses in maize, 202 Bean, classification, 241 disease resistance in, 139, 244, 245 flower structure, 243 inheritance of characters, 242, 244 M. A. C. Robust, 131 Beet, inheritance and breeding, 250 Belling, inheritance in velvet bean, 149 varietal crosses, maize, 203 Biffin, barley inheritance, 102, 103 disease resistance in wheat, 85, 134 immediate effect of pollination in wheat, 82 linkage in wheat crosses, 79 polonicum-wheat crosses, 79 species crosses, barley, 101 wheat pollination, 35 Bolley, flax, wilt resistance, 157, 286 Brand, alfalfa, Grimm, 216 Brassica, inheritance studies in, 251 Breeding, asparagus, 254 beets, 250 by crossing, 116 by selection, 115 cabbage, 252 clover, 215 319 320 INDEX Breeding, cotton, 177 cucurbits, 259 flax, 158 fruits, 266, 273, 278, 279 Hays, progeny test, 111 Hopkins, ear-to-row test, 111, 198 keeping records in, 111 maize, 196, 290 radish, 250 self-fertilized vegetables, 247 small grains, 111 soybeans, 148 timothy, Cornell station, 210, 211 at Svalof, 212 wheat, 135, 137 Buckwheat, breeding, 108 characters of, 107 original home of, 107 species, 107 Bull, the Concord grape, 265 Burnett, oat selections, 131 Bushnell, squash, selfing in, 68 breeding, 260 C Camerarius, on sexuality, 3 Caporn, inheritance, size characters oats, 96 Carleton, origin of rice, 108 Carrier, increase of seed size due to crossing, 201 Check plots, correcting yields by, 53, 55 estimating heterogeneity by, 53 in obtaining probable errors, 55 Cherries, self-sterility in, 269 Citranges, 277 Clover, pollination in, 207, 214 selection in, 292 species, 214 Coe, mutations in velvet beans, 150 Collins, origin of maize, 182 podded characters in maize, 189 varietal crosses in maize, 202, 203 Collins and Kempton, endosperm characters in maize, 185 Collins and Kempton, increase in seed size due to a cross, 201 Competition in variety test, 63 in rod row tests, 64 Correlations, morphological charac- ters and lodging, small grains, 127 value of, 125 Correns, endosperm characters of maize, 185 xenia in maize, 183 Cotton, chromosome number, 177 cultivated species, 173 disease resistance, 177, 178 flower structure, 174, 175 inheritance in, 176 mutations in, 177 natural crosses in, 38, 173 origin of, 173 Cowpea, inheritance in, 143, 144 Iron, a variety of, 131 origin, 143 resistance to disease, 144, 145 selection and crossing results, 145 Crane, inheritance in tomato, 245 Crop improvement, value of, 14, 15 Crops, mode of reproduction of, 33, 34 Crosses, handling of, 116 Crossing, artificial, 68 small grains, 69 technic of, 74 Crossing-over, 12, 27 Cucumber, heterosis in, 257 inheritance, 257 Cucurbit crosses, 7 classification, 255 Cytology, reduction division in plants, 17, 18 1) Darwin, heredity, 10 natural selection, 9 variability, 10 Date palm, sexuality, 3 De Candolle, origin and antiquity of vegetables, 235 INDEX 321 Dillman, selection in sorghum, 179 Disease resistance, in asparagus, 253 in beans, 139, 244 in cabbage, 252 in clover, 215 in cotton, 177 in cowpeas, 144, 145 in flax, 151 in oats, 95 in rice, 109 in timothy, 213 in tomatoes, 248 in watermelons, 258 in wheat, 76, 85, 134 Dorsey, breeding fruits, 279 mutations in fruits, 264 origin of fruits, 263 self-fertile and self-sterile fruits, 269 self-sterility in grape, plum, 270 Durst, disease resistant tomatoes, 248 E East, artificial crossing, 68 maize, ear-to-row tests, 198 self-fertilization in, 202 mathematical requirements of size inheritance, 163, 164 potato, clonal selection, 228 introduction to Europe, 220 pollination in, 224, 225 species in, 219 tobacco, color of flowers, 161 size of flower, 165 species crosses, 159 sterility in, 160 East and Hayes, hybrid vigor, 45, 47 maize, inheritance, 185, 187 pericarp color, 188 podded character, 189 seed and ear characters, 192 subspecies of, 182 East and Jones, factor stability, 32 loss mutations, 276 size inheritance, 30 East and Park, sterility in tobacco, 160 21 Edgerton, disease resistance in toma- toes, 248 Emerson, bean, inheritance in, 242 maize, auricle and ligule, 189 cob color, 188 endosperm characters, 187 squash, size inheritance in, 258 tomato, inheritance, 246 Emerson and East, size inheritance, 192 Engledow, linkage in wheat crosses, 79 Etheridge, oat classification, 89 Failyer and Willard, selection in sorghum, 179 Farrar, wheat breeding, 135 Fertilization, maize, 19 Field plot technic, 65, 266 Flax, breeding, 158 flower structure, 154 inheritance, 155, 156 natural crosses, 37 origin of, 153 species crosses, 153 wilt resistance, 158, 287 Fletcher, self-sterility in pears, 269 strawberry, varieties, 262 Focke, xenia, 183 Frear, crossing in uncovered wheat spikes, 69 Freeman, linkage in wheat, 80 seed characters of wheat, 82 sterility in wheat crosses, 79 Fruits, crosses in, 277 early breeders of, 264 origin, antiquity of, 261, 264 self-sterility and heterozygosity in, 267 Fruwirth, barley pollination, 36 flax, natural crosses, 37 maize, self-fertilization, 39 oats, natural crosses, 36 potatoes, inheritance in, 221 pure-line selection, 12 • rye, pollination in, 40 wheat, pollination, 35 winter-spring habit, barley, 103 322 INDEX G Gaines, disease resistance in wheat, 86 oat crosses, 93 seed characters in wheat, 82 spike density in wheat, 81 Gametes, production in flowering plants, 17 Garber, stem rust in oats, 95 Garber and Olson, stem characters of small grains, 127 Garner, tobacco mutations, 167 Garner and Allard, effect of length of day on flowering, 169 Genetics, factor stability, 32 inheritance factors, 20 inheritance, two independently inherited factors, 23 linkage, 26 method of studying, 16 size characters, 27 Germ plasm, constancy of, 10 Gilbert, potato characters, 223 Giltay, xenia in rye, 106 Goff , self-sterility in plum, 269 Goodspeed, parthenogenesis in tobacco, 160 size inheritance in tobacco, 165 Goss, early pea crosses, 7 Graham, inheritance in sorghum, 179 pollination of sorghum, 38 Grape, early breeding, 265 inheritance in, 272 self-sterility in, 269 Grasses, pollination in, 41, 207 economic species, 207 selection in, 293 Griff ee, FI wheat crosses, 43 Groth, inheritance in tomato, 245 Guignard, double fertilization, 183 H Hagedoorn, early wheat selections, 119 Hallet, early selections in small grains, 118 Hansen, fruit breeding, 278 Harlan, barley, natural crosses, 36 classification of barley, 98 Harlan and Anthony, the barley awn, 104 Harlan and Hayes, barley species crosses, 99 Harland, cowpea, inheritance, 143, 144 natural crosses, 39 Harris, soil heterogeneity, 51 Harshberger, home of maize, 181 Hartley and others, varietal crosses of maize, 203 Hasselbring, pure lines in tobacco, 166 Hayes, maize, self-pollination, 39 pericarp color, maize, 188 pure-line selection, tobacco, 122, 166 size characters, 165 tobacco mutations, 167 Hayes and Arny, replication, 59 Hayes and Beinhart, tobacco muta- tions, 167 Hayes and East, maize inheritance, 185, 192 maize, varietal crosses, 203 Hayes, East and Beinhart, size characters, tobacco, 162, 165 Hayes and Garber, winter hardiness in wheat, 87, 138 high protein maize, 195 Hayes and Harlan, barley, spike density, 104 Hayes and Jones, FI tomato crosses, 43 Hayes and Olson, FI varietal maize crosses, 203 Hayes and others, disease resistance in wheat, 85 sterility in wheat, 79 Hayes and Stakman, rust in timo- thy, 213 Hays, small grain breeding, 112 Head thrasher, small grains, 117 Heckel, mutations in potatoes, 220 Hector, inheritance in rice, 109 natural crosses in rice, 37 INDEX 323 Hedrick and Anthony, inheritance in grape and raspberry, 272 Hedrick and Wellington, inheritance in apples, 268, 271 Henkemeyer, chaff characters in wheat, 84 Heterogeneity, in field plots, 51 estimating, 53 inheritance in raspberry, 272 Hildebrand, rye pollination, 39 Hilson, inheritance in sorghum, 178 Homozygosis, from self-fertilization, 49 Hopkins, ear-to-row maize, 198 Hoshino, inheritance in rice, 109, 110 Houser, Fi tobacco crosses, 42 Howard and others, inheritance, beards in wheat, 85 chaff characters in wheat, 84 seed characters in wheat, 82 standing power in wheat, 88 natural crosses, in beans and peas, 39 flax, 37 tobacco, 36 wheat, 35 parthenogenesis in tobacco, 160 tobacco groups, 159 Hutcheson, pure-line selection, 122 Hutcheson and Wolf, maize varietal crosses, 203 Hybrid vigor, 47 Ikeno, inheritance, in rice, 108, 109 inheritance, in peppers, 247 natural crosses in rice, 37 Jardine, Kanred wheat, 76, 128 Jellneck, artificial wheat crosses, 71 Jenkins, number of seeds per tobacco plant, 36 Jensen, plant height, tobacco, 165 Jesenko, wheat-rye crosses, 106 Johannsen, the pure line, 11, 120 Johnson, cytology of fertilization, 5 sex in date palm, 3 tobacco breeding, 140 tobacco inheritance, 165 . Jones, crossing selfed strains, maize, 205 heterosis, 49 homozygosis on selfing, 49 increased seed size in a cross, 201 inheritance, pseudo-starchy, maize, 184 lethal endosperm, maize, 185 tomato, 246 self-fertilization, maize, 47 Jones and Gallastegui, podded char- acter, maize, 189 Jones and Oilman, disease resis- tance, cabbage, 252 self -sterility, cabbage, 251 Jones and others, varietal crosses, maize, 203 K Kajanus, inheritance in beet, 250 Karper and Conner, pollination, grain sorghums, 38 Kearney, cotton mutations, 177 Keeble and Pellew, inheritance in pea, 236 Kezer and Boyack, wheat crosses, 79, 80 Kiesselbach, competition in variety tests, 63 • ear-to-row breeding, maize, 199 varietal crosses, maize, 203 Kiesselbach and Ratcliff, selection in oats, 130 Kirchner, wheat pollination, 35 Knight, early crosses, 6 fruit breeding, 264 self -sterility in apple, 271 Knuth, maize pollination, 39 Koch, inheritance in rice, 109, 110 Koelreuter, early crosses, 5 Leake, cotton inheritance, 175 cotton, natural crosses, 38 324 INDEX Le Couteur, selection, small grains, 119 Legumes, crossing artificially, 72 Leidigh, self-fertilizing sorghums, 180 Leighty, wheat-rye crosses, 35, 106 Leighty and Hutcheson, blooming in wheat, 35, 69 Lewis and Vincent, heterosis in apples, 269 self-sterility in apples, 269 Lily, anther and pollen of, 18 Lindstrom, chlorophyll inheritance, maize, 190 Linkage, in cotton, 175 in oats, 97 in pea, 241 in tomato, 246 in wheat, 79, 80, 84 of characters, 26 Love and Craig, pure-line selections, oats, 123 chaff characters, wheat, 84 oat crosses, 92, 93, 94 rod-row method, 114 thrashing machine, 117 wheat, species crosses, 78 wheat-rye crosses, 106 Love and McRostie, hulled vs. hull- less, oats, 93 Lumsden, inheritance in muskmelon, 257 M MacDougal, inheritance in potatoes, 222 Macoun, degeneracy in potato, 230 hardy fruits, 277 Maize, chemical composition, 193 chlorophyll inheritance, 190 ear characters and yield, 197 ear-to-row breeding, 198 effects of self-fertilization, 46, 47 endosperm character inheri- tance, 185 fertilization in, 19 heterosis in, 200 home grown seed, 199 Maize, increase in seed size from crossing, 201 near relatives of, 181 origin of, 182 plant character inheritance, 187-196 podded condition of, 189 Malte and Macoun, crossing in vegetables, 251 Martin, clover, pollen germination, 215 Mayer-Gmelin, spike density in wheat, 81 McFadden, wheat-rye crosses, 106 McRostie, beans, disease resistance, 139, 245 Mendel's law, 12 Mercer and Hall, replication in plot test, 59 size and shape of plot, 60, 61 Montgomery, ear-to-row maize, 198 origin of maize, 181 rod-row method, 115 size of plot, 60 weight of seed planted, 126 Morrow and Gardner, varietal crosses, maize, 202, 203 Munson, evolution of fruits, 264 Muskmelon, inheritance in, 257 Mutations, cotton, 177 due to loss of factors, 276 fruits, 273 maize, 189 potatoes, 220, 229 tobacco, 167 velvet beans, 150 X Nabours, cross-overs in partheno- genesis, 276 Natural selection, 9 Naudin, on segregation, 7 on cucurbit classification, 254 Nawaschin, double fertilization, 183 New introductions, 113 Newman, correlations, 125 improvement by crossing, 134 selection methods, 127 INDEX 325 Nilsson, inheritance in potatoes, 221 oat, classification, 95 potato, degeneracy in, 232 rye, self-pollination, 40 Nilsson-Ehle, oat inheritance, 92, 93, 95, 96, 97, 133 oats, false wild, 97 wheat, disease resistance, 85 seed characters in, 82 spike density in, 81 winter-hardiness, 87 Norton, A. sterilis, oats, 90 rust resistance, asparagus, 254 time of blooming, oats, 71 0 Oats, awn development, 90 color of lemma, 93 color of stem, 92 disease resistance, 95 false wild, 97 improvement by crossing, 133 linkage in, 97 natural crosses, 36 open vs. side panicle, 95 origin of, 90 pubescence on grain, 94 selection in, 129, 130, 131 size characters, 96 species groups, 89 yellow factor, 92 Oliver, crossing methods, 71 Olson, Bull and Hayes, ear charac- ters and yield, maize, 198 Origin and development of crops, 1 Orton, cotton, wilt resistance, 177 cowpea, resistance and wilt and root-knot, 144 watermelon, wilt resistance, 258 Pea, classification of, 236 genetic factors in, 240 inheritance in, 236 linkage in, 241 natural crosses, 38 structure of flower, 237 Pear, self -sterility in, 269 Pearl and Miner, probable errors, 56 Pepper, characters, 246 inheritance in, 247 Piper, alfalfa, origin and species, 215 cowpea, breeding, 146 cowpea, origin of, 143 pea, natural crosses, 38 sorghum species, 178 soybeans, natural crosses, 39 Piper and Morse, origin of soybean, 146 Piper and others, alfalfa pollination, 40, 216 Plant breeding, development of art, 2 relation of biological principles to, 8 Plum, self-sterility in, 269 Pollination, artificial crosses, oats, 71 artificial self-pollination, 67 depollination with water, 73 tools for, 73 Pope, pollination of crop plants, 34 Price, cabbage, self -sterility, 251 inheritance in cabbage, 251 Price and Drinkard, tomato inheri- tance, 245 Pridham, oat crosses, 36, 93 Probable error, use of, 56 in eliminating strains, 57 pairing method of obtaining, 57 Pure line theory, 11, 120, 121 Pammel, crosses in cucurbits, 256 Parker, disease resistance in oats, 95 spike density in wheat, 81 Parnell and others, inheritance in rice, 109 natural crosses in rice, 37 Quanjer, diseases in potatoes, 231 R Radish, origin, inheritance, breed- ing, 249 Raspberry, inheritance, 272 326 INDEX Ray, species, first use of term, 9 Replication, value of, 58 correct method of, 60 reducing probable error by, 59 Rice, important characters of, 108 inheritance summary, 109 natural crosses, 37 origin, 108 Rimpau, barley pollination, 36 oat crosses, 36 Riolle, radish inheritance, 249 Roberts, on early plant breeders, 5-8 Roguing, seed plots, 180 . Riimker, von, xenia in rye, 106 inheritance in rye, 106 Russell and Morrison, natural crosses, soybean, 39 Rye, pollination, 39, 106 winter vs. spring, 106 xenia in, 106 S Salaman, potato inheritance, 221 Salmon, blooming in wheat, 69 Sargaret, cucurbits, classification, 254 cucurbits, crosses in, 7 Saunders, Marquis wheat, 136 wheat crosses, 36 Scott, evolution, 8 Seed, improved corn, 287 pure bred, 281 registered, 286, 293 what is good, 282 wilt resistant flax, 287 Selection, individual plant, 119, 128 in small grains, early workers, 118, 119 isolation of pure lines, 125 methods of, 114, 115 sorghum, sugar content, 179 Self-fertilization, in normally cross- fertilized species, 45, 205 Setchell, tobacco sections, 159 Sexuality, Camerarius proves fact, 3 further proof of, 4 Shamel, tobacco breeding, 165 Shamel and Cobey, tobacco breed- ing, 166 Shamel and others, bud selection in fruits, 274 Shaw, self-fertilization in beet, 250 Shaw and Norton, inheritance in beans, 242 Shirreff, early selections, 118 Shull, self-fertilization in maize, 202, 205 Smith, selection for chemical con- tent, maize, 193 Sorghum, breeding, 179 classification, 178 inheritance in, 178 origin of, 178 self-fertilization in, 38, 180 Soybeans, breeding, 148 characters of, 146 inheritance in, 147 natural crosses, 39 origin of, 146 Species, crosses, Fi, tobacco, 45 crosses, willow, 8 first use of term, 9 Spillman, cowpea inheritance, 143, 144 wheat, density of spike, 81 Spragg, M. A. C. Robust bean, 131 Spragg and Clark, Red Rock wheat, 128 Squash, breeding, 259 flower structure, 255 size inheritance, 258 Stakman and others, biologic forms of rust, 85 flax wilt resistance, 157 Stewart, bud selection in fruits, 275 tobacco breeding, 166 Stout, bud variations, 274, 276 Stuart, potato breeding, 223, 224, 226 Surface, oat inheritance, 92, 93 oat selections, 129 oat species crosses, 90, 94 Sutton, Brassica crosses, 251 self-sterility, fruits, 269 Swingle, citrus fruits, 277 INDEX 327 Tammes, flax inheritance, 155, 156 flax species crosses, 153 Thatcher, barley inheritance, 102 Thompstone, inheritance in rice, 109 natural crosses in rice, 37 Timothy, breeding, 210 flower structure, 209 variability in, 208 Tisdale, flax wilt, inheritance, 158 Tobacco, breeding, 140 classification in groups, 159 color of flowers, inheritance, 161 Fi crosses, vigor of, 42 mutations in, 167 natural crosses in, 36 number of seeds per plant, 36 parthenogenesis in, 160 quantitative characters, inheri- tance, 162 Tolaas and Bisby, disease-free pota- toes, 291 Tomato, characters of, 245 Fi crosses, vigor of, 42, 43 inheritance, 245, 246 natural crosses, 39 Townsend, self-fertilization in sorg- hum, 180 Trabut, oats, A. sterilis origin, 90 Tschermak, barley inheritance, 102 bean inheritance, 242 oats, natural crosses, 36 oats, species crosses, 90 pea, inheritance, 238, 239 rye, wild rye, 106 rye, winter vs. spring habit, 106 tomato, inheritance, 245 vegetable, mass selection, 250 wheat, species groups and crosses, 77, 78 U Ulrich, rye-pollination, 40 V Valleau, sterility in strawberry, 270 Variations, hybridization, 13 Variations, mutations, 21 new combination, 21 non-heritable, 20 Vegetables, cross-fertilized group, 248 origin and antiquity of, 235 self-fertilized group, 234 Velvet bean, breeding, 151 characters and inheritance, 149 mutations in, 150 origin, 149 Vilmorin, early selections, 119 selection in beets, 250 wheat species crosses, 77 Von Mons, selection in fruits, 264 DeVries, correlations, value of, 125 individual plant method of selection, 120 wheat, barley and oats pollina- tion, 35 xenia in maize, 183 W Waite, self-sterility in plum, 269 Waldron, alfalfa, natural crosses, 40 pollination of, 216 winter hardiness, 216 Waller, normal self-pollination in maize, 39 Watermelon, wilt resistance in, 258 Waugh, self-sterility in plum, 269 Webber, cotton inheritance, 177 cotton, natural crosses, 38, 173 pepper inheritance, 247 timothy breeding, 210 xenia in maize, 183 Webber and Orton, cowpea, disease resistance, 145 Weigmann, early crosses, 5 Weissmann, constancy of germ plasm, 10 Wellington, Fi tomato crosses, 42 heterosis and inheritance, cu- cumber, 257 parthenogenesis in tobacco, 160 raspberry inheritance, 272 Westgate and others, clover, seed setting, 214 328 INDEX Wheat, artificial crossing of, 69, 71 awns in wheat, 85 blooming in, 69 breeding, 134, 135 chaff characters, 84 classification of species, 77 disease resistance, 85, 87, 134 FI crosses, vigor, 43-45 flower structure, 34, 70 hardiness in, 87 improvement by crossing, 134 Kanred, 76, 128 linkage in crosses, 79, 80, 84 Marquis, 136 natural crosses in, 34-36 Polonicum crosses, 79 Red Rock, 128 seed characters, 81-84 size characters, 88 species crosses, 77, 78 spike density, 81 sterility in species crosses, 79 T. dicoccoides, wild, 78 wheat-rye crosses, 35, 106 White, endosperm characters, maize, 186 fruit, origin, and antiquity of, 261 inheritance in pea, 236, 239, 240 Wichura, species crosses, willow, 8 Wight, potato species, 219 plum species, 262, 265 Williams, ear-to-row, maize, 198 selection for stiff straw, wheat, 129 Williams and Welton, ear characters and yield, maize, 197 varietal crosses, maize, 203 weight of seed planted and yield, 124 Wilson, inheritance, potatoes, 222 Wilson and Warburton, cotton spe- cies, 175 Witte, timothy breeding, 212 Wittmack, origin of potatoes, 220 Wolf, seed size increase due to cross, maize, 201 Wood and Stratton, checks in correcting yield, 53 probable error by pairing method, 58 X Xenia in barley, 103 in maize, 183 in rice, 37 in rye, 106 law in maize, 184 DAY AND TO * OVERDUE. 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