546 



BREEDING 



BREEDING 



plant-breeding was given by the publication of Darwin's 

 famous works, particularly his "Animals and Plants 

 under Domestication," in 1868. His extensive re- 

 searches, masterful compilation and systematizatipn of 

 the existing knowledge may be said to have established 

 breeding on a systematic basis. 



Following Darwin, little advance was made in the 

 knowledge of the principles 

 of breeding until in 1900, 

 when Mendel's papers on plant 

 hybridization, describing his 

 now famous principles or laws 

 of inheritance, were rediscovered 

 independently and brought to 

 attention by DeVries, Correns 

 and Tschermak. The discovery 

 of these laws and the publication 

 of DeVries' "Mutation Theory" 

 in the same year, marked the 

 beginning of a new era in plant- 

 breeding. No matter what the 

 final conclusions may be regard- 

 ing Mendel's principles and the 

 mutation theory, the stimula- 

 tion which these two theories 

 have given to breeding has 

 already served greatly to modify 

 and extend knowledge, both in 

 scientific and practical direc- 

 tions. 



The great advance that has 

 been made in the discovery of 

 the underlying principles of 

 breeding puts experimentation 

 in this field on a much surer 

 basis and the breeder can now 

 approach his subject with defi- 

 nite understanding. 



637. Variations in size 

 of wheat plants grown 

 side by side in field. 



Classification of varieties. 



To understand clearly the character of organisms 

 with which breeding deals, careful definitions of the 

 different groups of cultivated plants which are ordi- 

 narily known as varieties are needed. One speaks of 

 varieties of wheat, corn, apples and pears, yet it is 

 known that these varieties differ from each other as 

 natural groups. In order to distinguish clearly these 

 differences, the following classification of varieties into 

 races, strains and clons has been proposed: 



Races are groups of cultivated plants that have well-marked 

 differentiating characters, and propagate true to seed except for 

 simple fluctuating variations. The different groups of beans, peas, 

 wheat, oats, ' corn, cotton, and the like, referred to commonly as 

 varieties, are thus in a more restricted sense races. Boone County 

 White, Learning, Golden Bantam, and so on, would be recognized 

 as races of com, and Turkey Red, Fulcaster, Fultz, as races of 

 wheat, and Early Paris, Dwarf Erfurt and Snowball as races of 

 cauliflower. 



Strains are groups of cultivated plants derived from a race, 

 which do not differ from the original of the race in visible taxonomic 

 characters. When the breeder, by a careful selection of Blue Stem 

 wheat, produces a sort of Blue Stem that differs from the original 

 race only in the quality of yielding heavily, it would be called a 

 strain of Blue Stem. 



Clons are groups of cultivated plants the different individuals 

 of which are simply transplanted parts of the same individual, the 

 reproduction being by the use of vegetative parts such as bulbs, 

 tubers, buds, grafts, cuttings, runners, and the like. The various 

 sorts of apples, potatoes, strawberries, chrysanthemums, and so on, 

 commonly denominated varieties, in a more restricted sense would 

 be clons. Clons of apples, pears, strawberries, potatoes, and the 

 jike, do not propagate true to seed, while this is one of the most 

 important characters of races and strains of wheat, corn, and others. 

 The term variety would thus be used in a general sense, and would 

 include races, strains and clons. 



Heredity. 



The laws of heredity are of primary importance to 

 the breeder. It is a general principle that like begets 

 like, but it is also true that like frequently gives rise to 



unlike. In general, by heredity is meant the tendency 

 which an organism manifests to develop in the form and 

 likeness of its progenitors, and the study of heredity 

 includes thus the inheritance of characters. It is of the 

 utmost importance that organisms in general reproduce 

 their kind, as otherwise the breeder would be confronted 

 with confusion, but it is of equal importance that the off- 

 spring does not always reproduce the parental charac- 

 ters. There are thus apparently two conflicting prin- 

 ciples in plant-breeding. On the one hand, the breeder 

 seeks to produce variations in order to get new types 

 as the foundations for improvement. On the other hand, 

 when such a variation from or improvement on the nor- 

 mal type is secured, he reverses the process and tries 

 to establish heredity and reduce the amount of varia- 

 tion, so that the aphorism, "like begets like, 1 ' will hold 

 true. 



In pedigree- or grade-breeding, and in breeding to 

 produce new varieties, the importance of hereditary 

 strength cannot be overestimated, as it is only by ren- 

 dering this power very great that any new form can be 

 brought to what is called a fixed type. 



In recent years, the ideas of fixity of type have been 

 greatly modified, and it is now held that fixity of 

 type is secured by purifying a race from all admixtures 

 so that any character represented in a race will be pure. 



Unit-characters. 



The modern studies of heredity have led to a new 

 conception of organic characters that should be clearly 

 understood by the breeder. A careful study of species or 

 varieties of plants or animals focuses the attention not 

 on the generality of the differences existing but rather 

 on the differences in certain characters; one observes 

 whether a plant is smooth or hairy, cut-leaved or entire- 

 leaved, much branched or simply branched, erect or 

 procumbent, tall or short, and the like. This leads to 

 the conception that a plant is not of simple organiza- 

 tion but is comprised of a combination of characters. 

 These characters or the physiological units which cause 

 them are now thought of as in considerable measure 

 independent of each other and as representing dis- 

 tinct organic units. The classical studies of Gregor 



638. Red cedar: a, Columnar form; b, Spreading form. 



Mendel on the hybridization of races of peas that 

 exhibited different characters established the fact that 

 at least certain characters are inherited separately and 

 may form permanent new combinations. 



A unit-character, then, may be defined as any char- 

 acteristic quality or set of qualities or expression of 

 character in an organism that is inherited as a whole 

 and independent of any other quality or set of qualities. 

 They are the organic units of inheritance. The units 

 that are considered in hybridization are not the 

 species or varieties themselves, but the unit-characters 

 of which they are composed. The origin of a new 



