BREEDING 



BREEDING 



547 



variety would then consist in the acquirement of a new 

 character by the organism or the loss of an old character 

 or of the production through hybridization of new 

 combinations of characters that already existed but 

 in different combinations. 



Nature of variation. 



AMiile, as indicated in the discussion of heredity, 

 organisms are usually reproduced in the likeness of 

 their parents, nevertheless it is well known that 

 all plants vary. Individual plants differ from one 

 another just as do men. The fact that plants can be 

 improved by selection depends upon the occurrence of 

 these so-called variations. One is accustomed to think 

 of plants as very stable and uniform. Casually looking 

 over a field of ox-eye daisies and admiring their beauty, 

 one distinguishes no apparent variabilit}'; all seem to be 

 alike. Nevertheless, if the plants are examined care- 

 fully and the different individuals studied, it is found 

 that each one possesses certain peculiarities. Some have 

 large flower-heads, others small flower-heads; some 

 have very many rays or petals, others comparatively 

 few; some have broad rays, others narrow rays. Some 

 plants are tall, others short. No two plants can be 

 found which do not differ from each other in some 

 noticeable character. Thej' present different facial 

 expressions, the same as do people or cattle, so that 

 different individuals may be recognized after one has 

 studied them and made their acquaintance. This is 

 one of the interesting studies which the breeder pur- 

 sues. Careful gardeners learn to recognize the individ- 

 ual plants that they handle day after daj' as the 

 shepherd recognizes the different members of his flock. 



The inheritance of a character ordinarily does not 

 mean its exact expression in the offspring as in the par- 

 ent. In considering variations from the standpoint of 

 the conception of unit-characters, it must be remem- 

 bered that only the determiners of a character are in- 

 herited and the expression of the character in the new 

 individual is influenced by the environment under which 

 the individual develops. It must also be remembered 

 that in the higher plants and animals with which the 

 breeder ordinarily has to deal, an individual results 

 from a fertilized egg-cell which contains the heritage 

 determiners of two parents and, as there are a very 

 large number of characters making up any individual 

 and as different individuals possess different determi- 

 ners which are brought together in fertilization, rarely or 

 never can one individual be conceived to be an exact 

 counterpart of another. 



Variaiiorts are of verj' great difference in magnitude 

 and kind; and while many different names have been 

 given to the different types of variation, the most 

 generally accepted usage at present is to classify all 

 variations either as fluctuations or mutations. 



Fluctuations are those variations that are .supposed 

 to be due to the direct action of environment and that 

 are not inherited. The variation in size as a result of 

 richness of soil, is such a fluctuating variation and, as 

 well recognized, is not a heritable character. A similar 

 illustration of such a variation is the difference in size 

 of oat or wheat plants due to crowding in the field 

 (Fig. 6.'37j. It is known that if a pole bean be trans- 

 ferred to the North, it tends to produce a bush type, 

 and if a cowpea be transferred to the North, it tends to 

 shorten up its vine and assume a bush habit. An in- 

 teresting illustration of such modifications is shown 

 in the ordinary red cedar, Junipcrus virgininnn fFig. 

 638). In the rich, moist soils of Pennsylvania, Mary- 

 land and Virginia, this tree forms a beautiful tall col- 

 umnar top with dense foliage (Fig. 6.38a). On the dry, 

 sterile, limestone hills cf Kansas, Nebraska, and Ken- 

 tucky, and in the sandy soil of Florida, the same tree 

 produces a spreading, scraggly top of entirely different 

 character (Fig. 638 6). If one of these trees is trans- 



planted while young, from sterile barren soil to moist 

 rich land, it assumes the tali columnar habit as a result 

 of the environment. 



Plant-breeders have sometimes assumed that such 

 modifications, which are the result of environment 

 (Fig. 639), are of great importance to them. This 

 matter, however, is in grave doubt. The informa- 

 tion at command indicates that these characters, which 

 are physiological adaptations, are not hereditary, and 

 are lost as soon as the plant is transferred again to its 

 normal environment. If, for example, it is desired to 

 produce a bush cowpea and the selection is undertaken 

 in the South with a viny variety, a search should be 

 made among the plants for the individual that ap- 

 proaches most nearly to the bush type, and it is probable 

 that this plant would be as likely to transmit this 

 character to its progeny as a similar bushy tj^pe selected 

 under northern conditions. As a matter of fact, it may 

 be that this tendency could be recognized much more 



639. Variation ia size of dandelion caused by growing at dif* 

 ferent altitudes, a. Plant grown in valley at low altitude; b, plant 

 grown on mountain at high altitude. (From Bonnier. j 



clearly in a southern location, where the plants normally 

 produce vines, than in a northern location. 



Mutations, on the other hand, are changes that are 

 more profound and effect the germinal cells of the 

 organism in such a way that the changes are inherited. 

 The most typical illustrations of mutations are the 

 striking large tjTie-variations th.at are known to gar- 

 deners as sports, and which ordinarily reproduce true 

 to seed. It mu.st not be understood, however, that 

 all mutations are large tv^pe-variations. This, it is 

 true, was in large measure the meaning given to muta- 

 tions by DeVries in his development of the mutation 

 theorj' of evolution, but the moie general interpretation 

 of biologists at present is to consider any type of varia- 

 tion that is inherited as a mutation. Many small varia- 

 tions, such as a slight difference in height of ear in corn, 

 may be regularly inherited, and in some instances 

 differences that are so slight as to be distinguished only 

 by careful biometrical analysis are regularly inherited, 

 generation after generation, even under very different 

 conditions. Recent scientific studies have emphasiAi 



