PLANT BREEDING AND EVOLUTION 



191 



SY 



Sg 



wY 





EGGS 

 Sg wY 



wg 



if properly bred and cared for. When the combinations which he 

 desires appear, as in Burbank's Shasta daisy and hybrid walnut, 

 he can proceed to preserve the favorable combination which he 

 knows is composed of stable characters. A favorite method, 

 already indicated, of preserving such desirable combinations of 

 characters is by grafting and budding the new variety on hardy 

 but otherwise less valuable stock. Since plants grown by asexual 

 means always come true to the 

 original parents, a new com- 

 bination of characters can be 

 preserved indefinitely by these 

 means. In other instances the 

 new combinations may prove 

 to breed true by seeds when 

 the germ cells were pure for 

 the characters combined in the 

 new type. The type may also 

 be maintained by careful con- 

 tinued selection. The scientific 

 breeder is thus able to forecast 

 results and plan his crossing in 

 accordance with definite laws. 

 In the above brief survey of 



FIG. 100. Diagram showing method of 



determining the composition of zygotes 



shown in Eig. 99 



The four classes of female gametes above 

 and of male gametes at the left. The pos- 

 sible combinations of the gamete factors 

 shown in squares. Considering S and V 

 dominant wherever they occur, we get the 

 ratio 9 SY to 3 Sg to 3 wY to 1 wg 



Mendel's work all details and 

 exceptions have been neces- 

 sarily omitted, but enough has 



been given to indicate to the student that, in the future, plant 

 breeding is likely to become a science and not a game of chance, 

 with results of profound importance to the improvement of plants 

 and animals for man's uses. If this prediction proves to be cor- 

 rect, Mendel will always be known as the founder of a scientific 

 method in plant and animal breeding. It is more than probable 

 also that nature repeats, in the wild, the same experiments which 

 man has learned to do in his culture beds and experiment 

 plots. Nearly related species in nature are known to hybridize, 

 and, such being the case, the hybrids must combine the char- 

 acters of the parents in new mixtures, and then these mixtures 



