576 



PLANT BREEDING 



more for size than the embryos developed by a much smaller 

 plant which has grown in less favorable conditions. To com- 

 pare plants as to what is really in their cells, the conditions 

 under which the plants were grown must be considered. It is 

 for this reason that it is much better to select seed Corn from the 

 field than to select it from the crib; for by the first method the 

 conditions under which the plants developed 

 can be considered and their genetic consti- 

 tution better estimated. 



In addition to Johannsen's investigations 

 of inheritance in Beans and a number of 

 other plants, there are many other experi- 

 ments that tend to establish the fact that 

 little is gained by selections in pure lines, 

 unless mutations happen to occur. For 

 example, as previously cited a long series of 

 selections in a pure line of Oats to increase 

 the yield secured no results. Likewise selec- 

 tions in pure lines of Wheat to intensity and 

 fix a desirable variation have given no re- 

 sults. In each generation plants having the 

 desirable variation were selected for seed, 

 but selections carried on this way for a 

 number of generations did not increase the 

 average of the variation. 



In vegetative propagation, where the pro- 

 geny does not develop from seed but grows 

 from vegetative structures taken from the 

 parent, some variations, such as bud sports, 

 that are often not inheritable through seed 

 can be perpetuated. Such is true in such 

 plants as Strawberries, which are propagated 

 by runners, and in fruit trees, which are 

 propagated by grafting. But modifications that are simply 

 responses to some peculiarity in the environment are not per- 

 petuated even by vegetative propagation. 



Although selection often fails to produce the desired results, 

 nevertheless, selection is one of the chief means of improving 

 plants. A mass of plants, such as a field of grain, is a mixture 

 of individuals most of which are heterozygous for one or more 



FIG. 485. Heads 

 of Wheat, showing 

 improvement in size 

 as a result of a five- 

 year selection in 

 which the plants 

 bearing the largest 

 heads were selected 

 each year for seed. 

 Redrawn from De 

 Vries. 



