22 



PLANT BREEDING. 



been broken. Tliroiigli the mixture of "blood" the centrifugal force 

 has found a new place to eentei- its efforts against the centripetal 

 or conservative forces of hereditj', and we have a few individuals 

 varying in the desired direction of larger jield. 



Variations thus newly created in r>eminally proj^agated varieties 

 are generalh' not stable. The force of heredity of the family, race, 

 and species powerfully combat the new characteristics and tend to 

 reduce the new forms to conformity with the old. Most of the plants 

 in the upper curve are very uncertain in their ability to transmit to 

 their progeny their new and superior qualities, and those few which 

 liave this qualitj^ of strongly transmitting the new valuable qualities 

 are all that have an especial value. Since, as a rule, thousands of 

 their fellows must be eliminated in order to get one good plant, 

 methods must be devised by which this work may be cheaply and 



Fig. 10.— Yields of Blue Stem. Fife, and hybrid plants after further selection — all grown from 

 seed from the best plants as, shown in fig. 9 at o—o. Yield of Blue Stem is indicated by the 

 broken line, that of Fife by the solid line, and that of the hybrid by the line marked — x—. 

 Each of the ten perpendicular lines stands for the average of 1(X) plants grown in nursery 

 centgener. 



effectively done on a large scale. To have many in the upper curve 

 the line must represent very large numbers. 



The results of a method used for this selection in breeding wheat 

 is shown in figure 10. The 10 plants standing highest in the Fife, 

 Blue Stem, and hj^brid curves (Fig. 9, o — o), are chosen as 30 

 mothers of nursery j^lots of 100 plants each, called, for convenience, 

 centgeners. The average yield per plant in each of the 30 centgeners 

 is a measure of the abilitj^ of the respective mother plants to produce 

 progeny which yields heavily. These average measures can in turn be 

 shown graphically, as in figure 10. Here it will be observed that the 

 same general curve prevails; also that some of the progenj^ of mother 

 plants whose individual yields ranked in the upper curve in figure 9 

 now falls below the double horizontal line. In other words, some 

 plants which j'ield well individual!}- Di-oduce progeny decidedly' low 



