220 Plant-Breeding 



tendency after it once appears, lest the plant acquire a too 

 strong habit of scattering. This initial variation is induced 

 by changing the conditions in which the plant has habit- 

 ually grown, as a change of seed, change of soil, tillage, 

 varying the food supply, crossing, and the like. 



As a matter of fact, however, nearly all plants that 



FIG. 56. Disease resistance in cowpeas. Showing a variety which 

 is immune (on the left) and a susceptible variety (on the right) 

 to cowpea wilt. 



have been long cultivated are already sufficiently variable 

 to afford a starting-point for breeding. The operator 

 should have a vivid mental picture of the variety which 

 he designs to obtain ; then he should select that plant in 

 his plantation which is nearest his ideal, and sow the 

 seeds of it. From the seedlings he should again select 

 his type, and so on, generation after generation, until 



