142 



BREEDING CROP PLANTS 



index is obtained by dividing the average leaf breadth by the 

 average length and multiplying by 100. The table shows that 

 by crossing, a form, No. 38, was obtained which combined some- 

 what the desirable features of the parents (Nos. 26 and 27) 

 and is superior in both number and size of leaves to the better 

 pure-line obtained by selection (No. 33). As an indication of 



the commercial reception of 

 this new form, it was esti- 

 mated that at least 10,000 

 acres of No. 38 were grown 

 in Wisconsin in 1919 cut of 

 a total of about 40,000 

 acres. Here we have an 

 example of crossing two 

 closely-related forms and 

 obtaining from the resul- 

 tant progeny a strain of 

 more commercial value 

 than either parent. 



The illustrations bring 

 out more clearly some of 

 the features of the parents 

 and progeny. 



Summary. In this 

 chapter concrete evidence 



FIG. 32.-Tobacco No. 38. This strain f the Value f crossing as 



was produced by crossing No. 26, which ex- a means of producing im- 



cels in leaf breadth, with No. 27, which is nrftvp j VJ , ri WiV<a nf <?plf 



homozygous for high leaf number. This new P^Ved varieties Ol SeJt 



strain is more desirable than any other pure fertilized crops has been 



line form obtained. It is widely grown in , j /-, u 1.4 



Wisconsin. (After Johnson, 1919.) presented. Crosses should 



be made with a definite 



purpose in view and the parents should be selected on the basis of 

 performance records. Just as a chemist requires a certain knowl- 

 edge of the elements which he synthesizes into compounds, so 

 also the plant breeder may make crosses much more intelligently 

 if he is thoroughly acquainted with the prospective parental 

 material. Promiscuous crossing as a means of producing im- 

 proved forms is discouraged. 



