Application of Biological Principles to Plant Breeding 119 



logical mixtures due to hybridity, strains characterized by 

 desirable new variations. 



Practical procedure in hybridization naturally varies 

 somewhat depending upon the exact object in view. I will 

 endeavor to illustrate the following phenomena as those of 

 most importance: (a) Recombinations of desirable char- 

 acters and their fixations, including the production of 

 blends; (b) production of desirable combinations in the first 



FIG. 50. Buds of Nicotiana tabacum showing time and method of castration 



hybrid generation and their continuation by asexual propa- 

 gation; (c) production of fixed first generation hybrids. 



If one is to begin at the real beginning in this discussion, 

 he should spend a moment in describing the mechanical 

 operations of crossing (Fig. 50). There are three important 

 steps. First it must be determined by experiment what 

 environmental conditions are best suited for normal seed 

 production. Second, an intimate knowledge of the flower- 

 ing habits and flower structure should be gained in order 



