Applicalion of Biological Principles lo Plant Breeding 131 



other and that the extra length of the longer llower is con- 

 trolled by three or four separately heritable factors. If 

 only a few of the egg cells and pollen cells can fuse on 

 account of the dissimilarity of their gametic constitution, 

 one would expect only those seeds to be formed that would 

 result from the fusion of the germ cells nearest alike. 

 Intermediates would therefore be more likely to occur than 

 extremes. 



There is one other possible way of accounting for con- 

 stant intermediate races. In crossing species of the genus 

 Nicotiana, I have had plants develop from carefully 

 guarded and supposedly hybrid seed that were exactly like 

 the maternal plant. These seeds must have resulted from 

 apogamy or polyembryony, that is, from the development 

 of an immature egg cell without fertilization. The phe- 

 nomenon was evidently induced by the extraordinary 

 irritation of the foreign pollen. The question then arises: 

 May not the difficulty of maturing sex cells in the F, genera- 

 tion of a wide cross sometimes cause apogamous seed 

 development and therefore a continued propagation of a 

 constant and uniform race ? 



These pieces of work illustrate the various distinct types 

 in the improvement of plants by hybridization. 



Intentionally, little has been said regarding the fixation 

 of desired character combinations when the new varieties 

 obtained are to be reproduced by seed. The reason for this 

 omission is that the selective method used after hybridiza- 

 tion is the same as that used upon crops whose small seeds 

 and tendency to vary makes it difficult or unnecessary to 

 produce artificial hybrids. The method is Vilmorin's and is 

 based upon the fact that one cannot tell the most productive 

 or otherwise desirable plant by inspection. The true basis 



