362 Plant Physiology 



descendants " varied from the common type, not only in 

 acquiring great power of growth and increased fertility 

 when subjected to self-fertilization, but in not profiting 

 from a cross with a distinct stock ; and this latter fact, if 

 trustworthy, is a unique case, as far as I have observed 

 in all my experiments." 



211. The need of further work. - The question of cross 

 and self fertilization (cross-breeding and in-breeding) is 

 now receiving renewed attention. The indications are 

 that with many plants in-breeding is for plant production 

 far less dangerous than has been supposed. It seems to 

 be conspicuously dangerous in the case of corn, some 

 reasons for which will be subsequently considered. Sha- 

 mel states, " In the breeding of tobacco it is well known 

 that cross-pollination within the limits of a single strain 

 produces inferior offspring, and only self-fertilization gives 

 offspring of the highest degree of vigor, though hybrids 

 between distinct strains of tobacco often display a vigor 

 superior to that of either parental strain. Examples 

 could be continued indefinitely, but even one instance, in 

 which long-continued in-breeding results in no injurious 

 effects, would be sufficient to discredit the old hypothesis." 



It is evident that in breeding studies each crop must be 

 examined with respect to this important point. It is to 

 be expected that much new evidence on the general prob- 

 lems of self and cross fertilization will be available, for 

 the more certain methods in recent years with pure-line 

 ancestry, the conception of unit characters, the develop- 

 ment of biometry - - all make possible far more definite 

 experimental conditions. 



212. Experiments with self-sterility in pear. - Waite 



