166 BOARD OF AGRICULTURE. 



some of you as another form of presenting the statements of 

 Knight and Darwin respecting the desirability, or rather 

 absolute necessity, of occasional cross-fertilization. 



5. If different kinds of pollen are applied to the stigma 

 simultaneously, that alone takes effect which has the greatest 

 sexual affinity. The presence of pollen of the same species, 

 as a rule, excludes hybrid-fertilization by other species ; but, 

 on the other hand, the pollen of a different variety can easily 

 prevent self-fertilization. The power of exclusion does not 

 exist, however, when impregnation has taken place. Since, 

 therefore, fertilization ensues more slowly from pollen of 

 slight affinity, pollen of greater affinity, which is applied later, 

 can act at the same time, and give rise to two kinds of seeds 

 in the same fruit. Hybrid-fertilization can take place in the 

 case of Nicotiana in two hours, in Malva and Hibiscus in three 

 hours, in Dianthus in from five to six hours, after the applica- 

 tion of their own pollen. Observation has shown that often 

 these short periods suffice to remove the hindrance just 

 referred to. 



6. The characteristic power of the pollen is exerted only 

 on the germ-vesicle and the germ resulting from impregna- 

 tion. All other changes would take place in their sequence 

 from any active pollen. Hybrid-fertilization does not, as 

 such, affect the mother, but the offspring. This statement, 

 which, like the others, I have endeavored to give as nearly 

 as possible in Nageli's own words, will at once call to your 

 minds the apparent exceptions in certain fruits.^ Without 

 attempting to defend the position, I will cite only one phrase 

 respecting this : " Artificial impregnation shows that this 

 fundamental proposition is true without any exception." 



7. The hybrid produced from the blending of two parent- 

 forms stands between the two so far as its systematic (or ex- 

 ternal) characters are concerned. For the most part, it occu- 

 pies a middle position ; less often it inclines more to one or 

 the other. The cases in which the hybrid resembles one 

 parent rather than the other are more striking in variety- 

 hybrids than in species-hybrids: in the former some non- 

 essential characters are seen side by side in the progeny, but 

 uncombined. 



8. The general rule that the characters of hybrids lie some- 



1 See note B at the end of the lecture. 



