114 president's address. 



of crossing and intercrossing, and the singular readiness with 

 which species far removed from each other, such as S. Lapponum 

 and S. purpurea, S. viminalis and S. daphnoides combined to pro- 

 duce seedlings. 



Another point of great importance is the imperfect nature of 

 hybrids. In hybrid willows the pollen is less regular than that 

 of pure species, that is to say, it is less regular in form. The 

 pollen of almost all the European willows is said to be remark- 

 able for its great regularity.* One grain is almost perfectly like 

 another in size, colour, constitution, and form. Pure species are 

 thus distinguished from hybrids, in whose pollen there are always 

 some that are abnormal. A tolerably correct opinion, therefore, 

 may be formed of the comparative fertility of hybrids from the 

 examination of the pollen. Pollen grains of willows were treated 

 with a solution of honey (as much as would lie on the point of 

 a knife), mixed with two ounces of water, and it appeared on 

 microscopic examination that the normally formed grains of hy- 

 brids, of a light colour and semi-transparent texture, constantly 

 developed pollen tubes. Fresh pollen placed in this mixture 

 frequently began, in the course of ten or twelve minutes, to put 

 forth pollen tubes. 



Amongst willow hybrids traces of a defective feeble develop- 

 ment are far more frequent than rankness of growth. "Wichura 

 gives an enumeration of many hybrids which languished miser- 

 ably for a few years, and then eventually died out. The possi- 

 bility must therefore be allowed that the vegetative growth of 

 other seemingly strong hybrids (such as 8. rubra, Huds., said to 

 have been proved experimentally to be a hybrid between pur- 

 purea and viminalis) is essentially weak, so as to prevent them 

 in the Darwinian struggle for life from competing with the parent 

 species. If all these circumstances, Mr. Berkeley observes, are 

 combined with the imperfection of the pollen, and the partial 

 sterility of the ovaries, the comparative defect of vital energy in 

 hybrids may be looked upon as proved. Nor does the luxuriant 



* I find that the easiest way to procure pollen is to gather the catkin-bearing shoots 

 before the anthers have begun to discharge their contents. They open well if kept in 

 water for a day or two iu the house. 





