STRUCTURAL A1S POTENTIAL SEGREGATION. 89 



cessive steps by which impr nation is reached. This last clause is 

 added, for we observe that 1 the language of recent botanists the 

 stamens and pistils of plars are not sexual organs; and "pollen 

 grains are asexual spores. " Phis, however, does not change the fact 

 that in order to secure fertili tion the pollen grain, after reaching the 

 stigma, must be able to send ut a pollen tube long.enough and pene- 

 trating enough to descend t h ugh the length of the pistil to the center 

 of the ovule, through the nuc us and embryo sac. The coordinations 

 required for securing these id many other steps in the process of 

 fecundation are maintained y impregnational selection, and so far 

 as they depend on the form nd structure of organs we may call the 

 process "structural intersele ion." The propagation of every sex- 

 ually reproducing plant an animal must depend on such coordi- 

 nations. 



Structural isolation arises len local varieties that have become so 

 far divergent in structure as > be incompatible are brought together 

 in the same district. Darwi suggested that difference in the length 

 of the pollen tubes and the p ils may be the cause preventing crosses 

 between certain species of pi its. 



9, The Potential >rm of Selection and Isolation. 



Potential selection. — Then, re characters more fundamental than 

 form and structure that mu be coordinated in order to secure the 

 fertilization of the ova that j >duce the next generation. The pollen 

 of one species of plants is usu ly either partially or entirely ineffective 

 if it falls upon the stigma of ; other species, even though both species 

 are of the same genus. Tl e are also certain species having two 

 kinds of stamens producing - o kinds of pollen ; and the pollen from 

 the short stamens is said to 1 most effective upon the stigmas of the 

 short styles, and the pollen frm the long stamens most effective upon 

 the stigmas of the long style As each flower produces either a long 

 style and short stamens or short style and long stamens, the dis- 

 criminate prepotence just de^ribed insures cross-fertilization. f But 

 our present interest is in the ict that in pollen grains there are char- 

 acters of an obscure nature Mich are of great importance in insuring 

 the required potency. Ther is reason to believe that in every species 

 depending on sexual reprodu ion there must be more or less potential 

 selection, by which the coorcnation of the sexual elements enabling 

 them to coalesce is maintain* . 



Potential isolation occurs 1 the two forms, prepotential isolation 

 and complete potential isolaU 1. Complete potential isolation exists 



* See Plant Structures, by John . Coulter; Appleton & Co., 1900; pp. 176, 177. 

 f See Plant Relations, by John Coulter, pp. 129, 130. 



