CHAP. VI. ON HETEROSTYLED PLANTS. 251 



the size of the pollen-grains and the length of the 

 pistil is manifest : for instance, I found that the dis- 

 tended grains of Datura arborea were '00243 of an 

 inch in diameter, and the pistil no less than 9*25 

 inches in length ; now the pistil in the small flowers 

 of Polygonum fagopyrum is very short, yet the larger 

 pollen-grains from the short styled plants had exactly 

 the same diameter as those from the Datura, with its 

 enormously elongated pistil. 



Notwithstanding these several considerations, it is 

 difficult quite to give up the belief that the pollen-grains 

 from the longer stamens of heterostyled plants have 

 become larger in order to allow of the development of 

 longer tubes ; and the foregoing opposing facts may 

 possibly be reconciled in the following manner. The 

 tubes are at first developed from matter contained 

 within the grains, for they are sometimes exserted 

 to a considerable length, before the grains have 

 touched the stigma ; but botanists believe that they 

 afterwards draw nourishment from the conducting 

 tissue of the pistil. It is hardly possible to doubt 

 that this must occur in such cases as that of the Da- 

 tura, in which the tubes have to grow down the whole 

 length of the pistil, and therefore to a length equal- 

 ling 3,806 times the diameter of the grains (namely, 

 00243 of an inch) from which they are protruded. 

 I may here remark that I have seen the pollen -grains 

 of a willow, immersed in a very weak solution of honey, 

 protrude their tubes, in the course of twelve hours, to 

 a length thirteen times as great as the diameter of the 

 grains. Now if we suppose that the tubes in some 

 heterostyled species are developed wholly or almost 

 wholly from matter contained within the grains, while 

 in other species from matter yielded by the pistil, we 

 nan see that in the former case it would be necessary 



