252 CONCLUDING REMARKS CHAP. VI. 



grains and the length of the pistil is manifest; for 

 instance, I found that the distended 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 Datura, in 

 which the tubes have to grow down the whole length 

 of the pistil, and therefore to a length equalling 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 spe- 

 cies from matter yielded by the pistil, we can see that 

 in the former case it would be necessary that the grains 



