Pontederia (sp.?) short-styled form : 87.
Pontederia other sp. mid-styled form : 86.
We here see that, with seven or eight exceptions out of the forty-three cases, the pollen-grains from one form are larger than those from the other form of the same species. The extreme difference is as 100 to 55; and we should bear in mind that in the case of spheres differing to this degree in diameter, their contents differ in the ratio of six to one. With all the species in which the grains differ in diameter, there is no exception to the rule that those from the anthers of the short-styled form, the tubes of which have to penetrate the longer pistil of the long-styled form, are larger than the grains from the other form. This curious relation led Delpino (as it formerly did me) to believe that the larger size of the grains in the short-styled flowers is connected with the greater supply of matter needed for the development of their longer tubes. (6/2. 'Sull' Opera, la Distribuzione dei Sessi nelle Piante' etc 1867 page 17.) But the case of Linum, in which the grains of the two forms are of equal size, whilst the pistil of the one is about twice as long as that of the other, made me from the first feel very doubtful with respect to this view. My doubts have since been strengthened by the cases of Limnanthemum and Coccocypselum, in which the grains are of equal size in the two forms; whilst in the former genus the pistil is nearly thrice and in the latter twice as long as in the other form. In those species in which the grains are of unequal size in the two forms, there is no close relationship between the degree of their inequality and that of their pistils. Thus in Pulmonaria officinalis and in Erythroxylum the pistil in the long-styled form is about twice the length of that in the other form, whilst in the former species the pollen-grains are as 100 to 78, and in the latter as 100 to 93 in diameter. In the two forms of Suteria the pistil differs but little in length, whilst the pollen-grains are as 100 to 75 in diameter. These cases seem to prove that the difference in size between the grains in the two forms is not determined by the length of the pistil, down which the tubes have to grow. That with plants in general there is no close relationship between the size of the pollen-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 species from matter yielded by the pistil, we can see that in the former case it would be necessary that the grains of the two forms should differ in size relatively to the length of the pistil which the tubes have to penetrate, but that in the latter case it would not be necessary that the grains should thus differ.