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.
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