I will give only one--not as a very striking case,
but as likewise illustrating one step in the separation of the sexes
of plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a small quantity of
pollen, and a rudimentary pistil; other holly-trees bear only female
flowers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put
the stigmas of twenty flowers, taken from different branches, under
the microscope, and on all, without exception, there were
pollen-grains, and on some a profusion of pollen. As the wind had set
for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and boisterous,
and therefore not favourable to bees, nevertheless every female flower
which I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having flown from tree to tree in
search of nectar. But to return to our imaginary case: as soon as the
plant had been rendered so highly attractive to insects that pollen
was regularly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what has been called
the "physiological division of labour;" hence we may believe that it
would be advantageous to a plant to produce stamens alone in one
flower or on one whole plant, and pistils alone in another flower or
on another plant.
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