Many flowers, on the other
hand, have their organs of fructification closely enclosed, as in the
great papilionaceous or pea-family; but in several, perhaps in all,
such flowers, there is a very curious adaptation between the structure
of the flower and the manner in which bees suck the nectar; for, in
doing this, they either push the flower's own pollen on the stigma, or
bring pollen from another flower. So necessary are the visits of bees
to papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits
be prevented. Now, it is scarcely possible that bees should fly from
flower to flower, and not carry pollen from one to the other, to the
great good, as I believe, of the plant. Bees will act like a
camel-hair pencil, and it is quite sufficient just to touch the
anthers of one flower and then the stigma of another with the same
brush to ensure fertilisation; but it must not be supposed that bees
would thus produce a multitude of hybrids between distinct species;
for if you bring on the same brush a plant's own pollen and pollen
from another species, the former will have such a prepotent effect,
that it will invariably and completely destroy, as has been shown by
Gartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but, the agency of insects is often required to cause
the stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and curiously in this very genus, which seems to
have a special contrivance for self-fertilisation, it is well known
that if very closely-allied forms or varieties are planted near each
other, it is hardly possible to raise pure seedlings, so largely do
they naturally cross.
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