It has already been shown that in about 32 out of the 55 genera in the list just
referred to, the perfect flowers are irregular; and this implies that they have
been specially adapted for fertilisation by insects. Moreover three of the
genera with regular flowers are adapted by other means for the same end. Flowers
thus constructed are liable during certain seasons to be imperfectly fertilised,
namely, when the proper insects are scarce; and it is difficult to avoid the
belief that the production of cleistogamic flowers, which ensures under all
circumstances a full supply of seed, has been in part determined by the perfect
flowers being liable to fail in their fertilisation. But if this determining
cause be a real one, it must be of subordinate importance, as four of the genera
in the list are fertilised by the wind; and there seems no reason why their
perfect flowers should fail to be fertilised more frequently than those in any
other anemophilous genus. In contrast with what we here see with respect to the
large proportion of the perfect flowers being irregular, one genus alone out of
the 38 heterostyled genera described in the previous chapters bears such
flowers; yet all these genera are absolutely dependent on insects for their
legitimate fertilisation. I know not how to account for this difference in the
proportion of the plants bearing regular and irregular flowers in the two
classes, unless it be that the heterostyled flowers are already so well adapted
for cross-fertilisation, through the position of their stamens and pistils and
the difference in power of their two or three kinds of pollen, that any
additional adaptation, namely, through the flowers being made irregular, has
been rendered superfluous.
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