On the other hand, if two plants of
the same trimorphic species happen to grow in an isolated spot, the chances are
two to one in favour of their not belonging to the same form; and in this case
they will legitimately fertilise one another, and yield the full complement of
vigorous offspring.
THE MEANS BY WHICH PLANTS MAY HAVE BEEN RENDERED HETEROSTYLED.
This is a very obscure subject, on which I can throw little light, but which is
worthy of discussion. It has been shown that heterostyled plants occur in
fourteen natural families, dispersed throughout the whole vegetable kingdom, and
that even within the family of the Rubiaceae they are dispersed in eight of the
tribes. We may therefore conclude that this structure has been acquired by
various plants independently of inheritance from a common progenitor, and that
it can be acquired without any great difficulty--that is, without any very
unusual combination of circumstances.
It is probable that the first step towards a species becoming heterostyled is
great variability in the length of the pistil and stamens, or of the pistil
alone. Such variations are not very rare: with Amsinckia spectabilis and Nolana
prostrata these organs differ so much in length in different individuals that,
until experimenting on them, I thought both species heterostyled. The stigma of
Gesneria pendulina sometimes protrudes far beyond, and is sometimes seated
beneath the anthers; so it is with Oxalis acetosella and various other plants.
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