Plants which are already well adapted by the structure
of their flowers for cross-fertilisation by the aid of insects often possess an
irregular corolla, which has been modelled in relation to their visits; and it
would have been of little or no use to such plants to have become heterostyled.
We can thus understand why it is that not a single species is heterostyled in
such great families as the Leguminosae, Labiatae, Scrophulariaceae, Orchideae,
etc., all of which have irregular flowers. Every known heterostyled plant,
however, depends on insects for its fertilisation, and not on the wind; so that
it is a rather surprising fact that only one genus, Pontederia, has a plainly
irregular corolla.
Why some species are adapted for cross-fertilisation, whilst others within the
same genus are not so, or if they once were, have since lost such adaptation and
in consequence are now usually self-fertilised, I have endeavoured elsewhere to
explain to a certain limited extent. (6/4. 'The Effects of Cross and Self-
fertilisation' 1876 page 441.) If it be further asked why some species have been
adapted for this end by being made heterostyled, rather than by any of the above
specified means, the answer probably lies in the manner in which heterostylism
originated,--a subject immediately to be discussed. Heterostyled species,
however, have an advantage over dichogamous species, as all the flowers on the
same heterostyled plant belong to the same form, so that when fertilised
legitimately by insects two distinct individuals are sure to intercross.
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