But
how the sexual elements of heterostyled plants came to differ from what they
were whilst the species was homostyled, and how they became co-adapted in two
sets of individuals, are very obscure points. We know that in the two forms of
our existing heterostyled plants the pistil always differs, and the stamens
generally differ in length; so does the stigma in structure, the anthers in
size, and the pollen-grains in diameter. It appears, therefore, at first sight
probable that organs which differ in such important respects could act on one
another only in some manner for which they had been specially adapted. The
probability of this view is supported by the curious rule that the greater the
difference in length between the pistils and stamens of the trimorphic species
of Lythrum and Oxalis, the products of which are united for reproduction, by so
much the greater is the infertility of the union. The same rule applies to the
two illegitimate unions of some dimorphic species, namely, Primula vulgaris and
Pulmonaria angustifolia; but it entirely fails in other cases, as with Hottonia
palustris and Linum grandiflorum. We shall, however, best perceive the
difficulty of understanding the nature and origin of the co-adaptation between
the reproductive organs of the two forms of heterostyled plants, by considering
the case of Linum grandiflorum: the two forms of this plant differ exclusively,
as far as we can see, in the length of their pistils; in the long-styled form,
the stamens equal the pistil in length, but their pollen has no more effect on
it than so much inorganic dust; whilst this pollen fully fertilises the short
pistil of the other form.
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