Several closely allied genera in this family probably owe their heterostyled
structure to descent in common; but as the genera thus characterised are
distributed in no less than eight of the tribes into which this family has been
divided by Bentham and Hooker, it is almost certain that several of them must
have become heterostyled independently of one another. What there is in the
constitution or structure of the members of this family which favours their
becoming heterostyled, I cannot conjecture. Some families of considerable size,
such as the Boragineae and Verbenaceae, include, as far as is at present known,
only a single heterostyled genus. Polygonum also is the sole heterostyled genus
in its family; and though it is a very large genus, no other species except P.
fagopyrum is thus characterised. We may suspect that it has become heterostyled
within a comparatively recent period, as it seems to be less strongly so in
function than the species in any other genus, for both forms are capable of
yielding a considerable number of spontaneously self-fertilised seeds. Polygonum
in possessing only a single heterostyled species is an extreme case; but every
other genus of considerable size which includes some such species likewise
contains homostyled species. Lythrum includes trimorphic, dimorphic, and
homostyled species.
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