Dimorphic heterostyled plants offer still more strongly marked facilities for
becoming dioecious; for they likewise consist of two bodies of individuals in
approximately equal numbers, and what probably is more important, both the male
and female organs differ in the two forms, not only in structure but in
function, in nearly the same manner as do the reproductive organs of two
distinct species belonging to the same genus. Now if two species are subjected
to changed conditions, though of the same nature, it is notorious that they are
often affected very differently; therefore the male organs, for instance, in one
form of a heterostyled plant might be affected by those unknown causes which
induce abortion, differently from the homologous but functionally different
organs in the other form; and so conversely with the female organs. Thus the
great difficulty before alluded to is much lessened in understanding how any
cause whatever could lead to the simultaneous reduction and ultimate suppression
of the male organs in half the individuals of a species, and of the female
organs in the other half, whilst all were subjected to exactly the same
conditions of life.
That such reduction or suppression has occurred in some heterostyled plants is
almost certain. The Rubiaceae contain more heterostyled genera than any other
family, and from their wide distribution we may infer that many of them became
heterostyled at a remote period, so that there will have been ample time for
some of the species to have been since rendered dioecious.
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