About the origin of such plants nothing is known. It is possible that they may
be descended from ancient lowly organised forms, which had from the first their
sexes separated; so that they have never existed as hermaphrodites. There are,
however, many other groups of species and single ones, which from being allied
on all sides to hermaphrodites, and from exhibiting in the female flowers plain
rudiments of male organs, and conversely in the male flowers rudiments of female
organs, we may feel sure are descended from plants which formerly had the two
sexes combined in the same flower. It is a curious and obscure problem how and
why such hermaphrodites have been rendered bisexual.
If in some individuals of a species the stamens alone were to abort, females and
hermaphrodites would be left existing, of which many instances occur; and if the
female organs of the hermaphrodite were afterwards to abort, the result would be
a dioecious plant. Conversely, if we imagine the female organs alone to abort in
some individuals, males and hermaphrodites would be left; and the hermaphrodites
might afterwards be converted into females.
In other cases, as in that of the common Ash-tree mentioned in the Introduction,
the stamens are rudimentary in some individuals, the pistils in others, others
again remaining as hermaphrodites.
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