It is
therefore probable that the two forms in this sub-class have been separated or
developed for this special end.
Various hermaphrodite plants have become heterostyled, and now exist under two
or three forms; and we may confidently believe that this has been effected in
order that cross-fertilisation should be assured. For the full and legitimate
fertilisation of these plants pollen from the one form must be applied to the
stigma of another. If the sexual elements belonging to the same form are united
the union is an illegitimate one and more or less sterile. With dimorphic
species two illegitimate unions, and with trimorphic species twelve are
possible. There is reason to believe that the sterility of these unions has not
been specially acquired, but follows as an incidental result from the sexual
elements of the two or three forms having been adapted to act on one another in
a particular manner, so that any other kind of union is inefficient, like that
between distinct species. Another and still more remarkable incidental result is
that the seedlings from an illegitimate union are often dwarfed and more or less
or completely barren, like hybrids from the union of two widely distinct
species.
INDEX.
Acanthaceae.
Acer campestre.
Adoxa.
Aegiphila elata.
--mollis.
--obdurata.
Alefeld, Dr.
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