Conversion of diclinous into hermaphrodite flowers.
Trees often have their sexes separated.
In the introductory chapter I briefly specified the various means by
which cross-fertilisation is favoured or ensured, namely, the separation
of the sexes,--the maturity of the male and female sexual elements at
different periods,--the heterostyled or dimorphic and trimorphic
condition of certain plants,--many mechanical contrivances,--the more or
less complete inefficiency of a flower's own pollen on the stigma,--and
the prepotency of pollen from any other individual over that from the
same plant. Some of these points require further consideration; but for
full details I must refer the reader to the several excellent works
mentioned in the introduction. I will in the first place give two lists:
the first, of plants which are either quite sterile or produce less than
about half the full complement of seeds, when insects are excluded; and
a second list of plants which, when thus treated, are fully fertile or
produce at least half the full complement of seeds. These lists have
been compiled from the several previous tables, with some additional
cases from my own observations and those of others. The species are
arranged nearly in the order followed by Lindley in his 'Vegetable
Kingdom.' The reader should observe that the sterility or fertility of
the plants in these two lists depends on two wholly distinct causes;
namely, the absence or presence of the proper means by which pollen is
applied to the stigma, and its less or greater efficiency when thus
applied.
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