Dioecious plants, however fertilised, have a great advantage over other
plants in their cross-fertilisation being assured. But this advantage is
gained in the case of anemophilous species at the expense of the
production of an enormous superfluity of pollen, with some risk to them
and to entomophilous species of their fertilisation occasionally
failing. Half the individuals, moreover, namely, the males, produce no
seed, and this might possibly be a disadvantage. Delpino remarks that
dioecious plants cannot spread so easily as monoecious and hermaphrodite
species, for a single individual which happened to reach some new site
could not propagate its kind; but it may be doubted whether this is a
serious evil. Monoecious plants can hardly fail to be to a large extent
dioecious in function, owing to the lightness of their pollen and to the
wind blowing laterally, with the great additional advantage of
occasionally or often producing some self-fertilised seeds. When they
are also dichogamous, they are necessarily dioecious in function.
Lastly, hermaphrodite plants can generally produce at least some
self-fertilised seeds, and they are at the same time capable, through
the various means specified in this chapter, of cross-fertilisation.
When their structure absolutely prevents self-fertilisation, they are in
the same relative position to one another as monoecious and dioecious
plants, with what may be an advantage, namely, that every flower is
capable of yielding seeds.
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