Cross-fertilisation is also ensured, in many cases, by mechanical
contrivances of wonderful beauty, preventing the impregnation of the
flowers by their own pollen. There is a small class of plants, which I
have called dimorphic and trimorphic, but to which Hildebrand has given
the more appropriate name of heterostyled; this class consists of plants
presenting two or three distinct forms, adapted for reciprocal
fertilisation, so that, like plants with separate sexes, they can hardly
fail to be intercrossed in each generation. The male and female organs
of some flowers are irritable, and the insects which touch them get
dusted with pollen, which is thus transported to other flowers. Again,
there is a class, in which the ovules absolutely refuse to be fertilised
by pollen from the same plant, but can be fertilised by pollen from any
other individual of the same species. There are also very many species
which are partially sterile with their own pollen. Lastly, there is a
large class in which the flowers present no apparent obstacle of any
kind to self-fertilisation, nevertheless these plants are frequently
intercrossed, owing to the prepotency of pollen from another individual
or variety over the plant's own pollen.
As plants are adapted by such diversified and effective means for
cross-fertilisation, it might have been inferred from this fact alone
that they derived some great advantage from the process; and it is the
object of the present work to show the nature and importance of the
benefits thus derived.
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