h the cross-fertilisation of
flowers is brought about, we find that some are comparatively simple in
their operation and needful adjustments, others highly complex. The
simple methods belong to four principal classes:--(1) By dichogamy--that
is, by the anthers and the stigma becoming mature or in a fit state for
fertilisation at slightly different times on the same plant. The result
of this is that, as plants in different stations, on different soils, or
exposed to different aspects flower earlier or later, the mature pollen
of one plant can only fertilise some plant exposed to somewhat different
conditions or of different constitution, whose stigma will be mature at
the same time; and this difference has been shown by Darwin to be that
which is adapted to secure the fullest benefit of cross-fertilisation.
This occurs in Geranium pratense, Thymus serpyllum, Arum maculatum, and
many others. (2) By the flower being self-sterile with its own pollen,
as in the crimson flax. This absolutely prevents self-fertilisation. (3)
By the stamens and anthers being so placed that the pollen cannot fall
upon the stigma, while it does fall upon a visiting insect which carries
it to the stigma of another flower. This effect is produced in a variety
of very simple ways, and is often aided by the motion of the stamens
which bend down out of the way of the stigmas before the pollen is ripe,
as in Malva sylvestris (see Fig. 28). (4) By the male and female flowers
being on different plants, forming the class Dioecia of Linnaeus. In
these cases the pollen may be carried to the stigmas either by the wind
or by the agency of insects.

[Illustration: FIG. 28.

Malva sylvestris, adapted for insect-fertilisation.

Malva rotundifolia, adapted for self-fertilisation.]

Now these four methods are all apparently very simple, and easily
produced by variation and selection. They are applicable to flowers of
any shape, requiring only such size and colour as to attract insects,
and some secretion of nectar to ensure their repeated visits, characters
common to the great majority of flowers. All these methods are common,
except perhaps the second; but there are many flowers in which the
pollen from another plant is prepotent over the pollen from
fertilisation, the same flower, and this has nearly the same effect as
self-sterility if the flowers are frequently crossed by insects. We
cannot help asking, therefore, why have other and much more elaborate
met