But we should always keep in mind that two somewhat opposed ends have to
be gained; the first and more important one being the production of
seeds by any means, and the second, cross-fertilisation.
The advantages derived from cross-fertilisation throw a flood of light
on most of the chief characters of flowers. We can thus understand their
large size and bright colours, and in some cases the bright tints of the
adjoining parts, such as the peduncles, bracteae, etc. By this means
they are rendered conspicuous to insects, on the same principle that
almost every fruit which is devoured by birds presents a strong contrast
in colour with the green foliage, in order that it may be seen, and its
seeds freely disseminated. With some flowers conspicuousness is gained
at the expense even of the reproductive organs, as with the ray-florets
of many Compositae, the exterior flowers of Hydrangea, and the terminal
flowers of the Feather-hyacinth or Muscari. There is also reason to
believe, and this was the opinion of Sprengel, that flowers differ in
colour in accordance with the kinds of insects which frequent them.
Not only do the bright colours of flowers serve to attract insects, but
dark-coloured streaks and marks are often present, which Sprengel long
ago maintained served as guides to the nectary. These marks follow the
veins in the petals, or lie between them. They may occur on only one, or
on all excepting one or more of the upper or lower petals; or they may
form a dark ring round the tubular part of the corolla, or be confined
to the lips of an irregular flower.
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