Marcgravia, in which the pedicels and
bracts of the terminal portion of a pendent bunch of flowers have been
modified into pitchers which secrete nectar and attract insects, while
birds feeding on the nectar, or insects, have the pollen of the
overhanging flowers dusted on their backs, and, carrying it to other
flowers, thus cross-fertilise them (see Illustration).

[Illustration: FIG. 31.--Humming-bird fertilising Marcgravia
nepenthoides.]

In Australia and New Zealand the fine "glory peas" (Clianthus), the
Sophora, Loranthus, many Epacrideae and Myrtaceae, and the large flowers
of the New Zealand flax (Phormium tenax), are cross-fertilised by
birds; while in Natal the fine trumpet-creeper (Tecoma capensis) is
fertilised by Nectarineas.

The great extent to which insect and bird agency is necessary to flowers
is well shown by the case of New Zealand. The entire country is
comparatively poor in species of insects, especially in bees and
butterflies which are the chief flower fertilisers; yet according to the
researches of local botanists no less than one-fourth of all the
flowering plants are incapable of self-fertilisation, and, therefore,
wholly dependent on insect or bird agency for the continuance of the
species.

The facts as to the cross-fertilisation of flowers which have now been
very briefly summarised, taken in connection with Darwin's experiments
proving the increased vigour and fertility given by cross-fertilisation,
seem amply to justify his aphorism that "Nature abhors
self-fertilisation," and his more precise statement, that, "No plant is
perpetually self-fertilised;" and this view has been upheld by
Hildebrand, Delpino, and other botanists.[150]

_Self-Fertilisation of Flowers._

But all this time we have been only looking at one side of the question,
for there exists an abundance of facts which seem to imply, just as
surely, the utter uselessness of cross-fertilisation. Let us, then, see
what these facts are before proceeding further.

1. An immense variety of plants are habitually self-fertilised, and
their numbers probably far exceed those which are habitually
cross-fertilised by insects. Almost all the very small or obscure
flowered plants with hermaphrodite flowers are of this kind. Most of
these, however, may be insect fertilised occasionally, and may,
therefore, come under the rule that no species are perpetually
self-fertilised.

2. There are many plants, however, in which special a