The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom | Page 6

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. There are, however, some exceptions to the rule of plants
being constructed so as to allow of or to favour cross-fertilisation, for
some few plants seem to be invariably self-fertilised; yet even these
retain traces of having been formerly adapted for cross-fertilisation.
These exceptions need not make us doubt the truth of the above rule,
any more than the existence of some few plants which produce flowers,
and yet never set seed, should make us doubt that flowers are adapted
for the production of seed and the propagation of the species.
We should always keep in mind the obvious fact that the production of
seed is the chief end of the act of fertilisation; and that this end can be
gained by hermaphrodite plants with incomparably greater certainty by
self-fertilisation, than by the union of the sexual elements belonging to
two distinct flowers or plants. Yet it is as unmistakably plain that
innumerable flowers are adapted for cross-fertilisation, as that the teeth
and talons of a carnivorous animal are adapted for catching prey; or
that the plumes, wings, and hooks of a seed are adapted for its
dissemination. Flowers, therefore, are constructed so as to gain two
objects which are, to a certain extent, antagonistic, and this explains
many apparent anomalies in their structure. The close proximity of the
anthers to the stigma in a multitude of species favours, and often leads,
to self-fertilisation; but this end could have been gained far more safely
if the flowers had been completely closed, for then the pollen would
not have been injured by the rain or devoured by insects, as often
happens. Moreover, in this case, a very small quantity of pollen would
have been sufficient for fertilisation, instead of millions of grains being
produced. But the openness of the flower and the production of a great

and apparently wasteful amount of pollen are necessary for
cross-fertilisation. These remarks are well illustrated by the plants
called cleistogene, which bear on the same stock two kinds of flowers.
The flowers of the one kind are minute and completely closed, so that
they cannot possibly be crossed; but they are abundantly fertile,
although producing an extremely small quantity of pollen. The flowers
of the other kind produce much pollen and are open; and these can be,
and often are, cross-fertilised. Hermann Muller has also made the
remarkable discovery that there are some plants which exist under two
forms; that is, produce on distinct stocks two kinds of hermaphrodite
flowers. The one form bears small flowers constructed for
self-fertilisation; whilst the other bears larger and much more
conspicuous flowers plainly constructed for cross-fertilisation by the
aid of insects; and without their aid these produce no seed.
The adaptation of flowers for cross-fertilisation is a subject which has
interested me for the last thirty-seven years, and I have collected a large
mass of observations, but these are now rendered superfluous by the
many excellent works which have been lately published. In the year
1857 I wrote a short paper on the fertilisation of the kidney bean (1/1.
'Gardeners' Chronicle' 1857 page 725 and 1858 pages 824 and 844.
'Annals and Magazine of Natural History' 3rd series volume 2 1858
page 462.); and in 1862 my work 'On the Contrivances by which
British and Foreign Orchids are Fertilised by Insects' appeared. It
seemed to me a better plan to work out one group of plants as carefully
as I could, rather than to publish many miscellaneous and imperfect
observations. My present work is the complement of that on Orchids, in
which it was shown how admirably these plants are constructed so as to
permit of, or to favour, or to necessitate cross-fertilisation. The
adaptations for cross-fertilisation are perhaps more obvious in the
Orchideae than in any other group of plants, but it is an error to speak
of them, as some authors have done, as an exceptional case. The
lever-like action of the stamens of Salvia (described by Hildebrand, Dr.
W. Ogle, and others), by which the anthers are depressed and rubbed on
the backs of bees, shows as perfect a structure as can be found in any
orchid. Papilionaceous flowers, as described by various authors--for
instance, by Mr. T.H. Farrer--offer innumerable curious adaptations for

cross-fertilisation. The case of Posoqueria fragrans (one of the
Rubiaceae), is as wonderful as that of the most wonderful orchid. The
stamens, according to Fritz Muller, are irritable, so that as soon as a
moth visits a flower, the anthers explode and cover the insect with
pollen; one of the filaments which is broader than the others then
moves and closes the flower for about twelve hours,