The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom | Page 9

that I have spoken for the sake of brevity, and
shall continue to do so, of crossed and self-fertilised seeds, seedlings,
or plants; these terms implying that they are the product of crossed or
self-fertilised flowers. Cross-fertilisation always means a cross between
distinct plants which were raised from seeds and not from cuttings or
buds. Self-fertilisation always implies that the flowers in question were
impregnated with their own pollen.
My experiments were tried in the following manner. A single plant, if it
produced a sufficiency of flowers, or two or three plants were placed
under a net stretched on a frame, and large enough to cover the plant
(together with the pot, when one was used) without touching it. This
latter point is important, for if the flowers touch the net they may be
cross-fertilised by bees, as I have known to happen; and when the net is
wet the pollen may be injured. I used at first "white cotton net," with
very fine meshes, but afterwards a kind of net with meshes one-tenth of
an inch in diameter; and this I found by experience effectually excluded
all insects excepting Thrips, which no net will exclude. On the plants
thus protected several flowers were marked, and were fertilised with
their own pollen; and an equal number on the same plants, marked in a
different manner, were at the same time crossed with pollen from a
distinct plant. The crossed flowers were never castrated, in order to
make the experiments as like as possible to what occurs under nature
with plants fertilised by the aid of insects. Therefore, some of the
flowers which were crossed may have failed to be thus fertilised, and
afterwards have been self-fertilised. But this and some other sources of
error will presently be discussed. In some few cases of spontaneously

self-fertile species, the flowers were allowed to fertilise themselves
under the net; and in still fewer cases uncovered plants were allowed to
be freely crossed by the insects which incessantly visited them. There
are some great advantages and some disadvantages in my having
occasionally varied my method of proceeding; but when there was any
difference in the treatment, it is always so stated under the head of each
species.
Care was taken that the seeds were thoroughly ripened before being
gathered. Afterwards the crossed and self-fertilised seeds were in most
cases placed on damp sand on opposite sides of a glass tumbler covered
by a glass plate, with a partition between the two lots; and the glass was
placed on the chimney-piece in a warm room. I could thus observe the
germination of the seeds. Sometimes a few would germinate on one
side before any on the other, and these were thrown away. But as often
as a pair germinated at the same time, they were planted on opposite
sides of a pot, with a superficial partition between the two; and I thus
proceeded until from half-a-dozen to a score or more seedlings of
exactly the same age were planted on the opposite sides of several pots.
If one of the young seedlings became sickly or was in any way injured,
it was pulled up and thrown away, as well as its antagonist on the
opposite side of the same pot.
As a large number of seeds were placed on the sand to germinate, many
remained after the pairs had been selected, some of which were in a
state of germination and others not so; and these were sown crowded
together on the opposite sides of one or two rather larger pots, or
sometimes in two long rows out of doors. In these cases there was the
most severe struggle for life among the crossed seedlings on one side of
the pot, and the self-fertilised seedlings on the other side, and between
the two lots which grew in competition in the same pot. A vast number
soon perished, and the tallest of the survivors on both sides when fully
grown were measured. Plants treated in this manner, were subjected to
nearly the same conditions as those growing in a state of nature, which
have to struggle to maturity in the midst of a host of competitors.
On other occasions, from the want of time, the seeds, instead of being

allowed to germinate on damp sand, were sown on the opposite sides of
pots, and the fully grown plants measured. But this plan is less accurate,
as the seeds sometimes germinated more quickly on one side than on
the other. It was however necessary to act in this manner with some
few species, as certain kinds of seeds would not germinate well when
exposed to the light; though the glasses containing them were kept on
the chimney-piece on one side of a room, and some way from the two