The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom | Page 5

crossed and self-fertilised.--Comparison of the
fertility of flowers fertilised with their own pollen and with that from
other flowers on the same plant.--Self-sterile plants.--Causes of
self-sterility.--The appearance of highly self-fertile
varieties.--Self-fertilisation apparently in some respects beneficial,
independently of the assured production of seeds.--Relative weights
and rates of germination of seeds from crossed and self-fertilised
flowers.

CHAPTER X.
MEANS OF FERTILISATION.
Sterility and fertility of plants when insects are excluded.--The means
by which flowers are cross-fertilised.--Structures favourable to
self-fertilisation.--Relation between the structure and conspicuousness
of flowers, the visits of insects, and the advantages of
cross-fertilisation.--The means by which flowers are fertilised with
pollen from a distinct plant.--Greater fertilising power of such
pollen.--Anemophilous species.--Conversion of anemophilous species
into entomophilous.--Origin of nectar.--Anemophilous plants generally
have their sexes separated.--Conversion of diclinous into
hermaphrodite flowers.--Trees often have their sexes separated.

CHAPTER XI.
THE HABITS OF INSECTS IN RELATION TO THE
FERTILISATION OF FLOWERS.
Insects visit the flowers of the same species as long as they can.--Cause
of this habit.--Means by which bees recognise the flowers of the same
species.--Sudden secretion of nectar.--Nectar of certain flowers
unattractive to certain insects.--Industry of bees, and the number of

flowers visited within a short time.--Perforation of the corolla by
bees.--Skill shown in the operation.--Hive-bees profit by the holes
made by humble-bees.--Effects of habit.--The motive for perforating
flowers to save time.--Flowers growing in crowded masses chiefly
perforated.

CHAPTER XII.
GENERAL RESULTS.
Cross-fertilisation proved to be beneficial, and self-fertilisation
injurious.--Allied species differ greatly in the means by which
cross-fertilisation is favoured and self-fertilisation avoided.--The
benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.--The evil effects not due to the
combination of morbid tendencies in the parents.--Nature of the
conditions to which plants are subjected when growing near together in
a state of nature or under culture, and the effects of such
conditions.--Theoretical considerations with respect to the interaction
of differentiated sexual elements.--Practical lessons.--Genesis of the
two sexes.--Close correspondence between the effects of
cross-fertilisation and self-fertilisation, and of the legitimate and
illegitimate unions of heterostyled plants, in comparison with hybrid
unions.
INDEX.
...
THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE
VEGETABLE KINGDOM.

CHAPTER I.

INTRODUCTORY REMARKS.
Various means which favour or determine the cross-fertilisation of
plants. Benefits derived from cross-fertilisation. Self-fertilisation
favourable to the propagation of the species. Brief history of the subject.
Object of the experiments, and the manner in which they were tried.
Statistical value of the measurements. The experiments carried on
during several successive generations. Nature of the relationship of the
plants in the later generations. Uniformity of the conditions to which
the plants were subjected. Some apparent and some real causes of error.
Amount of pollen employed. Arrangement of the work. Importance of
the conclusions.
There is weighty and abundant evidence that the flowers of most kinds
of plants are constructed so as to be occasionally or habitually
cross-fertilised by pollen from another flower, produced either by the
same plant, or generally, as we shall hereafter see reason to believe, by
a distinct plant. Cross-fertilisation is sometimes ensured by the sexes
being separated, and in a large number of cases by the pollen and
stigma of the same flower being matured at different times. Such plants
are called dichogamous, and have been divided into two sub-classes:
proterandrous species, in which the pollen is mature before the stigma,
and proterogynous species, in which the reverse occurs; this latter form
of dichogamy not being nearly so common as the other.
Cross-fertilisation is also ensured, in many cases, by mechanical
contrivances of wonderful beauty, preventing the impregnation of the
flowers by their own pollen. There is a small class of plants, which I
have called dimorphic and trimorphic, but to which Hildebrand has
given the more appropriate name of heterostyled; this class consists of
plants presenting two or three distinct forms, adapted for reciprocal
fertilisation, so that, like plants with separate sexes, they can hardly fail
to be intercrossed in each generation. The male and female organs of
some flowers are irritable, and the insects which touch them get dusted
with pollen, which is thus transported to other flowers. Again, there is a
class, in which the ovules absolutely refuse to be fertilised by pollen
from the same plant, but can be fertilised by pollen from any other
individual of the same species. There are also very many species which

are partially sterile with their own pollen. Lastly, there is a large class
in which the flowers present no apparent obstacle of any kind to
self-fertilisation, nevertheless these plants are frequently intercrossed,
owing to the prepotency of pollen from another individual or variety
over the plant's own pollen.
As plants are adapted by such diversified and effective means for
cross-fertilisation, it might have been inferred from this fact alone that
they