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

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.
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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 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