CHAPTER VII.

SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED PLANTS.

Number of species and plants measured.--Tables given.--Preliminary remarks on the offspring of plants crossed by a fresh stock.--Thirteen cases specially considered.--The effects of crossing a self-fertilised plant either by another self-fertilised plant or by an intercrossed plant of the old stock.--Summary of the results.--Preliminary remarks on the crossed and self-fertilised plants of the same stock.--The twenty-six exceptional cases considered, in which the crossed plants did not exceed greatly in height the self-fertilised.--Most of these cases shown not to be real exceptions to the rule that cross-fertilisation is beneficial.--Summary of results.--Relative weights of the crossed and self-fertilised plants.

CHAPTER VIII.

DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS.

Greater constitutional vigour of crossed plants.--The effects of great crowding.--Competition with other kinds of plants.--Self-fertilised plants more liable to premature death.--Crossed plants generally flower before the self-fertilised.--Negative effects of intercrossing flowers on the same plant.--Cases described.--Transmission of the good effects of a cross to later generations.--Effects of crossing plants of closely related parentage.--Uniform colour of the flowers on plants self-fertilised during several generations and cultivated under similar conditions.

CHAPTER IX.

THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS.

Fertility of plants of crossed and self-fertilised parentage, both lots being fertilised in the same manner.--Fertility of the parent-plants when first crossed and self-fertilised, and of their crossed and self-fertilised offspring when again 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.

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The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 03

19th Century English Literature

Charles Darwin

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Charles Darwin

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