[126]

Heliotropism. Tropæolum majus Sept. 6th

Figure B

[127]

[127v]

5

Introduction

female organs are adapted for the production of seed & the propagation of the species. We should always keep bear in mind the obvious fact that the production of seed is the chief end of the act of fertilisation; and that flowers are constructed for this purpose which 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 the adaptations for cross-fertilisation, are as manifest as that the teeth and talons of a carnivorous animal serve are adapted for catching prey, or that the plumes, wings and hooks of a seed serve 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 stamens to the stigma as in the case of a multitude of species, favours and often

[Cross and self fertilisation, p. 3: "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."]

[128]

N.E. window Tropæolum Fig 207

Sept. 7th

Heliotropism towards very obscure light

Window Tracing D.

Tropaeolum majus

Sept. 7th

(Window)

(1/2 scale only 4 10 sets of lettering all marked by Blue lines.

4 darkened

(I do not know when the 9°38' had better be engraved or inserted)

[128v]

17

Introduction

luteus and Ipomoea purpurea, both of which, unlike the Linaria and Dianthus, are highly self-fertile if insects are excluded. Some flowers on a single plant of both species were fertilised with their own pollen, and others on the same plants were crossed with pollen from a distinct individual; both the plants of course being protected by a net from insects.

The crossed and self-fertilised seeds were sown on opposite sides of the same pots, and treated in all respects alike: and the plants when grown up were measured and compared. With both species, as in the cases of the Linaria and Dianthus, the crossed seedlings were conspicuously superior in height and in other ways to the self-fertilised. I therefore determined to begin a long series of experiments with various plants, which & then were continued for the ensuing following eleven years; and we shall see that contrary to what might have been anticipated in a large majority of the cases the crossed plants beat the self-fertilised

[Cross and self fertilisation, p. 3: "Mimulus luteus and Ipomoea purpurea, both of which, unlike the Linaria and Dianthus, are highly self-fertile if insects are excluded. Some flowers on a single plant of both species were fertilised with their own pollen, and others were crossed with pollen from a distinct individual; both plants being protected by a net from insects. The crossed and self-fertilised seeds thus produced were sown on opposite sides of the same pots, and treated in all respects alike; and the plants when fully grown were measured and compared. With both species, as in the cases of the Linaria and Dianthus, the crossed seedlings were conspicuously superior in height and in other ways to the self-fertilised. I therefore determined to begin a long series of experiments with various plants, and these were continued for the following eleven years; and we shall see that in a large majority of cases the crossed beat the self-fertilised plants. Several of the exceptional cases, moreover, in which the crossed plants were not victorious, can be explained."]