In Pot 1, and it should be remembered that very large pots had been used, the oval clump of crossed plants was 10 by 4 1/2 inches across, with the tallest stem, as yet young, 5 1/2 inches in height; whilst the clump of self-fertilised plants, on the opposite side of the same pot, was only 3 1/2 by 2 1/2 inches across, with the tallest young stem 4 inches in height. In Pot 2, the clump of crossed plants was 18 by 9 inches across, with the tallest young stem 8 1/2 inches in height; whilst the clump of self-fertilised plants on the opposite side of the same pot was 12 by 4 1/2 inches across, with the tallest young stem 6 inches in height. The crossed plants during this season, as during the last, flowered first. Both the crossed and self-fertilised plants being left freely exposed to the visits of bees, manifestly produced much more seed than their grand-parents,--the plants of the original clump still growing close by in the same garden, and equally left to the action of the bees.

5. ACANTHACEAE.--Thunbergia alata.

It appears from Hildebrand's description ('Botanische Zeitung' 1867 page 285) that the conspicuous flowers of this plant are adapted for cross-fertilisation. Seedlings were twice raised from purchased seed; but during the early summer, when first experimented on, they were extremely sterile, many of the anthers containing hardly any pollen. Nevertheless, during the autumn these same plants spontaneously produced a good many seeds. Twenty-six flowers during the two years were crossed with pollen from a distinct plant, but they yielded only eleven capsules; and these contained very few seeds! Twenty-eight flowers were fertilised with pollen from the same flower, and these yielded only ten capsules, which, however, contained rather more seed than the crossed capsules. Eight pairs of germinating seeds were planted on opposite sides of five pots; and exactly half the crossed and half the self-fertilised plants exceeded their opponents in height. Two of the self-fertilised plants died young, before they were measured, and their crossed opponents were thrown away. The six remaining pairs of these grew very unequally, some, both of the crossed and self-fertilised plants, being more than twice as tall as the others. The average height of the crossed plants was 60 inches, and that of the self-fertilised plants 65 inches, or as 100 to 108. A cross, therefore, between distinct individuals here appears to do no good; but this result deduced from so few plants in a very sterile condition and growing very unequally, obviously cannot be trusted.]



Brassica oleracea, crossed and self-fertilised plants. Great effect of a cross with a fresh stock on the weight of the offspring. Iberis umbellata. Papaver vagum. Eschscholtzia californica, seedlings from a cross with a fresh stock not more vigorous, but more fertile than the self-fertilised seedlings. Reseda lutea and odorata, many individuals sterile with their own pollen. Viola tricolor, wonderful effects of a cross. Adonis aestivalis. Delphinium consolida. Viscaria oculata, crossed plants hardly taller, but more fertile than the self-fertilised. Dianthus caryophyllus, crossed and self-fertilised plants compared for four generations. Great effects of a cross with a fresh stock. Uniform colour of the flowers on the self-fertilised plants. Hibiscus africanus.

[6. CRUCIFERAE.--Brassica oleracea.


The flowers of the common cabbage are adapted, as shown by H. Muller, for cross-fertilisation, and should this fail, for self-fertilisation. (4/1. 'Die Befruchtung' etc. page 139.) It is well known that the varieties are crossed so largely by insects, that it is impossible to raise pure kinds in the same garden, if more than one kind is in flower at the same time. Cabbages, in one respect, were not well fitted for my experiments, as, after they had formed heads, they were often difficult to measure. The flower-stems also differ much in height; and a poor plant will sometimes throw up a higher stem than that of a fine plant. In the later experiments, the fully-grown plants were cut down and weighed, and then the immense advantage from a cross became manifest.

Charles Darwin

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