Some flowers on the crossed plants were crossed with pollen from another plant, and the capsules thus produced contained a rather greater weight of seed than those on the self-fertilised plants again self-fertilised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Seeds from the foregoing plants, fertilised in the manner just stated, were sown on the opposite sides of a small pot (1) and came up crowded. The four tallest crossed seedlings, at the time of flowering, averaged 8 inches in height, whilst the four tallest self-fertilised plants averaged only 4 inches. Crossed seeds were sown by themselves in a second small pot, and self-fertilised seeds were sown by themselves in a third small pot so that there was no competition whatever between these two lots. Nevertheless the crossed plants grew from 1 to 2 inches higher on an average than the self-fertilised. Both lots looked equally vigorous, but the crossed plants flowered earlier and more profusely than the self-fertilised. In Pot 1, in which the two lots competed with each other, the crossed plants flowered first and produced a large number of capsules, whilst the self-fertilised produced only nineteen. The contents of twelve capsules from the crossed flowers on the crossed plants, and of twelve capsules from self-fertilised flowers on the self-fertilised plants, were placed in separate watch-glasses for comparison; and the crossed seeds seemed more numerous by half than the self-fertilised.

The plants on both sides of Pot 1, after they had seeded, were cut down and transplanted into a large pot with plenty of good earth, and on the following spring, when they had grown to a height of between 5 and 6 inches, the two lots were equal, as occurred in a similar experiment in the last generation. But after some weeks the crossed plants exceeded the self-fertilised ones on the opposite side of the same pot, though not nearly to so great a degree as before, when they were subjected to very severe competition.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

Crossed seeds from the crossed plants, and self-fertilised seeds from the self-fertilised plants of the last generation, were sown thickly on opposite sides of a small pot, Number 1. The two tallest plants on each side were measured after they had flowered, and the two crossed ones were 12 and 7 1/2 inches, and the two self-fertilised ones 8 and 5 1/2 inches in height; that is, in the ratio of 100 to 69. Twenty flowers on the crossed plants were again crossed and produced twenty capsules; ten of which contained 1.33 grain weight of seeds. Thirty flowers on the self-fertilised plants were again self-fertilised and produced twenty-six capsules; ten of the best of which (many being very poor) contained only .87 grain weight of seeds; that is, in the ratio of 100 to 65 by weight.

The superiority of the crossed over the self-fertilised plants was proved in various ways. Self-fertilised seeds were sown on one side of a pot, and two days afterwards crossed seeds on the opposite side. The two lots of seedlings were equal until they were above half an inch high; but when fully grown the two tallest crossed plants attained a height of 12 1/2 and 8 3/4 inches, whilst the two tallest self-fertilised plants were only 8 and 5 1/2 inches high.

In a third pot, crossed seeds were sown four days after the self-fertilised, and the seedlings from the latter had at first, as might have been expected, an advantage; but when the two lots were between 5 and 6 inches in height, they were equal, and ultimately the three tallest crossed plants were 11, 10, and 8 inches, whilst the three tallest self-fertilised were 12, 8 1/2, and 7 1/2 inches in height. So that there was not much difference between them, the crossed plants having an average advantage of only the third of an inch. The plants were cut down, and without being disturbed were transplanted into a larger pot. Thus the two lots started fair on the following spring, and now the crossed plants showed their inherent superiority, for the two tallest were 13 inches, whilst the two tallest self-fertilised plants were only 11 and 8 1/2 inches in height; or as 100 to 75.

The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 33

19th Century English Literature

Charles Darwin

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Charles Darwin

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