In all the foregoing experiments, seedlings from flowers crossed by pollen from a distinct plant (though in the later generations more or less closely related) were put into competition with, and almost invariably proved markedly superior in height to the offspring from self-fertilised flowers. I wished, therefore, to ascertain whether a cross between two flowers on the same plant would give to the offspring any superiority over the offspring from flowers fertilised with their own pollen. I procured some fresh seed and raised two plants, which were covered with a net; and several of their flowers were crossed with pollen from a distinct flower on the same plant. Twenty-nine capsules thus produced contained on an average 4.86 seeds per capsule; and 100 of these seeds weighed 36.77 grains. Several other flowers were fertilised with their own pollen, and twenty-six capsules thus produced contained on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains. So that a cross of this kind appears to have increased slightly the number of seeds per capsule, in the ratio of 100 to 91; but these crossed seeds were lighter than the self-fertilised in the ratio of 86 to 100. I doubt, however, from other observations, whether these results are fully trustworthy. The two lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of nine pots, and were treated in every respect like the plants in the previous experiments. The remaining seeds, some in a state of germination and some not so, were sown on the opposite sides of a large pot (Number 10); and the four tallest plants on each side of this pot were measured. The result is shown in Table 2/12.

TABLE 2/12. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 82 : 77 4/8. Pot 1 : 75 : 87. Pot 1 : 65 : 64. Pot 1 : 76 : 87 2/8.

Pot 2 : 78 4/8 : 84. Pot 2 : 43 : 86 4/8. Pot 2 : 65 4/8 : 90 4/8.

Pot 3 : 61 2/8 : 86. Pot 3 : 85 : 69 4/8. Pot 3 : 89 : 87 4/8.

Pot 4 : 83 : 80 4/8. Pot 4 : 73 4/8 : 88 4/8. Pot 4 : 67 : 84 4/8.

Pot 5 : 78 : 66 4/8. Pot 5 : 76 6/8 : 77 4/8. Pot 5 : 57 : 81 4/8.

Pot 6 : 70 4/8 : 80. Pot 6 : 79 : 82 4/8. Pot 6 : 79 6/8 : 55 4/8.

Pot 7 : 76 : 77. Pot 7 : 84 4/8 : 83 4/8. Pot 7 : 79 : 73 4/8.

Pot 8 : 73 : 76 4/8. Pot 8 : 67 : 82. Pot 8 : 83 : 80 4/8.

Pot 9 : 73 2/8 : 78 4/8. Pot 9 : 78 : 67 4/8.

Pot 10 : 34 : 82 4/8. Pot 10 : 82 : 36 6/8. Pot 10 : 84 6/8 : 69 4/8. Pot 10 : 71 : 75 2/8. Crowded plants.

Total : 2270.25 : 2399.75.

The average height of the thirty-one crossed plants is 73.23 inches, and that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to 106. Looking to each pair, it may be seen that only thirteen of the crossed plants, whilst eighteen of the self-fertilised plants exceed their opponents. A record was kept with respect to the plant which flowered first in each pot; and only two of the crossed flowered before one of the self-fertilised in the same pot; whilst eight of the self-fertilised flowered first. It thus appears that the crossed plants are slightly inferior in height and in earliness of flowering to the self-fertilised. But the inferiority in height is so small, namely as 100 to 106, that I should have felt very doubtful on this head, had I not cut down all the plants (except those in the crowded pot Number 10) close to the ground and weighed them. The twenty-seven crossed plants weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20 1/2 ounces; and this gives a ratio of 100 to 124.

A self-fertilised plant of the same parentage as those in Table 2/12 had been raised in a separate pot for a distinct purpose; and it proved partially sterile, the anthers containing very little pollen.

The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 23

19th Century English Literature

Charles Darwin

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Charles Darwin

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