The five crossed plants in Pots 1 and 2 were covered with a net, and produced 121 capsules; the five self-fertilised plants produced eighty-four capsules, so that the numbers of capsules were as 100 to 69. Of the 121 capsules on the crossed plants sixty-five were the product of flowers crossed with pollen from a distinct plant, and these contained on an average 5.23 seeds per capsule; the remaining fifty-six capsules were spontaneously self-fertilised. Of the eighty-four capsules on the self-fertilised plants, all the product of renewed self-fertilisation, fifty-five (which were alone examined) contained on an average 4.85 seeds per capsule. Therefore the cross-fertilised capsules, compared with the self-fertilised capsules, yielded seeds in the proportion of 100 to 93. The crossed seeds were relatively heavier than the self-fertilised seeds. Combining the above data (i.e., number of capsules and average number of contained seeds), the crossed plants, compared with the self-fertilised, yielded seeds in the ratio of 100 to 64.

These crossed plants produced, as already stated, fifty-six spontaneously self-fertilised capsules, and the self-fertilised plants produced twenty-nine such capsules. The former contained on an average, in comparison with the latter, seeds in the proportion of 100 to 99.

In Pot 3, on the opposite sides of which a large number of crossed and self-fertilised seeds had been sown and the seedlings allowed to struggle together, the crossed plants had at first no great advantage. At one time the tallest crossed was 25 1/8 inches high, and the tallest self-fertilised plants 21 3/8. But the difference afterwards became much greater. The plants on both sides, from being so crowded, were poor specimens. The flowers were allowed to fertilise themselves spontaneously under a net; the crossed plants produced thirty-seven capsules, the self-fertilised plants only eighteen, or as 100 to 47. The former contained on an average 3.62 seeds per capsule; and the latter 3.38 seeds, or as 100 to 93. Combining these data (i.e., number of capsules and average number of seeds), the crowded crossed plants produced seeds compared with the self-fertilised as 100 to 45. These latter seeds, however, were decidedly heavier, a hundred weighing 41.64 grains, than those from the capsules on the crossed plants, of which a hundred weighed 36.79 grains; and this probably was due to the fewer capsules borne by the self-fertilised plants having been better nourished. We thus see that the crossed plants in this the first generation, when grown under favourable conditions, and when grown under unfavourable conditions from being much crowded, greatly exceeded in height, and in the number of capsules produced, and slightly in the number of seeds per capsule, the self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Flowers on the crossed plants of the last generation (Table 2/1) were crossed by pollen from distinct plants of the same generation; and flowers on the self-fertilised plants were fertilised by pollen from the same flower. The seeds thus produced were treated in every respect as before, and we have in Table 2/2 the result.

TABLE 2/2. Ipomoea purpurea (Second Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 87 : 67 4/8. Pot 1 : 83 : 68 4/8. Pot 1 : 83 : 80 4/8.

Pot 2 : 85 4/8 : 61 4/8. Pot 2 : 89 : 79. Pot 2 : 77 4/8 : 41.

Total : 505 : 398.

Here again every single crossed plant is taller than its antagonist. The self-fertilised plant in Pot 1, which ultimately reached the unusual height of 80 4/8 inches, was for a long time taller than the opposed crossed plant, though at last beaten by it. The average height of the six crossed plants is 84.16 inches, whilst that of the six self-fertilised plants is 66.33 inches, or as 100 to 79.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 17

19th Century English Literature

Charles Darwin

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Charles Darwin

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