CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
Seeds from the crossed plants of the last generation (Table 2/2) again crossed, and from the self-fertilised plants again self-fertilised, were treated in all respects exactly as before, with the following result:—
TABLE 2/3. Ipomoea purpurea (Third Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 74 : 56 4/8. Pot 1 : 72 : 51 4/8. Pot 1 : 73 4/8 : 54.
Pot 2 : 82 : 59. Pot 2 : 81 : 30. Pot 2 : 82 : 66.
Total : 464.5 : 317.
Again all the crossed plants are higher than their antagonists: their average height is 77.41 inches, whereas that of the self-fertilised is 52.83 inches, or as 100 to 68.
I attended closely to the fertility of the plants of this third generation. Thirty flowers on the crossed plants were crossed with pollen from other crossed plants of the same generation, and the twenty-six capsules thus produced contained, on an average, 4.73 seeds; whilst thirty flowers on the self-fertilised plants, fertilised with the pollen from the same flower, produced twenty-three capsules, each containing 4.43 seeds. Thus the average number of seeds in the crossed capsules was to that in the self-fertilised capsules as 100 to 94. A hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of the self-fertilised seeds weighed only 37.63 grains. Many of these lighter self-fertilised seeds placed on damp sand germinated before the crossed; thus thirty-six of the former germinated whilst only thirteen of the latter or crossed seeds germinated. In Pot 1 the three crossed plants produced spontaneously under the net (besides the twenty-six artificially cross-fertilised capsules) seventy-seven self-fertilised capsules containing on an average 4.41 seeds; whilst the three self-fertilised plants produced spontaneously (besides the twenty-three artificially self-fertilised capsules) only twenty-nine self-fertilised capsules, containing on an average 4.14 seeds. Therefore the average number of seeds in the two lots of spontaneously self-fertilised capsules was as 100 to 94. Taking into consideration the number of capsules together with the average number of seeds, the crossed plants (spontaneously self-fertilised) produced seeds in comparison with the self-fertilised plants (spontaneously self-fertilised) in the proportion of 100 to 35. By whatever method the fertility of these plants is compared, the crossed are more fertile than the self-fertilised plants.
I tried in several ways the comparative vigour and powers of growth of the crossed and self-fertilised plants of this third generation. Thus, four self-fertilised seeds which had just germinated were planted on one side of a pot, and after an interval of forty-eight hours, four crossed seeds in the same state of germination were planted on the opposite side; and the pot was kept in the hothouse. I thought that the advantage thus given to the self-fertilised seedlings would have been so great that they would never have been beaten by the crossed ones. They were not beaten until all had grown to a height of 18 inches; and the degree to which they were finally beaten is shown in Table 2/4. We here see that the average height of the four crossed plants is 76.62, and of the four self-fertilised plants 65.87 inches, or as 100 to 86; therefore less than when both sides started fair.
TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised plants having had a start of forty-eight hours).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 3 : 78 4/8 : 73 4/8. Pot 3 : 77 4/8 : 53. Pot 3 : 73 : 61 4/8. Pot 3 : 77 4/8 : 75 4/8.
Total : 306.5 : 263.5.
Crossed and self-fertilised seeds of the third generation were also sown out of doors late in the summer, and therefore under unfavourable conditions, and a single stick was given to each lot of plants to twine up. The two lots were sufficiently separate so as not to interfere with each other’s growth, and the ground was clear of weeds. As soon as they were killed by the first frost (and there was no difference in their hardiness), the two tallest crossed plants were found to be 24.5 and 22.5 inches, whilst the two tallest self-fertilised plants were only 15 and 12.5 inches in height, or as 100 to 59.
I likewise sowed at the same time two lots of the same seeds in a part of the garden which was shady and covered with weeds. The crossed seedlings from the first looked the most healthy, but they twined up a stick only to a height of 7 1/4 inches; whilst the self-fertilised were not able to twine at all; and the tallest of them was only 3 1/2 inches in height.
Lastly, two lots of the same seeds were sown in the midst of a bed of candy-tuft (Iberis) growing vigorously. The seedlings came up, but all the self-fertilised ones soon died excepting one, which never twined and grew to a height of only 4 inches. Many of the crossed seedlings, on the other hand, survived; and some twined up the stems of the Iberis to the height of 11 inches. These cases prove that the crossed seedlings have an immense advantage over the self-fertilised, both when growing isolated under very unfavourable conditions, and when put into competition with each other or with other plants, as would happen in a state of nature.