The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 3

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 compet.i.tion with each other or with other plants, as would happen in a state of nature.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.

Seedlings raised as before from the crossed and self-fertilised plants of the third generation in Table 2/3, gave results as follows:--

TABLE 2/5. Ipomoea purpurea (Fourth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 84 : 80.

Pot 1 : 47 : 44 1/2.

Pot 2 : 83 : 73 1/2.

Pot 2 : 59 : 51 1/2.

Pot 3 : 82 : 56 1/2.

Pot 3 : 65 1/2 : 63.

Pot 3 : 68 : 52.

Total : 488.5 : 421.0.

Here the average height of the seven crossed plants is 69.78 inches, and that of the seven self-fertilised plants 60.14; or as 100 to 86. This smaller difference relatively to that in the former generations, may be attributed to the plants having been raised during the depth of winter, and consequently to their not having grown vigorously, as was shown by their general appearance and from several of them never reaching the summits of the rods. In Pot 2, one of the self-fertilised plants was for a long time taller by two inches than its opponent, but was ultimately beaten by it, so that all the crossed plants exceeded their opponents in height. Of twenty-eight capsules produced by the crossed plants fertilised by pollen from a distinct plant, each contained on an average 4.75 seeds; of twenty-seven self-fertilised capsules on the self-fertilised plants, each contained on an average 4.47 seeds; so that the proportion of seeds in the crossed and self-fertilised capsules was as 100 to 94.

Some of the same seeds, from which the plants in Table 2/5 had been raised, were planted, after they had germinated on damp sand, in a square tub, in which a large Brugmansia had long been growing. The soil was extremely poor and full of roots; six crossed seeds were planted in one corner, and six self-fertilised seeds in the opposite corner. All the seedlings from the latter soon died excepting one, and this grew to the height of only 1 1/2 inches. Of the crossed plants three survived, and they grew to the height of 2 1/2 inches, but were not able to twine round a stick; nevertheless, to my surprise, they produced some small miserable flowers. The crossed plants thus had a decided advantage over the self-fertilised plants under this extremity of bad conditions.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.

These were raised in the same manner as before, and when measured gave the following results:--

TABLE 2/6. Ipomoea purpurea (Fifth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 96 : 73.

Pot 1 : 86 : 78.

Pot 1 : 69 : 29.

Pot 2 : 84 : 51.

Pot 2 : 84 : 84.

Pot 2 : 76 1/4 : 59.

Total : 495.25 : 374.00.

The average height of the six crossed plants is 82.54 inches, and that of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every crossed plant exceeded its antagonist in height. In Pot 1 the middle plant on the crossed side was slightly injured whilst young by a blow, and was for a time beaten by its opponent, but ultimately recovered the usual superiority. The crossed plants produced spontaneously a vast number more capsules than did the self-fertilised plants; and the capsules of the former contained on an average 3.37 seeds, whilst those of the latter contained only 3.0 per capsule, or as 100 to 89. But looking only to the artificially fertilised capsules, those on the crossed plants again crossed contained on an average 4.46 seeds, whilst those on the self-fertilised plants again self-fertilised contained 4.77 seeds; so that the self-fertilised capsules were the more fertile of the two, and of this unusual fact I can offer no explanation.

CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.

These were raised in the usual manner, with the following result. I should state that there were originally eight plants on each side; but as two of the self-fertilised became extremely unhealthy and never grew to near their full height, these as well as their opponents have been struck out of the list. If they had been retained, they would have made the average height of the crossed plants unfairly greater than that of the self-fertilised. I have acted in the same manner in a few other instances, when one of a pair plainly became very unhealthy.

TABLE 2/7. Ipomoea purpurea (Sixth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 93 : 50 1/2.

Pot 1 : 91 : 65.

Pot 2 : 79 : 50.

Pot 2 : 86 1/2 : 87.

Pot 2 : 88 : 62.

Pot 3 : 87 1/2 : 64 1/2.

Total : 525 : 379.

The average height of the six crossed plants is here 87.5, and of the six self-fertilised plants 63.16, or as 100 to 72. This large difference was chiefly due to most of the plants, especially the self-fertilised ones, having become unhealthy towards the close of their growth, and they were severely attacked by aphides. From this cause nothing can be inferred with respect to their relative fertility. In this generation we have the first instance of a self-fertilised plant in Pot 2 exceeding (though only by half an inch) its crossed opponent. This victory was fairly won after a long struggle. At first the self-fertilised plant was several inches taller than its opponent, but when the latter was 4 1/2 feet high it had grown equal; it then grew a little taller than the self-fertilised plant, but was ultimately beaten by it to the extent of half an inch, as shown in Table 2/7. I was so much surprised at this case that I saved the self-fertilised seeds of this plant, which I will call the "Hero," and experimented on its descendants, as will hereafter be described.