The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 34

The eight crossed plants now averaged 40.96, and the eight self-fertilised plants 53.87 inches in height, or as 100 to 131; and this excess chiefly depended, as already stated, on the unusual tallness of two of the self-fertilised plants in Pot 2. The self-fertilised had therefore lost some of their former great superiority over the crossed plants. In three of the pots the self-fertilised plants flowered first; but in Pot 3 at the same time with the crossed.

The case is rendered the more strange, because the crossed plants in the fifth pot (not included in the two last tables), in which all the remaining seeds had been thickly sown, were from the first finer plants than the self-fertilised, and had larger leaves. At the period when the two tallest crossed plants in this pot were 6 4/8 and 4 5/8 inches high, the two tallest self-fertilised were only 4 inches. When the two crossed plants were 12 and 10 inches high, the two self-fertilised were only 8 inches. These latter plants, as well as many others on the same side of this pot never grew any higher, whereas several of the crossed plants grew to the height of two feet! On account of this great superiority of the crossed plants, the plants on neither side of this pot have been included in the two last tables.

Thirty flowers on the crossed plants in Pots 1 and 4 (Table 6/79) were again crossed, and produced seventeen capsules. Thirty flowers on the self-fertilised plants in the same two pots were again self-fertilised, but produced only seven capsules. The contents of each capsule of both lots were placed in separate watch-gla.s.ses, and the seeds from the crossed appeared to the eye to be at least double the number of those from the self-fertilised capsules.

In order to ascertain whether the fertility of the self-fertilised plants had been lessened by the plants having been self-fertilised for the three previous generations, thirty flowers on the crossed plants were fertilised with their own pollen. These yielded only five capsules, and their seeds being placed in separate watch-gla.s.ses did not seem more numerous than those from the capsules on the self-fertilised plants self-fertilised for the fourth time. So that as far as can be judged from so few capsules, the self-fertility of the self-fertilised plants had not decreased in comparison with that of the plants which had been intercrossed during the three previous generations. It should, however, be remembered that both lots of plants had been subjected in each generation to almost exactly similar conditions.

Seeds from the crossed plants again crossed, and from the self-fertilised again self-fertilised, produced by the plants in Pot 1 (Table 6/79), in which the three self-fertilised plants were on an average only a little taller than the crossed, were used in the following experiment. They were kept separate from two similar lots of seeds produced by the two plants in Pot 4 in the same table, in which the crossed plant was much taller than its self-fertilised opponent.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 1, TABLE 6/79).

Crossed and self-fertilised seeds from plants of the last generation in Pot 1 in Table 6/79, were placed on sand, and after germinating, were planted in pairs on the opposite sides of four pots. The seedlings when in full flower were measured to the base of the calyx. The remaining seeds were sown crowded on the two sides of Pot 5; and the four tallest plants on each side of this pot were measured in the same manner.

TABLE 6/80. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 1, table 6/79).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 29 2/8 : 30 2/8.

Pot 1 : 36 2/8 : 34 6/8.

Pot 1 : 49 : 31 3/8.

Pot 2 : 33 3/8 : 31 5/8.

Pot 2 : 37 3/8 : 38 2/8.

Pot 2 : 56 4/8 : 38 4/8.

Pot 3 : 46 : 45 1/8.

Pot 3 : 67 2/8 : 45.

Pot 3 : 54 3/8 : 23 2/8.

Pot 4 : 51 6/8 : 34.

Pot 4 : 51 7/8 : 0.

Pot 5 : 49 4/8 : 22 3/8.

Pot 5 : 46 3/8 : 24 2/8.

Pot 5 : 40 : 24 6/8.

Pot 5 : 53 : 30.

Crowded plants.

Total : 701.88 : 453.50.

The fifteen crossed plants average 46.79, and the fourteen (one having died) self-fertilised plants 32.39 inches in height; or as 100 to 69. So that the crossed plants in this generation had recovered their wonted superiority over the self-fertilised plants; though the parents of the latter in Pot 1, Table 6/79, were a little taller than their crossed opponents.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 4, TABLE 6/79).

Two similar lots of seeds, obtained from the plants in Pot 4 in Table 6/79, in which the single crossed plant was at first shorter, but ultimately much taller than its self-fertilised opponent, were treated in every way like their brethren of the same generation in the last experiment. We have in Table 6/81 the measurements of the present plants. Although the crossed plants greatly exceeded in height the self-fertilised; yet in three out of the five pots a self-fertilised plant flowered before any one of the crossed; in a fourth pot simultaneously; and in a fifth (namely Pot 2) a crossed plant flowered first.

TABLE 6/81. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 4, Table 6/79).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 46 : 30 2/8.

Pot 1 : 46 : 28.

Pot 2 : 50 6/8 : 25.

Pot 2 : 40 2/8 : 31 3/8.

Pot 2 : 37 3/8 : 22 4/8.

Pot 3 : 54 2/8 : 22 5/8.

Pot 3 : 61 1/8 : 26 6/8.

Pot 3 : 45 : 32.

Pot 4 : 30 : 24 4/8.

Pot 4 : 29 1/8 : 26.

Pot 5 : 37 4/8 : 40 2/8.

Pot 5 : 63 : 18 5/8.

Pot 5 : 41 2/8 : 17 4/8.

Crowded plants.

Total : 581.63 : 349.36.

The thirteen crossed plants here average 44.74, and the thirteen self-fertilised plants 26.87 inches in height; or as 100 to 60. The crossed parents of these were much taller, relatively to the self-fertilised parents, than in the last case; and apparently they transmitted some of this superiority to their crossed offspring. It is unfortunate that I did not turn these plants out of doors, so as to observe their relative fertility, for I compared the pollen from some of the crossed and self-fertilised plants in Pot 1, Table 6/81, and there was a marked difference in its state; that of the crossed plants contained hardly any bad and empty grains, whilst such abounded in the pollen of the self-fertilised plants.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

I procured from a garden in Westerham, whence my plants originally came, a fresh plant differing in no respect from mine except in the colour of the flowers, which was a fine purple. But this plant must have been exposed during at least four generations to very different conditions from those to which my plants had been subjected, as these had been grown in pots in the greenhouse. Eight flowers on the self-fertilised plants in Table 6/81, of the last or fourth self-fertilised generation, were fertilised with pollen from this fresh stock; all eight produced capsules containing together by weight 5.01 grains of seeds. The plants raised from these seeds may be called the Westerham-crossed.

Eight flowers on the crossed plants of the last or fourth generation in Table 6/81 were again crossed with pollen from one of the other crossed plants, and produced five capsules, containing by weight 2.07 grains of seeds. The plants raised from these seeds may be called the INTERCROSSED; and these form the fifth intercrossed generation.

Eight flowers on the self-fertilised plants of the same generation in Table 6/81 were again self-fertilised, and produced seven capsules, containing by weight 2.1 grains of seeds. The SELF-FERTILISED plants raised from these seeds form the fifth self-fertilised generation. These latter plants and the intercrossed are comparable in all respects with the crossed and self-fertilised plants of the four previous generations.

From the foregoing data it is easy to calculate that:

Ten Westerham-crossed capsules would have contained 6.26 grains weight of seed.

Ten intercrossed capsules would have contained 4.14 grains weight of seed.