The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 15

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

Some flowers on a crossed plant of the last or second generation were fertilised, without being castrated, by pollen taken from a plant of the same variety, but not related to my plants, and brought from a nursery garden (whence my seeds originally came) having a different soil and aspect. The flowers on the self-fertilised plants of the last or second generation (Table 4/29) were allowed to fertilise themselves spontaneously under a net, and yielded plenty of seeds. These latter and the crossed seeds, after germinating on sand, were planted in pairs on the opposite sides of six large pots, which were kept at first in a cool greenhouse. Early in January their heights were measured to the tips of their leaves. The thirteen crossed plants averaged 13.16 inches in height, and the twelve (for one had died) self-fertilised plants averaged 13.7 inches, or as 100 to 104; so that the self-fertilised plants exceeded by a little the crossed plants.

TABLE 3/30. Bra.s.sica oleracea.

Weights in ounces of plants after they had formed heads.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants from Pollen of fresh Stock.

Column 3: Self-fertilised Plants of the Third Generation.

Pot 1 : 130 : 18 2/4.

Pot 2 : 74 : 34 3/4.

Pot 3 : 121 : 17 2/4.

Pot 4 : 127 2/4 : 14.

Pot 5 : 90 : 11 2/4.

Pot 6 : 106 2/4 : 46.

Total : 649.00 : 142.25.

Early in the spring the plants were gradually hardened, and turned out of their pots into the open ground without being disturbed. By the end of August the greater number had formed fine heads, but several grew extremely crooked, from having been drawn up to the light whilst in the greenhouse. As it was scarcely possible to measure their heights, the finest plant on each side of each pot was cut down close to the ground and weighed. In Table 4/30 we have the result.

The six finest crossed plants average 108.16 ounces, whilst the six finest self-fertilised plants average only 23.7 ounces, or as 100 to 22.

This difference shows in the clearest manner the enormous benefit which these plants derived from a cross with another plant belonging to the same sub-variety, but to a fresh stock, and grown during at least the three previous generations under somewhat different conditions.

THE OFFSPRING FROM A CUT-LEAVED, CURLED, AND VARIEGATED WHITE-GREEN CABBAGE CROSSED WITH A CUT-LEAVED, CURLED, AND VARIEGATED CRIMSON-GREEN CABBAGE, COMPARED WITH THE SELF-FERTILISED OFFSPRING FROM THE TWO VARIETIES.

These trials were made, not for the sake of comparing the growth of the crossed and self-fertilised seedlings, but because I had seen it stated that these varieties would not naturally intercross when growing uncovered and near one another. This statement proved quite erroneous; but the white-green variety was in some degree sterile in my garden, producing little pollen and few seeds. It was therefore no wonder that seedlings raised from the self-fertilised flowers of this variety were greatly exceeded in height by seedlings from a cross between it and the more vigorous crimson-green variety; and nothing more need be said about this experiment.

The seedlings from the reciprocal cross, that is, from the crimson-green variety fertilised with pollen from the white-green variety, offer a somewhat more curious case. A few of these crossed seedlings reverted to a pure green variety with their leaves less cut and curled, so that they were altogether in a much more natural state, and these plants grew more vigorously and taller than any of the others. Now it is a strange fact that a much larger number of the self-fertilised seedlings from the crimson-green variety than of the crossed seedlings thus reverted; and as a consequence the self-fertilised seedlings grew taller by 2 1/2 inches on an average than the crossed seedlings, with which they were put into compet.i.tion. At first, however, the crossed seedlings exceeded the self-fertilised by an average of a quarter of an inch. We thus see that reversion to a more natural condition acted more powerfully in favouring the ultimate growth of these plants than did a cross; but it should be remembered that the cross was with a semi-sterile variety having a feeble const.i.tution.

Iberis umbellata.

VAR. KERMESIANA.

This variety produced plenty of spontaneously self-fertilised seed under a net. Other plants in pots in the greenhouse were left uncovered, and as I saw small flies visiting the flowers, it seemed probable that they would be intercrossed. Consequently seeds supposed to have been thus crossed and spontaneously self-fertilised seeds were sown on opposite sides of a pot. The self-fertilised seedlings grew from the first quicker than the supposed crossed seedlings, and when both lots were in full flower the former were from 5 to 6 inches higher than the crossed!

I record in my notes that the self-fertilised seeds from which these self-fertilised plants were raised were not so well ripened as the crossed; and this may possibly have caused the great difference in their growth, in a somewhat a.n.a.logous manner as occurred with the self-fertilised plants of the eighth generation of Ipomoea raised from unhealthy parents. It is a curious circ.u.mstance, that two other lots of the above seeds were sown in pure sand mixed with burnt earth, and therefore without any organic matter; and here the supposed crossed seedlings grew to double the height of the self-fertilised, before both lots died, as necessarily occurred at an early period. We shall hereafter meet with another case apparently a.n.a.logous to this of Iberis in the third generation of Petunia.

The above self-fertilised plants were allowed to fertilise themselves again under a net, yielding self-fertilised plants of the second generation, and the supposed crossed plants were crossed by pollen of a distinct plant; but from want of time this was done in a careless manner, namely, by smearing one head of expanded flowers over another. I should have thought that this would have succeeded, and perhaps it did so; but the fact of 108 of the self-fertilised seeds weighing 4.87 grains, whilst the same number of the supposed crossed seeds weighed only 3.57 grains, does not look like it. Five seedlings from each lot of seeds were raised, and the self-fertilised plants, when fully grown, exceeded in average height by a trifle (namely .4 of an inch) the five probably crossed plants. I have thought it right to give this case and the last, because had the supposed crossed plants proved superior to the self-fertilised in height, I should have a.s.sumed without doubt that the former had really been crossed. As it is, I do not know what to conclude.

Being much surprised at the two foregoing trials, I determined to make another, in which there should be no doubt about the crossing. I therefore fertilised with great care (but as usual without castration) twenty-four flowers on the supposed crossed plants of the last generation with pollen from distinct plants, and thus obtained twenty-one capsules. The self-fertilised plants of the last generation were allowed to fertilise themselves again under a net, and the seedlings reared from these seeds formed the third self-fertilised generation. Both lots of seeds, after germinating on bare sand, were planted in pairs on the opposite sides of two pots. All the remaining seeds were sown crowded on opposite sides of a third pot; but as all the self-fertilised seedlings in this latter pot died before they grew to any considerable height, they were not measured. The plants in Pots 1 and 2 were measured when between 7 and 8 inches in height, and the crossed exceeded the self-fertilised in average height by 1.57 inches.

When fully grown they were again measured to the summits of their flower-heads, with the following result:--

TABLE 4/31. Iberis umbellata.

Heights of plants to the summits of their flower-heads, in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants of the Third Generation.

Pot 1 : 18 : 19.

Pot 1 : 21 : 21.

Pot 1 : 18 2/8 : 19 4/8.

Pot 2 : 19 : 16 6/8.

Pot 2 : 18 4/8 : 7 4/8.

Pot 2 : 17 6/8 : 14 4/8.

Pot 2 : 21 3/8 : 16 4/8.

Total : 133.88 : 114.75.

The average height of the seven crossed plants is here 19.12 inches, and that of the seven self-fertilised plants 16.39, or as 100 to 86. But as the plants on the self-fertilised side grew very unequally, this ratio cannot be fully trusted, and is probably too high. In both pots a crossed plant flowered before any one of the self-fertilised. These plants were left uncovered in the greenhouse; but from being too much crowded they were not very productive. The seeds from all seven plants of both lots were counted; the crossed produced 206, and the self-fertilised 154; or as 100 to 75.

CROSS BY A FRESH STOCK.

From the doubts caused by the two first trials, in which it was not known with certainty that the plants had been crossed; and from the crossed plants in the last experiment having been put into compet.i.tion with plants self-fertilised for three generations, which moreover grew very unequally, I resolved to repeat the trial on a larger scale, and in a rather different manner. I obtained seeds of the same crimson variety of Iberis umbellata from another nursery garden, and raised plants from them. Some of these plants were allowed to fertilise themselves spontaneously under a net; others were crossed by pollen taken from plants raised from seed sent me by Dr. Durando from Algiers, where the parent-plants had been cultivated for some generations. These latter plants differed in having pale pink instead of crimson flowers, but in no other respect. That the cross had been effective (though the flowers on the crimson mother-plant had NOT been castrated) was well shown when the thirty crossed seedlings flowered, for twenty-four of them produced pale pink flowers, exactly like those of their father; the six others having crimson flowers exactly like those of their mother and like those of all the self-fertilised seedlings. This case offers a good instance of a result which not rarely follows from crossing varieties having differently coloured flowers; namely, that the colours do not blend, but resemble perfectly those either of the father or mother plant. The seeds of both lots, after germinating on sand, were planted on opposite sides of eight pots. When fully grown, the plants were measured to the summits of the flower-heads, as shown in Table 4/32.

TABLE 4/32. Iberis umbellata.

Height of Plants to the summits of the flower-heads, measured in inches: 0 signifies that the Plant died.

Column 1: Number (Name) of Pot.

Column 2: Plants from a Cross with a fresh Stock.

Column 3: Plants from Spontaneously Self-fertilised Seeds.

Pot 1 : 18 6/8 : 17 3/8.

Pot 1 : 17 5/8 : 16 7/8.

Pot 1 : 17 6/8 : 13 1/8.

Pot 1 : 20 1/8 : 15 3/8.

Pot 2 : 20 2/8 : 0.

Pot 2 : 15 7/8 : 16 6/8.

Pot 2 : 17 : 15 2/8.

Pot 3 : 19 2/8 : 13 6/8.

Pot 3 : 18 1/8 : 14 2/8.

Pot 3 : 15 2/8 : 13 4/8.