The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Some flowers on the crossed plants of the last generation were again crossed with pollen from another crossed plant, and produced fine capsules. The flowers on the self-fertilised plants of the last generation were allowed to fertilise themselves spontaneously under a net, and they produced some remarkably fine capsules. The two lots of seeds thus produced germinated on sand, and eight pairs were planted on opposite sides of four pots. These plants were measured to the tips of their leaves on the 20th of October of the same year, and the eight crossed plants averaged in height 8.4 inches, whilst the self-fertilised averaged 8.53 inches, so that the crossed were a little inferior in height, as 100 to 101.5. By the 5th of June of the following year these plants had grown much bulkier, and had begun to form heads. The crossed had now acquired a marked superiority in general appearance, and averaged 8.02 inches in height, whilst the self-fertilised averaged 7.31 inches; or as 100 to 91. The plants were then turned out of their pots and planted undisturbed in the open ground. By the 5th of August their heads were fully formed, but several had grown so crooked that their heights could hardly be measured with accuracy. The crossed plants, however, were on the whole considerably taller than the self-fertilised. In the following year they flowered; the crossed plants flowering before the self-fertilised in three of the pots, and at the same time in Pot 2. The flower-stems were now measured, as shown in Table 4/29.
TABLE 3/29. Brassica oleracea. Measured in inches to tops of flower-stems: 0 signifies that a Flower-stem was not formed. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 49 2/8 : 44. Pot 1 : 39 4/8 : 41. Pot 2 : 37 4/8 : 38. Pot 2 : 33 4/8 : 35 4/8. Pot 3 : 47 : 51 1/8. Pot 3 : 40 : 41 2/8. Pot 3 : 42 : 46 4/8. Pot 4 : 43 6/8 : 20 2/8. Pot 4 : 37 2/8 : 33 3/8. Pot 4 : 0 : 0. Total : 369.75 : 351.00. The nine flower-stems on the crossed plants here average 41.08 inches, and the nine on the self-fertilised plants 39 inches in height, or as 100 to 95. But this small difference, which, moreover, depended almost wholly on one of the self-fertilised plants being only 20 inches high, does not in the least show the vast superiority of the crossed over the self-fertilised plants. Both lots, including the two plants in Pot 4, which did not flower, were now cut down close to the ground and weighed, but those in Pot 2 were excluded, for they had been accidentally injured by a fall during transplantation, and one was almost killed. The eight crossed plants weighed 219 ounces, whilst the eight self-fertilised plants weighed only 82 ounces, or as 100 to 37; so that the superiority of the former over the latter in weight was great.
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. Brassica 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 competition. 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 constitution. Iberis umbellata.
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