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.

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 analogous manner as occurred with the self-fertilised plants of the eighth generation of Ipomoea raised from unhealthy parents. It is a curious circumstance, 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 analogous 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 assumed 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.

The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 52

19th Century English Literature

Charles Darwin

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Charles Darwin

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