Considering all the evidence with respect to the plants in Table3/ 22, a cross between two flowers on the same plant seems to give no advantage to the offspring thus produced, the self-fertilised plants being in weight superior. But this conclusion cannot be absolutely trusted, owing to the measurements given in Table 3/21, though these latter, from the cause already assigned, are very much less trustworthy than the present ones.]

SUMMARY OF OBSERVATIONS ON Mimulus luteus.

In the three first generations of crossed and self-fertilised plants, the tallest plants alone on each side of the several pots were measured; and the average height of the ten crossed to that of the ten self-fertilised plants was as 100 to 64. The crossed were also much more fertile than the self-fertilised, and so much more vigorous that they exceeded them in height, even when sown on the opposite side of the same pot after an interval of four days. The same superiority was likewise shown in a remarkable manner when both kinds of seeds were sown on the opposite sides of a pot with very poor earth full of the roots of another plant. In one instance crossed and self-fertilised seedlings, grown in rich soil and not put into competition with each other, attained to an equal height. When we come to the fourth generation the two tallest crossed plants taken together exceeded by only a little the two tallest self-fertilised plants, and one of the latter beat its crossed opponent,--a circumstance which had not occurred in the previous generations. This victorious self-fertilised plant consisted of a new white-flowered variety, which grew taller than the old yellowish varieties. From the first it seemed to be rather more fertile, when self-fertilised, than the old varieties, and in the succeeding self-fertilised generations became more and more self-fertile. In the sixth generation the self-fertilised plants of this variety compared with the crossed plants produced capsules in the proportion of 147 to 100, both lots being allowed to fertilise themselves spontaneously. In the seventh generation twenty flowers on one of these plants artificially self-fertilised yielded no less than nineteen very fine capsules!

This variety transmitted its characters so faithfully to all the succeeding self-fertilised generations, up to the last or ninth, that all the many plants which were raised presented a complete uniformity of character; thus offering a remarkable contrast with the seedlings raised from the purchased seeds. Yet this variety retained to the last a latent tendency to produce yellow flowers; for when a plant of the eighth self-fertilised generation was crossed with pollen from a yellow-flowered plant of the Chelsea stock, every single seedling bore yellow flowers. A similar variety, at least in the colour of its flowers, also appeared amongst the crossed plants of the third generation. No attention was at first paid to it, and I know not how far it was at first used either for crossing or self-fertilisation. In the fifth generation most of the self-fertilised plants, and in the sixth and all the succeeding generations every single plant consisted of this variety; and this no doubt was partly due to its great and increasing self-fertility. On the other hand, it disappeared from amongst the crossed plants in the later generations; and this was probably due to the continued intercrossing of the several plants. From the tallness of this variety, the self-fertilised plants exceeded the crossed plants in height in all the generations from the fifth to the seventh inclusive; and no doubt would have done so in the later generations, had they been grown in competition with one another. In the fifth generation the crossed plants were in height to the self-fertilised, as 100 to 126; in the sixth, as 100 to 147; and in the seventh generation, as 100 to 137. This excess of height may be attributed not only to this variety naturally growing taller than the other plants, but to its possessing a peculiar constitution, so that it did not suffer from continued self-fertilisation.

The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 40

19th Century English Literature

Charles Darwin

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Charles Darwin

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