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Show 92 ON THE ADVANTAGE [CHAP. IV. the visits of bees to papilionaceous flowers, that I ha':e found, by experiments published elsewhere, that thmr fertility is greatly dilninished if these visits be prevented. Now, it is scarcely possible that bees should fly from flower to flower, and not carry pollen from one to the other, to the great good, as I believe, of the plant. ~ees will act like a camel-hair pencil, and it is quite su:ffici~nt just to touch the anthers of one flower and then the stig-ma of another with the sa1ne brush to ensure fertilisation ; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species; for if you bring on the same brush a plant's own pollen and pollen from another species, the foriner will have such a prepotent effect, that it ·will invariably and completely destroy, as has been shown by Gartner, any influence from the foreign pollen. When the stainens of a flower suddenly spring towards the pistil, or slowly move one after the other towards it, the contrivance seems adapted solely to ensure s~lf-fertilisation; and no doubt it is useful for this end : but, the agency of insects is often required to cause the stamens to spring forward, as Kolreuter has shown to be the case with the barberry; and curiously in this very genus, which seems to have a special contrivance for self-fertilisation, it is well known that if very closely-allied forms or varieties are planted near each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross. In many other cases, far from there being any aids for self-fertilisation, there are special contrivances, as I could show from the writings of C. C. Sprengel and from my own observations, which effectually prevent the stign1a receiving pollen from its own flower: for instance, in Lobelia fulgens, there is a really beautiful and elaborate contrivance by which every one of the infinitely numerous pollen~granules are swept out of the conjoined anthers of each flower, before the stign1a of that individual flower is ready to receive them; and as this flower is never visited, at least in my garden, by insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, I raised plenty of seedlings; and whilst CHAP. IV.) OF INTERCROSSING. 93 another species of Lobelia growing close by, which is visited by bees, seeds freely. In ve.ry many other cases though there be no special mechanical contrivance to pre: vent the stigma of a :flower receiving its own gollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the anth~rs bu~st before the stigma is ready for fertilisation, or the stigma Is ready before the pollen of that :flower is ready, so that these plants have in fact separated sexes and must habitually be crossed. How strange are thes~ facts ! llow strange that the pollen and stigmatic surface of the same flower, though pla~~d s? close together, as if for the very purpose of self-fertihsatlon, should in so many cases be mutually useless to each other ! How simply are these facts explained on the view of an occasional cross with a distinct individual being advantageous or indispensable! If several varieties of the cabbage, radish, onion, and of some other plants, he allowed to seed near each other a !arge ~ajority, as I have found, ?f the seedlings thu~ raise~ will turn out mongrels : for Instance, I raised 233 seedhng cabbages from some plants of different varieties grow~g ~ear each other, and of these only '78 were true to their kind, an~ ~orne even of these were not perfectly true. Yet the pistil of each cabbage-fl.ower is surrounded not only by its own six stamens, but by those of the many other :flowers on the same plant. .IIow, then, comes it that such a va~t number ?f the seedhngs are mongrelized? I suspect that It must arise from the pollen of a distinct variety having a prepotent effect over a flower's own po~len; a~d that this is part of the general law of good bmng denved from the intercrossing of distinct individuals of the same species. When distinct speoies are crossed the case is directly the reverse, for a plant's own pollen is always prepotent over foreign pollen; but to this subject we shall return in a future chapter. In the case of a gigantic tree covered with innumerable :flowers, it may be objected that pollen could seldom be carried froin tree to tree, and at most only from flower to :flower on the same tree, and that flowers on the same tree can be considered as distinct individuals only in a |
