This section is from the book "The Gardener's Monthly And Horticulturist V26", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
A distinguished botanist sends us the following: "In August number of Gardeners' Monthly, p. 229, you say 'composite plants are unfavorable to cross-fertilization.' Please explain, for it is the general impression that the flower heads of nearly all com-positae are specially adapted for cross-fertilization. The pollen matures long before the stigma, and the provisions for a cross seem to be excellent".
[Let us first understand what we mean by cross-fertilization. Mr. Darwin tells us. In his work on cross-fertilization, p. 22, he says he covered certain flowers to prevent the entrance of winged insects, then raises the objection that thrips run from one flower to another carrying pollen, but he says this is not cross-fertilization. Previously, at p. 10, he observes that "cross-fertilization always means a cross between distinct plants which have been raised from seeds, and not from cuttings." Hence even a mass of plants, as in the case of many herbaceous species, which often cover a large tract of ground by stolons or running root stocks, will not give flowers that can properly cross-fertilize anything around them. He illustrates this at p. 399 by a reference to the horse chestnut, which may produce " fifty thousand flowers" in a season. The bees going from flower to flower, he says, do no more than aid to self-fertilize. It is only the few flowers which may receive pollen on the first alighting of a bee to that tree which are cross-fertilized. At p. 61 he emphasizes this by giving as the result of his experiments that flowers crossed by other flowers on the same plant produce progeny even more inferior than when the flower is fertilized by its own pollen.
Now a large number of composite plants are perennial, and extend from year to year by their root stocks, extending the area from year to year indefinitely. For fertilizing purposes these plants are, in Mr. Darwin's view, practically but one plant. The extension by root-growth is an arrangement favorable to self-fertilization.
In many composite flowers, the sunflower for instance, there are many hundreds of florets. The florets open in circles from day to day. The pollen cells burst almost immediately on the opening of the floret. Often it is pushed out by the growth of the pistil, which is frequently covered by the pollen. The stigma is not then in condition to profit by pollen, but the pollen remains there until it is. The stigmas expand and the interior surface is apparently bare of pollen, but there is generally pollen enough, in centaurea for instance, falling into the cleft during the act of expansion. A single grain, among all the many thousands produced, is sufficient. But supposing this does not occur. Suppose on the second day the expanded stigmas have no pollen, this either falls from the next day's pollen crop or it is knocked down by the visiting insect pollen-gatherers. Any person may watch a head of the sunflower and see for himself that the florets are all fertilized by the pollen of the same head.
Of the thousands of florets in a; sunflower head we must fall back on the chance that on the first visit of the bee it may strike a floret it is not visiting for honey, to get one cross-fertilized, and not then if the bee had come from another flower on the same plant, instead of from a distinct plant - a chance that would rarely happen, and if it did, it would still more rarely happen that that cross-fertilized seed, among the thousands of self-crossed ones, would be the one to grow.
Physiologically, as well as popularly, the head of an aster, or any composite plant, is but a single flower; and though it were a fact that every floret was pollemzed by its neighbor, that could not, in any sense, be cross-fertilization.
True, Mr. Darwin does say in this same book, "compositae are well adapted for cross-fertilization," but he evidently had in his mind at that moment what our esteemed correspondent also has, that the mere fact of the stamens maturing a day before the pistils is the whole of the story. We have seen by Darwin's own observations that this is but a very small part of it. Adaptation to cross-fertilization implies much more. The truth is, composite flowers are specially adapted to self-fertilization. - Ed. G. M].