Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/301

. IX. he visited a site where Vanilla creeps over almost every tree, and although the plants had been covered with flowers, yet only two seed-capsules were produced. So again with an Epidendrum, 233 flowers had fallen off unimpregnated and only one capsule had been formed; of the still remaining 136 flowers, only four had their pollinia removed. In New South Wales Mr. Fitzgerald does not believe that more than one flower out of a thousand of Dendrobium speciosum sets a capsule; and some other species there are very sterile. In New Zealand over 200 flowers of Coryanthes triloba yielded only five capsules; and at the Cape of Good Hope only the same number were produced by 78 flowers of Disa grandiflora. Nearly the same result has been observed with some of the species of Ophrys in Europe. The sterility in these cases is very difficult to explain. It manifestly depends on the flowers being constructed with such elaborate care for cross-fertilisation, that they cannot yield seeds without the aid of insects. From the evidence which I have given elsewhere we may conclude that it would be far more profitable to most plants to yield a few cross-fertilised seeds, at the expense of many flowers dropping off unimpregnated, rather than produce many self-fertilised seeds. Profuse expenditure is nothing unusual under nature, as we see with the pollen of wind-fertilised plants, and in the multitude of seeds and seedlings produced by most plants in comparison with the few that reach maturity. In other cases the paucity of the flowers that are impregnated may be due to the proper insects having become rare under the incessant changes to which the world is subject; or to other plants which are more