Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/309

. IX. superfluity owing to the many special contrivances for its safe transportal from plant to plant, and for placing it securely on the stigma. Thus we can understand why the Orchideæ are more highly endowed in their mechanism for cross-fertilisation, than are most other plants.

In my work on the "Effects of Cross and Self Fertilisation in the Vegetable Kingdom," I have shown that when flowers are cross-fertilised they generally receive pollen from a distinct plant and not that from another flower on the same plant; a cross of this latter kind doing little or no good. I have further shown that the benefits derived from a cross between two plants depends altogether on their differing somewhat in constitution; and there is much evidence that each individual seedling possesses its own peculiar constitution. The crossing of distinct plants of the same species is favoured or determined in various ways, as described in the above work, but chiefly by the prepotent action of pollen from another plant over that from the same flower. Now with the Orchideæ it is highly probable that such prepotency prevails, for we know from the valuable observations of Mr. Scott and Fritz Müller, that with several Orchids pollen from their own flower is quite impotent, and is even in some cases poisonous to the stigma. Besides this prepotency, the Orchideæ present various special contrivances—such as the pollinia not assuming a proper position for striking the stigma until some time has elapsed after their removal from the anthers—the slow curving forwards and then backwards of the rostellum in Listera and Neottia—the