[Illustration: OUTLINE OF A WHITE DOGWOOD FLOWER (_Cornus florida_), SHOWING BUD-SCALES DEVELOPED TO BRACTS.]
The Darwinian view, that form results from external conditions of which the plant avails itself in a struggle for existence, is still widely accepted as a leading factor in the origin of species. Those which can assume the strongest weapons of defense continue to exist under the changed conditions. The weaker ones do not survive, and we only know of them as fossils. This is termed the doctrine of natural selection.
The origin and development of plant-life, or, as it is termed, evolution, has made rapid advancement as a study during the century. That there has been an adaptation to conditions in some respects, as contended by Mr. Darwin and his followers, must be correct. The oak and other species of trees must have been formed before mistletoe and other parasites could grow on them. In the common Dodder—species of _Cuscuta_—the seeds germinate in the ground like ordinary plants. As soon as they find something to attach themselves to, they cut loose from mother earth and live wholly on the host. As a speculation it seems plausible that all parasites have arisen in this way. Some, like the mistletoe, having the power, at length, to have their seeds germinate on the host-plant, have left their terrestrial origin in the past uncertain. A number of parasites, however, do not seem to live wholly on the plants they attach themselves to. These are usually destitute of green color. The Indian pipe, snow plant of the Pacific Coast, and Squaw root of the Eastern States are examples; the former called ghost-flower from its paleness. These plants have little carbonaceous matter in their structure, and hence are regarded as having formed a kind of partnership with fungi. This is known now as symbiosis, or living together of dissimilar organisms, each dependent mutually. The fungus and the flowering plant in these cases are necessary to the existence of each other. They demand nitrogen instead of carbonhydroids. The Squaw root, _Conopholis Americana_, though attached to the subterranean portions of the trunks of trees, is probably sustained by the fungus material in the old bark, or even in the wood, rather than by the ordinary food of flowering plants. Lichens, as it is now well known, are a compound of fungi and water weeds (algæ), and this doctrine of symbiosis is regarded as one of the great advances of the century.