PEANUT.
A pod magnified.
Modification in connection with behavior is further illustrated by the grapevine and Virginia creeper. The whole leading shoot is here pushed aside by the development of a bud at the base of the leaf, that takes the place of a leading shoot. The original leader then becomes a tendril, and serves in the economy of the plant by clinging to trees or rocks, or in coiling around other plants in support. Great progress has been made in this department of botany within recent years. Darwin has shown that the tendrils of some plants continue in motion for some time in order to find something to cling to. The grapevine especially spends a long time in this labor if there is difficulty in reaching a host. The plant preserves vital power all this time, but no sooner is support found, than nutrition is cut off, and the tendril dies, though, hard and wiry, it serves its parent plant as a support better dead than alive. The amount of nutrition spent in sustaining motion is found to be enormous. A vine that can find ready means of support grows with a much more healthy vigor than one that has difficulty in finding it. Many plants present illustrations.
Much advance has been made in the knowledge of the motions of plants as regards their various forms. Growth in plants is not continuous; but is a series of rests and advances. In other words it is rhythmic. The nodes, or knots, in the stems of grasses are resting-places. When a rest occurs, energy may be exerted in a different direction, and a change of form result. This is well illustrated by the common Dogwood of northern woods, Cornus florida on the eastern, and Cornus Nuttallii on the western slope of the American continent. On the approach of winter the leaf is reduced to a bud scale, and then rests. When spring returns these scales resume growth and appear as white bracts. In the annexed illustration the scales that served for winter protection to the buds are seen at the apex of the bracts. In other species of Dogwood the bud scales do not resume growth. Energy is spent in another direction. In this manner we have an insight as to the cause of variation, which was not perceived even so recently as Darwin’s time. We now say that variation results from varying degrees of rhythmic growth—force; and that this again is governed by varying powers of assimilation.
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.
It is but fair to say that the doctrine of evolution by the influence of external conditions in the change of form, though widely accepted at this time, is not without strong opponents, who point to the occasional development or suppression of parts on the same plant, though the external conditions must be the same. For instance, there are flowers that have all their parts regular, as in the petals of a buttercup; and irregular, as in the snap-dragon or fox-glove. But it has been noted that irregular flowers have pendulous stalks, while the regular ones are usually erect. But once in a while, on the same plant, flowers normally drooping will become erect. In these cases the flowers are regular. In the wild snap-dragon or yellow toad-flax, Linaria vulgaris, one of the petals is developed into a long spur; the other four petals have, in early life, become connate and transformed into parts of the flower wholly unlike ordinary petals. But now and then the original petals will all develop spurs, resulting in the condition technically known as peloria.