Page:Popular Science Monthly Volume 25.djvu/665

Rh is probably represented in certain species of Gerardia. Here is found the first indication of the parasitic habit. While the roots are attached to those of other plants, its green leaves are well developed, and it takes only the crude material into its system and there elaborates it; and at the same time it absorbs matter by means of the other roots with which it is provided.

The mistletoe comes second. In this plant we find the root absorbing nourishment from the branch on which it has located; the stem provided with green leaves, to which it can bring the sap to a proper state for assimilation, but no connection with the soil. The next step would be for the plant to loose its connection with earth or branch, take the fully elaborated sap, and by gradual stages lose all its foliage organs. Then the fully formed parasitic dodder results.

Proceeding in the opposite direction we find the beech-drop, a plant which lives in the rich mold of beech-woods, taking part of its food from the decaying leaves, and part from the roots of the beech-trees which it penetrates with its own rootlets. This plant is entirely destitute of green leaves, is of a brownish color, and may be considered one step on the road taken by the Indian pipe.

The Indian pipe, again, is a little plant which lives in the débris of forests, finding its food in the mass of decaying vegetable mold. While it is not probable that its roots are connected with those of the trees under which it grows, it is certain that the rich matter there found contains the constituents it requires for its growth. It, like the dodder, is destitute of green leaves, and for the same reason, namely, because it finds its food already prepared for it and has only to absorb it. But it differs in taking the food from the dead and decayed matter, instead of from the living. Plants of this kind are known as saprophytes, and are most common among the fungi. Here, then, in the saprophytic Indian pipe we have one end of a line of habit of living which has its other end in the perfect parasitism of the dodder.

In attempting to trace the origin of any particular habit peculiar to any one species, it is always necessary to examine the near relatives and see in what respects they resemble and in what ones they differ from the plant under consideration. The dodder belongs to the Convolvulaceæ, or the morning-glory family, and one of the most striking features of this family is found in their habits of twining. But what a vast difference there is in appearance between the morning-glory, with its large leaves, its root, and its conspicuous flowers, and the dodder, with its yellow stem, complete absence of green leaves, and lack of root! How is the change to arise which will bring the dodder to its present condition?

Evolutionists acknowledge that all changes in either plants or animals are the results of changes in conditions or surroundings. When once a change has occurred which is beneficial in a certain way, the