Page:Popular Science Monthly Volume 35.djvu/550

528 against which a plant has to contend to maintain its existence, their defenses are more numerous than we would suspect, and more important than we might at first believe.

In analyzing M. Stahl's results, we perceive that some families possess, as a whole, similar methods of protection; the grasses, sedges, and horse-tails, silicification; the rough-leaved orders, hairs; the Amaryllidece, Asparageæ, orchids, and Onagraceæ, raphides; the gentians, bitter substance; the rose family, geraniums, legumes, and heaths, tannic acid; the nightshades, alkaloids; the labiates, ethereal oils; mosses, mechanical means (by silicification); and liverworts, chemical means, and one genus of them, Riccia, mechanical means also.

Different genera in the same family sometimes present quite diverse means. Among the lilies are genera (Scilla and Ornithogallus) having raphides; others, alliaceous compounds; lilies, tulips, and crown imperial, poisons. There are also differences between the species of the same genus; thus, one species of Sedum is protected by tannin, and another by an alkaloid. And in the same plant there are often very notable differences between the leaves, fruit, and root.

M. Stahl asserts that he has not found a single phanerogamous species, living in a wild condition, that is not armed in some way against slugs and snails. Such armor is wanting only among cultivated plants, or, rather, among some of them. It appears as if at the moment when man cultivates a species of plant, or takes it under his protection, using all possible means to facilitate its existence and remove its enemies, the plant gives up its own means of maintaining the struggle, surrendering its defensive armor at man's invitation. The common lettuce is a striking example of this fact. It is a favorite viand, as all know, of the Gasteropods of the garden. Nothing protects it against their attacks, and its smooth, tender, and succulent leaves make it a ready prey to them; yet it is the descendant, modified by cultivation, of the Lactuca scariola, which has chemical constituents so distasteful to snails, and so constant, that they will not eat it even after it has been treated with alcohol.

The defensive armor of plants is most frequently situated upon their surface, or where the attack begins. This is particularly the case with the mechanical weapons and such chemical ones as tannin, special juices, etc.

When we consider how varied are these armors of plants and how generally spread they are among all the orders, and that without them some species would not be able to exist, it is hard to deny that there is some special adaptation in them, or to suppose that they are merely accidental. The case is undoubtedly one of natural selection; and the fact that the protection is gained