Page:Popular Science Monthly Volume 35.djvu/546

524 in which this species, with Helix hortensis and Limax agrestis, a voracious all-feeder, were put in presence of several plants having strong odors and pronounced flavors, showed that their tastes as toward living plants were very different. These experiments tend to show that the living plants are protected to a greater or less extent by the presence of some constituent disagreeable to the snails, which we may regard as a defensive armor to them. The dead parts of the plants were preferred, although as a rule dried vegetable is less alimentary than fresh, because the disagreeable substance had been removed or weakened by evaporation. Other experiments show that this kind of armor is, as a rule, the most effective.

When a drop of the juice of sorrel, garlic, saxifrage, or nasturtion is put upon the tegument of a snail, the animal manifests pain and exudes abundance of its mucous secretion; yet it is not thus affected by a drop of water. When snails avoid plants marked by such juices, we have a right to regard the plants as defended by a chemical armor. The offensive substance may also be important to the nutrition of the plant, but that is not the question we are dealing with here. Many plants are evidently lacking in this means of defense; for, of some plants, all the animals experimented upon have been found to prefer fresh to dead parts. Others are never touched by them, whether living or dead. Hence we may conceive that an infinite variety may exist in the degrees of chemical armoring between total absence of protection and complete protection.

Plants containing perceptible tannin are disagreeable to nearly all animals. Only swine will eat acorns as if they regard them as food. Other animals reject them, except when they can not get anything else. Leguminous plants containing tannin in weak proportions are eaten by horses and cattle, but snails are not fond of them. But the garden snail, which lets fresh clover alone, will eat it freely after the tannin has been extracted with alcohol. It is also probably tannin that inspires snails with respect for vetches, saxifrage, and stone-crop. Many water-plants, likewise, strong in tannin, are respected by water-snails, while the treatment with alcohol converts them into savory dishes for the same animals. Other plants, like dock, sorrel, and begonia, contain oxalic acid in notable quantities, and are obnoxious to them when too freely mixed with their food. It is worthy of remark that if carrot, of which snails are fond, is soaked in solutions of tannin or oxalic acid, they will avoid it in proportion as it is strongly impregnated with the offensive substance.

Strongly acid substances are often found on the surface of the leaves of plants. M. Stahl casually perceived that a leaf of Œnothera caused a very pronounced acid sensation on contact with