Page:Popular Science Monthly Volume 35.djvu/547

Rh the tongue, which was due to the presence of a superficial acid. On examination he found the same property present in other plants of the Onograceæ and Papilionaceæ. The acid is secreted by numerous one-celled cylindrical hairs. It consists of a mixture of oxalic, acetic, and malic acids, and, being very disagreeable to slugs and snails, constitutes an efficacious protection against their ravages. A simple contact of its tentacles or teguments with the secretory hairs is enough to cause the animal to draw back and go somewhere else to indulge its cravings. But if the leaves are washed, and the hairs cleansed of the acid secretion, they will be eaten at once.

Many plants are furnished with strong and pungent ethereal oils or similar substances. Prof. Tyndall thinks that these essences help to protect the plant against excessive heat. Without disputing this, M. Stahl finds that they are also efficient in defense against animals. This was proved with respect to rue, calamus, peppermint, dictamnus, and crane's-bill; and snails would at once turn out of the way to avoid a crushed leaf of the latter when placed in their road. Bitter leaves were avoided when fresh; when dead, even those of the gentian were relished, although the fresh ones were rejected by very hungry animals. The expressed juices were very disagreeable to them. The bitter was evidently the unpleasant quality, for the plants in question were free from tannin. The liverworts, according to W. Pfeffer's researches, contain fat substances, the function of which is unknown, but to which Mr. Stahl ascribes a protecting agency. It is certain that, though they are easily accessible to all animals, they very rarely present any traces of having been attacked by them; and land snails respect them in a very marked manner. Even after fourteen days of fasting, Helix hortensis could not resolve to eat the thallus of Pellia. But there are genera (Lunaria and Marchantia) of which the less delicate snails will consent to eat a little. When the thalluses are treated with alcohol, the mollusks accept them readily; and there are some, like Plagiochila, that they will even eat fresh, in spite of their disagreeable smell, because of the much sugar that is in them. But most plants of the order are avoided, because of the unpleasant taste and smell given them by their