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Page:Popular Science Monthly Volume 35.djvu/545

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THE DEFENSIVE ARMOR OF PLANTS.

M. Stahl's experiments were made in his own garden and in the woods in the neighborhood, and bore direct reference to the attitude of snails toward the plants. The questions were asked. What plants do snails prefer; what ones do they avoid, and why do they avoid them? The results of his study may be verified by almost any one. Several species of snails were observed; including special feeders, those which live wholly on mushrooms, and omnivorous snails, which, while preferring certain species, eat more or less of all kinds of plants, and sometimes accommodate themselves to animal food.

Pieces of mushroom were offered to the snails, a part of them fresh, others after having been macerated in alcohol, dried by evaporation, and washed. The different species varied in their behavior toward the food. The omnivorous snails would not eat, or would only touch the fresh pieces, but readily devoured those which had been treated with alcohol; but a special feeder ate the fresh pieces and left the others. Hence the author concluded that there exists in the fresh mushroom a substance soluble in alcohol that attracts some animals and repels others. It must not, however, be believed that the special feeders can only live on particular food, for they are capable of accommodating themselves to other kinds when it is necessary. That the ingredient soluble in alcohol was the essential element of the food was proved by the special feeders, which avoided the macerated and dried food, but returned to it when it had been soaked again in the alcohol by which that ingredient had been abstracted.

Some light is cast upon the bearing of this experiment by reflecting on the enormous quantities of food which the omnivorous snails in a state of nature require. A vine-snail or a slug will eat a quarter or a third of its weight of carrot or potato in twelve or twenty-four hours. Although their needs are but slight, they can hardly find enough to assuage their hunger, on account of the mechanical or chemical defenses which most plants offer against them. Thus, the garden snail causes immense destruction of the filbert-leaves in the spring; but it would cause more if these leaves did not contain certain chemical substances, for it eats them more greedily after they have been treated with alcohol. Though this sort of protection is only relative, it will appear very considerable when we reflect upon the abundance and fertility of some species of snail.

Examining a garden near Jena after a warm rain in April, of forty-four snails of the species hortensis, fruticum, and arbustorum, ten were found upon living plants, while the thirty-four others were eating dead leaves. These three species, therefore, most usually attack dead plants. Helix pomatia, on the other hand, was observed almost exclusively upon living species. Ex-