fats. It is not always easy to determine of what other uses disagreeable or toxic chemical constituents may be to the plant; the point that concerns us in this discussion is, that they protect it from being eaten, and of this there can be hardly any doubt.
M. Stahl's study of the mechanical defenses of plants is no less interesting than that of their chemical armor. Many of the weapons of this character are obvious and well known; but some of them are more difficult of discovery, while a great variety prevails among them. In the large majority of cases the mechanical defense consists of a hardening of some parts of the plants, which may be general, so as to form a kind of carapace, or local, in the production of hard special organs, such as hairs, thorns, or needles, making it harder for animals to reach the plants. Sometimes the mechanical weapons are associated with chemical qualities, as in the nettle, crane's-bill. Primula sinensis, blessed thistle, etc. They either serve to prevent or impede the access of snails and slugs, to make it harder for them to take hold of the alimentary part, or to cause pain during the eating.
Hairy plants certainly offer more obstacles to snails going about on them than do glabrous plants. If we place a snail upon a comfrey-plant, it will find itself very uncomfortable, unable to get any hold on the leaves, and continually brought to a stop by the disagreeable contact of the hairs with its tentacles; and a free snail or slug will be hard to find on this plant. Other hairy plants possess immunity in less marked degrees; and M. Stahl's conclusions from his experiments as a whole are that chemical armor is more efficient than hairs. In some cases downy plants were preferred, while chemically armored species were always respected. So, when glabrous and downy species of the same family were tested, downy ones were eaten, while smooth ones were left alone. Hence, the hairs afford only an inefficient defense. M. Stahl accounts for this by supposing that, while the smooth plants are protected by disagreeable chemical constituents, the hairy plants are without this armor, or else present attractive qualities of odor or taste, against which their hairs are only an imperfect set-off.
Some plants are defended by the calcification of their superficial cells. The snails would not eat the leaves of Erysimum cheirantoides (treacle-mustard) when fresh, or even when treated with alcohol, but attacked them readily after the carbonate of lime had been dissolved out by acetic acid. The same was observed with other plants having a similar property. The grasses are protected against attacks from many animals by the silicification of the walls of their cells, without which the new enemies that would be added to supplement the assaults of their present foes would make an end of the whole family. This may be tested