Many plants produce bitter substances in their green parts, and so secure at least some measure of protection, as is the case with the majority of mosses, the ferns, and species of _Plantago_ and _Linaria_. Others, again, deposit silicic acid in their cell-walls, or develop in addition a very thick epidermis, so that they afford at the best an unpleasant food, e.g. many grasses, the horse-tails, the rhododendron, and the bilberry. Others, again (_Alchemilla vulgaris_), have cup-shaped leaves, which retain rain and dew for a long time, and this protects them from grazing animals, which are unwilling to touch wet grass and plants. Especially widely distributed and diverse is the protection of plants by sharp thorns and spines. It is extremely interesting to note in how many different and advantageous ways this armature is disposed. Obvious at once is the fact that thorns and spines only occur on those parts which are naturally exposed to attack. Thus we find them particularly strong in young plants, and on the lower parts of older ones. The holly, for instance, has crenate, spinose leaves only to the height to which grazing animals can reach; beyond that the leaves are smooth-edged and spineless, like those of the camelia. It is almost the same with some wild pear-trees, which are quite covered with thorns as long as they are low, but afterwards grow a thornless crown. Similarly, low bushes, when they are armed with thorns or the like at all, are covered with them all over, like the rose-bush. When the leaves of a plant are spinose the spines are disposed on the parts usually attacked; and thus we understand why the enormous floating leaves of _Victoria regia_ should have on their under surface long, pointed spines which, especially at the upturned margin, attain a length of several inches; it is from water animals--water snails--that danger threatens them. Thorns are developed in the most diverse ways. In many of the bushes on the coast of the Mediterranean true leaves are wanting altogether, the green branches and twigs being themselves the assimilating parts, and these are so stiff and rigid, so like some kind of thorn, that they suffice to scare off any greedy herbivore. Among our own bushes the Broom (_Spartium scoparium_) may be taken as an example of this class.