Again, in many plants the whole leaf-edge is transformed into a spiny wall, which may be enlarged by indentations and lobate projections, as in the holly, and, in a much higher degree, in the thistles (Carduus), in Eryngium, in Acanthus, and in many Solanaceæ. Often, too, there are barbed hooks on the leaf-edge, which work like a saw; or the leaf-edge, though without spines, may be made sharp by deposits of silicic acid, as in the sedges, whose sharp edges are moved to and fro in the mouths of ruminants, and thus injure the mucous membrane. The hook-bristles of the fig-cactus (Opuntia), which, though small, are abundantly provided with barbs, must also be mentioned; for they are to be found in great numbers surrounding the buds of these plants, and most effectively protect them from being eaten away by animals (Fig. 19).
To this category, too, belong the short, prickly bristles of the rough-leaved plants, which cover the whole plant as with an overcoat of sharp needles; of these we may mention the adder's tongue (Echium vulgare), the comfrey (Symphytum officinale), and the borage (Borago officinalis).
Very well known are the stinging-hairs of the Urticaceæ, long hairs (Fig. 20) with an elastic base, but with glass-like, brittle, rounded heads, which break off at the lightest touch, whereupon the sharp point of the broken hair penetrates the skin of the creature which has touched it, and the poisonous contents of the hair are poured into the wound. Even our large stinging-nettle (Urtica dioica) can cause intense irritation, and evoke the 'nettle-rash,' named after it, on the human skin; but there are many tropical species of nettle, e.g. Urtica stimulata in Java, and others, which have an effect similar to that of snake-poison and produce tetanoid spasms, and so on. In addition to formic acid these hairs contain an undefined ferment, a so-called Enzyme. It need scarcely be said that these stinging-hairs must have much more severe effects on the mucous membrane of the mouth of grazing animals than on the human skin, and that they are therefore an excellent protection for the plants. As a matter of fact we never find our nettle patches eaten away, and even the donkey, which eats thistles freely, turns away from the stinging-nettle. But even these stinging-hairs, like all other protective devices, do not afford an absolute protection. The caterpillars of several of our diurnal butterflies feed exclusively on the stinging-nettle, and they eat up the leaves, stinging-hairs and all. This is the case with five species of the genus Vanessa, namely: Vanessa io, the 'peacock,' Vanessa urticæ, the small tortoiseshell, Vanessa prorsa, Vanessa C. album, the C. butterfly, and Vanessa atalanta, the admiral.
Fig. 19. Barbed bristles
of Opuntia rafinesquii; enlarged.
Fig. 20. Vertical section through
a piece of a leaf of the Stinging-nettle
(Urtica dioica), bearing two stinging-hairs;
magnified 85 times; adapted
from Kerner and Haberlandt.
We are all familiar with our mulleins (Verbascum), those beautiful flower-spikes with the thick, soft felt leaves, which grow on stony or sandy soil. Harmless as they look, they are much disliked by animals as food, for the thick hairy felt which covers them breaks up in the mouth, and sticks in the folds of the mucous membrane, causing burning sensations and other discomforts. They, too, are therefore spared by grazing animals, but they have smaller enemies, like the caterpillars of the genus Cucullia, which, however, never completely destroy them, but only eat large holes in their leaves.
Let us now consider in somewhat greater detail the true thorns, the most conspicuous protection of many plants. It is very remarkable that these are always so placed, and so regulated as to their length and character, as to afford protection to the most important and the most exposed parts of the plant. Thus many bushes, which would otherwise be in danger of being completely devoured by cattle, are stiff with thorns which are nothing else than pointed, hard twigs without, or with very little foliage. Among these are the sloes, the buckthorn (Rhamnus), the sea-buckthorn (Hippophäe), and the barberry (Berberis). In the last-named three thorns arise in a group, and protect the young bud from danger in three directions (Fig. 21).
