But, strangely enough, there are not a few poisonous plants in which we, at least, are unable to detect any such warning sign. Among these are the blue aconite (Aconitum), the black hellebore (Helleborus niger), the meadow-saffron (Colchicum autumnale), species of Gentian, of spurge (Euphorbia), and others. Yet these are avoided by deer, roe-deer, chamois, hares, and marmots, and our cattle, horses, and sheep also usually leave them untouched. A case has, however, been reported from the valley of the Aur, on the lower Rhine, which seems to contradict this. On the rocky grass-slopes of the valley the poisonous hellebore (Helleborus viridis) grows in great abundance, and the sheep of that region, which were wont to graze on the slopes, avoided these plants. But some sheep from another part were imported into the valley, and these ate the hellebore, with the result that many died. If these poisonous plants, then, were furnished with a warning sign such as a disagreeable odour, not perceptible to us, we should have to assume that the imported sheep had a less acute sense of smell than the others, which is not impossible in domesticated animals. If there were no such warning sign, then it must have been not an instinct but a continuous tradition which prevented the native sheep from touching the inedible plants.
A more naïve interpretation of nature than that of our day would have regarded the fragrant ethereal oils developed in the seeds of many plants, as in those of fennel, cummin, and other Umbelliferous plants, as a peculiarity designed for the use and profit of man. But these ethereal substances are obviously a means of protection against the depredations of seed-eating birds, for a sparrow which was allowed to eat three or four seeds of cummin died very soon afterwards.
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.
In other cases the spines are found on the leaves themselves, but there is great diversity in their mode of arrangement. In many tropical plants, such as the Yucca and the Aloe, the point of the long, reed-shaped leaf is transformed into a spine, and this is the case in many of our native grasses. Kerner von Marilaun notes that, in the Southern Alps, two such grasses, Festuca alpestris and Nardus stricta, occur frequently in certain localities, and they prick the muzzles of the cattle so badly that they return bleeding from the pasture. This prevents these Alpine runs from being made full use of, so the grasses are as far as possible extirpated by man, and, curiously enough, also by the cattle themselves, for they seize the grass at the base of the tuft with their teeth, pull it out, and let it fall, so that it withers. Kerner saw thousands of such pieces of turf which had been pulled up by the cattle lying dried and bleached by the sun on some of the Alpine grazing grounds in the Tyrolese Stubaithal.