Kerner and Stahl have proved that tannin is distasteful to snails, goats, rabbits, sheep, and cattle. It is therefore not strange that natural selection has brought about the accumulation of this disagreeable substance in organs and at stages of growth needing special protection; and, in fact, it is most abundant in young leaves, flowers, and unripe fruits.
Wigand has shown that red color often accompanies tannin, and it is also true that red-leaved plants commonly contain oxalic acid. Pick suggests that the union of elements of the oxalic acid and of the red coloring matter, resulting in the formation of crystals of calcium oxalate, prevents the formation of such an excess of the acid as would prevent the action of the ferments by which starch is made diffusible. Raphides are bundles of calcium oxalate crystals. Stahl says that the single-pointed crystals which exist in certain irises are an effectual protection against snails, and that in the pickerel weed there are, besides these, raphides and cells rich in tannin—perhaps a suggestion that raphides may have arisen as the further development by natural selection of such solitary crystals. For these delicate, sharp rods are a formidable defense, piercing the skin of a would-be destroyer of the tissues containing them like so many needles. (Taste Jackin-the-pulpit leaves.)
So there is law in it all. Tannin abounds in plant-tissues. Where it is, red color often appears; and where there is red color, oxalic acid is frequently found; where oxalic acid is, raphides may be formed; and, finally, where there is tannin or raphides or both, there are substances generally disliked by herbivora—a long story, which the red color of the exposed parts of many plants doubtless tells briefly but effectively to their enemies.
For example, various members of the orpine family are not eaten by large animals, because their leaf-tips—the most available parts—contain tannin, "as shown by their dark-red color." Otto Kunze says that the Javanese surround their coffee plantations with a living hedge of red-leaved plants, so keeping off the swine, which abhor this color. The brightly tinted leaves of young oaks, maples, etc., are seldom eaten; and in the tropics, where there is the severest struggle for existence, gay leaves are most abundant.
The mottled leaves of arum, lady's-thumb, some everlastings, and prince's pine, and probably of adder's tongue and cyclamen, are protected by raphides; those of begonia have sour, and of coleus and wild ginger, bitter sap. Such variegations of leaf-surface, which may be imperceptible to the larger animals, may have much significance to smaller ones. Species of caterpillars and of beetles are often confined to particular orders or even genera of plants, presumably because, like the cockatoo to the kanary nut, they have adapted themselves to the peculiar characters of these plants,