PHOTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 441 



This is the family of the Boraginese, which has been thus named, indeed, in 

 consequence of its characteristic armour. Examples of the equipment described 

 are furnished in abundance particularly by species of the Viper's Bugloss (Echium), 

 from which the pointed bristles in fig. 117 " are taken, and of the genera Onosma, 

 Comfrey (Symphytum), and Borage (Borago). 



On the leaves on Nettles, Loasaceae, Hydrophyllese, and Euphorbiacese, occurs 

 a very peculiar mode of protection against the attacks of large herbivorous 

 animals, in the formation of stinging hairs or bristles. These stinging hairs are 

 formed of single large cells like the pointed bristles of Boraginese. They expand 

 like a club at the lower end, and are much elongated above. Only in Wigandia 

 urens, which belongs to the Hydrophylleae, is the upper free end finely 

 pointed; in the species of the genus Jatropha, in Loasaceae, and in nettles, the 

 extremity is swollen into a small head, which is bent to one side. At the knee- 

 shaped bend the cell- wall of the stinging hair is extremely thin (figs. 117^-^'^), 

 so that the slightest contact suffices to break off the head. As the head is broken 

 off obliquely, a very sharp point is produced, and the opening formed by the 

 rupture is not horizontal, but oblique, so that the broken end resembles the 

 poison-tooth of a snake or the nozzle of a hypodermic syringe. The breaking, 

 independently of the extreme thinness of the cell-wall below the head, is helped 

 by the brittleness of the hairs, and this is caused by the silicification, sometimes 

 by the calcification, and in Jat7^opha by the lignification, of the cell-wall. This 

 modification of the cell-wall, however, is restricted to the upper part of the 

 hairs. The cell-wall of the club-like swelling at the base of the stinging hair 

 is neither silicified nor calcified, but consists of unaltered cellulose, and yields to 

 an external pressure, so that by such a pressure the outflow of the cell-contents 

 is assisted. By these means, also, the stinging hair is enabled to become turgid, 

 which property certainly plays a very important part in the outflow or outspurt 

 of the cell-contents from the silicified or calcified funnel-shaped apex after the 

 head has been broken off. When by a pressure from above the brittle end 

 of the hair is splintered, and the head broken off", the point formed at the place 

 of rupture penetrates into the body causing the pressure, provided this is soft, 

 as, for example, the skin of men and animals; and the contents are injected 

 into the wound so formed. In the fluid contents of the stinging hair a substance 

 occurs together with formic acid, resembling the unorganized ferments or enzymes, 

 and it is this which produces the violent inflammation round the wound formed 

 by the puncture. The painful sensation felt immediately after the puncture, 

 which is popularly called " burning ", on account of its resemblance to that 

 produced by a burn, is indeed caused by the formic acid; but a series of other 

 phenomena which are observed after the puncture, can only be placed to the 

 account of the enzyme, which acts like a poison. When numerous stinging 

 hairs penetrate the skin in close proximity, a wide area becomes reddened, and 

 inflammatory swellings, with violent pain, are produced. Even the European 

 nettles, viz. Urtica dioica and urens, give rise to unpleasant burning and 



