122 THE EVOLUTION THEORY 



In other cases the spines are found on tlie leaves themselves, hut 

 there is threat diversity in their mode of arrangement. In many 

 tropieal 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, Feshwa alpestris and Nardus 

 strict a, occur frequently in certain localities, and they prick the 

 muzzles of the cattle so badly that the}'- 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. 



Again, in many plants the whole leaf-edge is transformed into 

 a spinj- wall, which may be enlarged by indentations and lobate 

 projections, as in the holly, and, in a much higher degree, in the 

 thistles {Canlmin), in Eryngium, in Acanthui<, and in many Solanace^e. 

 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 bi'istles of the 

 rough-leaved plants, which cover the whole plant as with an over- 

 coat of sharp needles; of these we may mention the adder's tongue 

 (EcJiium vulgare), the comfrey {Symphytwm officinale), and the borage 

 [Borago officinalis). 



Very well known are the stinging-hairs of the Urticaceae, 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 [TJrtica 

 clioica) can cause intense irritation, and evoke the 'nettle-rash,' 

 named after it, on the human skin; but there are many tropical 



