3o8 THE BIOLOGY OF FLOWERING PLANTS 



On the other hand, they offer no protection against grass- 

 hoppers. 



Specialised hairs, only partly mechanical in their action, 

 are the stinging hairs such as those of the nettle. The long 

 tapering point of the unicellular hair is hardened by a deposit 

 of chalk, the rounded tip by silica ; just below the tip is a 

 weak spot. Pressure breaks off the tip and the fine point 

 enters the skin of an animal touching the plant ; at the same 

 time the soft bulb-like base of the hair is compressed and 

 the contents injected into the wound. The well-known 

 inflammation follows quickly. The nature of the toxic sub- 

 stance causing it is not known ; it is probably a protein. 

 The similar stinging hairs of some tropical plants belonging 

 to the Urticaceae, e.g. Urera, Laportea, and to the Loasacese, 

 e.g. Loasa, Blumenbachia, are much more violent and may 

 have dangerous results. 



The silicification of cell walls may make it difficult for an 

 animal to eat a plant. This is well shown by many grasses 

 and sedges, where the epidermal cells along the leaf margin 

 are strongly silicified and also rough. The blade of 

 Aira ccespitosa or that of Phragmites communis cuts the 

 fingers if drawn through them. The leaves of many 

 bamboos and other grasses of warmer zones cut like razors, 

 inflicting deep wounds. 



Perhaps the most important protection against snails 

 and slugs is the presence of raphides, the bundles of needle- 

 like crystals of calcium oxalate deposited in the cells of 

 many plants, as, for example, the rhubarb, the cuckoo- 

 pint, and the wood hyacinth. The needles lie parallel 

 in the cell, but are scattered if the tissue is chewed and 

 penetrate the delicate mouth parts of the snail in every 

 direction, choking them effectively. Stahl, who has 

 investigated this point exhaustively, finds that raphide- 

 bearing plants are never touched by snails or slugs, and are 

 avoided by other animals. If swallowed the raphides may 

 have the fatal effect of powdered glass. 



Chemical Protection. — Of chemical means, of protection 

 the most widespread are the bitter and astringent tannins. 



