144 PROTECTION OF THE GREEN TISSUE 



they are large; for even the smallest is much 

 larger than these, and the largest look like needles 

 with their heads buried in the leaf. The bristle 

 itself is formed of a single cell, which is on a sort 

 of pedestal of regularly arranged cells, surround- 

 ing its base. The wall of this long cell is hard- 

 ened by deposits of silica, and often thickened 

 by little knots (Fig. 39, 6). The Borrage family 

 is provided with this form of bristles, as may be 

 seen in the Bugloss (Echium), the Comfrey (Sym- 

 phytum), etc. 



A very peculiar form of protection against the 

 attacks of larger plant-eating animals is the pos- 

 session of stinging hairs, which may be seen on 

 the leaves of Nettles and some other plants. 

 These are formed of large cells, like the sharp 

 bristles of the Borrage, round and enlarged below, 

 and long-drawn-out above. The top is generally 

 enlarged and bent in the form of a knee (Fig. 

 39, 3). Here the cell-wall of the hair is very 

 thin, so that the smallest pressure is able to 

 break it. As the breaking follows an oblique 

 line, a sharp point is made. If this brittle end 

 of the hair is shattered by a pressure from above, 

 the sharp point formed at the place of breaking 

 presses into the opposing body ; if this be soft and 



