440 PROTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 



side, and in a particular manner, such as does not suit the purposes of the 

 grazing animal, and is by no means welcome to it. By firmly stroking the 

 margin of such a scabrous leaf, a wound is produced, the silicified points on 

 the margin acting like the teeth of a fine saw. It is readily intelligible that 

 grazing animals will shun such scabrous leaves; indeed, it is a matter of observa- 

 tion tiiat they seldom (and then only when impelled by great hunger) eat sedges 

 {e.g. Carex stricta and C. acuta), and those grasses which possess particularly 

 sharp-edged leaves. 



Still worse than the barbs of scabrous leaves are the barbed bristles (figs. 117 ^ 

 and 117^), which, it is true, but rarely occur in plants; indeed, almost exclusively 

 on the branches of opuntias. They are always found surrounding the buds, 

 which rise like warts, with fine bristles above the green tissue in opuntias or 

 prickly pears. If such a spot be ever so lightly touched, small stiff bristles 

 will certainly remain sticking in the skin of the hand, and will produce a very 

 unpleasant itching sensation. On trying to pull out these small brown bristles 

 the matter is only made worse, for they then penetrate much deeper into the 

 skin, and may produce violent pain and inflammation. The reason of all this 

 is at once evident on examining one of the bristles under the microscope. Each 

 bristle is composed of numerous rigid, fusiform cells, arranged in spiral rows; 

 at the upper end each of these cells is wedged in between the others, but the 

 very hard, backwardly-directed, pointed end is free, and thus the whole structure 

 is set with barbs. When once the point of the bristle has penetrated the skin, 

 it is held there by the barbed cells. With the slightest pressure they are easily 

 moved forward in one direction, but on trying to produce a movement in the 

 opposite direction, the free ends of the cells resist the attempt, and it is unavoid- 

 able that the forcible extraction of one of these bristles should injure a larger 

 area of the skin than would have been thought possible from the small size of 

 the structure. 



Another form of weapon originating from the epidermal cells consists of 

 stiff" hairs or bristles, with hard silicified cell-wall and sharp apex, which prick 

 and wound like needles, though only unicellular; they are called pointed bristles. 

 They usually project from the surface of the green leaves, closely crowded together, 

 and their points are turned in the direction from which an attack might be expected. 

 They appear gigantic in comparison with barbs, for even the smallest are much 

 longer than these, and the largest resemble pins with their heads imbedded 

 in the leaf -blade. This comparison becomes the more fitting since the pointed 

 bristles are surrounded at their base by very regularly arranged cells which 

 rise above the surface like a cushion, or often like a short white cone. The 

 bristle itself on the end of this pedestal is formed of a single cell, which, when 

 fully developed, loses its protoplasm and becomes filled with air. The wall of 

 this elongated cell is hardened by the deposition of silica, and is usually unequally 

 thickened by small knobs (fig. 117^). Although pointed bristles are developed 

 in numerous groups of the vegetable kingdom, one group is especially so armed. 



