142 PROTECTION OF THE GREEN TISSUE 



The prickly organs , which are not to be regarded 

 as metamorphosed ribs, but which originate in the 

 epidermis of the leaf, are sometimes many-celled, 

 sometimes one-celled. One form of the one-celled 

 prickles are barbs, formed of oblique conical cells, 

 which stand out from the margin of the leaf. 

 They end in a firm, hard point, which is usually 

 somewhat hooked (Fig. 39, 7, 8). Leaves with 

 their margins thickly covered with such barbs look 

 like a saw under the microscope, and they are really 

 able under some circumstances to act like a saw. 

 If we stroke one of these barbed leaves very gently 

 from the side towards which the barbs point, they 

 do not cut into the opposing hand, but they also 

 do not bend, but make a firm resistance. With 

 increased pressure of the hand, the leaf itself is 

 bent, but, as this is also well stiffened, the pressing 

 hand experiences a resistance which would not be 

 expected from so tender a leaf. If the hand is 

 passed violently over the sharp margin, a bloody 

 cut is made, in which the flinty teeth act precisely 

 like the teeth of a fine saw. It is natural that 

 grazing animals should shun these sharp leaves, 

 and, in fact, they only eat them under great stress 

 of hunger. 



Another form of arms, which has its origin in 



