PROTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 



439 



Prickle structures, which are not to be regarded as metamorphosed terminations 

 of leaf-ribs, but which originate from the epidermis of the leaf, are sometimes 

 multicellular, sometimes unicellular. The former are termed prickles (aculei), 

 the latter bristles (setce). In this series of weapons, barbs are particularly 

 worthy of notice. These are formed by obliquely directed conical cells, which 

 project from the margin of the leaf, and terminate in a hard silicified, generally 

 somewhat bent, apex (figs. 117'' and 117^). Leaves, whose margins are thickly 

 beset with these cells, exhibit, under the microscope, a saw-like appearance. It is 



Fig. 117.— Weapons of Plants. 



^ Baxhei bristles otOpuntia Rafinesquii; x25. 2 xjpper portion of this barbed bristle ; xlSO. " Vertical section through a 

 part of the leaf, based with stinging hairs, of the Stinging Nettle (Urtica dioica); x86. * Capitate termination of a 

 stinging hair; xl50. « The capitate termination broken off; xl60. « Pointed bristles of JHchium Italicum; x40. 

 ^ Margin of a scabrous leaf, beset with barbs, of a Sedge (Carex stricta) ; x 200. 8 Margin of a scabrous leaf, beset with 

 barbs, of a Grass (Festuca arundinacea) ; x 180. 



to be noted that leaves of this kind can, under certain circumstances, really act 

 as saws. If such leaves are very gently stroked in the direction opposite to 

 that of the points, they do not, of course, immediately cut the hand, but they 

 do not bend, and under increasing pressure, the lamina of the leaf becomes 

 arched. Since the leaf is also well stiffened, a resistance is encountered which 

 could scarcely have been expected from so fragile a leaf. If a surface on which 

 portions of these leaves have been laid be shaken, the bits move in a direction 

 opposed to that of the points of the barbs. Movement in the opposite direction 

 is impossible, because it is opposed by these apices. When such leaf-portions 

 get into the mouths of ruminants, they can easily move forward to a particular 



