140 PROTECTION OF THE GREEN TISSUE 



other plants among the sods of Festuca alpestris, 

 often prick themselves so severely that they come 

 home from the pasture with their noses all running 

 with blood. It is curious that when such grasses 

 are easily uprooted the sheep themselves undertake 

 their destruction. 



Another bristly grass (Nardus stricta), when it 

 grows on the heaths, is pulled out of the ground 

 by the sheep, who seize the clods near the ground, 

 uproot them, and again let them fall, so that the 

 grass soon withers and dies. It is absurd to sup- 

 pose that the flocks undertake with forethought 

 this improvement of their pasture, but we can 

 understand that they may pull up the bristly grass 

 in order to enjoy the other sprouting plants beneath 

 it, without the danger of wounding their mouths 

 with the sharp points. . . . 



Another form of leaf armed with spines is that 

 belonging to the Thistles and their allied forms. 

 Thistle leaves are often three, four, or five-divided, 

 and variously incised and lobed. If the ends of 

 all these divisions are metamorphosed into sharp 

 points, little is left of the green tissue of the leaf; 

 : only a small green space remains in the centre, 

 from which yellow and white thorns stand out on 

 every side (Fig. 38). 



