THE IMPORTANCE OF HAIRS IN PLANT LIFE 75 





noted that 

 internode. 



the line of hairs on the stem alternates with each 



PROTECTION. Hairs are often developed on the stom or 

 leaves, or on both, in order to protect the plant from animals. 

 Stinging Nettles are let severely alone, and some people have 

 supposed that the Dead-nettle has gained some advantage to 

 have leaves which very closely resemble those of the Stinging 

 Nettle. If a Nettle is grasped from below, the hairs which point 

 forwards are merely compressed and do not sting. 



These hairs contain an acid fluid, which causes the irritation. 

 The leaves of the common Elm have on 

 their under surface, especially along the 

 ribs, hairs which sting, though not so 

 severely as those of the Nettle. This tree 

 is often planted in hedgerows, and is no 

 doubt protected from browsing animals 

 by these hairs. The teeth of the Holly are 

 a protection to the tree. Lord Avebury 

 notices in this connection that the upper 

 leaves which are out of the reach of animals 

 tend to lose their spines, and old trees 

 are often almost entirely without them. 

 (British Flowering Plants, p. 281.) 



Another plant which has stiff, almost 

 prickly, leaves protecting it from animals 

 is the Viper's Bugloss. 



In some plants prickles are developed instead of hairs. These 

 are often protective in function; they may be also an aid in 

 climbing. 



Flowers are sometimes protected from creeping insects by 

 hairs on the involucre, as in Crepis pakidosa. In the Composite 

 it is very common to find the bracts of the involucre with stiff 

 hairs, e.g. Knapweed. The stomata of leaves are often pro- 

 tected from dust by hairs. In London parks the hairs on the 

 under surface of the leaves of the Wych Elm are often found 

 covered with soot particles, which are thus prevented entering 

 the stomata. 



FIG. 42. Under surface of 

 leaf of Elm, showing hairs 

 along the ribs. 



