THE WOMK OF LEAVES 



221 



on which side their greatest development occurs and 

 how this is affected by light. 



The leaf, more than any other part of the plant, 

 needs protection 

 against animals, 

 insects and para- 

 sites. Devices for 

 this purpose are 

 found in great 

 variety. Make a 

 practical study of 

 the following 

 forms of protec- 

 tion. 



( a ) Prickles, 

 spines, thorns, 

 hairs, etc. These 

 protective weap- 

 ons are borne by 

 both leaf and 

 stem, and may be outgrowths 

 from the surface (prickles of 

 Eose and Thistle, stinging hairs 

 of Nettle, which break off in 

 the wound and discharge an irritating poison into it, 

 hairs of Mullein, which are irritating to the mouth, 

 etc.), or the spiny tip of the. leaf (Thistle, Spanish 

 Bayonet, etc.), or the teeth of the edge of the leaf 



130. Horizontal (trail- 

 ing) branch from the 

 same plant from 

 which the upright 

 branch shown in Figs. 

 128 and 129 was taken. 



