LEAVES or PECULIAR FORMS AND USES. 



103 



128. Prickly Leases. — In many whole groups of plants 

 the leaves are sufB.ciently prickly or spiny to serve the plant 

 as a protection against browsing animals. Oftentimes the 

 prickles are borne on the midrib or the principal veins only, 

 as in some kinds of nightshade (Fig. 88). At other times the 

 tip of the midrib, or the tips of that and other veins become 

 spiny, as in the thistle. In many acacias, and in some euphor- 



FiG. 90. —Branch of a Euphor- 

 bia, witK Spiny Stipules. 



Fig. 91. — Compound Leaf of Pea,' 

 the leaflets at the upper end in 

 the form and doing the work of 

 tendrils. 



bias (Fig. 90), the stipules form slender spines. Sometimes, 

 in the barberry, for example, whole leaves become narrowed 

 and hardened into spines. (Fig. 89.) 



129. Leaves as Aids to Climbing. — Some pinnately com- 

 pound leaves, like those of the pea, terminate in a tendril 



1 In young seedlings, as the student has already learned during the germination 

 experiments, only one pair of leaflets will he found. 



