LEAVES OF PECULIAR FORMS AND USES. 103 
128. Prickly Leaves.—In many whole groups of plants 
the leaves are sufficiently 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. 91. — Compound Leaf of Pea,t 
the leaflets at the upper end in 
Fic. 90. — Branch of a Euphor- the form and doing the work of 
bia, with Spiny Stipules. 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 be found. 
