90 



ELEMENTS OP BOTANY. 



117. Stipules. — Although they are absent from many- 

 leaves, and disappear early from others, stipules form a part 

 of what the botanist regards as an ideal or model leaf.^. When 

 present they are sometimes found as little bristle-shaped 

 objects, at the base of the leaf as in the apple leaf (Fig. 71), 

 sometimes as leaf-like bodies, for example in the pansy (Fig. 

 72), and in many other forms, one of which is that of spinous 

 appendages, as shown in the common locust (Fig. 76). 



118. Relation of Venation to Shape of Leaves. — As soon 

 as the student begins to observe leaves somewhat widely, he 

 can hardly fail to notice that there is a general relation 

 between the plan of venation and the shape of the leaf. How 

 may this relation be stated? In most cases the principal 

 veins follow at the outset a pretty straight course, a fact for 

 which the student ought to be able to give a reason after he 



has performed Exp. 25. 



On the whole the arrangement of the 



\ ins seems to be such as to stiffen the 



1 if most in the parts that need most 



pport, and to reach the region near the 



irgin by as short a course as possible 



I om the end of the petiole. 



119. Parallel - Veined Leaves. — The 

 1 aves of many great groups of plants, 

 such as the lilies, the sedges, and the 

 grasses, are commonly parallel-veined, that 

 is, with the veins running nearly parallel, 

 lengthwise through the blade, as shown in 

 Fig. 73, or with parallel veins proceeding 

 from a midrib and then sending off parallel veinlets, as 

 shown in Fig. 74. 



Fig. 73. — Parallel- 

 Veined Leaf of Sol- 

 omon's Seal. 



1 Unless the elm twigs used in the previous study were cut soon after the Tinfold- 

 ing of the leaves in spring, the stipules may not have been left in any recognizable 



