THEIR STRUCTURE AND FORMS. 



100 



base, as in many umbelliferous plants ; sometimes it forms a 

 sheath, occasionally it is bordered with appendages, &c. Peti- 

 oles ma} r assume special functions, to be hereafter considered. 

 The woody and vascular tissue runs lengthwise through the 

 petiole, in the form usually of a definite number of parallel 

 threads, to be ramiiied in the blade. The ends of these tlnv:i<U 

 are apparent on the base of the leafstalk when it falls off, and on 

 the scar left on the stem, as so many round dots (Fig. 81, 85, 

 91), of a uniform number and arrangement in each species. 



203. Partial Petioles are the divisions of the petiole in a 

 compound leaf. The footstalk of a leaflet takes the diminutive 

 name of PETIOLULE. 



204. Stipules (157) are lateral appendages, one each side of 

 the base of the petiole, sometimes free 



from it and from each other (Fig. 

 142), sometimes attached by one 

 edge to its base (Fig. 211), some- 

 times united with each other into a 

 single body (Fig. 212) in various 

 wa} T s or degrees. In the latter case, 

 they usually appear to be within the 

 base of the leaf or leafstalk ; or, as 

 in the Plane-tree, they may be joined 

 into one over against the leaf, as 

 if opposite to it, but their normal 

 position is supposed to be lateral or 

 marginal to the petiole. Sometimes 

 they are foliaceous in appearance and 

 in function ; sometimes they are dry 

 and colorless or scale-like, reduced to 

 mere epidermal tissue, and evidently 

 functionless ; sometimes (as in Mag- 

 nolia, Fig. 81, Fig-tree, and Beech), 

 they serve as bud-scales, and fall when 

 the leaves develop ; sometimes they are 

 reduced to a mere bristle, or take the 

 form of a spine, as in the Locust (Robinia) . Between salient 

 expansions or wing-like margins of the base of the petiole, 

 such as those of the Saxifrage tribe, and stipules adnate to the 

 margins of the petiole, as in most Rosacese, there is no clear 

 limitation. But presence or absence of stipules generally runs 



FIG. 2H. Clover-leaf, with adnate stipules. 212. Ochreate stipules (ocbrea or ocrea 1 . 

 of Polygoumn orientale, sheathing the stem for some distance, and ending in a spread- 

 ing border. 



