STRUCTURE AND FORMS OF STIPULES. 83 



and broad. Some petioles, in place of ending in a lamina, form a 

 tendril or cirrhus (If 201), so as to enable the plant to climb. 



Stipules. 



158. At the place where the petiole joins the axis, a sheath 

 (vagina) is sometimes produced, which embraces the whole or part of 

 the circumference of the stem (fig. 182 g). This sheath is formed by 

 the divergence of the vascular bundles which separate so as to form a 

 hollow cavity towards the stem. The sheath is occasionally developed 

 to such a degree as to give a character to the plants. Thus, in the 

 Khubarb tribe, it is large and membranous, and has received the name 

 of ochrea or boot (fig. 132 #); while in Palms it forms a kind of net- 

 work, to which the name of reticulum has been given (^[ 57) ; and in 

 umbelliferous plants, it constitutes the pericladiwn (vtpl, around, and 

 xA<jf, a branch). In place of a sheath, leaves are occasionally pro- 

 duced at the base of the petiole (fig. 189 s s), 



which have been denominated stipules (stipula, 

 straw or husk). These stipules are often two 

 in number, and they are important as sup- 

 plying characters in certain natiu-al orders. 

 Thus they occur in the Pea and Bean family, 

 in Rosaceous plants, and the Cinchona bark 

 family. They are rarely met with in Endo- 

 gens, or in Exogens with sheathing petioles, 

 and they are not common in Exogens with 



opposite leaves. Plants having stipules, are stipulate; those having 

 none, are exstipulate. 



159. Stipules are formed by some of the vascular bundles diverg- 

 ing as they leave the stem, and becoming covered with parenchyma, 

 so as to resemble true leaves. Like leaves they are large or 

 small, entire or divided, deciduous or persistent, articulated or non- 

 articulated. Their lateral position at the base of the petiole, distin- 

 guishes them from true leaves. In the Pansy, the true leaves are 

 stalked and crenate, while the stipules are large, sessile, and pinnatifid. 

 In Lathyrus aphaca, and some other plants, the true pinnate leaves 

 are abortive, the petiole forms a tendril, and the stipules alone are 

 developed, performing the office of leaves. 



160. When stipules are attached separately to the stem at the base 

 of the leaf, they are called caulinary. Thus, in fig. 189, r is a branch 

 of Salix aurita, with a leaf, f, having a bud, &, in its axil, and two 

 caulinary stipules, s s. When stipulate leaves are opposite to each 



Fig. 189. Portion of a branch, r, of Salix aurita, bearing a single petiolate leaf,/, which has 

 been cnt across, s s, Stipules, b, Bud in the axil of the leaf. 



