STETTCTUEB AND FOEMS OF STIPULES. 97 



lamina, form a tendril or cirrus (p. 120), so as to enable the plant 

 to climb. 



Stipules. 



At the place where the petiole joins the axis, a sheath (vagina) is 

 sometimes produced, which embraces the whole or part of the cir- 

 cumference of the stem (fig. 147 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 

 Rhubarb order, it is large and membranous, and has received the name 

 of ochrea or boot (fig. Ii7 g) ; while in Palms it forms a kind of net- 

 work, to which the name of reticulum has been given (p. 32) ; and in 

 umbelliferous plants it constitutes the perieladium (■rsg;, around, and 

 xkdhoi, a branch). In place of a sheath, leaves are occasionally pro- 

 duced at the base of the petiole (fig. 205 s s), 



which have been denominated stipules (stipula, ^ 'y^v\ 



straw or husk). These stipules are often two _^<-^^jSv|\ 

 in number, and they are important as sup- Ni^:^?^^;;^) 

 plying characters in certain natural orders. ^^>^^M^ 

 Thus they occur in the Pea and Bean family, ||s 



in Rosaceous plants, and the Cinchona bark * -^ 



family. They are rarely met with in Mono- ^ 



cotyledons, or in Dicotyledons with sheath- 

 ing petioles, and they are not common in ^^ " 

 Dicotyledons with opposite leaves. Plants having stipules are stipu- 

 late; those having none are exstipulate. 



Stipules are formed by some of the vascukr bundles diverging as 

 they leave fce 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 distinguishes 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, perform- 

 ,. ing the office of leaves. 



When stipules are attached separately to the stem at the base of 

 the leaf, they are called caulinary. Thus, in fig. 205, r is a branch 

 of SaliK aurita, with a leaf, /, having a bud, 6, in its axU, and two 

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

 other, at the same height on the stem, it occasionally happens that the 



a 



Fig. 205. Portion of a branch, r, of Salix aurita bearing a single petlolate leaf, /, whicih 

 bas been cut across. . s s, Caulinary stipules. 2), Bud. iu the axil of the leaf, 



H 



