106 MORPHOLOGY OP LEAVES. 



through a natural order. Yet what are called stipules in one order 

 ma}- pass for expansions or appendages of the petiole in another. 

 In Spergularia, some stipules are connate around the base of the 

 pair of leaves, including them as well as the stem in the sheath. 1 



205. Stipules, which are normally a pair, may unite into one 

 body, either adnate to the inner face of the leaf, as in some 

 species of Potamogeton, or united opposite the leaf, as in Plane- 

 tree, or united inter se in a sheath, as in Polygonum. Also when 

 the leaves are opposite and the stipules thus brought into prox- 

 imity, the adjacent half stipules of the two leaves may coalesce, 

 and present the appearance of only two stipules to two leaves, 

 as in many Rubiaceae. A notch or fork at the apex often 

 indicates the composition. 



206. Sheathing stipules, like those of Polygonum (Fig. 212), 

 are said to be ochreate, or (better) ocreate ; the sheath, thus 

 likened to a leggin or the leg of a boot, is an OCHREA, as written 

 by Willdenow, or better OCRBA. 



207. The LIGULE of Grasses (Fig. 150) is seemingly a thin and 

 scarious extension of the lining to the sheath which answers to 

 petiole in such leaves : it projects at the junction of the sheath 

 and blade, there forming a kind of ocrea ; and it is generally 

 regarded as a sort of stipule. 



208. Stipels (Stipella) are as it were stipules of leaflets, which 

 are common in certain tribes of Papilionaceous Legnminosje, e. g. 

 in the Phaseoleae, in Wistaria, Locust, &c. ; also in Staplrylea. 

 They are small and slender, and, unlike stipules, they are single 

 to each leaflet, except to the terminal one, which has a pair. As 

 leaves furnished with stipules are said to be stipulate, so leaflets 

 with stipels are stipulate. 



209. Some unusual modifications of leaves as foliage. In 

 leaves as illustrated thus far, it is the lamina or blade which is 

 expanded to do the work of foliage ; which is expanded hori- 

 zontally, so as to present upper and under surfaces, one to the 

 sky, the other to the ground ; which is bilaterally symmetrical 

 or substantially so, the two lateral halves being nearly if not 

 quite alike ; and which is affixed to the stem at the basal margin, 

 or some part of it, with or without a petiole. Various deviations 

 or apparent deviations from this pattern occur. Some of them are 

 of comparative!}' small account and simple explanation, such as 



210. Inequilateral Leaves, being unsymmetrical by the much 

 greater development of one side. This is illustrated in the 

 whole genus Begonia (as in Fig. 161), consisting of many spe- 



1 As pointed out by Prof. A. Dickson, in Nature, xviii. 507. 



