54 MORPHOLOGY, OE COMPARATIVE ANATOMY. 



they are united by their inner margins above the petiole, so as to 

 form a compound axillary stipule ; in the Polygonacese they are 

 not only united on this side, but also by their outer margins on the 

 other side of the stem, thus forming a short tubular sheath round 

 the latter, called an ocrea (fig. 65). All the above cases relate to 

 stipules of single leaves ; but similar coherence or lack of disunion 

 occurs in the stipules of opposite leaves, where it is not uncommon 

 to find the two stipules which stand between the leaves, at back 

 and front, more or less confluent into a single leaf-like or scale- 

 like body (interpetiolar stipule), so as to form a kind of whorl with 

 the true leaves. 



This interpetiolar confluence of the stipules is very characteristic of the 

 Order Rubiacese ; and the apparent whorls of the Stellatce (Galium, &c.) 

 often exhibit a confluence of the highly developed leaf-like stipules. 



At the summit of the sheath of the leaf of Grasses exists a little 

 membranous scale, connecting the blade with the epidermis of the 

 stem; it is either entire or forked at the top (figs. 59 * & 60*); 

 this structure, called the ligule, is a mere excrescence from the 

 stalk. 



The stipules of some plants fall off at an early period. This is the case 

 with the interpetiolar stipules of various Rubiaceous plants. It also 

 occurs commonly when the stipules form the outermost envelopes of the 

 leaf-buds, as in Magnoliacese, Ficus elastica, the Beech tree, &c. 



Small secondary stipules exist at the base of the partial petioles 

 of some compound leaves, especially of LeguminosaB (Desmodium) ; 

 they are called stipels (stipellce). 



For convenience of description the stipule has been here treated 

 as if it were uniformly of the same nature, varying only in form, 

 position, &c. In point of fact, however, the morphological nature 

 of the stipules varies in different plants : sometimes they represent 

 mere excrescences from the petiole ; at other times they consist of 

 the lower leaflets of a compound leaf (Laihyrus'), or they may be 

 leaves formed on a contracted and rudimentary axillary branch. 



Lamina. The lamina or blade (b, fig. 54) of the leaf constitutes 

 the most important part of the structure, and exhibits the greatest 

 . variety in its forms, which latter require to be studied in detail, as 

 they often furnish the principal characters for the discrimination 

 of species of Flowering Plants and Ferns. It is ordinarily a flat 

 plate, possessing an upper and lower surface, turned respectively 

 towards the sky and the earth, two margins, a base, and an apex. 



In plants of succulent habit the thickness of the leaves is often so great 

 that the sides are as broad as the surfaces, or they are more or less 

 confounded in a cylindrical, prismatic, or some similar form (Mesembryan- 



