177. In Grasses, when the sheathing base of the leaf may answer to petiole, the summit of the sheath commonly projects as a thin and short membrane, like an ocrea: this is called a LIGULA or LIGULE. 178. When stipules are green and leaf-like they act as so much foliage. In the Pea they make up no small part of the actual foliage. In a related plant (Lathyrus Aphaca, Fig. 173), they make the whole of it, the remainder of the leaf being tendril. 179. In many trees the stipules are the bud-scales, as in the Beech, and very conspicuously in the Fig-tree, Tulip-tree, and Magnolia (Fig. 179). These fall off as the leaves unfold. 180. The stipules are spines or prickles in Locust and several other Leguminous trees and shrubs; they are tendrils in Smilax or Greenbrier. § 4. THE ARRANGEMENT OF LEAVES. 181. =Phyllotaxy=, meaning leaf-arrangement, is the study of the position of leaves, or parts answering to leaves, upon the stem. [Illustration: Fig. 181. Alternate leaves, in Linden, Lime-tree, or Basswood.] [Illustration: Fig. 182. Opposite leaves, in Red Maple.] 182. The technical name for the attachment of leaves to the stem is the _insertion_. Leaves (as already noticed, 54) are _inserted_ in three modes. They are _Alternate_ (Fig. 181), that is, one after another, or in other words, with only a single leaf to each node; _Opposite_ (Fig. 182), when there is a pair to each node, the two leaves in this case being always on opposite sides of the stem; _Whorled_ or _Verticillate_ (Fig. 183) when there are more than two leaves on a node, in which case they divide the circle equally between them, forming a _Verticel_ or whorl. When there are three leaves in the whorl, the leaves are one third of the circumference apart; when four, one quarter, and so on. So the plan of opposite leaves, which is very common, is merely that of whorled leaves, with the fewest leaves to the whorl, namely, two. [Illustration: Fig. 183. Whorled leaves of Galium.] 183. In both modes and in all their modifications, the arrangement is such as to distribute the leaves systematically and in a way to give them a good exposure to the light. [Illustration: Fig. 184. A piece of stem of Larch with two clusters (fascicles) of numerous leaves.] [Illustration: Fig. 185. Piece of a branch of Pitch Pine, with three leaves in a fascicle or bundle, in the axil of a thin scale which answers to a primary leaf. The bundle is surrounded at the base by a short sheath, formed of the delicate scales of the axillary bud.]