THE LEAF. 41 



trous if the three angles are sharp (fig. 40), triangular if they are about 

 right angles, and trigonous when the angles are 

 obtuse or rounded off. When the surface 

 presents a great number of longitudinal ridges, 

 it is called ribbed (tig. 42) : numerous longitu- 

 dinal grooves render it furrowed (snlcatus). In 

 some cases the projecting angles of stems are 

 winyed (alatus), as in many Thistles ; in other 

 cases the stem or branch is flattened, so as to 

 resemble a leaf, in which case the term cladode 

 is applied, as in Ruscus (fig. 43). Such leaf-like 

 branches or cladodes are distinguishable from 

 true leaves by their axillary position, mode of 

 origin, internal structure, and by the circumstance 

 that they bear flowers. Foliaceous cladodes of 



The apex of a stem or branch is usually pointed Pu*cu aculeatu*. 



or conical, but it may be globose or concave, as in the flower-stalk of a Rose. 



d. The surface of a stem may be smooth (Icecis) or striate (striatus), i.e. 

 marked with tine grooves and ridges. It may be devoid of epidermal 

 appendages or glabrous (glaber), or furnished with a more or less dense 

 coat of hairs, bristles (setosus),or thorns (epfaofttt). Similar terms are 

 still more commonly applied to the surfaces of leaves. 



e. Ramification. A stem is either simple or branched; if the ramifica- 

 tion is excessive, it is called much-branched (ramosissimus). The branches 

 may be erect, spreading (patens), outstretched (divaricatus), deflexed (de- 



flexus), OT pendulous (pendulus). These qualities especially affect the crown 

 or head of trees. 



f. Dimensions. Different terms are applied to plants with woody stems, 

 according to their size and mode of branching. A tree (arbor) is a plant 

 with a woody trunk and branched head. A shrub or bush (frutex) is a 

 kind of dwarf tree, where the main trunk is little developed, but the 

 lateral branches very much so. Under-shrub (fruticulus) is the dimi- 

 nutive of this. 



Sect. 4. THE LEAF. 



Leaves are the lateral organs issuing from the ascending portion 

 of the stem and its branches below their growing points, and in 

 general are flat, expanded plates, produced directly from the 

 superficial part of the stem, and from which, after a certain term 

 of existence, they are removed, either by breaking off at a distinct 

 joint, or by decay. 



The simplest leaves occur as flat plates traversed by a nerve, as in 

 Mosses. In many Algae and cellular Cryptogams processes of the thallus 

 may be seen resembling leaves in form and arrangement, but not in struc- 

 ture. The term phyllome is used in a comprehensive sense to signify any 

 leaf, or modification of a leaf, springing from a caulome or axis. In some 

 cases, as in Cactus (figs. 35-37), the true leaves are absent, their office 

 being filled by the green stem. Normal leaves, belonging to the vege- 

 tative system, are alone taken into account in this chapter ; the modified 

 foliar organs composing flowers must be treated separately. 



