86 



FOEMS OF SIMPLE LEAVES. 



sist of one or more leaflets (foliola) separately attached to the petiole 

 or leaf-stalk (fig. 156). In a single leaf the blade may be either' 

 attached to a petiole or sessile on the stem ; while in a compound leaf 

 the blades or leaflets are separately attached to the petiole. In the 

 earliest stage of growth aU leaves are simple and undivided, and it is 

 only during the subsequent development that divisions appear, which 

 may commence at the base or at the apex of the leaf The forms 

 which the difierent kinds of simple and compound leaves assume 

 are traced to the character of the venation, and to the amount of 

 parenchyma produced. 



SiSiPLE Leaves. — When the parenchyma is developed symme- 

 trically on each side of the midrib or stalk, the leaf is equal (fig. 164); 

 if otherwise, the leaf is unequal or oblique (fig. 151), as in Begonia. 

 If the margins are even and present no divisions, the leaf is entire (irir- 

 teger), as in figs. 164 and 165 ; if there are slight projections of cellular 

 or vascular tissue beyond the margin the leaf is not entire (fig. 151) ; 

 when the projections are irregular and more or less pointed, the leaf 

 is dentate or toothed (fig. 170); when they lie regularly over each 





Fig. 164. Fig. 165. Fig. 166. 



Fig 16r. Fig. 158. 



Fig. 15B. 



other, like the teeth of a saw, the leaf is seirate (figs. 151, 169); when 

 they are rounded, the leaf is crenate (fig. 174). If the divisions extend 

 more deeply than the margin, the leaf receives different names accord- 

 ing to the nature of the segments : thus, when the divisions extend 

 about half-way down (figs. 149, 159), it is cleft (fissus), and its lines of 

 separation are called fissures (fissura, a cleft) ; when the divisions 

 extend nearly to the base or to the midrib (fig. 185), the leaf is 

 partite, and its lines of separation are called partitions. 



These divisions take place in simple leaves exhibiting different 

 kinds of venation, and give rise to marked jforms. Thus, if they 

 occur in a feather-veined leaf (fig. 152), it becomes either pinnatifd 

 (pinna, a wing or leaflet, and fissus, cleft), when the segments extend 



Fig 164. Lyrate leaf of Barbarea. Fig. 156. Panduriform, a fldaie-shapcd leaf of 

 Rumex pulcher. Fig. 156. Compound leaf, temate, the leaflets being obcordate. 



Fig. 167. Compound leaf ; quaternate, the leaflets being rotundate-ouneiform, or wedge- 

 shaped with rounded apices. Fig. 168. Two-lobed leaf, somewhat cordate at the base, 

 emarginate, and mucronate. Fig. 159. Palmate leaf, the divisions acute and serrated at 

 their margins. Radiating venation. 



