DIV. I 



MORPHOLOGY 



109 



their origin into the leaf-base (Fig. 125 A and B, g) and the upper 

 leaf (Fig. 125 A, B, o). From the leaf-base the stipules (g) arise or it 

 forms aleaf-sheath or a thickened pulvinus. Frequently it undergoes 

 no special further development and is not distinguishable in the 

 mature leaf. The leaf -blade (Fig. 116 A, sp) is developed from the 

 upper leaf, and so also when this is present is the leaf-stalk (A, st). 

 The latter develops relatively late by intercalary growth and is thus 

 intercalated between the already present leaf-blade and leaf-base ; it 

 is never inserted directly on the stem. 



(a) The Leaf-blade. External Form (Fig. 127). The leaf -blade, 







FIG. 126. Leaf of Crataegus with reticulate 

 venation, (f nat. size. After NOLL.) 



Fin. 127. Diagram of a foliage leaf. 

 .4, Surface view. B, Transverse 

 section ; s, plane of symmetry. 

 (After STRASBURGER.) 



which is as a rule definitely dorsiventral and of a deeper green colour 

 on the upper side, may be entire or divided (Fig. 122 C), or composed 

 of a number of leaflets. Such compound leaves arise by a process 

 of branching from the margins of the primordia (Fig. 122 A), or 

 occasionally, as in the Palms, by splitting of the young lamina as it 

 unfolds. The leaves of Monocotyledons are usually simple, while 

 compound leaves are common among Dicotyledons. 



A leaf-blade is termed PELTATE when the leaf-stalk appears to be inserted 

 centrally (Fig. 242). The margin of simple leaves (Figs. 123 Ib, 124 sp} may be 

 ENTIRE or slightly divided, and in the latter case is described as SERRATE, DENTATE, 

 etc. If more deeply divided the leaf is described as LOBED when the divisions do 



