THE ANGIOSPERMAE : LEAVES 1003 



Anatomy of the Leaf. 



I. The Epidermis. — The epidermal cells of leaves are very frequently 

 tabular, that is to say, they are flattened in the anticlinal direction and are 

 broader than they are deep. A characteristic feature is likewise their wavy 

 or undulatory outline, in surface view, especially in the Dicotyledons. 

 In soft-leaved Monocotyledons the epidermal cells are generallv elongated, 

 with practically straight walls, but in a few families with hard leaves, such 

 as Gramineae and Cyperaceae, the waviness of the outline reaches an extremitv 

 of complexity, resembling the suturing between the bony plates of the skull 

 (Fig. 990). Waviness is usually more pronounced in sun leaves than in shade 



i t ,;f, ^ , J J 



f^^ 



Fk;. yyo. — Carex pendula. Superficial \ ic-v\ of 

 epidermal cells of the leaf show ing suturing of 

 the walls. 



leaves, even in plants of the same species, and it has been attributed to tiie 

 progressive hardening of the cuticle from the middle of the cell surface 

 outwards to the margin, its consistencv becoming uniform only at maturity, 

 when stretching stops. The increase of the cell-diameter by the stretching 

 of the radial walls will naturally stop first at the points where the hardening 

 first reaches the margins, while other parts of the wall will continue to expand. 

 This view is supported by the fact that waviness affects only the outer contours 

 of the cells, that is to say, on the cuticularized surface. It is not shown by 

 the inner edges of the radial walls, where they meet the inner tangential walls. 

 The degree of w'aviness is related to ecological conditions, being usually less 

 in plants of dry situations and vice versa. This is probably related to the 

 extent of leaf growth, which is greater in moist conditions. 



It is quite exceptional to find chloroplasts or, as a natural consequence, 

 starch grains in the epidermis. Instances to the contrary may, however, be 

 found among plants which habitually grow in deep shade and among sub- 

 merged aquatics, where the epidermis has largely lost its protective functions. 



The foliar epidermis resembles that of the stem in being covered with 



