THE LEAF AND ITS FUNCTIONS 



65 



is more commonly broken into teeth of various sizes. The 

 vein-system (Fig. 31), is either parallel where the veins run side 

 by side with no conspicuous branches; or netted, where they divide 

 and anastomose repeatedly. The petiole and stipules vary 

 greatly in their development. 



Internal Structure (Fig. 33). — Internally, the structure of the 

 leaf is highly differentiated. A transverse section cut at right 

 angles to the surface of the blade (Fig. 34) displays three impor- 

 tant tissues : The epidermis, or protective covering ; the mesophyll, 



Fig. 33. — A small piece of a typical leaf-blade, seen in three planes and highly 

 magnified. A, upper epidermis, covered by the cuticle (in black). B, palisade 

 layer. C, spongy layer. D, lower epidermis, covered by cuticle. E, stoma (in 

 one case seen in section). F, air space. G, vein, cut lengthwise. 



constituting the major portion of the leaf substance, and the 

 veiiis, each of which is a separate fibro-vascular bundle and repre- 

 sents a final branch of the vascular system which runs through 

 root and stem. 



The epidermis covers the entire leaf surface and is usually 

 but one cell-layer in thickness. Its cells are generally thin- 

 walled and are filled with a transparent cell-sap. Spread over 

 the outside wall is a thin, waxy, water-proofing layer, the cuticle, 

 extending from cell to cell and forming a continuous skin which 

 covers the epidermis. It is usually much thicker on the upper 

 than on the lower surface of the leaf. The epidermis is not an 

 unbroken layer but is provided with minute openings, the 

 stoniata (singular, stoma), through which an exchange of gases 

 between the tissues of the leaf and the outside air may take 



