10 



STRUCTURE OF THE LEAF 



from such leaves as the live-for-ever, blue flag or hyacinth. The 

 cells of the epidermis are very compactly put together and may be 

 stripped off by means of a penknife as a thin white skin. Ex- 

 amined under a microscope, such preparations show the stomata 

 as minute openings guarded by two rather lens-shaped cells, 

 termed the guard cells (Fig. 6). Chloroplasts usually occur 

 only in the guard cells, the other epidermal cells having a watery, 

 colorless cell content. In addition to these features. Fig. 7 shows 

 that the outer portion of the walls of the epidermal cells is modi- 

 fied so as to form a delicate, skin-like layer over the outer sur- 



FiG. 7. Fig. 8. 



Fig. 7. Section across a stoma shown in Fig. 6: s, stoma; g, guard cell; 

 c, cuticle; a, air chamber. — H. O. Hanson. 



Fig. 8. Hairs from the epidermis of squash leaf. 



face of the epidermis. This part of the cell wall is called the 

 cuticle. It contains a fatty substance, cutin, that renders the 

 epidermis nearly impervious to gases and fluids, so that the leaf 

 is entirely surrounded by a water- and gas-proof coat. In many 

 leaves hairs of various forms grow out from_the epidermal cells 

 (Fig. 8). 



5. The Mesophyll. — The cells within the epidermis have, for 

 the most parts, a greenish color due to numerous green bodies, 

 chloroplasts, contained in the cells (Fig. 5, ch). These green 

 cells of the leaf, termed the mesophyll, consist of one or more 

 layers of rather elongated and compact cells on the upper side 

 of the leaf (the palisade mesophyll, Fig. 5, p), and below they 



