THE LEAF AND ITS FUNCTIONS 



167 



with a very thin cuticle — a noncellular layer composed of cutin, which is a 

 waxlike, impermeable material secreted by the epidermal cells. 1 The 

 presence of this coating greatly diminishes loss of water by evaporation. 

 Just beneath the cuticle lies the epidermis, a single layer of cells that is 

 continuous over the entire surface of the leaf. The upper surface is usually 

 smoother than the lower, and one or both may be more or less densely 

 covered with microscopic hairs. Small openings, the stomata, penetrate 

 the cuticle and epidermis and communicate with air spaces within the 

 leaf. 



upper 

 epidermis 



palisade 

 parenchyma 



spongy 

 parenchyma 



lower 

 epidermis 



air spaces 



stoma and 

 guard cell 



Fig. 12.2. Cross section of a leaf, including a vein. (Modified from Turtox chart, courtesy 

 General Biological Supply House, Inc.) 



The space between the two epidermal layers is filled, except where the 

 veins occur, with parenchymous tissue, which is differentiated into two 

 layers. The upper layer, one or two cells deep, is composed of tall, finger- 

 shaped cells standing vertically beneath the epidermis. This is the palisade 

 parenchyma. Its cells are closely packed together but with air spaces so 

 arranged that each cell has at least one face in contact with the air. The 

 palisade cells have a large central vacuole filled with cell sap, surrounded 

 by a thin layer of protoplasm containing numerous small green plastids 

 (chloroplasts) , which contain chlorophyll. The lower and thicker layer 

 of the parenchyma consists of a mass of thin-walled, loosely arranged cells, 

 the spongy parenchyma, enclosing abundant air spaces that communicate 

 with those of the palisade parenchyma. The cells of the spongy paren- 



1 A cutin layer covers all exposed surfaces of seedlings, herbaceous plants, and the 

 young twigs of woody plants, as well as leaf surfaces. 



