THE CELL STRUCTURE OF A LEAF 



153 



upper epidtermis of leaf 



shows that the stomata open directly into air chambers which pene- 

 trate between and around the loosely arranged cells of spongy tis- 

 sue composing the under part 

 of the leaf. The position of the 

 stomata varies in different kinds 

 of leaves. Most have stomata 

 only in the under epidermis, 

 but some, as the water lily, 

 have them in the upper epi- 

 dermis only. Still others have 

 them in both surfaces. The 

 under surface of an oak leaf of 

 ordinary size contains about 

 2,000,000 stomata. Under the 

 upper epidermis is a layer of 

 green cells closely packed to- 

 gether (called collectively the 

 palisade layer) . These cells are 

 more or less columnar in shape 

 and have tiny green bodies in 

 them. Air can easily pass 

 through the stomata and be- 

 tween the cells of the spongy 

 tissue until it reaches the pali- 

 sade layer. In a section of a leaf cut through a vein, we find 

 the veins to be composed of a number of tubes made up of, and 



strengthened by, thick- 



walled cells. The veins 

 are a continuation of 

 the tubes of the stem 

 which form the frame- 

 work of the blade of the 

 leaf. 



Practical Exercise 6. 



Study the opposite diagram 

 carefully and draw one for 

 your workbook that will show 

 all the structures mentioned 



-Stoma. 



guard cell 



nucleus 



.chloropktst 



C. -vacuole 



lov/er epicCe^mis of leaf 



Compare the upper and lower surfaces of the leaf. 



epictermis 



tissue 



epidermis 



Stomal v duaroC Cell 



A cross section through a leaf, seen through the com- 

 pound microscope. State the use of the vein, the stoma, 

 the air spaces, the palisade layer, the epidermis. 



