THE LEAF. 241 



through the epidermis into the intercellular spaces of 

 the parenchyma. These thin-walled cells which guard 

 the opening separate when swollen with moisture, 

 and close together, so as to cover it, when dry. They 



FIG. 408. 



are called stomata, or breathing-pores. In some plants, 

 as the under surface of the leaves of the white lily, 

 there are about sixty thousand of these stomata to 

 the square inch ; while in the epidermis of the upper 

 surface there are only about three thousand to the 

 square inch. They vary in different plants from less 

 than a thousand to one hundred and seventy thousand 

 to the square inch of surface. Examine the epider- 

 mis from any part of a plant, from the stem, or from 

 sepals, petals, etc. You will often find it furnished 

 with stomata, but you will look in vain for them in 

 the leaves of water-plants. 



The lower side of the leaf has generally more 

 hairs than the upper side. These hairs are continua- 

 tions of epidermal cells, and vary much in structure. 



Fig. 409 represents a magnified portion of the 

 epidermis of a cabbage-leaf. The oblong slits are 

 stomata, while the pointed, protruding bodies are 



