116 



BIOLOGY 



a hard cell wall forming a layer that is impervious to liquids 



and even to gases. They form a covering of the leaf which 



prevents the entrance of water, and 

 protects it from too great a loss of 

 water by evaporation. Through the 

 epidermis are numerous openings 

 known as stomata (Gr. stoma = 

 mouth), st, that serve as breathing- 

 pores. If a bit of the epidermis is 

 stripped from the leaf, it will ap- 

 pear as shown in Figure 58. The 

 cells of the epidermis are irregular 

 in shape, due to the irregular growth 

 of the leaf, and among them are nu- 

 merous pores. Each pore is sur- 

 rounded by two crescent-shaped cells, 

 guard cells, so related to each other 

 that the pore itself lies between the 

 two crescent cells. The guard cells 

 are capable of expansion and con- 

 traction under different conditions. 

 As they expand, they straighten out 



and close the opening of the stomata; and when they contract 



they shorten slightly and the opening 



of the stomata is enlarged; Fig. 59. 



In this way they can change the size 



of the breathing pores of the plant 



and thus regulate the amount of air 



that passes in and out of the leaf. 



These stomata occur in the epidermis 



of the petiole and all over the leaf, less 



abundantly on the upper side than on 



the under side. In the leaves of water o c < s uard cells - 



plants, however, the stomata are chiefly on the upper side of the 



leaves, where they are in contact with the air when the plant 



FIG. 58. THE EPIDERMIS 

 SHOWING THE STOMATA 



A, from the leaf blade; B, from 

 the petiole. 



FIG. 59. DIAGRAMMATIC 

 CROSS SECTION OF A 



STOMA 



