114 GENEliAL BOTANY 



The epidermis (Fig. 61, />) is usually composed of a single 

 layer of cells without green chloroplasts, which inclose and pro- 

 tect the more delicate mesophyll cells over the entire surface 

 of the leaf. Its cells are distinctive in that their outer walls are 

 covered with a waxy secretion which makes them almost com- 

 pletely impervious to water and air. The epidermis thus forms 

 an effective protection against excessive evaporation from the 

 mesophyll cells, which would wither and destroy the leaf. The 

 lack of green chloroplasts in the epidermis also enables the sun- 

 light to penetrate easily its translucent cell Avails and illuminate 

 the green mesophyll cells below it. 



The stomata are highly modified cells of the epidermis which 

 regulate to a certain extent the flow of gases into the leaf and 

 the evaporation of water from the internal air spaces between the 

 mesophyll cells. Each stoma is composed of two guard cells 

 containing chloroplastids, between which there is a narrow slit, 

 or pore, averaging about .008 mm., or ^-Q-Q of an inch, in width 

 (Figs. 62, b and c). Although the pore between the guard cells 

 is so very small, ample provision is made for the exchange of 

 gases with the external air and for the evaporation of water from 

 the leaf, since it is estimated that the number of stomata in the 

 epidermis of common leaves averages from 100 to 700 per 

 square millimeter. Stomata are usually open in daylight, when 

 the mesophyll cells are manufacturing sugar and starch, and arc 

 partially or wholly closed at night, during the period of leaf 

 inactivity. The exact mechanism for the control of stomata is 

 not thoroughly understood, although the guard cells are known 

 to be sensitive to light. 



The mesophyll cells vary, in size, form, and arrangement, 

 in different kinds of leaves. In the more typical horizontal 

 leaves (Fig. 62, a) the mesophyll cells of the upper surface next 

 to the epidermis are greatly elongated, with their long axes per- 

 pendicular to the leaf surface. On account of their palisade- 

 like form and arrangement this layer of mesophyll cells is 

 termed the palisade layer. In vertical or erect leaves, such 

 as those of narcissus and many lilies, the palisade layer often 

 extends entirely around the leaf as a continuous layer beneath 



