THE COVERING TISSUES 89 



less transparent. The epidermal membrane (Plate 

 32) is seldom more than one layer of cells in thickness; 

 and the individual cells are set so closely together 

 that air and water cannot gain access to the inner 

 tissues except through specialized organs called stomata, 

 (singular, stoma}. Stomata are distributed among 

 the epidermal cells, especially those of the lower leaf- 

 surface. Each stoma consists of two semicircular 

 cells surrounding an oval opening or breathing pore. 

 Stomata are of importance in the respiratory processes 

 of the plant. Certain leaves, on sectional view, show 

 several layers of cells apparently similar in structure 

 to the epidermal cells to which they are attached. 

 These layers are termed subepidermal cells and occur 

 frequently in leaves of tropical plants, where they 

 serve as a protection against the excessive heat to 

 which such plants are exposed. The individual epi- 

 dermal cells, as seen on surface view (Plate 33), are 

 polygonal in shape, or else show a very irregular 

 outline, because of their wavy walls. Sectional views 

 show that the upper or exposed surface of the cell 

 is thicker-walled than the surfaces in contact with 

 other cells. The cell walls frequently show markings 

 which are of use in identification. Striations are 

 markings or lines parallel with the wall of the cell 

 as seen on surface view. These markings may pos- 

 sibly be due to the deposition of cutin or other pro- 

 tective substances in successive layers. The thicken- 

 ing of the exposed wall may be partially extended 

 into the side walls of epidermal cells, thus affording 

 greater protection. Beaded cell walls are side walls 

 which have become thickened at irregular intervals 

 around their circumference or margin. Roughening 



