THE LEAF 29 
of water being lost in this manner. This restriction of transpi- 
ration is one of the most important functions of the epidermis. 
Another function is the prevention of the entrance of disease- 
producing organisms into the interior of the leaf. The epi- 
dermis also protects the soft interior from mechanical injury. 
In the epidermis of the leaf are numerous small openings, 
the stomata. These are shown 
in Fig. 18, as seen in the sur- 
face of the epidermis. ‘The 
small oval openings are the 
stomata, while the large, ir- 
regularly shaped cells are ordi- 
nary epidermal cells. A cross 
section of a stoma is shown in 
Fig. 17, in the lower epidermis. 
Each stoma is surrounded 
by two kidney-shaped cells 
called guard cells. Under cer- 

tain conditions these guard 
cells move in such a way as 
to open or close the stomata. 
The stomata allow for the 
exchange of oxygen and car- 
bon dioxide between the cells 
in the interior of the leaf and 
the external atmosphere, and 

ET 
C 
"ANN 
Fia. 18. Surface view of epidermis of 

a dicotyledonous leaf 
The oval openings are the stomata. 
Each is surrounded by two kidney- 
shaped guard cells containing chloro- 
plastids. The large irregular cells are 
ordinary epidermal cells. An epidermal 
hair is shown at the left. (x 150) 
also permit the passage of 
water vapor from the interior of the leaf to the outside air. 
Distribution of stomata. Stomata are usually found only in 
the lower epidermis, or much more abundantly in the lower than 
in the upper epidermis. This would seem to be explained in part 
by the fact that stomata in the lower surface are less likely to 
be closed by rain or by dust than are those in the upper epidermis. 
In leaves which float on the surface of the water stomata occur 
only in the upper epidermis. Submerged plants do not possess 
stomata. 
