CELLULAR STRUCTURE OF LEAVES 247 
A surface view of a small portion of epidermis stripped off and 
highly magnified is shown in Figure 230. The epidermal cells in 
this view are irregular in shape, but so closely fitted together 
that no openings occur except through the stomata. A stoma 
(singular of stomata) is a definite structure, consisting of two 
curved cells, known as guard cells, which are so fitted together 
as to enclose a slit-like opening. The guard cells are so named 
because they regulate the size of the opening. Some plants, 
such as the Grasses of which Corn is 
a familiar example, have a peculiar 
type of guard cells as Figure 231 
shows. In this case the guard cells 
are enlarged at the ends, and re- 
semble dumbbells in shape. How- 
ever, this difference in shape seems 
to have nothing to do with their 
behavior, for they open and close 
their slit-like opening. just as the 
ordinary type of guard cells is able 
to do. 
By changes taking place within 
the guard cells, the stomata are 
opened and closed, but the causes 
of such changes are not definitely 
known. The guard cells have 
. . Fie. 231. — A much enlarged 
chloroplasts, and there is consider- : : : 
: surface view of the epidermis of 
able evidence that the chloroplasts Gom, showing one: stoma. g, 
have something to do with bringing guard cells; 1, slit-like opening; 
about these changes. Since chloro- ¢, epidermal cells. The chloro- 
plasts make sugar and have the plasts are in the ends of the guard 
: cells. 
power to transform sugar into starch 
or starch into sugar, it is evident that they can alter the concentra- 
tion of the sugar in the cell sap and in this way alter the turgor pres- 
sure of the guard cells. For example, if the chloroplasts of the 
guard cells manufacture much sugar which is allowed to concen- 
trate in the cell sap, then by the principle of osmosis the guard 
cells draw in water forcibly and develop a high internal pressure 
which tends to expand them and alter their shape. On the other 
hand, if the chloroplasts remove the sugar from the cell sap by 
changing it into starch, which is insoluble, the result may be 
