68 A TEXTBOOK OF GENERAL BOTANY 
night and open in the day; but when the rate of evaporation 1s 
rapid, they also close during the day. When the guard cells are 
turgid the stomata are open, while as the guard cells lose their 
turgidity the stomata close. 
Mechanism of opening and closing of stomata. ‘’o understand 
how changes in the turgidity of the guard cells cause the open- 
ing and closing of the stomata it will be necessary to study the 
structure of the guard cells. There are a number of types of 
guard cells, but fer simplicity we may consider only what is 
probably the most widely dis- 
tributed one. A transverse 
section through a guard cell 
(Fig. 50) shows that the wall | 
which is farthest from the sto- 
matal pore (dorsal wall) is very 
thin, while that which is next 
to the pore (ventral wall) is 
much thicker. When the guard _ 
cells become turgid their walls 
naturally tend to stretch; but 
the ventral walls, owing to their great thickness, stretch much 
less than do the dorsal walls. The result is that the cells become 
bent, with the concave side toward the pore. This is the princi- 
pal cause of the opening of stomata. When the guard cells lose 
their turgidity they straighten cut and come together, thus clos- 
ing the stomata. This type of bending can be illustrated by 
means of a rubber tube or an elongated toy balloon, to one side 
of which adhesive tape has been attached. When the tube is 
inflated it will bend, the side to which the adhesive tape has 
been applied being the concave side (Fig. 52). 
The ventral wall is not uniformly thickened. In the center, 
as seen in cross section, it is thin. Away from the center it is 
greatly thickened, and along the edges it projects into the 
stomatal pore. The central part of the wall also bulges into 
the pore. The stomatal pore is thus not of uniform diameter, 
but consists of two more or less inclosed chambers which are 

Fic. 51. Combined cross section and 
surface view of a stoma 
