THE LEAF 
69 
connected by means of a narrow passage. The fact that the 
ventral wall is composed of two thickened ridges connected by 
a thin strip has distinct advantages. These ridges are so thick 
that they offer resistance to any tendency to stretch them, so 
that when the cell becomes turgid and 
the thin dorsal wall is stretched, the thin 
part of the ventral wall bends and there 
is a tendency for it to be drawn away 
from the center of the stomatal opening, - 
thus enlarging that opening. This type 
of movement can be imitated roughly by 
means of two boards fastened together at 
one end by a piece of leather to represent 
the thin part of the ventral wall, and at 
the other end by a rubber band to repre- 
sent the dorsal wall (Fig. 53). If the 
rubber band is stretched, it will be seen 
that the other ends of the boards are drawn 
to it. When the guard cell loses its turgid- 
ity, the dorsal wall contracts and the thin 
part of the ventral wall bulges out, and 
it is this part which closes the stomatal 
passage. It is an advantage to have the 
passage closed by the thin part of the ven- 
tral walls (Fig. 50), as thin walls can be 
pressed much closer together than could 
thick, rigid ones. 
Factors affecting opening and closing of 
stomata. The guard cells contain well- 
developed chloroplastids, while these are 












Fic. 52. Apparatus. to 
unitate the opening and 
closing of a stoma 
The two .large rubber 
tubes with adhesive tape 
on the adjacent sides are 
inflated by means of the 
rubber bulb 
usually not present in the other cells of the epidermis, except 
in some plants that grow in moist, shady situations. During 
the day the contents of the guard cells have a high osmotic 
pressure. Consequently, when the surrounding cells are well 
supplied with water, the guard cells draw water from them 
and become turgid, and the stomata open. If, however, the 
