Green Leaves at Work 107 
stomata gape open; and during times of drought, 
when it is desirable that the plant’s fluids shall be 
saved, they close. This timely opening and shut- 
ting is effected by a mechanism extremely simple, 
yet perfect in its working. 
Each breathing pore is like a double door, 
Fic. 20.—A cell of the leaf-skin and one stoma of a fern 
(Pteris cretica) 
whose leaves to left and right are cells. And 
these cells, like their neighbors in leaf-skin and 
tissue, become distended in moist weather, and 
shrink in time of drought. As soon as they be- 
come flaccid they collapse, like an empty pair of 
bellows, and their sides bulge like the bellows- 
leather. This bulging brings the walls of the two 
