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, 



FIG. 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 



