108 Field, Forest, and Wayside Flowers 
stoma cells into contact, so that the double door is 
shut (Fig. 21). 
But when damp weather causes the cells to swell 
again, they stand erect and their sides are drawn 
apart. Then the double door is open, and the 
Fic. 21.—Closed stoma of a Cycas. 
(From the Vegetable World.) 
superabundant moisture in the leaf can pass out 
freely. 
Each stoma opens into one of the spaces in the 
leaf-tissue. 
In general these little holes are irregularly 
placed, but on grass-blades and lily-leaves they are 
ranged in long, straight rows. The number of 
them in a square inch of leaf-surface varies from 
two hundred in the foliage of the mistletoe to two 
hundred thousand in that of the lilac. In the 
white lily they are unusually large, and easily seen 
by a simple microscope of moderate power, and 
some one has had the patience to compute that 
on the lower surface of the leaf there are sixty 
thousand of these little breathing-pores to every 
