io8 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 



FIG. 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 



