CH. VIl] STOMATA. 103 



leaf and examined under the microscope, it will be evident 

 by what means the water passed this membrane. 



FIG. 47. 



THBEE STOMATA WITH SURROUNDING EPIDERMIC CELLS (E) : 

 (?, G, guard cells of a stoma. 



Scattered thickly among the ordinary epidermic cells 

 are structures known as stomata, shown in fig. 47. Each 

 stoma is made of a pair of kidney-shaped cells called 

 guard cells, fitting together with their concave sides 

 inwards and leaving an oval cleft by which the inter- 

 cellular spaces of the leaf communicate with the external 

 air; it should be noted that the guard cells differ from 

 ordinary epidermic cells in possessing chloroplasts. Each 

 stoma is at first a single cell which is divided into 

 two compartments by a cross wall. The cross wall finally 

 splits into two layers between which the opening of the 

 stoma lies. The remarkable form of the guard cells is 

 well seen in transverse section (fig. 46). 



The stomata have the power of opening and shutting 



