THE GROUPS OF TISSUES, OB TISSUE-SYSTEMS. 45 



always absent. The breathing-pores on leaves are gener- 

 ally confined to the lower surface, and when present on the 

 upper they are usually much fewer in number; there are, 

 however, some exceptions to this. 



79. In the light, under certain conditions of moisture 

 and temperature, the guard-cells become curved away from 

 each other in their central portions, thus opening the slit 

 and allowing free communication between the external air 

 and that in the intercellular spaces and passages of the leaf. 



The number of breathing-pores has been determined for many 

 leaves. The following table will give an idea of their abundance on 

 some common leaves : 



