38 



BOTANT. 



76. They occur on aerial leaves and stems most abun- 

 dantly, being sometimes exceedingly numerous, and are 

 exceptionally found elsewhere, as on the parts of the flow- 

 ers. On submerged or underground stems and leaves they 

 are found in less numbers, and from true roots they are 

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

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



Fig. 34. — A bit of the epideimis of Wild Cucumber (Echinocystis), showing 

 breathing-pores at 8, s, s. At g^ p, the epidermal cells are irregular; at v, over a 

 vein, they are more regular. Magnified SSO times. 



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

 however, some exceptions to this. 



77. 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. 



