38 BOTANY. 
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 
Fic. 24.—A bit of the epidermis of Wild Cucumber (Echinocystis), showing 
breathing-pores at s, s,s. Atg, g, the epidermal cells are irregular; at v, over a 
vein, they are more regular. Magnified 250 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 betweén the external air 
and that in the intercellular spaces and passages of the leaf. 
