XX 



ON WALLS, ROCKS, AND STONY PLACES 



Several of our flowering plants are to be seen most frequently on 

 walls and rocks, or in other situations where there is hardly a trace 

 of soil of any kind. Some of these thrive in such dry spots, with 

 often such free exposure to the rays of the spring or summer sun, 

 that it is difficult to understand how they manage to survive 

 the periods of drought through which they live until we become 

 acquainted with certain pecuharities of their form and structure. 



In the first place we must recall the fact that plants lose a 

 considerable amount of moisture by evaporation from the transpirat- 

 ing surfaces of their leaves, and that this loss must necessarily be 

 greatest when the air is warm and dry unless there is some means 

 by which the transpiration is automatically regulated according 

 to the requirements of the plant and to the varying conditions 

 under which it has to exist. 



The leaves of plants are covered with a thin skin or epidermis 

 which consists of a single layer of cells, and which is practically 

 impermeable to moisture. In this epidermis, however, on one or 

 both sides of the leaf, are minute pores {stomata) through which 

 water vapour is free to pass ; and beneath the porous epidermis is a 

 loose, cellular tissue, with air-spaces, from which the moisture can 

 readily pass, in the form of vapour, to these stomata. 



Each of the stomata is bordered by a pair of crescent-shaped 

 guard-cells, placed with their concave sides towards each other, 

 and joined at the ends. Further, the guard-cells are capable of 

 changing their form, becoming straighter, and thus reducing or 

 even closing the aperture between them ; and becoming again 

 curved, opening or enlarging the pore. The former change takes 

 place during darkness, thus preserving the plant from the cooling 

 effects of evaporation during the chilly nights ; and also during 



