Green Leaves at Work 105 



stances (salts) which have come up with it from 

 the soil remain behind in the leaf cells. These 

 will enter into the composition of the living proto- 

 plasm which is filling the new cells of shoot and 

 root. 



Though we know how plants make starch, com- 

 paratively little has been learned about the more 

 vital process of protoplasm-making. But it is be- 

 lieved that in green plants this work, too, can go 

 on only in the presence of light. 



As the water from the roots is to go directly to 

 the leaf -laboratories, Nature has taken care that 

 the precious fluid shall not be wasted on the way. 

 So the trunks and branches of trees and shrubs 

 are wrapped in a skin of cork, which prevents the 

 ascending nutritive water from evaporating. But 

 once in the leaves it is desirable, in most cases, 

 that the water may evaporate and give up its 

 chemical and mineral treasures. So the leaf, broad 

 and thin, -exposes the largest possible proportion of 

 surface to the light and air. 



Over the whole leaf veins, cells, and all there 

 is stretched a transparent skin. A powerful micro- 

 scope shows this skin to be itself a sheet of cells, 

 often very irregular in form and generally destitute 

 of chlorophyll. 



