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. 
