Leaves and their Work 171 



covered with beech-forest, all this gas, even though it 

 amounts .to billions of tons, would be gone in eight 

 years ! For one acre of beechwood takes about a ton 

 of cmhon every year ; and to supply one ton of carbon, 

 three and a half tons of carbon-dioxide gas are neces- 

 sary. This is taken up not only by the leaves, but 

 by all the green parts of a plant, leaves, buds, stems, 

 and fruit, so long as these remain green ; for it is only 

 in the cells which contain leaf-green that starch is 

 manufactured from the gas. These green cells lie im- 

 mediately underneath the thicker-walled but transparent 



Surface leaf-cells, with pores, magnified. 



cells of the surface which compose the skin ; and through 

 the skin the gas finds its way into them. 



The leaf-pores, by which water escapes, are openings 

 in the skin formed by two curved, lip-like cells, which 

 gape open in hot, bright weather, and close more or 

 less in rain, damp, and darkness ; and it is when they 

 open most widely that the manufacture of food goes 

 on most briskly. For it is then that most carbon 

 is separated, and most food is pumped up from the 

 roots, as that is the time when the plant transpires 

 most, and in this way both kinds of food are received 

 together. When there is much transpiration, and water 

 containing dissolved food is pumped up rapidly, then 

 also much carbon is received, and vice versa. 



