94 Introduction to Botany. 



and although it is employed by plants in making their food, 

 it is constantly being replenished by the breathing of ani- 

 mals and plants, by the disintegration of plant and animal 

 remains, by volcanic activity, and by the burning of wood 

 and coal. Since carbon dioxide exists in such small per- 

 centage in the atmosphere, vast amounts of the latter must 

 be sifted by the leaf before sufficient carbon can be ob- 

 tained to build the body of a good-sized plant. A tree 

 having a dry weight of, say, five thousand kilograms would 

 contain twenty-five hundred kilograms of carbon, and to 

 obtain this, twelve million cubic meters of air must have 

 been deprived of carbon dioxide. The spaces between the 

 cells of the palisade and spongy parenchyma allow a broad 

 expanse of free cell surface for the absorption and giving 

 off of gases, the free surfaces acting much as do the gills 

 of a fish in absorbing the small percentage of oxygen from 

 the water. 



72. Action of the Stomata. — When there is sufficient 

 light to enable the chloroplasts to do their work, there 

 being at the same time plenty of water in the soil to satisfy 

 the demands from the leaf, the stomata stand wide open 

 (Fig. 40, a) and permit the ingress of carbon dioxide ; but 

 if the water supply is running low so that the plant is in 

 danger of drying up, the stomata close (Fig. 40, b), even 

 when the leaf is well illuminated. The stomata as a rule 

 close in darkness, but rather from physical than physio- 

 logical reasons. The conditions governing the action of 

 the stomata appear to be about as follows : when the leaf 

 is illuminated, the chloroplasts in the guard cells manufac- 

 ture substances which become dissolved in the cell sap, 

 and so alter its density and constitution. This results in 

 an osmotic inflow from the nei2;hboring tissues, which in- 

 creases the turgidity of the guard cells, causing them to 



