NUTRITION. 165 



uously so long as the supply outside exists. Substances ab- 

 sorbed by the plant and not utilized accumulate, and their 

 solutions soon attain the same degree of saturation within the 

 plant as outside, when they cease to be absorbed. It is for 

 this reason that two plants growing upon the same soil may 

 contain very unequal quantities of any important material. 

 Plants thus exert a sort of selective action, but this selection 

 is dependent upon purely physical laws, and is not directly 

 under the control of the plant. 



226. Carbon dioxide. — Carbon dioxide, as such, is not 

 found in nature. * It instantly combines with water to form 

 a gas known as carbonic acid gas, and this is ordinarily 

 meant when carbon dioxide is spoken of. This gas exists in 

 small quantities in the atmosphere, rarely exceeding one part 

 in twenty-five hundred, except in secluded spaces. The 

 constant currents in the atmosphere make its distribution 

 practically uniform. On account of its ready solubility, this 

 gas also exists in abundance in soil waters and in the larger 

 bodies of water constituting streams, lakes, or pools. In a 

 soil containing carbon compounds it is constantly being pro- 

 duced by decomposition. The water which passes through 

 the soil therefore has a larger percentage of this gas than the 

 air, sometimes containing as much as one per cent. 



227. Absorption. — Water plants readily absorb the dis- 

 solved gas by such surfaces as are exposed to the water. 

 Floating plants have opportunity to obtain it both from the 

 water and from the atmosphere. Land plants, although 

 their roots are surrounded by a comparatively concentrated 

 solution of carbonic acid, do not take up appreciable quan- 

 tities by these organs. On the contrary, the absorption of 

 this gas seems to depend entirely upon those cells which 

 contain chlorophyll. The stomata, which allow the internal 

 intercellular spaces free communication with the outside air, 

 are important organs, not only in regulating transpiration, 



