NUTRITION. 139 



many additional compounds, of no use in forming food, are taken in. 

 These are all found in the ash, when a plant is completely burned, 

 though not necessarily in the same form in which they were absorbed. 



187. Selective action. — The compounds which exist in 

 the water in various, though small amounts are not taken into 

 the plant in the same proportions as they exist in the water. 

 Substances which are utilized by the plant and which, there- 

 fore, disappear as such within it by having their chemical 

 composition altered or by being stored up in a different form 

 and so removed from solution, will enter the plant contin- 

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

 sorbed and not utilized accumulate in the water inside the 

 plant, and these solutions soon attain the same degree of con- 

 centration as those outside. Then they are no longer ab- 

 sorbed. It is for this reason that two plants growing upon 

 the same soil may contain very unequal quantities of any im- 

 portant material. Plants thus exert a sort of selective action, 

 but this selection is dependent upon purely physical laws, and 

 is only indirectly under the control of the plant. 



188. Carbon dioxid. — Carbon dioxid is a gas, which is 

 always present in the air, in which, however, it exists in small 

 quantities, rarely exceeding one part in twenty-five hundred. 

 An abundant supply of it is constantly being returned to the 

 air by the breathing of animals and plants, by burning of fuel 

 and by slow decomposition of dead bodies of plants and 

 animals. 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. 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. 



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

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



