716 PART IV. — THE PHYSIOLOGY OF PLANTS. 



the essential mineral elements of the food, and of ascertaining the 

 physiological significance of these elements in the metabolism of 

 the plant. 



The following is a brief account of the non-essential mineral 

 constituents of the food. 



Silicon, is absorbed from the soil as silica (SiO.^) or as silicates. 

 It cannot be regarded as of nutritive importance, since plants 

 which are usually rich in silica can be brought to an apparently 

 normal development under conditions which render the absorption 

 of silica impossible. It is usually deposited it the cell- walls, as iil 

 Diatoms, Equisetum, many Grasses, etc. 



Iodine and Bromine are found in the many marine plants, 

 especially in Algae, and are prepared from them ; it is not known 

 that they are of any value in the economy of the plant. 



Sodium, being universally distributed, is found in plants. 



Lithium occurs in the ash of several plants, particularly in 

 Tobacco. 



Zinc, Copper, and other metals, though they are not commonly 

 present in the ash of plants, are nevertheless taken up by plants 

 from soils which are rich in them ; from this it appears that 

 plants may absorb substances which are not necessary and may 

 be even injurious. 



3. Anaholism. Under this term are included all the chemical 

 processes going on in the plant which lead to the formation of 

 complex substances from simpler ones (p. 669). Of these, those 

 which are undergone by the food of the plant constitute assimi- 

 lation. 



In the case of plants which contain chlorophyll, the first step 

 in the assimilation of the food is the construction of a carbon- 

 molecule which contains hydrogen and oxygen. The process may 

 be represented by the following equation 



That some process of the kind takes place is proved by the fact 

 that when green plants are placed under the necessary conditions, 

 that is, when they are supplied with carbon dioxide, with water 

 and with salts from the soil, and are exposed to light, they gain in 

 weight in consequence of an increase in the amount of their dry 

 organic substance, and they give off oxygen. Moreover, the 

 volume of the free oxygen evolved is actually equal to that of the 

 carbon dioxide absorbed, as indicated in the equation. 



