146 BACTERIA 



elements do not enter the plant as such, but combined with 

 other substances or dissolved in water. Potassium occurs in 

 salt form combined with various organic acids (tartaric, 

 oxalic, etc.), calcium and magnesium as salts of lime and 

 magnesia in combination both with organic and inorganic 

 acids. Iron contributes largely to the formation of the 

 green colouring matter of plants, and is also derived from 

 the soil. Phosphorus, one of the chief constituents of seeds, 

 generally occurs as phosphate of lime. Sulphur, which is 

 an important constituent of albumen, is derived from the 

 sulphates of the soil. In addition to the above, there are 

 other elements, sometimes described as non-essential con- 

 stituents of plants. Amongst these are silica (to give stiff- 

 ness), sodium, chlorine, iodine, bromine, etc. All these 

 elements contribute to the formation or quality of the 

 protoplasm of plants. 



The gases essential to plants are four: Carbon dioxide 

 (carbonic acid). Hydrogen, Oxygen, and Nitrogen. By the 

 aid of the green chlorophyll corpuscles, and under the influ- 

 ence of sunlight, we know that leaves absorb the carbon 

 dioxide of the atmosphere, and effect certain changes in it. 

 The hydrogen, as we have seen, is obtained from the water. 

 Oxygen' is absorbed through the root from the interstices 

 of the soil. Each of these contributes vitally to the exist- 

 ence of the plant. The fourth gas, nitrogen, which consti- 

 tutes more than two thirds of the air we breathe (79 per 

 cent, of the total volume and yy per cent, of the total weight 

 of the atmosphere), is, perhaps, the most important food 

 required by plants. Yet, although this is so, the plant can- 

 not absorb or obtain its nitrogen in the same manner in 

 which it acquires its carbon — viz., by absorption through 

 the leaves — nor can the plant take nitrogen into its own sub- 

 stance by any means as nitrogen, with the exception of the 

 fiesh-feeding plants (insectivorus). Hence, although this 

 gas is present in the atmosphere surrounding the plant, the 



