324 Biological Chemistry. 



THE UTILIZATION OF NITROGEN OF INORGANIC 

 COMPOUNDS. 



It is easy to demonstrate that plants can grow when 

 the only source of nitrogen at their disposal is that present 

 in inorganic salts. If a seedling is supported in a vessel 

 with the roots immersed in a solution containing the neces- 

 sary elements, and the upper part of the plant free in the air, 

 it will grow normally, provided that the nutrient solution 

 employed is one adapted to the plant under investigation. 

 This method of investigating plant growth is known as 

 that of water-culture. The elements which appear to be 

 necessary for the normal growth of the higher plants are 

 the following : Nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, and iron, and sometimes chlorine. It 

 is not proposed to consider in detail the function of all 

 these elements. Nitrogen, sulphur, and phosphorus form 

 part of the various organic constituents of the plants, 

 such as the proteins, nucleo-proteins and lipoids ; and 

 magnesium, as has already been stated, enters into the 

 molecule of the two chlorophylls. For the purpose of 

 making water-cultures, the composition of the nutrient 

 medium can be varied for different plants. One solution, 

 which is very largely employed, contains the following 

 salts in the proportions given : 



Potassium nitrate .1 part. 



Potassium hydrogen phosphate (KH 2 P0 4 ) . . 1 ,, 



Magnesium sulphate .*"' V '' . .1 



Calcium nitrate . ., . . . . 4 parts. 



with a trace of iron salts. 



A solution containing salts in these proportions is 

 generally known as Knoop's fluid, and is employed in 

 concentrations of O'l per cent, of solids for young plants, 



