ELEMENTARY CONS-TlTtJENTS OF PLANTS 171 



To both the solutions add a few drops of ferric chloride solution. 



2. Essential elementary constituents of plants. — ^The follow- 

 ing is a brief account of the elements which are absolutely 

 necessary for the nutrition of plants. 



(i) Carbon. — Carbon is one of the essential constituents of 

 protoplasm, and enters very largely into the composition of the 

 cell-wall, and many reserve foods of plants. The amount present 

 in plants usually amounts to between 40 and 50 per cent, of the 

 dry matter within them. The greater portion of it is derived 

 from the carbon dioxide of the atmosphere, but in some cases, 

 and perhaps in all, a certain amount of carbon is taken from the 

 soil in the form of organic compounds. 



Fungi among the lower, and Dodder {Cuscuta), (p. 573), 

 Broom-rape {Orobanche), (p. 574), and Bird's-nest orchis {Neoitia) 

 among the higher plants, obtain their carbon in the form of 

 organic carbon compounds from living animals and plants, or 

 from the decaying remains of these organisms. 



(ii) Hydrogen and oxygen are found combined with carbon 

 and other elements in the protoplasm, cell-wall, sugars, fats, and 

 other compounds present within the plant. 



Hydrogen is a constituent of water, and in this form is chiefly 

 absorbed from the soil. Between 5 and 6 per cent, of the dry 

 matter of a plant is hydrogen. 



The amount of oxygen present in the dry matter of plants 

 averages between 35 and 45 per cent. It is absorbed in a free 

 state from the air in the respiration-process, and is also taken up 

 from the soil in nitrates, sulphates, carbonates and phosphates. 



(iii) Nitrogen. — This element enters into the composition of 



