120 PHYSIOLOGICAL BOTANY. 



4. CONSTITUENTS OF THE PLANT-FOOD. 



It has been discovered by water culture, which consists 

 in growing plants with their roots immersed in water hold- 

 ing known substances in solution, and analysis, that the 

 elements which are essential to the life of plants are Carbon, 

 Hydrogen, Oxygen, Nitrogen, Sulphur, Phosphorus, Potas- 

 sium, Calcium, Magnesmm, and Iron. 



Carbon, which is an essential constituent of organic com- 

 pounds, is absorbed by green plants in combination in the 

 form of the carbonic acid which is found in a small quantity 

 in the atmosphere. 



Hydrogen, which is a constituent of every organic com- 

 pound, is absorbed by all plants in combination usually in 

 the form of water. 



Oxygen, which is a constituent of some organic com- 

 pounds, is absorbed by plants either free, or in combination 

 in the form of water or of salts. 



Nitrogen, which is an essential constituent of albuminous 

 substances, is absorbed by plants in combination in the 

 form of nitrates or of compounds of ammonia. 



Sulphur, which is a constituent of albuminous substances, 

 Mustard oil, etc., is taken up by plants in the form of 

 sulphates. 



Phosphorus, which is a constituent of some organic com- 

 pounds, is derived from phosphates. It is said that this 

 element bears an important relation to certain metabolic 

 processes of plants. 



Potassium, which bears a very important relation to the 

 function of chlorophyll and to the storing-up of carbohy- 

 drates, is derived from its salts. 



Calcium and Magnesium, which are both important for the 

 normal development of plants, are derived from their salts. 



