General PJiysiology of tJie Plant. 191 



of the atmosphere (p. 42). Parasites and saprophytes (p. 

 100) and carnivorous plants (p. 213), however, obtain their 

 supply of carbon from organic compounds present in living 

 or dead animals and plants. 



Importance. Carbon forms fully half the dry weight of 

 the plant, and forms an essential constituent of proteids, 

 carbohydrates, fats, and many transition substances (p. 26). 



HYDROGEN. 



Origin. From the water obtained from the soil and 

 from the air : also from ammonia and its compounds present 

 in the soil. 



Importance. As in the case of the carbon, hydrogen 

 is an essential constituent of proteids, carbohydrates, fats, 

 and manv transition substances. It also, of course, forms 

 an essential constituent of the omnipresent water. 



OXYGEN. 



Origin. Most of the oxygen used in the formation of 

 organic compounds in the plant is obtained from water 

 and salts containing oxygen. The oxygen, on the other hand, 

 required in the oxidising of protoplasm in the liberation of 

 energy, is obtained from the boundless supply of free oxygen 

 in the air. 



Importance. Again an essential constituent of the 

 substances mentioned under hydrogen, as well as forming a 

 necessary condition of plant life in its free form for purposes 

 of oxidation (respiration, p. 35). 



NITROGEN. 



Origin. The nitrogen found in combination in the 

 plant body is entirely obtained from nitrogen in combination 

 in the soil, as nitrates, nitrites, &c. Notwithstanding the 

 enormous quantity of free nitrogen in the air, none of it 

 seems to be used directly by the plant as food. 



