THE SIMPLER ORGANISMS 83 



The four in the first column, carbon, hydrogen, oxygen 

 and nitrogen constitute over 99 per cent of the living sub- 

 stance, the others being present in very small amounts. 



In nature these elements are found everywhere in the 

 crust of the earth, combined as simple mineral salts, which, 

 being more or less soluble, are found also in the waters of 

 the earth. That these salts will maintain the life of the 

 green plant cell may readily be determined by supplying 

 them to it as food. The commonly used food solution for 

 green plants has the following composition: 



Distilled water (H 2 O) 1,000. grams 



Potassium nitrate (KNO 3 ) i. 



Sodium chloride (NaCl) 0.5 " 



Calcium sulphate (CaSO 4 ) 0.5 



Magnesium sulphate (MgSo 4 ) 0.5 " 



Potassium phosphate (K 2 HPO 4 ) . . 0.5 



Ferrous sulphate (Fe 2 SO 4 ) a trace 



Here we have all of the twelve elements listed above ex- 

 cept carbon, and this the green plant obtains from the car- 

 bon dioxide (CO 2 ) of the air, either direct, if it be a terrestrial 

 plant, or dissolved in the water, if it be aquatic. On such a 

 solution of the simplest mineral compounds green plants 

 thrive. These elements are recombined in the living body 

 into compounds of very much greater variety and complex- 

 ity, the more important of which fall into two great classes, 

 according as they possess or lack nitrogen in their composi- 

 tion: 



I. Carbohydrates and fats: non-nitrogenous compounds, 

 containing carbon, oxygen and hydrogen, but no nitrogen. 



II. Proteins : nitrogenous compounds of great complexity. 

 These substances, formed in, and constituting the bodies 



of plants, are the primary food of animals. 



Energy. The forces that operate upon living bodies are 

 those that operate upon the non-living: gravitation, heat, 



