Natural History 605 



from the air. The raw material they take in by means of their 

 roots is no less indispensable, and this consists of water and 

 mineral salts from the soil. Water enters the roots from the soil 

 solution, drawn in through the thin-walled and delicate root hairs 

 which arise near the root tip and are continually renewed as the 

 root grows. With this stream of water the plant receives its sup- 

 ply of mineral salts. From the air there passes into the leaves a 

 slow current of carbonic acid gas. This is present in the atmo- 

 sphere in the small proportion of three parts in ten thousand; 

 and the loss due to constant absorption by the plant is made good 

 by the respiration of animals and the burning of fires. 



What the Green Plant Does 



The raw materials with which the plant works are thus water 

 and mineral salts from the soil and carbon dioxide from the air. 

 The green leaf is the laboratory in which the chemical syntheses 

 take place. 



What goes on in this laboratory? Mainly the manufacture 

 of such chemical substances as starches, sugars, and oils, which 

 are essential to living beings. The bulk of living substance is 

 composed of four ubiquitous elements carbon, hydrogen, nitro- 

 gen, and oxygen besides other elements such as phosphorus and 

 sulphur. Nitrogen compounds are supplied in solution by the 

 root of the plant, carbonic acid gas is drawn from the air by the 

 leaves. The plant breaks down the various compounds under 

 the influence of sunlight, and builds up the chemical elements into 

 fresh forms, whose great peculiarity is that they possess potential 

 energy. All life depends on the peculiar green substance "chloro- 

 phyll." It is only the plant that manufactures this essential 

 chlorophyll. Animals take all their organic compounds ready- 

 made from plants, and rebuild them into their own living matter, 

 and use them as a source of chemical energy. The plant manu- 

 factures these organic compounds. The making of the leaf-green 

 pigment called chlorophyll is in most plants carried through with 



