VITAL PROCESSES IN PLANTS 11 



compounds ; and the soil or water, which furnishes the 

 other elements. The raw food material is converted 

 into complex substances by the protoplasm itself, and 

 these are made use of for growth, reproduction, and 

 other vital processes. It is, in fact, the coal which 

 keeps the machinery in motion. 



Now, if the roots of an Alpine plant are buried 

 in a frozen soil, one source of food- supply is, for a 

 time at least, cut off, and the plant must depend on 

 such reserves of water and mineral salts as it has in 

 •hand. On the other hand, the leaves, if covered with 

 snow, are in darkness, and cannot assimilate, and 

 here another source of raw material fails for a time. 

 Plants can, however, by storing up reserves, survive 

 for a long period, during which both sources of food- 

 supply are cut off; but if this period is unduly 

 prolonged for any reason, the plant, having used up 

 all its reserves, may die, for the machinery can no 

 longer continue in motion. 



In plants, then, as in animals, food is essential to 

 the life of the protoplasm, and is built up into its 

 substance to form new protoplasm. On the other 

 hand, protoplasm being unstable, is constantly break- 

 ing down, and the simpler substances which result are 

 returned to the atmosphere. In man and all the 

 higher animals, and also in plants, there is a twofold 

 exchange with the atmosphere. Man breathes by 

 means of his lungs — that is to say, he is constantly 

 exhaling carbon dioxide and water vapour, and inhaling 

 oxygen — a process known as respiration. If animals 



