222 FUNCTIONS OF THE ROOT 



Metabolism refers to all chemical changes which occur in or are 

 produced by living matter. These are further subdivided into 



(1) Anabolism or constructive metabolism, including all processes 

 in which complex compounds are built up from simpler ones by the aid 

 of living organisms. The final stage of all anabolic processes is the 

 formation of protoplasm. In the case of plants, the most characteristic 

 anabolic process is the synthesis of carbohydrates from carbon dioxide 

 and water. This is the first portion of the process of assimilation. 



(2) Katabolism or destructive metabolism, including all chemical 

 changes resulting in the formation of the more simple from the complex. 

 A type of such katabolic processes is the absorption of oxygen and the 

 liberation of carbon dioxide ; this occurs most markedly in animals, 

 but is also an essential function of plants. The process is known as 

 respiration. 



The products of metabolism are classed as plastic products, which 

 can be utilised again in anabolism, and waste products, which cannot 

 be again used, and in which are either excreted or secreted in the in- 

 soluble form within the body of the plant or animal. 



The main parts of a plant are 



(1) The roots. 



(2) The stem. 



(3) The leaves. 



(4) The flowers and seeds. 



A brief account of the functions of these will now be given : 



1. The Roots. The radicle formed from the seed naturally grows 

 downward, i.e., in the direction towards the force of gravitation or other 

 force acting upon it. Soon, however, it branches, and the secondary 

 roots generally grow laterally. From these roots, in turn, other 

 branches are formed, and in the neighbourhood of the growing point 

 of a root, root-hairs are sent out among the particles of the soil. These 

 root-hairs have thin walls through which water can readily pass, 

 -carrying with it the dissolved matter which it may contain, provided 

 1ihat matter be capable of passing through the membranous walls (i.e., 

 "be a crystalloid). The constituents of the sap, often including free 

 vegetable acids, pass outwards from the root-hairs and aid in the solu- 

 tion of certain constituents of the soil, which, when dissolved, diffuse 

 into the root. 1 All the crystalloid constituents of soil-water in this way 

 enter the root and are carried in the sap to other parts of the plant, 

 where they are absorbed, if required, in order to build up the tissues of 

 the plant. If not so utilised they remain in the sap and thus pre- 

 vent the entrance, by diffusion, of additional matter of the same kind. 



Diffusion and Osmosis. 



A few words may here be said about the phenomena of diffusion 

 and osmotic pressure. Diffusion, as is well known, refers to the process 



1 Recent experiments lead to the conclusion that the solvent action of root-hairs 

 upon soil is mainly to be attributed to the carbon dioxide evolved by them and not 

 o the escape of their acid sap. 



