THE ROOTS. 225 



carbon dioxide and water. This is the first portion of 

 the process of assimilation. 



(2) Katabolism or destructive metabolism, including all chemi- 

 cal 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 libera- 

 tion 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 which are either excreted or 

 secreted in the insoluble 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 a 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 

 that 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 solution of certain constituents of the soil, 

 which, when dissolved, diffuse into the root. 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 



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