THE CONSTITUENTS OF PLANTS. 13 



oxygen being then consumed. The quantity of oxygen 

 set free by young plants is, however, greatly in excess of 

 that consumed. The food directly supplied by the atmos- 

 phere is, therefore, chiefly carbon. 



The carbon absorbed by the plant, in connection with 

 the hydrogen and oxygen of the water, forms starch, dex- 

 trine, sugar, fat, cellulose, substances serviceable in the 

 growing plant, and to be stored away for future use in 

 seeds, roots, and stems ; and the same elements, together 

 with nitrogen and a little sulphur, form the albuminoids. 

 All of these are vegetable substances with which we are 

 familiar, and which are termed "organic.'' 



Pood obtained from the Soil. — Eoots are of two 

 kinds, — the " tap root," the chief use of which is to sus- 

 tain the plant in an upright position; and the "fiibrous 

 root," which is engaged mainly in absorbing food from 

 the soil. These roots are made up of cells, through the 

 walls of which solid matter cannot pass ; all food absorbed 

 by the root must be in solution. The surface membrane 

 of the root, unlike that of the leaf, is not full of holes ; 

 the absorption of solids is, therefore, impossible. The 

 absorption of the solution by the root is obtained by 

 means of what is called "diffusion." By this means 

 water is absorbed, and, with the water, the dissolved 

 food elements usually contained in the ash of plants. The 

 absorption of food is, however, not confined to the taking 

 up of ready formed solutions. 



The root grows at its tip, and it is through the delicate 

 cells located there that the plant absorbs nutriment. The 

 tips are protected by a sheath, or cover, of dead or dying 

 cells, which protects the delicate fibres, and permits the 



