22 OUTLINES OF FORESTRY 



itself, is called assimilation. At the commence- 

 ment of its life the plant gets its nourishment 

 from the protoplasm surrounding the nucleus. It 

 is not long, however, before it exhausts this stock 

 of food, and it must then get all its nourishment 

 either from the soil or from the atmosphere ; or, 

 in other words, from outside the seed. 



This latter nourishment of the plant comes 

 from a variety of materials, derived either from 

 the air or from the ground, the most important 

 of which are as follows : 



1. Moisture. 



This moisture is mainly taken up by the roots 

 of the plant from the soil; but it is, in some 

 cases, absorbed directly from the air by the leaves. 



2. Carbonic acid. 



Carbonic acid is a gaseous substance, formed of 

 carbon combined with an invisible gas called oxy- 

 gen. The carbonic acid is absorbed by the leaves 

 of the plant, and, in the presence of sunshine, is 

 broken up into carbon and oxygen. The oxygen 

 is given oft* from the surfaces of the leaves, and 

 the carbon is retained by the plant to form its 

 woody fibre. In the case of large vegetable forms 

 like forest-trees, the amount of carbonic acid taken 

 from the air and converted into woody fibre must 



