NOURISHMENT OF PLANTS. 45 



Nature, by means of rain and dew, decay and pu- 

 trefaction, and by physical, chemical, and volcanic 

 forces, provides that the three universal means of 

 nourishment, water, carbonic acid, and ammonia, 

 shall not be wanting to plants ; and man also, with- 

 out exactly intending it, contributes his share, by the 

 processes of respiration and combustion. The air 

 contains an inexhaustible provision of these substan- 

 ces, since the processes by which they are generated 

 on the earth never suffer intermission. The air 

 alone would accordingly suffice for the nourishment 

 of plants, if they could but find in the soil the neces- 

 sary inorganic salts in solution. 



But as a building advances more rapidly when 

 several parts are worked upon at the same time, so 

 the growth of a plant proceeds more quickly and 

 luxuriantly when it can absorb nourishment from 

 several different sources, — not only by its leaves, but 

 at the same time also by its roots ; that is to say, 

 when the soil contains in sufficient quantity all 

 those materials which are essential to its nourish- 

 ment. Art must assist wherever this is not the 

 case ; and this assistance is rendered by manifring-, a 

 subject which will be discussed in the following 

 chapter. 



