THE FOOD OF PLANTS. 267 



It is a well understood fact that without plants, 

 animals could not exist upon our planet. In the 

 wonderful economy of things it is absolutely essen- 

 tial that there should be some intermediate or con- 

 necting link between ourselves and the mineral 

 kingdom, and plants constitute this important link 

 in the chain of life. The three kingdoms, animal, 

 vegetable, and mineral, are correlated, and involved 

 in a cycle of changes, which are unintermitting, 

 and wonderful in their nature. We are incapable 

 of being nourished by any form of mineral sub- 

 stances, but such nourish plants, and are transformed 

 by them into vegetable tissues and products ; and 

 subsisting as we do upon plants, we draw support 

 indirectly from the insensible rocks. The plant 

 consumes the rock dust, and attracts to itself the 

 carbon of air and earth ; we transform these into 

 flesh and bones", and, as a last step in this perpet- 

 ual circulation of matter, after death they relapse 

 again into their dead inorganic condition. 



It was formerly thought by chemists that plants 

 lived upon humus, a compound entirely organic in 

 its nature, and when some of the metals were found 

 in the ash of plants, they were regarded as acci- 

 dental ingredients, or extraneous bodies which 

 somehow intruded themselves into the incinerated 

 mass. In our time, we know that these mineral 

 bodies enter the vegetable structure as food, and 



