GERMINATION. 261 



carbon. There is, however, no proof whatever that 

 this really is the case ; on the contrary, as has al- 

 ready been stated, there is every reason to believe 

 that decaying vegetable matters merely yield carbon 

 to growing plants, by generating carbonic acid gas. 



681. The food of plants, then, is carbonic acid 

 gas, water, and ammonia, partly derived from the 

 air, and partly from the soil; and certain earthy and 

 saline substances, derived almost wholly from the 

 soil. Let us consider the circumstances under which 

 plants are able to absorb and assimilate these sub- 

 stances, and what other conditions are requisite to 

 their growth. 



682. The important influence which water has on 

 the changes which vegetable substances are liable to 

 undergo, has already been alluded to, when speaking 

 of albumen, fibrin, and the other similar substances 

 which enter into the composition of seeds. All these 

 substances, which under the joint action of air and 

 moisture, are so liable to undergo various changes, 

 become comparatively fixed and unchangeable, when 

 well dried and protected from the action of water. 

 During the germination of seeds, a great change 

 takes place in the nature of the substances composing 

 them. Carbonic acid is formed by the combination 

 of a portion of the carbon which they contain, with 

 the oxygen of the air. The conditions requisite to 

 the germination of seeds are, therefore, moisture, a 

 certain degree of heat, and the presence of air. 



683. Light appears to be prejudicial, and, there- 



