GROWTH OP PLANTS. 271 



degree of warmth-, carbonic acid, ammonia, and cer- 

 tain earthy and saline substances, are required ; 

 whilst light, which in the first stage of its growth 

 appears prejudicial, is now required to assist in those 

 complicated chemical changes which are going on, 

 when the compound organic substances, such as 

 woody fibre, gum, starch, gluten, &c., are formed in 

 the organs of the plant by the combination of the 

 elements of water, carbonic acid, and ammonia. 



710. Hence during the day, or whilst plants are 

 exposed to the influence of light, the carbonic acid 

 absorbed by them is decomposed, the carbon alone 

 being retained in their structure, whilst the oxygen 

 is given off. 



711. During the night, or when light is withdrawn, 

 this process of decomposing carbonic acid and acquir- 

 ing carbon goes on very slowly, and accordingly some 

 of the carbonic acid then absorbed by the roots is 

 given off again unchanged, by other parts of the sur- 

 face of the plants, in place of undergoing decomposi- 

 tion. For plants are at all times receiving from the 

 soil carbonic acid, which must necessarily enter their 

 system with the water they absorb from the [soil 

 through their roots ; but they can only derive nourish- 

 ment from it when, in consequence of the influence 

 of light, they are able to efi'ect its decomposition, and 

 so, by retaining the carbon, to increase the quantity 

 of organic matter which they contain. 



712. In the third stage of the existence of a plant, 

 or during the formation of seed, the same general 



