NUTRITION. 55 



The direct rays of the sun are necessary to the 

 effect : no other light, however strong, will suf- 

 fice. The course of the phenomena connected 

 with the respiration of plants appears to be the 

 following. The water which enters the plant 

 by the roots contains carbonic acid, which is 

 carried with it into the green parts ; it is there 

 decomposed under the influence of the sun's 

 rays the carbon is fixed in the plant, and the 

 oxygen escapes. The carbonic acid which is 

 formed from the oxygen of the air in all those 

 portions of the plant which are not green, is 

 partly dispersed in the atmosphere, partly dis- 

 solved in water, which water at last reaches the 

 plant again, and thus is ultimately absorbed by 

 the roots, drawn up to the leafy parts and there 

 decomposed. The water taken up by the roots 

 holds besides its carbonic acid, a certain quan- 

 tity of soluble matter containing carbon : this 

 carbon is also carried with the sap into the 

 green parts, it combines during the night with 

 the oxygen which had been previously absorbed 

 by them, and the following day such of this car- 

 bonic acid thus formed in the leaves as has not 

 been given out during the night is decomposed 

 by the solar light, as if the carbon could not be 

 usefully deposited in the nutritive juices unless 



