ELABORATION OF THE FOOD. 577 



important relations with the atmosphere : in the one, occurring when 

 they are exposed to the sun's light, oxygen is liberated and carbon is 

 fixed, which must be regarded as a process of assimilation ; in the 

 other, oxygen is absorbed and carbonic dioxide is exhaled, as in the 

 respiration of animals. The plant is thus subjected to two opposing 

 forces in connexion with the chloropliyllian and the general respira- 

 tion the one tending to add, the other to abstract material ; and, ac- 

 cording to the proportion between these two forces, governed as they 

 are by the variations of light and temperature, a plant will either 

 emit oxygen or carbonic dioxide in variable proportions. In a 

 feebly illuminated spot a plant may remain nearly in equilibrium 

 for months. In absolute darkness, the eliminating force being 

 the only one in operation, the plant can only live upon its own con- 

 stituents, emit carbonic dioxide by their combustion, and finally 

 perish without increase of weight (Boussingault). Fixation of 

 carbon is absolutely necessary for the production of new ternary 

 compounds (C H 0), but elimination of carbonic dioxide appears ab- 

 solutely requisite for the maintenance of the life of the plant. The 

 elimination of carbonic dioxide is increased by heat, and is most 

 conspicuous at night or in darkness, but it never entirely ceases 

 during the life of the leaf. In the daytime the quantity eliminated 

 being small, it is entirely taken up and again deoxidized by the 

 chlorophyll. 



According to Corenwinder the season of vegetation may be 

 divided into two periods : the first that of growth, when the 

 nitrogenous matter is in excess but rapidly diminishing, the period 

 when the true respiratory process is most vigorous ; the second, 

 the period of maturity, when the process of assimilation dependent 

 on the deoxidation of carbonic dioxide by chlorophyll is at its 

 height, the relative proportion of carbonaceous to other constituents 

 being then greatest. 



The passage of gases, of whatever nature and in whichever direction, is 

 dependent on the laws of diffusion ; the cuticle of the leaf in these cases 

 acts as a dialyzer or filter, checking evaporation, but permitting the pas- 

 sage of gases. 



Effect of Deoxidation. The assimilative process, in which 

 oxygen is liberated, accompanied by accumulation of carbon in 

 the tissues, is evidently related to the formation of the remarkable 

 series of neutral ternary compounds which constitute the great 

 bulk of the substance of plants, and, further, to the production of 

 the more obscure and far more complex and varied series of sub- 

 stances formed by a further removal of oxygen from the compounds 

 of the first class. 



The composition of the principal constituents of cellular tissues, and 



