General Physiology of the Plant. 203 



to liberate the potential energy stored up in them. Oxygen 

 then takes part both in anabolism and katabolism ; that em- 

 ployed in the former process is obtained from the compounds 

 of oxygen in the soil (water and oxy- salts), that employed in 

 the latter process is obtained from the atmosphere in solu- 

 tion in water. In the one case it performs a nutritive, in 

 the other a respiratory function. 



The absorption of carbonic acid is a purely nutritive 

 process, taking place in sunlight and in the presence of 

 chlorophyll (p. 40). 



What becomes of the various food-stuffs absorbed by the 

 plant ? They become transformed into organic compounds 

 under certain conditions. This transformation takes place 

 in two stages primarily the formation of certain compara- 

 tively simple organic compounds, which may be termed 

 primary products of assimilation, which are afterwards built 

 up into higher organic compounds. 



Assimilation. The process known as assimilation con- 

 sists in the decomposition of the carbonic acid and water 

 absorbed by chlorophyll-bearing cells and the building up 

 of the products of decomposition into some comparatively 

 simple anastate. This process is accompanied by the libe- 

 ration of oxygen. Probably the anastate so formed is 

 formic aldehyde. In that case the primary change might 

 be represented thus : 



CO 2 -f H,O=CH 2 O(formic aldehyde) + O 2 



by the abstraction of one atom of oxygen from water and one 

 from carbonic acid. The carbonic oxide and hydrogen are 

 then said to be in a nascent condition, and would readily 

 unite to form formic aldehyde with the liberation of a mole- 

 cule of oxygen. These changes take place in the chloro- 

 phyll-bearing parts and in sunlight only. By the formation 

 of such anastates the dry weight of the plant is increased 

 considerably. The conditions of assimilation have already 

 been discussed in sections v. and vi. of Chap. II. 



