132 THE CELL 



Whilst the absorption of oxygen and the giving up of carbon 

 dioxide indicate the beginning and end of a series of complicated 

 processes which belong chiefly to retrogressive or destructive 

 metabolism (catabolism), the absorption and elaboration of carbon 

 dioxide in the cell afford us an insight into the opposite pro- 

 cess, progressive metabolism (anabolism), or the reproduction of 

 organic substance. This process, in contradistinction to respira- 

 tion, is termed assimilation. 



Respiration of oxygen and assimilation of carbon dioxide are in 

 every respect opposite processes. The former is a fundamental 

 phenomenon common to nearly the whole organic kingdom, the 

 latter is confined to the vegetable kingdom alone, and even here 

 occurs only in such cells as contain chlorophyll or xanthophyll in their 

 protoplasm. The respiration of oxygen conduces to oxidation 

 decomposition processes, whilst on the contrary the assimilation 

 of carbon dioxide causes the reduction of the latter, and the 

 synthetic formation of complex molecular organic substances. 

 These are carbo-hydrates, especially starches, which are found 

 deposited in the form of small granules in the green portions of 

 plants (chlorophyll corpuscles and chlorophyll bands). 



The individual stages of the synthetic processes which take 

 place in the plant-cell during the assimilation of carbon dioxide 

 are as yet unknown. Only so much may be said : carbon dioxide 

 and water form the initial material for the synthesis ; further, as 

 a result of the reduction of the carbon dioxide and water, oxygen is 

 evolved, and is given off largely in the form of a gas. This trans- 

 formation can only take place in protoplasm when chlorophyll is 

 present ; but it is possible that other chemical substances are also 

 concerned in the process. Finally, carbon dioxide assimilation 

 can only occur under the influence of light. Heat is necessary 

 in order to liberate the oxygen from the molecules of carbon 

 dioxide and water. In this point also carbon dioxide assimilation 

 and oxygen respiration are opposed : in the latter heat is evolved 

 through oxidation, which is a process of combustion, and vital 

 force is set free ; in the former heat is used up in reducing the 

 carbon dioxide, and as potential heat is rendered latent in the 

 assimilation products. The heat required for this process is af- 

 forded by the sun's rays. 



If an aquatic plant is introduced into water containing carbon 

 dioxide, and is placed in the sunlight, innumerable small bubbles 

 of gas are soon seen to rise ; if these are collected in a bell-jar, 



