ASSIMILATION OF CARBON BY PLANTS 161 



and most important role in the life of the plant therefore belongs 

 to carbohydrates. 



This process of the building of carbohydrates from inorganic 

 substances of the surrounding medium has been designated 

 ''photosynthesis," for it proceeds only with the absorption of 

 light, which supplies the necessary energy. 



The initial substances for photosynthesis are carbon dioxide 

 and water, and the whole process may be summarized in the 

 following simplified reaction: 



6CO2 + 6H2O -> CeHisOe + 6O2 



This reaction shows that during the process of photosynthesis 

 the plant liberates free oxygen into the surrounding atmosphere. 

 For the animal world, this oxygen is of major importance, for 

 it serves as a source of renewal of the oxygen in the atmosphere, 

 making it fit for respiration, in spite of the continuous consump- 

 tion of oxygen by animals and nongreen plants, especially by 

 bacteria and fungi and in combustion processes. The liberation 

 of oxygen by plants has played a great part in the history of the 

 process of photosynthesis. It was first observed by Priestley 

 (1772) in aquatic plants, which are especially convenient for 

 such observations; oxygen, being little soluble in water, appears 

 in the form of bubbles easily detected in sunlight. The capacity 

 of plants to ''improve" the air was fully appreciated by early 

 investigators long before the significance of photosynthesis for 

 the nutrition of plants was recognized. 



The simplified reaction of photosynthesis, given above, shows 

 that the process of assimilation of carbon is simultaneous with 

 the assimilation of the elements of water and that its essential 

 products are carbohydrates. In fact, it is not difficult to prove 

 by means of cUrect analyses that the carbohydrate content of 

 leaves increases considerably during the daylight and diminishes 

 significantly at night when, owing to the absence of light, the 

 formation of carbohydrates is checked, while their consumption 

 through respiration and their outflow into other parts of the 

 plant continue. Since by photosynthesis about 90 per cent of 

 the total dry substance of the plant is formed, very often the 

 whole process has been called "assimilation." This term is fully 

 justified, since assimilation of carbon, hydrogen, and oxygen is 

 closely connected with the assimilation or transformation into 



