CARBON ASSIMILATION. 



e : 



A Review of Recent Work on the Pigments of the 

 Green Leaf and the Processes connected with them. 



Chapter I. < 

 Introduction. 



'nr^HE elements contained in the plant are such as are always 

 -'- present in ample quantity in air or soil, but in the plant 

 they exist in compounds possessing greater energy than the 

 simpler substances in the surroundings. 



The fundamental characteristic of the green plant is that during 

 its life it produces these compounds of greater energy, the energy 

 required for this being obtained from the radiant energy of the 

 sun. 



One of the chief elements thus built up into the plant is carbon, 

 which is provided by the carbon dioxide of the air. It is in this 

 sense, and this only, that we use the term carbon assimilation. 



We consider the use of this rather non-committal expression 

 preferable to that of such terms as photosynthesis, photolysis of 

 carbon dioxide, etc., which suggest definite theories of the nature of 

 the processes. 



The main feature of carbon assimilation is the photochemical 

 reaction or reactions in which, by means of the green pigment of 

 the leaf, radiant energy is transformed into chemical energy. 



A discussion of photochemical reactions is, however, outside 

 the scope of this review because, unfortunately, no information is 

 available from plant physiological investigations which throws any 

 light on the particular photochemical reaction which is the essential 

 feature of carbon assimilation. 



The study of photochemical reactions forms an important 

 branch of physical chemistry at present undergoing rapid develop- 

 ment, and one which no doubt deserves more attention from plant 

 physiologists than it has so far received, not only in connection 

 with carbon assimilation but also in regard to many other processes. 



