i] INTRODUCTION 5 



protoplasm throughout the cell, and since many of these reactions can 

 take place both simultaneously and independently, the protoplasm must 

 have some form of organized structure. Though many phenomena of " life " 

 may be accounted for by the physical and chemical properties of such 

 substances as proteins, it is impossible to say, with our present knowledge, 

 how far all " living " phenomena may yet be explained in this way. 



Some of the main lines of metabolic syntheses which take place in 

 the plant will next be considered. A fundamental fact which should be 

 borne in mind is that the green plant synthesizes all the complex 

 materials of which it is composed from the simple compounds, carbon 

 dioxide, water and certain inorganic salts. The most important factor, } 

 perhaps, which figures in plant metabolism, is chlorophyll. The green 

 pigments of chlorophyll are esters of complex organic acids containing 

 the elements carbon, hydrogen, oxygen, nitrogen and magnesium. They 

 have the remarkable power of absorbing the radiant energy of the sun's 

 rays and of transforming it into chemical energy, by means of which 

 carbon dioxide and water are combined to form some organic compound, 

 possibly formaldehyde, from which a simple carbohydrate is readily 

 synthesized. 



If now the initial and final products of carbon assimilation be 

 considered in detail, it will be seen that the process is one of reduction : 



6CO 2 +6H 2 O = C 6 H 12 O 6 + 6O 2 . 



This is confirmed by the fact that oxygen is evolved in the process. 

 Moreover, the plant accumulates a store of energy, since the final 

 product, the carbohydrate, has a higher potential energy than the system, 

 water and carbon dioxide. Hence carbon assimilation, in addition to 

 providing a basis of organic material as a starting-point for all the main 

 metabolic functions, also provides a source of chemical energy by means 

 of which reactions in other directions are brought about. 



The setting free of this accumulated energy constitutes the process 

 of respiration, which is, in reality, an oxidation of carbohydrate taking 

 place in tissues throughout the plant. It is the converse of carbon 

 assimilation, in that oxygen is absorbed and carbon dioxide and water 

 are formed. Thus these two processes, both so fundamental and essential 

 to the metabolism of the green plant, are constantly taking place side 

 by side in the same cell. 



The first-formed carbohydrate, which is probably a hexose, is 

 condensed in the plant, on the general lines we have previously indicated, 

 to form more complex disaccharides and polysaccharides, such as maltose, 



