6 INTRODUCTION [ch. 



If now the initial and final products of carbon assimilation be con- 

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

 6C02 + 6H20 = C6Hi206 + 602. 



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

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

 duct, 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 mean& 

 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 con- 

 densed in the plant, on the general lines we have previously indicated^ 

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

 cane-sugar, starch, cellulose, etc. Some of these products, such as the 

 disaccharides, form true solutions and may be present in the cell-sap ; 

 others, such as cellulose and starch, are present in the solid state, though 

 they contain considerable quantities of water. Others, again, such as 

 dextrin and gum, are present in the colloidal state. Thus, given an 

 initial carbohydrate and a source of energy, we may proceed to indicate 

 the other main lines of syntheses in the plant. 



The next most important line of syntheses is probably that which 

 gives rise to the nitrogen-containing constituents of the plant. Nitrogen 

 is absorbed by the green plant in the form of nitrates and ammonium 

 salts, but the processes which lead to the synthesis of some of the simplest 

 nitrogen-containing compounds, such as the amino-acids, are still very 

 obscure. Aliphatic and aromatic acids of various kinds are abundantly 

 present in the tissues, but the reactions by which the NHg groups are 

 introduced are by no means clear. There is little doubt, however, that 

 once the amino-acids are formed, condensation takes place as already 

 indicated, and more complex molecules, termed polypeptides, arise. Such 

 polypeptides have now been synthesized artificially by the condensation 

 of amino-acids. From the polypeptides, by further stages of condensation, 

 the albumoses, peptones, and finally proteins are produced. 



