THE CONSTKUCTION OF PKOTEINS 171 



observers to be shared by certain lowly Algae, but the 

 evidence as to their activity in this direction is conflicting. 

 It may be that they are capable of a similar symbiotic 

 relationship with certain of the nitrogen-fixing bacteria of 

 the soil already mentioned, but it is more probable that it 

 is carried out by bacteria living simultaneously, but not 

 symbiotically, in the soil with them. Several of these 

 organisms appear to be associated symbiotically with each 

 other, though not with any green plant. 



When we turn to the method of construction of protein 

 by a green plant we find ourselves in possession of very 

 little accurate information as to the stages which are 

 involved. We find that nitrates especially are absorbed 

 by the root-hairs from the soil, and that a continuous 

 stream of them passes into the plants. This naturally 

 is associated with a transportation of the nitrates through 

 the root and stem. They can be detected in varying 

 quantities in these regions, but the amount seems to diminish 

 as the termination of the stem is approached, and little 

 can be found to be present in the leaves. It may be inferred 

 that a gradual decomposition takes place as they pass 

 along the axis, and that this is completed in the leaves. 



A theory has been advanced to explain this disappear- 

 ance, which may be mentioned here. It is that the nitrates 

 are decomposed by the organic acids of the plant, and in 

 particular by oxalic acid. Simultaneously the sulphates 

 which are absorbed undergo a similar fate. The resulting 

 bodies, the nitric and sulphuric acids, unite with some form 

 of non-nitrogenous organic substance, possibly formal- 

 dehyde, or a fairly simple carbohydrate, to form protein. 

 From what has already been advanced, however, it is evident 

 that this scheme of construction is purely hypothetical. 



When we search for a form of nitrogen compound that 

 is nearer protein in its composition than these simple salts, 

 it is natural to look at the products of the decomposition of 

 protein material to see if these furnish any clue to a possible 

 constructive process. When proteins are digested in the 



