166 VEGETABLE PHYSIOLOGY 



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. 



Wh'en 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 

 none 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 form- 

 aldehyde, 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 it these furnish any clue to a possible 

 constructive process. When proteins are digested in the 

 animal organism under the influence of the strong ferment 

 of the pancreatic secretion, we find that among the pro- 

 ducts of the decomposition certain nitrogenous compounds 

 occur which are crystalline and capable of diffusing through 

 animal and vegetable membranes. These substances, the 



