160 VEGETABLE PHYSIOLOGY 



CHAPTEB XI 



THE CONSTRUCTION OF PROTEINS 



THE simple compounds containing nitrogen which we have 

 seen to be absorbed by the roots of green plants, are as 

 unavailable for direct nutrition as the carbon dioxide taken 

 in from the air. The nitrogenous organic material which 

 is actually assimilated by the protoplasts is thought to be 

 some form of protein. With very few exceptions, and these 

 occurring only among micro-organisms, gelatin and similar 

 bodies cannot be made to support vegetable living sub- 

 stance, though they can be made use of by animals to 

 supplement, but not to replace, their protein supplies. 



In studying the story of the construction of proteins 

 from the nitrates and ammonia-compounds taken into the 

 plant, we meet with even greater difficulties than those which 

 are presented by the photosynthesis of carbohydrates. These 

 difficulties are connected with the stages which occur in 

 the course of the construction, with the mechanism which 

 is concerned in the transformation, and with the condi- 

 tions under which the building up of protein takes 

 place. 



At the outset of the study we find ourselves in com- 

 plete ignorance as to the chemical nature of protein. We 

 know that it is the most complex material found in the plant 

 with the exception of the living substance itself, but we 

 know hardly anything about its molecular structure or the 

 arrangement or grouping of its constituent atoms. De- 

 structive analysis has revealed its percentage composition 

 within certain limits, although, as there are many kinds of 

 protein and all of them are extremely difficult to prepare in 



