THE CONSTRUCTION OF PROTEINS 165 



compounds of ammonia, which have to be converted into 

 nitrates before green plants can utilise them. 



For the synthesis of proteins we have accordingly two 

 certain starting points, to which may be added another 

 which is confined to a small group of plants, if not indeed 

 to a single organism. We have already alluded to the fact 

 that certain plants, chiefly belonging to the Leguininosse, 

 have the power of using the nitrogen of the atmosphere for 

 the purpose of constructing organic food. This utilisation 

 of it is, however, not carried out by the green plant 

 independently, but only when its roots are associated 

 symbiotically with a fungus which usually forms peculiar 

 tubercular outgrowths upon the root-branches. It is 

 apparently the latter organism which effects the first fixa- 

 tion of the nitrogen. The leguminous plant alone is as 

 powerless in this direction as any other green plant. How 

 the fixation takes place, what part of it is due to the direct 

 metabolism of the fungus, and how far the protoplasm of 

 the green plant is concerned in the early stages, are at 

 present quite uncertain. It seems, however, probable that 

 the fixation is carried out by the fungus alone, without any 

 influence or aid derived from the green plant A few other 

 similar organisms can under appropriate conditions carry 

 on a similar fixation without being in symbiotic union with 

 any green plant. If this view is correct, the leguminous 

 plant is supplied by the fungus with a food material which 

 has already been worked up from the simple form in which 

 the elements of it are absorbed ; but how far the manu- 

 facture has proceeded that is to say, in what condition 

 the nitrogenous material is actually presented to, and 

 absorbed by, the tissues of the root is at present un- 

 certain. 



The power of fixation of free nitrogen thus possessed 

 by the organisms mentioned has been stated by several 

 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 



