178 VEGETABLE PHYSIOLOGY 



described. It cannot be classed with the latter as a process 

 of photosynthesis, and it is only indirectly dependent 

 upon the action of the chlorophyll apparatus. Unlike the 

 construction of carbohydrates, it is not confined to green 

 plants indeed the fungi can commence the synthesis at a 

 lower stage than the latter, beginning the construction with 

 compounds of ammonia, which have to be converted into 

 nitrates before green plants can utilise them. 



For the synthesis of proteids 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 of -the Leguminosae have the power of using 

 the nitrogen of the atmosphere for the purpose of construct- 

 ing 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 forms peculiar tubercular outgrowths upon the root- 

 branches. It is apparently the latter organism which 

 effects the first fixation 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 manufacture 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 uncertain. 



