170 VEGETABLE PHYSIOLOGY 



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 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 LeguminoseB, 

 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 inde- 

 pendently, but only when its roots are associated symbioti- 

 cally with a micro-organism which usually forms peculiar 

 tubercular outgrowths upon the root-branches. It is 

 apparently the micro-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 micro-organism, and how far the proto- 

 plasm 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 micro-organism 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 in the soil without 

 being in symbiotic union with any green plant. If this 

 view is correct, the leguminous plant is supplied by the 

 micro-organism 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. 



The power of fixation of free nitrogen thus possessed 

 by the organisms mentioned has been stated by several 



