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 certam plants, chiefly belonging to the Leguminosae, 
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, im 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 
