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 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 
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 Alge, but the 
evidence as to their activity in this direction is conflicting. 
It may be that they are capable of a similar symbiotic 
