Sources of Nitrogen in Plants. 357 
absorption and fixation of free nitrogen, so as to render them 
available to the plant? 
If Hellriegel’s results are confirmed, the last question is 
answered generally, the further enquiry narrowing itself into, 
How can the fungus act in the fixation &c. of free nitrogen ? 
But the previous question will still remain to be answered, 
the evidence at present being distinctly against the view 
that the fungus aids directly in absorbing food-materials, 
and in favour of the supposition that it stimulates the plant 
to greater metabolic activity. It is only fair to add that the 
possibility that the combined fungus and stimulated cells — i. e. 
the root-tubercles — may act, as a whole, as a compound 
organism possessing the power of making use of the nitrogen, 
is not to be set aside as absurd so long as the question of a 
nitrifying organism can be entertained at all. 
