Sources of Nitrogen in Plants . 333 
minishes with the poverty or richness in these consti- 
tuents.* 
(3) ‘ The fungus of the Mycorhiza conveys to the tree 
not only the necessary water and the mineral nutritive sub- 
stances of the soil, but also organic matters taken direct 
from the humous and decomposing vegetable remains.’ Only 
by the mediation of the fungus is the tree enabled to employ 
directly such organic matters. 
(4) The theory, superseded in the doctrine of the nu- 
trition of plants, of the direct nutrition of green plants by 
humus, is therefore again brought to the front in the light 
of the Mycorhiza, although in another sense than formerly, 
and the significance of the humus and the covering of dead 
leaves on the soil needs further investigation and consideration. 
This is the substance of Frank’s renewed and extended 
theoretical statements. They speak for themselves ; and it 
must be admitted that he proposes to raise a very large 
superstructure on the foundation of his anatomical investi- 
gations, and that, in doing so, he assumes a proportionately 
heavy responsibility. 
The next important communication on the subject is again 
by Frank, in the Berichte der deutschen botanischen Gesetl- 
schaft for 1887 k In this he expressly sums up once more 
the chief points already insisted upon, and then proceeds 
to add other facts. 
He finds a peculiar formation of pigment associated with 
some kinds of Mycorhiza, reminding us of the pigments 
formed by certain Schizomycetes and Saccharomycetes, and 
not unknown in connection with higher fungi (no cases of 
the latter are cited, but the author would no doubt accept 
the green dye in wood affected by Peziza aeruginosa as an 
example). These pigments stain the membranes and con- 
tents of the fungus hyphae as well as the surrounding media. 
Frank then proposes to classify all the known forms of 
Mycorhiza as follows : — 
1 Ueber neue Mycorhizaformen, pp. 395-408. 
A a % 
