Sources of Nitrogen in Plants . 329 
different fungus. The reason Frank’s fungus has been over- 
looked is probably that those who investigate roots use 
the seedlings, water-cultures, &c. 
Beeches, hornbeams, oaks, and hazels, removed from the 
ground in spring, when one to two years old, and already 
bearing Mycorhiza, can be easily grown as water-cultures, 
with the result that the roots go on growing free from the 
fungus— or, rather, that new laterals are formed as water-roots, 
and the mycelium does not spread on to these. The fungus 
could not be cultivated. 
As to the question, what is the systematic position of the 
fungus? no clear answer can be given. It is no doubt 
the mycelium of a subterranean form — one of the Tuberaceae 
or Gasteromycetes, perhaps. But new forms are discovered 
every day, and the presence of a mycelium does not neces- 
sarily imply the presence of the perfect fungus fructification ; 
mycelia may go on growing and sterile for years. 
We now come to Frank’s views as to the biological sig- 
nificance of the Mycorhiza. The organic union between 
root and mycelium, their harmonious growth, and the close 
physiological relations which must exist between them, all 
point to this being a new case of symbiosis. From the side 
of the root, we must regard the fungus as a parasite, which 
takes from the former food-supplies of the nature of car- 
bonaceous assimilated material : its minerals & c. must be 
taken by the fungus itself from the soil, the free hyphae 
acting like root-hairs. We may regard the thickening and 
other changes produced in the root as similar to the al- 
terations met with in hypertrophy, & c., — here in a slight 
degree only — due to a stimulus exerted by the parasite on 
the host. The roots are by no means killed, however, and 
that they preserve their capacity to serve the tree is proved 
by the well-being of the latter. We must conclude that the 
root-fungus, in the mycelium stage at least, is not injurious 
to the root and tree. 
Under such conditions we must look for a contrary benefit 
derived from the fungus by the tree, and Frank sees this 
