ON THE MYCORHIZAS OF FOREST TREES 
71 
in some way from the fungus, and it is probable that a repetition of 
the work would show that digestion does take place. No cytological 
work has been done on the Acer mycorhiza, but it must be said that 
the endophyte in these mycorhizas is very similar to that described 
by Shibata, and acts much the same, so that it is quite possible that 
the same physiological processes occur. 
Janse (8) advanced the hypothesis that the endophytes of endo- 
trophic mycorhizas have the ability to fix free nitrogen, just as do the 
bacteria in leguminous tubercles. He had no direct evidence of this, 
and the few cultural experiments which he performed gave negative 
results, but his statements induced other workers to take up the 
problem from that point of view. Nobbe and Hiltner (17) performed 
very extensive cultural experiments in which they grew plants of 
Podocarpus, both with and without mycorhizal nodules, in nitrogen- 
free sand, and obtained results which were very favorable to Janse's 
hypothesis. These results, however, can no longer hold, since Spratt 
(23) has shown that nitrogen fixing bacteria inhabit the same nodules. 
Later Ternetz (25) found nitrogen fixation in cultures of fungi which 
she obtained from endotrophic mycorhizas, but since her method of 
isolating the fungi, by allowing them to grow out of the roots in 
hanging drop cultures, is open to criticism, the work should be sub- 
jected to verification before final acceptance. It can at least be said 
there is no evidence that would warrant us in suspecting the Acer 
endophytes of having nitrogen fixing ability. 
Stahl's hypothesis of the absorption of salts from the soil certainly 
could not apply to the Acer mycorhizas, since there are very few 
communications of the endophyte with the soil. Also, these myco- 
rhizas are not always accompanied by a low water supply, since I 
have collected well developed, living mycorhizas from the mud at the 
bottom of a kettle-hole which is full of water during all wet seasons. 
Not only is it impossible that the endophyte is of importance in the 
absorption of salts, but the rootlet itself is inhibited from absorbing 
them to any great extent, since it is deprived of root hairs. The 
endophyte, of course, gets all, or a large percentage of its food from 
the root. It must, therefore, be considered as an internal parasite. 
It is possible, however, that, in those cases, in which the endophyte 
occupies only a small percentage of the cortical cells, the root may 
receive sufficient benefit from the digestion of fungous hyphae to justify 
applying the term symbiosis to the association. 
