Rctyner.—Obligate Symbiosis in Calluna vulgaris . 107 
That the inability to form roots, shown by the sterile seedlings, is 
not due either to lack of water or to lack of aeration in the substratum, but 
to absence of infection by the mycorrhizal Fungus, has been fully demon¬ 
strated by subsequent cultures (p. 120). 
2 . Effect of organic nutrient on uninfected seedlings . Uninfected 
seedlings germinated on blotting-paper or agar, and supplied with distilled 
water or with an appropriate solution of mineral salts, do not form roots, and 
show yellowing or discoloration of the leaves at an early stage. 
Seedlings infected normally with the Fungus germinated under similar 
conditions, and supplied with distilled water only, form a well-developed 
root-system and several leaves (Text-fig. 3). 
The failure to form roots, therefore, in the absence of infection, is 
not due to lack of inorganic food material, since the only differentiating 
Text-fig. 2. Sterile seedlings from 
agar cultures. Five months after sowing ; 
four months after planting (cf. Text-fig. 3). 
Camera lucida drawings. 
Text-fig. 3. Seedling from germi- 
nator, infected normally. Five months 
after sowing; about four months after 
germination (cf. Text-fig. 2). Camera 
lucida drawing. 
feature in the two cultures at germination consists in the presence or 
absence of the Fungus in the tissues of the respective seedlings. 
Since also a mycelium is developed without a supply of food 
material from external sources, beyond the traces of impurity present, 
it seems clear that, at this stage and under the conditions described, 
the micro-organism must obtain the greater part of its food material 
from the seedlings, the only other source of supply being the air. 
As the latter show no sign of injury, and the leaves remain green, one 
is tempted to suggest that the Fungus possesses, in some degree, the power 
of nitrogen fixation. 
The remarkable vitality shown by seedlings kept for long periods 
on blotting-paper moistened with tap-water may possibly have some 
significance as indirect evidence in support of the same view. 
Such seedlings form a well-developed root-system and several leaves, 
