The Ecology of C alluna vulgaris. 67 
In the case of such plants as Calluna and its Ericaceous 
allies, the relation of the roots to the soil is evidently rendered more 
complex by the presence of a mycorhizal fungus, of which the full 
significance to the plant is still in need of demonstration. The 
details of fungal infection, and the ability of the plant to germinate 
and grow without its fungal partner have also yet to be described 
in the case of Calluna. 
An obligate association of definite races of bacteria with the 
roots of plants has already been suggested 1 , and it would seem 
not unreasonable to expect that such associations would become 
much more complex in the case of a plant, the roots of which 
constantly contain mycorhiza. It is claimed that several species 
of nitrogen-fixing fungi have been isolated and cultivated 
from the mycorhiza-bearing roots of various Ericaceous plants, 
including Calluna vulgaris, but absolute demonstration that this 
was the case could not be made, since the seedlings were never 
obtained in a sterile condition. 2 Similar relations are suggested 
far Vaccinium corymbosum, 3 and have indeed already been inferred 
for most plants which form root associations with either fungi or 
bacteria. 
Before the main problem of the relations of the mycorhizal 
fungus to the plant and of the former to the soil and its bacterial 
flora can be attacked, it is necessary, therefore, to learn whether 
the association of the plant with its mycorhizal fungus is an 
obligate one, and for that purpose to determine the following 
points:— 
(i) . At what stage in the life-history of the plant, and from 
what source, does infection of the roots take place ? 
(ii) . Is it possible (a) to germinate, ( b ) to grow Calluna vul¬ 
garis without infection of the roots under sterile 
conditions ? 
Having a knowledge of these facts, it should be possible to 
infer the existence, or not, of fundamental relations between the 
plant and special micro-organisms of the soil; to determine if any 
of the latter are indispensable for its growth, and whether the soil 
preferences and peculiarities of Calluna are directly associated 
1 Gottheil, O. “ Botanische Beschreibung einiger Bodenbaktericn.” 
Centralbl. f. Bakteriologie, Bd. 7, p. 430. 
* Ternetz, C. “ Ueber die Assimilation des atmospharischen Stickstoffes 
durch Pilze.’’ Jahrb. f. vviss. Bot., Bd. 44, 1907, p. 353. 
3 Coville, F. V. “ Experiments in Blueberry Culture.” U.S. Dept. 
Agric. Bureau of Plant Industry, Bull. No. 193, 1911. 
