102 
Rayner.—Obligate Symbiosis in Callnna vulgaris . 
dependence on the part of the former. The Fungus has been isolated, 
grown in pure culture outside the plant, and used with success for the 
reinoculation of sterile seeds ( 17 ). 
A theory of tuberization, as a general consequence of infection by 
mycelium, was elaborated by Bernard as a result of his earlier researches ( 18 ). 
In 1909 Burgeff published a monograph on the root Fungi of the 
Orchids, recording researches in the course of which he repeated the experi¬ 
ments and confirmed the results of Bernard, and made further contributions 
to our knowledge of the metabolism and physiology of the endophytes ( 19 ). 
The conclusions of Burgeff differ somewhat from those of Bernard with 
respect to the degree of degeneration shown by the Fungus when grown 
outside the plant, as tested by subsequent inoculation. 
According to the latter author, the longer the Fungus is grown as 
an independent organism outside the plant, the more markedly does it lose 
its power of causing germination and inducing root formation. Burgeff, on 
the contrary, claims that one of his Fungi, after growing for twenty-six 
months on artificial media, had the same capacity for effecting germination 
as when first isolated from the plant. 
For purposes of comparison with the more specialized types, Gallaud 
( 20 ) investigated a large number of Phanerogams, the roots of which contain 
an endophytic mycelium, but in which the relations between plant and 
hyphae are apparently more simple and unspecialized than in the case 
of the Orchids. 
The researches of Gallaud appear to be of special importance in regard 
to the evolution of the more specialized types of root symbiosis, and some 
of the facts are discussed from this point of view in a subsequent part 
of this paper (p. 127). 
It is of interest to note that, when selecting endotrophic types for 
examination, Gallaud rejected the members of Ericaceae for the purpose, as 
possessing a mycorrhiza more nearly related to the ectotrophic forms. 
So far from this being the case, I hope to demonstrate in the following 
pages that the conditions existing in Calluna vulgaris (and probably 
common to all ericaceous species) connote a wider distribution of the endo¬ 
phyte in the tissues than has been recorded hitherto for any mycorrhizal 
Fungus, involving in some respects the most highly specialized relation 
between the two symbionts that has been yet described for a flowering 
plant and a Fungus. 
The Fungi concerned in Endophytic Mycorrhiza . Historical. 
Up to the present, the Orchids have provided the only case in which 
absolute proof of the identity of the endophytic Fungus has been established 
by the successful inoculation of sterile seeds or seedlings from a pure 
culture (p. 101). 
