Rayner.—Obligate Symbiosis in Calluna vulgaris . 99 
The facts established supply the data requisite for a direct attack upon 
the original ecological problem ; they throw further light on the rather vexed 
question of the significance of mycorrhiza, and they have demonstrated for 
Calluna , and probably for most ericaceous species, an unsuspected symbiotic 
relation, in some respects unique among flowering plants. 
The record of the results of the present investigation, since it is 
concerned more especially with the isolation and biology of the mycorrhizal 
Fungus of Calluna , is prefaced by a brief historical review of previous work 
on endotrophic mycorrhiza. 
Mycorrhiza—Historical. 
The peculiar association of the vegetative stage of a Fungus with 
the roots of the higher plants, known by this name, has been familiar to 
botanists since the middle of the last century, and although many cases have 
since been carefully investigated, especially from the cytological point of 
view, comparatively little is yet known with certainty of the bionomics 
of the relationship, and still less of the life-histories and systematic position 
of the Fungi concerned. 
Early observers were usually content to record the presence of hyphae 
in or upon the roots of various plants without attempting to investigate the 
exact relations between the two organisms. 
In the early part of the nineteenth century the presence of mycelium 
in plant tissues was recorded by Schleiden, and in 1882 Kamienski drew 
attention to the external investment of hyphae on the roots of Monotropa 
Hypopitys ( 4 ). 
In 1887 a great impetus was given to research by Frank, who first 
made use of the term mycorrhiza and formulated a definite theory of 
symbiosis, the possibility of which had already been suggested by Pfefler, 
Kamienski, Treub, and Goebel. 
Frank’s observations were made on a large number of plants, and 
he distinguished two forms of union: ectotrophic mycorrhiza , in which the 
Fungus forms an external investment on the root but does not penetrate the 
cells—especially characteristic of many forest trees; and endotrophic my cor- 
rhiza , in which the cells of the plant are actually invaded by fungal hyphae. 
Based on experimental study of ectotrophic mycorrhiza in forest trees, 
Frank formulated his well-known theory of the symbiotic role of the Fungus ; 
namely, that root-hairs are frequently absent and are replaced functionally 
by fungal hyphae, which are responsible for the absorption by the plant 
of mineral salts or organic nitrogen compounds from the soil—a debt repaid 
by the transference of carbohydrates from plant to Fungus. 
Subsequently, a similar function was claimed for the Fungus of endo¬ 
trophic mycorrhiza, the absorption of nitrogen compounds from the Fungus 
involving, in this case, their digestion by the cells of the plant ( 5 ). 
