IOO 
Rayner.—Obligate Symbiosis in Calluna vulgaris . 
Frank and his pupil Schlicht drew attention to the widespread occurrence 
of endotrophic mycorrhiza, especially in the Natural Orders Orchidaceae, 
Epacridaceae, and Ericaceae. 
In the case of Ericaceae, Frank described and figured the very fine 
hair-like roots of certain heath plants, the cells of which are constantly 
filled with fungal hyphae (6). 
In such roots he emphasizes the complete absence of root-hairs, the 
disappearance or reduction in amount of cortical tissue, and observed the 
‘ epidermal ’ layer of large cells filled with* knot-like ’ masses of mycelium, 
branches from which penetrate the external walls. 
Frank recognized the possible existence of more specialized relations 
between Fungus and flowering plant in the case of certain groups, such as the 
Ericaceae and Orchidaceae, but pointed out that comparative cultures 
of infected and uninfected plants of this type were not yet available for 
discussion. 
The conclusions of Frank as to the role of the Fungus in ectotrophic 
mycorrhiza were challenged by Sarauw ( 7 ) and by Moller (8), the latter 
of whom pointed out that ectotrophic mycorrhiza was often well developed 
in soils poor in humus. 
It was also shown subsequently that root-hairs were not uncommonly 
formed by plants possessing ectotrophic mycorrhiza, and Von Tubeuf ( 9 ) 
demonstrated that the endotrophic type was often characteristically 
developed in forest trees. 
On the whole, since Frank’s time, views as to the significance of 
ectotrophic mycorrhiza have diverged from his theory without entirely 
abandoning it. 
Subsequent to the researches of Frank, the endotrophic forms attracted 
more attention. Among a number of publications dealing more especially 
with their cytology and general significance may be mentioned those 
of Groom ( 10 ), Thomas ( 11 ), Janse ( 12 ), Magnus ( 13 ), Shibata ( 14 ), and 
Peklo ( 15 ). The first of these authors agreed substantially with the view 
of Schlicht and recognized a series of transition forms leading from the 
condition observed in Ericaceae—which, in his view, approached the ecto¬ 
trophic type present in forest trees—to the highly specialized relations found 
in Thismia Aseroe. 
In 1900 Stahl published his well-known work on the comparative 
biology of autotrophic and mycotrophic plants ( 16 ). 
He assigned to the fungal partner of mycotrophic plants the role 
of obtaining mineral salts from the soil, and pointed out that such salts are 
especially valuable to plants which, because they grow in humus, or for 
other reasons (e. g. slow transpiration), are unable to absorb water with 
sufficient rapidity to satisfy their requirements for mineral salts. 
The experimental work on which Stahl based his conclusions w6uld 
