Obligate Symbiosis in Calluna vulgaris . 1 
BY 
M. CHEVELEY RAYNER, 
Lecturer in Botany at University College , Reading. 
With Plate VI and four Figures in the Text. 
Introductory, 
~> an important member of the moorland flora of Northern Europe, 
JC\. Calluna vulgaris is commonly associated with soils of a definite type, 
especially when it occurs as the dominant or sub-dominant species of moor¬ 
land associations. As such, it is especially characteristic of dry heath 
soils, deficient in lime and often acid in reaction. 
So obviously is this the case, that the current hypothesis to explain the 
edaphic relations of this and other calcifuge ericaceous species assumes that 
their local dominance connotes soil conditions inimical to the growth of 
other plants. 
This hypothesis, however, did not adequately explain the success 
of small Calluna communities in the special case selected, 2 and a close 
scrutiny of ecological records revealed similar inconsistencies elsewhere. 
Experimental evidence has already been brought forward indicating 
that the ‘ lime-shy ’ habit is not a direct reaction to excess of calcium 
carbonate as such in the soil, but is associated with correlated peculiarities 
of calcareous soils (1). It seemed possible that detailed experimental 
investigation of a special case might throw light on the significance of the 
‘ calcifuge ’ habit in general. 
A detailed record of the soil conditions found in the special case 
referred to above has already been published (2). 
The observations recorded in this earlier paper, taken in conjunction 
with the results of water-culture experiments on seedlings of Calluna 
extending over a year, served to strengthen the impression that the soil pre¬ 
ferences of the plant are probably intimately connected with the biological 
relations of the roots with one or more micro-organisms. 
1 Thesis approved for the degree of Doctor of Science in the University of London. 
2 The investigation of which this paper gives an account was undertaken as the result of an 
inquiry into the precise ecological conditions—edaphic and biological—which are associated with 
the presence of small well-defined communities of the Common Ling (Calluna vulgaris ) in a restricted 
area of the Wiltshire Downs. 
[Annals of Botany, Vol. XXIX. No. CXIII. January, 1915.] 
H 
