240 The Ecology of Calluna vulgaris. 
other Ericaceae, owing to the presence of mycorhiza, it seems 
possible that a clue is to be sought for in the biological rather than 
in the chemical or physical peculiarities in the soil. 
Summary. 
Calluna vulgaris occurs in various places on the chalk downs of 
the South of England. The data hitherto recorded make it 
difficult to account for the existence of isolated and often sharply 
defined Calluna communities in these situations, such communities 
being apparently invasions and occurring on areas distinguished 
by the presence of heavy loamy soil, rich in mineral constituents 
although poor in lime. 
Although the thickness of the overlying deposits, and con¬ 
sequently the amount of calcium carbonate present in the rooting 
medium apparently determine the distribution of the Calluna, 
functional roots were found in places growing into the chalk rubble, 
and small lumps of chalk occur occasionally within the rooting area. 
The soil on which Calluna is growing contains a relatively high 
proportion of magnesia. It is suggested that this may be of signifi¬ 
cance either chemically, or in so far as it affects the physical 
or biological conditions of the soil, but until the results of experi¬ 
mental work are available, no definite conclusion can be arrived at. 
With this exception no evident edaphic factor has as yet been 
found to account for the successful competition of the heather. 
The soil is neutral in reaction. 
Calluna is associated with several species alien to the surrounding 
vegetation. These, as well as the flora of the neighbouring chalk 
down and of the clay-with-flints are being recorded. 
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3. Schimper, A. F. W. Plant Geography, Eng. Ed., p. 656. 
4. Contejean, Ch. Geographic botanique, 1881. 
5. Bonnier, G. Bull. Soc. Bot. France, XX., VI., 1879. 
6. Warming, Eug. Ecology of Plants, Eng. Ed., pp. 143, 212. 
7. Graebncr, P. Die Heide Norddeutschlands, pp. 142-144. 
8. ,, ,, Lehrbuch der allgemeinen Pflanzengeographie, r 1910, p. 
236. 
9. Voelcker, J. A. Jour. R. Agric. Soc., Vols. 68 and 70. 
10. Hall, A. D. and Russell, E. J. Report of Board of Agric. on Soils of Kent, 
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