6o 
M. Cheveley Ray tier. 
fertile loam, and the conditions generally give the impression that 
the heath flora has become established on the more fertile soils of 
the area, in competition with the surrounding vegetation. 
Mechanical and chemical analyses support this view and 
indicate that the soil monopolized by Calluna is in every respect a 
good fertile loam with a high percentage of available mineral 
matter ( loc. cit., pp. 236, 237). 
A low percentage of calcium carbonate excepted, there is no 
evident factor to account for the successful competition of the 
heather; the soil reaction is neutral, and the only feature of note 
chemically is the rather high ratio of magnesia to lime. 
With regard to the significance of this ratio as it affects plant 
growth, there is at present a conflict of opinion and the evidence 
adduced by different observers is somewhat contradictory. 
Two distinct problems were suggested as a result of these 
observations :— 
(i.) The nature of the soil conditions which allows Calluna to 
compete successfully with other plants on small areas 
of fertile soil. 
(ii.) The significance of calcium carbonate as a factor deter¬ 
mining the spread of Calluna from those sharply 
defined areas to the surrounding “ down ” soil. 
These problems do not appear to be capable of explanation in 
terms of current hypotheses, nor do these hypotheses throw clear 
light on the causes which determine the calcicole and calcifuge 
habit exhibited in varying degrees by so many plants. 
The present investigation was undertaken primarily in order 
to study the biological significance of the sharp delimitation of 
Calluna under the conditions described, and secondarily, in order 
to ascertain whether the facts obtained in the course of the inves¬ 
tigation might throw light on the larger question of the ecology 
of calcicole and calcifuge plants in general. 
(a). Experimental work was begun with two objectives in view. 
Sand and water cultures were grown in the hope of obtaining 
evidence as to the effect of alteration in the proportions of lime and 
magnesia in the culture solutions. 
Water culture experiments carried on continuously for twelve 
months pointed to the conclusion that further work in this direction 
was premature at this stage of the enquiry and unlikely to lead to 
definite results. They were therefore discontinued : the results 
obtained are briefly summarized at the end of this paper. 
