TRANSACTIONS OF SECTION K 677 
from calcium carbonate. Yet it resembles the English forest on chalk exactly, 
except as regards floristic detail; and the floristic differences are purely geo- 
graphical in origin, with the exception perhaps of the presence of Viola hirta in 
the English wood, and of the ferns in the French, which may be due to edaphic 
influences. 
From the genetic standpoint, too, both are essentially similar; both are stable 
associations, degenerating, if subjected to the ravages of grazing animals, into 
similar park- or pasture-land, from which they regenerate themselves if pro- 
tected by a simple mise en défense. 
It would thus seem necessary to regard the beech-forests of the Cevennes and 
of the English chalk as associations or sub-associations of some one formation 
and to discard the idea of a plant-formation on calcareous soils. 
4. The Ecology of Calluna vulgaris. 
By Miss M. Cusvetey Rayner, B.Sc. 
A starting-point for the experimental work described in this paper was 
found in the occurrence of sharply defined communities of Calluna vulgaris on 
the Wiltshire Downs, the tendency of the plant to spread being apparently 
determined by definite factors. 
As a preliminary step, the vegetation was mapped and a series of soil analyses 
made of the soils inside and outside a Calluna area. Experimental work was 
undertaken with the object of throwing light on the soil preferences of Calluna, 
and possibly on the edaphic relations of ‘ calcifuge’ plants in general. 
Experimental Lesults. 
In water-cultures Calluna seedlings grow equally well in neutral or faintly acid 
solutions, but are very sensitive to changes in the concentration of the culture 
fluids. 
Artificial cultures have, up to the present, lent no support to the view that the 
MOT oe : : 
CO ratio has any significance, as was suggested by the soil analyses. 
In pot cultures, using soil from the chalk down immediately outside the 
heather area, germination capacity is lowered and rate of germination much 
retarded as compared with what occurs when soil is used from within the area. 
Growth is inhibited in seedlings grown in the former soil; root-growth is 
checked and the tips of the roots show curvatures. Bacterial colonies, especially 
associated with the root-tip, and often forming a sheath round it, are a prominent 
feature of such roots in quite young seedlings. Partial sterilisation of the un- 
favourable soil does not aftect this result. 
In sterile agar cultures, using soil extracts from the same two soils, untreated 
or imperfectly sterilised seeds show a zone of growth due to micro-organisms 
associated with the seed coats. The character of these growths differs markedly 
in the two sets of plates. In the extract from heather soil they are chiefly 
composed of mycelium; in that from chalk-down soil bacterial cultures are the 
most prominent feature. 
Using data so obtained, the problem may be stated as follows: Are the soil 
preferences and peculiarities of Calluna specific to the plant, or are they indirectly 
affected by the association of a mycorhizal fungus with the roots, the growth of 
which, in its turn, is determined by the nature of the soil and its bacterial flora. 
pie ttowing points require elucidation before the main problem can be 
attacked :— 
1. At — eaee and from what sources does infection of the root take 
place? . 
2. Is it possible (a) to germinate, (b) to grow Calluna without infection 
under sterile conditions? 
3. Can it be shown that definite races of bacteria are associated with the 
roots, which, given certain conditions, become directly or indirectly 
pathogenic to the plant, or are the appearances observed of a 
secondary nature? 
