The Ecology o/Calluna vulgaris. 75 
There is no doubt that the abnormal growth exhibited by the 
roots in unfavourable soil is directly correlated with the presence of 
an investing sheath of bacteria on these roots, but the evidence is 
not conclusive that the organism concerned is the inducing cause 
of the unhealthy condition. 
Summary. 
1. As shown in a previous communication, small and sharply 
defined communities of Calluna vulgaris occur on the Wiltshire and 
Berkshire Downs, associated with soils on clay-with-flints. 
The distribution of these heather communities is anomalous, in 
that they have monopolized areas of fertile soil. 
Experimental work was undertaken to investigate the nature of 
the factors which limit the spread of Calluna from these areas to 
the surrounding down soils and indirectly to throw light on the 
significance of the calcifuge habit as shown by the soil relations of 
Calluna and its allies. 
2. As demonstrated by pot cultures in soil from (a) a heather 
area, ( b) soil overlying chalk, Calluna vulgaris grows normally in 
the former, abnormally in the latter. 
3. Abnormality of growth is exhibited in (a) reduced germin¬ 
ation capacity, (b) retarded germination, ( c ) arrest of root and 
curvatures of growing region, (d) arrest of shoot, (e) small size and 
red colouration of leaves. 
4. Intimately connected with these abnormalities is the 
presence of colonies of bacteria on the roots, especially around the 
tip, and also a marked diminution of vigour in the growth of the 
mycorhizal fungus. 
5. The abnormalities of growth can be induced in seedlings 
growing in “ heather ” soil by watering with filtered extracts of the 
unfavourable soil, if the treatment is continued over a considerable 
period. The unfavourable factors are presumably of a chemical 
nature. 
6. The result of germinating unsterilized or imperfectly steri¬ 
lized seeds in agar media, made up with extracts of the two soils 
named, shows that the development of the fungal and bacterial 
elements of the micro-flora associated with the seed-coat is deter¬ 
mined qualitatively by the nature of the soil extract used. 
With an extract of the heather soil, the mycelial constituent is 
predominant; with an extract of the down soil, colonies of bacteria 
