76 M. Cheveley Rayner. 
constitute the prominent feature of the growths associated with the 
seed-coat. 
7. Seedlings of Calluna vulgaris are infected by the mycorhizal 
fungus immediately after germination. 
Infection of the primary root takes place by a growth of 
mycelium from the seed-coat, the latter being infected while still in 
ovary. 
8. The resting embryo and endosperm are free from infection. 
9. The fungus isolated from the seed-coat is morphologically 
identical with one obtained in agar cultures of pieces of healthy root. 
10. Seeds can be sterilized and seedlings germinated in a 
sterile condition, i.e., free from fungal or bacterial infection. 
The germination and early stages of growth of such sterile 
seedlings are normal, but in the absence of infection complete 
arrest of root-formation occurs. 
11. The evidence at present available points to the conclusion 
that the relation between Calluna and its mycorhizal fungus is an 
obligate one, and that successful growth of the plant is ultimately 
bound up with infection of the roots at an early stage by the fungus, 
and with the subsequent healthy growth of the latter; hence, the 
soil preferences exhibited by the plant depend on the maintenance 
of a biological balance between the roots and the constituents of 
the microflora which beset them. 
12. On this view, the inability of Calluna to flourish in the soil 
investigated is due to a disturbance of the relations maintained 
under favourable soil conditions between the roots of the plant and 
its mycorhizal fungus. 
The bacterial colonies associated with the roots, more especially 
with the regions where the fungus is usually prominent, are to be 
regarded either as pathogenic agents, or as indicators of soil 
conditions unfavourable to the fungus. The evidence available 
points to the bacterial colonies as directly correlated with the 
abnormal growth displayed by the roots, but is not at present 
conclusive that they are the immediate cause of that condition. 
13. Water cultures of Calluna have not up to the present 
thrown any light on the significance or not of the relatively high 
ratio of MgO to CaO in the soil in the area investigated. 
They confirm previous observations as to the inability of the 
plant to thrive in any but very weak solutions, 
