98 Rayner.—Obligate Symbiosis in Calluna vulgaris. 
Subsequent experimental work led to the following conclusions, the data 
in support of which have been recorded ( 3 ) : 
1. Seeds of Calluna vulgaris may be sterilized, and seedlings germi¬ 
nated in a sterile condition, i. e. free from fungal or bacterial infection. 
2. In seeds removed with aseptic precautions from unopened fruits, the 
embryo and endosperm are sterile ; the testa is infected with a Fungus 
identical with the mycorrhizal Fungus of the root. 
3. Germination and the early stages of growth of sterile seedlings are 
normal, but, in the absence of infection, arrest of root formation occurs, with 
subsequent inhibition of growth. 
4. Infection of the primary root of the seedling takes place subsequent 
to germination, by a growth of mycelium from the seed-coat, the latter being 
infected while still in the ovary. 
5. Pot cultures in soils, favourable and unfavourable respectively to 
the growth of the plant in the field, demonstrate that Calluna grows 
normally in the former, abnormally in the latter. Abnormality of growth 
is exhibited in : 
(a) reduced germination capacity ; (b) retarded germination; ( c) arrest 
of seedling root and curvatures of growing region ; (d) arrest of shoot; 
(1 e ) small size and red coloration of leaves. 
6 . Intimately connected with these abnormalities is the presence of 
colonies of Bacteria on the roots, especially round the tips. These Bacteria 
are to be regarded either as pathogenic agents, or as indicators of soil con¬ 
ditions unfavourable to the Fungus or to the plant. The presence of 
bacterial colonies is directly correlated with the abnormal growth displayed 
by the roots, but evidence is not conclusive that they are the immediate 
cause of this condition. 
7. The evidence available points therefore to the conclusion that the 
relation between the plant and its mycorrhizal Fungus is an obligate one. 
Successful growth is bound up with infection of the roots at an early stage 
by the Fungus, and with the subsequent healthy growth of the latter. 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. 
Assuming these conclusions to be valid, it is apparent that before any 
special case involving the soil preferences of the plant can be attacked 
directly—before, indeed, such a problem can be clearly formulated—-the 
ground must be cleared by an investigation into the precise relations exist¬ 
ing between the plant and the Fungus which plays such an important part in 
its life-history. 
The work recorded in the present paper was commenced with this end 
in view. Owing to the somewhat unexpected nature of the results, it 
has been carried out as an independent research. 
