Mycorrhiza in the Ericaceae. M. Cheveley Rayner. 65 
When it is, criticism is invited, e.g. ten seedlings in one experi- 
ment, eleven seedlings in another. Numbers so low in the case 
of seeds so small as those of Calluna raise grave doubts as to 
the general conditions of the cultures. 
Again, the proof offered by Christoph of the identity of a 
fungus, isolated by him from clean pieces of Calluna root, with 
the endophyte is unsatisfactory. So far as is possible to judge 
from the description of morphological characters, the right 
fungus was isolated from the root although spores were not 
produced. Positive proof of identity can only be supplied by 
inoculation into a pure culture seedling growing under controlled 
conditions with subsequent production of mycorrhiza. 
The method adopted was to sow unsterilized seeds in pots of 
sterilized soil, and transfer the seedlings so obtained to pots of 
sterilized soil inoculated with the fungus he had isolated from 
roots. 
According to Christoph the first roots of such seedlings re- 
mained sterile, the numerous laterals which developed became 
typically infected. On my view, such seedlings would always 
become infected at germination, their establishment as rooted 
plantlets depending upon this. Even assuming the seed coat to 
be free from infection by the specific root fungus, the method 
used by Christoph is obviously incapable of providing the strict 
proof of identity required. 
Attention has already been drawn by me (loc. cit. p. 6.) to 
the possibility of replacing the stimulus to development normally 
provided by the fungus by stimuli of a chemical nature, e.g. 
by the addition of organic substances to the rooting medium 
in which seedlings are growing. It is even conceivable that an 
appropriate organic substance might be present in sterilized 
peat and so provide an explanation of the discordant experi- 
mental results recorded by Christoph. This possibility is now 
being fully explored. 
In conclusion, may I add one word as to the experimental 
difficulties experienced in work of the kind under discussion? 
After many years’ work I am still far from a complete under- 
standing of the co-ordination shown by plant and endophyte 
in Ericaceae. There is, for example, a close and, at present, 
unexplained correlation between germination of the seed and 
activity of the mycelium on the seed-coat. Again, the presence 
of the endophyte in the tissues of the shoot and within the un- 
opened fruit makes its isolation theoretically a simple matter; 
in practice this is difficult, and I am still ignorant of the exact 
conditions which determine the ability or otherwise of the 
fungus to grow out from the tissues. 
Owing to the extreme delicacy of the balance between seed- 
M.S, 5 
