74 
M. Cheveley Rayner. 
The evidence available as a result of these cultures points to 
the conclusion that roots are not formed by the seedlings unless 
infection with a fungus associated with the seed-coat takes place: 
and that therefore the relations between the plant and its endo- 
trophic fungal partner is an obligate one, under the conditions 
supplied, viz., a supply of inorganic salts, in a solution favourable to 
the healthy growth of unsterilized seedlings. 
Since roots are not formed, it is impossible to demonstrate 
with certainty how far the susceptibility of the roots to particular 
salts and to the concentration and the reaction of the culture media 
are specific characters of the plant. 
It is proposed to deal more fully with these and similar cultures 
and with the effects of inoculation from a pure culture of the fungus 
in a subsequent paper. 
The evidence is conclusive that the resting embryo of Calluna 
is uninfected; also that, by effective sterilization of the seed-coat, 
seedlings can be germinated in a sterile condition and can be main¬ 
tained alive under aseptic conditions for six months or longer. 
Infection of the plant by the fungus takes place under normal 
conditions immediately after germination and it is believed that a 
functional root-system is not formed by the seedling unless such 
infection occurs. 
Hence, the nutrition of Calluna is bound up with that of its 
mycorhizal fungus: and the soil preferences of the plant and the 
soil conditions which limit its growth are determined indirectly, by 
the ability of the fungus to flourish under the conditions supplied. 
In short, the maintenance of a definite biologic relation between 
the roots and the fungus which infects them acts as a limiting 
factor to the spread of the plant. 
The refusal of Calluna to grow in the calcareous soil investigated, 
and presumably in similar soils elsewhere, is due to a disturbance 
of these conditions. On this view, the habit usually described as 
“ calcifuge ” may be, in this case, characteristic of the fungus rather 
than of the plant. 
Directly connected with this disturbance is the presence of 
colonies of bacteria closely associated with the roots, especially with 
the regions where the fungus is usually prominent. 
It is suggested that the bacteria are to be regarded either as 
pathogenic agents, or as indicators of soil conditions unfavourable 
to the fungus and therefore ultimately to the plant. 
