66 
M. Cheveley Rayner. 
Daring part of the time occupied by the experiment, growth of 
the roots had apparently been almost normal and resulted in the 
formation of a more fully developed root system. 
Several points of interest are suggested by these observations: 
(i) . Since the unfavourable and favourable soil conditions 
respectively can be supplied in a filtered, unboiled soil 
extract, they are either of a chemical or biological 
nature. 
(ii) . If the conditions he biological, and the injurious effect 
can he produced by supplying an organism present in 
the unfavourable soil simultaneously with the conditions 
which favour its growth, all the plants so treated might 
fairly be expected to behave in the same way. 
Unfavourable symptoms appear, however, much more slowly 
in plants growing in large pots, thereby suggesting that the un¬ 
favourable factor is chemical in nature and that as soon as a certain 
concentration in the soil is reached, an unheathy condition of the 
roots is induced owing to disturbance of their normal relations with 
the micro-organisms present,—a view confirmed by the results of 
the treatment when an extract of “heather” soil is used for 
watering seedlings growing in “ chalk” soil. 
Whether the unfavourable effect is a direct result of the 
bacterial colonies which beset the roots, or whether it is indirect 
and due to the effect of these colonies on the micorhizal fungus, 
remains an open question, as does also the important point as to 
whether the pathogenic effect of the bacteria is dependent on a 
change in the chemical condition of the soil, the organism being 
present in both cases, or whether the latter is peculiar to the soil 
on chalk. 
This experimental work, combined with examination of the 
roots of large numbers of seedlings grown under different soil 
conditions, suggests that the relation between the roots of Calluna 
and the micro-flora of the soil in which it is growing is an intimate 
one ; that the balance between these biological factors may be very 
delicate, and only maintained in equilibrium, i.e., in such relations 
that the result is either beneficial or indifferent to the plant, under 
definite soil conditions. 
It is only by attacking this larger problem that a clue can be 
found to the soil conditions which determine growth in any given 
locality such as the one under consideration, where the soil differs 
in respect to certain constituents over a small area. 
