7o 
M. Cheveley Rayner. 
Since a majority of the seeds in each plate were infected, the 
Petri-dishes were opened and the growths examined microscopically. 
In the “chalk” soil cultures, well-defined bacterial colonies were 
present round all seeds but three, in a total of about one hundred 
seeds. 
Hyphae were absent from the outer limits of many of these 
colonies. In others mycelium was present but generally showed 
attenuated growth and the hyphae were invested with a sheath of 
bacteria (Plate III, Fig. 6). 
Masses of bacteria occurred in close association with the 
mycelium, which was rarely present in sufficient amount to alter 
the macroscopic appearance of the colony. 
In the “heather” soil cultures a vigorous zone of mycelium 
was present around every infected seed. Mycelial growth round 
“sterilized” seeds was apparently pure but had not reached the 
sporing stage. 1 
Bacterial colonies, closely resembling those in the “chalk” 
soil cultures were present in many of the growths. These were 
not sufficiently prominent in any single case to affect the macro¬ 
scopic appearance, which was always of a mycelial character. 
The close association of bacteria with the hyphae, which was a 
striking feature in the other plates, was not observable. 
At this stage of development the difference in the two sets of 
plates was very striking :— 
The cultures for which an extract of “ heather ” soil had been 
used were practically free from bacterial growth ; in those for which 
an extract of the “chalk” soil was used they were the most 
prominent feature. 
It was the striking contrast presented by this set of plates, in 
conjunction with the appearances noted in the roots of young 
seedlings growing in soil cultures, that first suggested the possibility 
of a biological relation between the mycorhizal fungus and soil 
bacteria. 
It is significant that the mycelium grows vigorously in a medium 
made up with an extract of the one soil and is practically eliminated 
by bacterial growth in that made with an extract of the other soil, 
the seeds in both cases having had precisely similar treatment. 
The appearance of the hyphae in the latter case strongly suggests 
that given certain conditions of nutrition, bacteria, constantly 
1 A fungus invariably present in these colonies has since then been 
identified with the species occurring as mycorhiza in the roots. 
