The Ecology of Calluna vulgaris. 71 
associated with the seed, can multiply and become pathogenic to 
the fungus which is also present on the seed coat. The same 
inference is suggested by the behaviour of the mycorhizal hyphae 
on the roots in pot cultures of the “chalk” soil. 
It is recognised, of course, that these pathogenic relations may 
be secondary in both cases, and determined by inability of the 
mycelium to thrive under the conditions supplied. In either case 
they seem to be pertinent to the inquiry. A large proportion of 
the seeds in these cultures germinated, and no difference was 
apparent either in rate of germination or in germination capacity. 
This difference of behaviour as compared with soil cultures may be 
due to the position of the seeds on the surface of the media, but 
requires further investigation. The seedlings grew for a month or 
more, but soon showed symptoms of starvation in all the cultures, 
due no doubt to the paucity of food material in the soil extracts. 
The next attempt to sterilize seed was successful. 
Similar extracts were prepared, made up with agar, sterilized, 
and placed in sterile Petri dishes. Seeds were soaked for thirty 
minutes in water, precautions being taken that the seed coats 
should be thoroughly wetted. They were then immersed for short 
periods (two minutes or less) in a 1% solution of corrosive sublimate, 
washed as before in many changes of sterile water and sown, fewer 
seeds being placed in any one dish. Care was taken to make the 
conditions as aseptic as possible and the cultures were placed under 
a bell-glass in a small greenhouse, all the surroundings having 
previously been washed down with a strong solution of Jeyes’ fluid 
in water. 
At the end of nineteen days three plates contained each a 
number of germinating seeds and were absolutely free from micro¬ 
organisms. 
No trace of fungal or bacterial growth appeared round any of 
these seeds, about 50% of which germinated at the usual rate. 
The early stages of growth of these sterile seedlings are other¬ 
wise normal and precisely similar to those of infected seedlings. 
The seedlings were transferred singly to special culture tubes 
fifty-three days after sowing, i.e., about five weeks after germination. 
Many of them formed several leaves and developed the rudiments 
of roots while still in the seed dishes (Text-fig. 1, A—F). 
The cotyledons and young leaves were green and the seedlings 
appeared to be perfectly healthy, with the exception that they had 
made no attempt to send down roots into the culture medium, or 
