The Ecology of Calluna vulgaris. 69 
resting seed, but in favourable cases the presence of hyphae and 
more rarely of isolated spores can be demonstrated. The evidence 
at present available, points to infection of the coat by spores as 
relatively rare. This may, to some extent, account for the 
differential results obtained when attempting to sterilize seed. 
Effective sterilization is doubtless more difficult in the case of seeds, 
to which spores adhere so closely that they are not dislodged by 
washing or centrifuging. Attempts to sterilize the seed by immersion 
in hot water at 70"C., 60 ft C., and 52°C. respectively for fifteen minutes 
led to all the seeds being killed by the treatment. Seeds soaked in 
•01% corrosive sublimate for five minutes always gave a large pro¬ 
portion of infected seeds when sown. 
The following method was then tried and although sterile 
seedlings were not obtained, it yielded interesting results in another 
direction. 
Soil extracts of the two soils were prepared, using 25 grams of 
fresh soil to 300c.c. distilled water; agar, in the proportion of T5% 
was added and the media sterilized in the usual way. 
Seed was sterilized by soaking for thirty minutes in *01% 
corrosive sublimate, washed repeatedly with sterile water and sown 
with a sterile pipette in Petri dishes of the above media, which were 
kept for some days at a temperature of 37°C. At the end of five 
days the two sets of plates were strikingly different in appearance, 
this observation applying both to the sterilized cultures and to the 
controls for which unsterilized seed was used. 
The results are summarized in Table II. 
TABLE II.— Seed Cultures in Agar Soil Extracts. 
Temp. 37°C. 5 Days. 
Seed Treated 
Corr. Sub. -01% 
Control 
(Seed Untreated). 
(A) 
“ Heather ” 
Soil 
Extract. 
Many seeds apparently 
sterile. 
Some with a zone of my¬ 
celium. 
Plates sterile except in the 
immediate neighbour¬ 
hood of seeds. 
Majority of the seeds 
surrounded by a zone 
of mycelium. 
A few seeds apparently 
unaffected. 
(B) 
“Chalk” 
Soil 
Extract. 
Almost every seed or group 
of seeds with a zone of 
bacterial growth. 
Mycelium almost or en¬ 
tirely absent. 
Plates otherwise sterile. 
Majority of seeds sur¬ 
rounded by bacterial 
colonies. 
Mycelium, if present, 
always accompanied 
by vigorous bacterial 
growth. 
