108 HEART-ROT 
process of wood destruction in these cultures are described in 
the sections dealing with these phenomena. In one of these 
cultures two fructifications have now been growing for 
more than four years (fig. 44). 
3. On small sterilized roots of larch and Weymouth pine 
growth was rapid, especially at a temperature of 20°-22° C. 
Small bunches of such roots in test-tubes became smothered 
in mycelium to a distance of an inch in fifteen days. 
4, Altogether over 220 cultures were attempted on soils. 
Special care was taken with these because, on discovering 
that mycelium would grow readily on sterilized soil, I thought 
I might find that it preferred some soil to others, and in 
this way immunity and susceptibility to heart-rot might 
be in some measure explained. Some results of value have 
been obtained from these experiments, but the problem of 
immunity and susceptibility has certainly not been solved 
by them. No growth was obtained on unsterilized soil, 
and growth on autoclaved soil was much more abundant 
than on dry sterilized soil, so that the method of treating 
soil prior to infection is a matter of great importance. 
After some preliminary trials my procedure was as follows : 
For each sample of soil twelve or more test-tubes were 
prepared. A plug of cotton-wool was placed in the bottom 
of each and dry soil poured in to a depth of about 2 in. 
The test-tubes were plugged in the usual way. To four of 
them sterilized water was added without further treatment 
till infection. Four more were moistened with sterilized 
water and autoclaved for twenty minutes at 2 atm. pressure. 
Yet another four were sterilized in the hot-air sterilizer for 
twenty-five minutes at 140° C., and sterilized water was 
added when they were cold. Soils were inoculated some- 
times from mycelium, but generally by pouring in a drop or 
two of water containing conidia. The soils on which such 
sets of cultures were made are shown in the following 
table. 
