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, 

 arid 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. 



