SOIL FUNGI 279 



5.0 and only poorly at pH 7.0 and pH 3.5; 1 ' 58 this has an important 

 bearing upon mycorrhiza formation. In neutral or slightly acid soils, 

 these fungi are not virulent and either cannot form at all or form only 

 with difficulty endo-and ecto- trophic mycorrhiza. The most abundant 

 mycorrhiza of evergreens are found in soils of pH 4.0 to 5.0. Rhizoctonia 

 silvestris and Mycelium radicis atrovirens grow well in neutral or slightly 

 alkaline media. When the reaction of the soil is near neutrality, 

 pseudo-nrycorrhiza will be formed. Melin succeeded in isolating a 

 series of fungi and demonstrated that the same types of mycorrhiza are 

 produced in pure culture as in the soil; these facts established experi- 

 mentally allow a better insight into the role of mycorrhiza in plant 

 nutrition and plant distribution. 



For establishing whether a certain organism forms a mycorrhiza with a given 

 plant, sand or forest humus can be used as a substrate. 169 In the first case, 

 150-gm. portions of pure, washed, dry sand are placed in 300-cc. Erlenmeyer 

 flasks. Forty-nine cubic centimeters of the following medium is then added 

 to each flask: 



NH 4 C1 0.50 gram CaCl 2 0.10 gram 



or NaCl . 10 gram 



KNO3 0.95 gram MgS0 4 -7H 2 30 gram 



Glucose 0.50 gram FeCl 3 0.01 gram 



KH2PO4 1.00 gram Distilled water 1000 cc. 



The flasks are plugged with cotton and sterilized in flowing steam for 25 

 minutes on 3 consecutive days. The humus medium is prepared by saturating 

 with water 70 grams of mixed forest humus material (upper part of slightly de- 

 composed layer). The flasks are sterilized in steam three times, and washed 

 twice with sterile distilled water by adding 50 cc. of water to each flask, shaking, 

 and allowing it to rest for 24 hours, then pouring off the excess of water. This 

 process is repeated. The contaminated cultures (examined after 14 days) are 

 discarded. 



The seeds are sterilized by moistening with water, keeping one minute in 

 1 : 1000 mercury bichloride solution, then washing in sterile water. They are 

 then allowed to germinate upon sterile agar (1.2 per cent agar in distilled water), 

 the individual seeds being removed from one another so that the infected seed 

 do not infect the sterile ones. After 10 to 15 days, the germinated seeds re- 

 maining sterile are transferred by means of a sterile platinum needle to the flasks. 

 Contaminated flasks (as shown microscopically and culturally) are discarded. 

 The flasks are then inoculated with the cultures of the fungi in question. 



168 Melin, E. Uber den Einflusz der Wasserstoffionenkonzentration auf die 

 Virulenz von Kiefer und Fichte. Bot. Notiser, 1924, 38-48. 



169 Melin, E. Untersuchungen iiber die Larix-Mykorrhiza. Synthese der 

 Mykorrhiza in Reinkultur. Svensk. Bot. Tidskr., 16: 1922. 



