EXPERIMENT STATION BULLETINS. 589 



districts of Michigan, a fungus restricted by such moisture requirements 

 must necessarily have a spore which will germinate and produce infection 

 in a short time, if it is to become an im.portant leaf spotting organism. The 

 spores of the root rot organism under natural conditions do not ordinarily 

 produce infection on the above-ground parts because of the drying out of 

 the leaves and the leaf petioles before the germ tubes have had time to form 

 and penetrate the host tissue. 



Thermal Death-Point — 



The ability of the fungus mycelium to withstand low temperatures has 

 been tested by placing flask cultures outside of a window when the out-door 

 temperature ranged from -18° to -28°C. Transfers proved that the mycelium 

 was not killed after several days' exposure. 



In the higher range of temperatures the mycelium is killed by subjecting 

 it to 35° for a period of 72 hours. A ten-day exposure at 32° stopped growth 

 but was not sufficient to produce death. Shorter exposures at lower tem- 

 peratures indicate that the thermal death-point of the mycelium is very 

 close to that of the spores which at a 10-minute exposure are killed at 49°. 



In determining the thermal death-point of spores the capillary tube meth- 

 od, as described by Novy*, has been used. Capillary tubes, five inches in 

 length and about 2mm. in cross section, were drawn out from a piece of 

 thin-walled glass tubing. These were inserted in a spore suspension until 

 they were properly filled, then sealed at each end. The tubes were immersed 

 in water baths held at constant temperatures. At the end of the time of 

 treatment they were cooled in water at a temperature of about 21 °C. The 

 tip of one end was then broken off, using aseptic precautions, and the con- 

 tents discharged into melted agar by applying heat to the unbroken end. 

 The experiment was run twice in duplicate, first with an interval of 5 degrees 

 and later with an interval of 1 degree. Results are combined in the follow- 

 ing table: 



TABLE 5. THERMAL DEATH-POINT OP SPORES. 



The thermal death-point of spores, at an exposure of 10 minutes, lies 

 between 48 and 49 degrees. Only a few spores germinated at 48 degrees, 

 indicating that there was some slight variation in the treatment which the 

 spores in any one tube received or that some of the spores had slightly greater 

 potentialities for withstanding heat than did others. 



* Novy, F. G. Laboratory work in Bacteriology. 1899. Ann Arbor Mich. (George Wahr.) 



