84 BACTERIA IN RELATION TO PLANT DISEASES. 
philic species. Usually even the most resistant spores, if freely exposed, are destroyed 
by one to two hours exposure to 150° C., of dry heat, or by thirty minutes exposure 
on each of three consecutive days to streaming steam at 100° C. Some very 
resistant spores have survived a single steaming or boiling of five or six hours 
duration (eight hours in one of Tyndall’s experiments), and it is not unlikely that 
some slowly germinating sorts may be able to resist discontinuous steamings for 
three days. It is possible also that there may be some sorts able to germinate and 
again assume a resistant spore form in less than twenty-four hours although this is 
not probable. Some spores are destroyed by a short boiling at 100° C., and all 
spores are quickly destroyed by steam under pressure, 7. ¢., in an autoclave. A 
temperature of 110° C. for ten or fifteen minutes is sufficient. Exposure of media 
to higher temperatures and for longer periods should be carefully avoided. . It must 
be remembered, however, in using autoclaves, that all’ of the air must be replaced 
by steam before the apparatus is closed, otherwise the temperature to which the 
medium is exposed. will not correspond to that indicated by the pressure gage. 
The most convenient autoclaves known to the writer aré the large sizes of the 
*Fic. 70.—Earliest stage of fruit spot on green plums, due to Bacterium pruni (Erw.Sm.). The 
bacteria have entered through the stoma. ‘They disappear farther in, and also a few micra to either 
side of this stoma, as shown by an examination of the serial sections. Material fixed in strong alco- 
hol, infiltrated with paraffin, and cut on the microtome in series. Section stained with carbol-fuchsin 
and drawn directly from the microscope with the aid of a camera lucida. 
