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 sursdved 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, i. e.^ in an autoclave. A 



Fig. 70.* 



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 are the large sizes of the 



*FiG. 70. — Earliest stage of fruit spot on green plums, due to Baclcriuin fruni (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. 



