82 BACTERIA IN RELATION TO PLANT DISEASES. 



per gallon. The wrought-steel cylinders cost about $10 each. A good quality 

 of resistant-glass test-tubes may be had from Greiner & Friedrichs, Stiitzerbach, 

 Germany. One sort has a faint-blue longitudinal stripe blown into the glass, 

 another kind has the letter " R " etched on the upper part of each tube. Tubes 

 without any distinguishing mark should not be purchased, as they are likely to 

 become mixed with ordinary non-resistant tubes. The cost of these tubes, duty 

 free, is about $16 per thousand. Good Petri dishes may be obtained from the same 

 firm, and also from E. H. Sargent & Co., Chicago. 



The temperature demands of bacteria are extremely variable. Whole groups 

 of them are able to live under conditions which would be impossible for the higher 



Fig. 68* 



plants and animals. Many of the northern forms, especially those which grow in 

 water, are adapted to low temperatures. The organisms of dung-heaps and thermal 

 springs, and the tropical forms, often grow at high temperatures. 



For a very few species it has been known that prolonged freezing or repeated 

 freezing and thawing destroys the weaker individuals and finally all. (See Bibliog., 

 XXXIII, especially papers by Sedgwick & Winslow, and by Park ; consult also an 

 earlier paper by Prudden, Bibliog., XL, VI.) For the bacteria as a whole, however, 

 it has been assumed that ordinary freezing or even very intense cold simply inhibits 



*Fic. 68. Petri-dish poured plate of Bacillus tracheifhilus. The 10 cc. of nutrient agar was 

 inoculated with a carefully measured loop of a fluid culture. The fluid culture was then exposed 

 in block-tin test-tubes to the temperature of liquid air, after which another plate (fig. 69) was made. 



