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 sepaaee 
freezing and thawing destroys the weaker individuals and finally all. (See Bibliog., 
XXXII, especially papers by Sedgwick & Winslow, and by Park; consult also an 
earlier paper by Prudden, Bibliog., XLVI.) 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 tracheiphilus. 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. 
