52 BACTERIA IN RELATION TO PLANT DISEASES. 
tuted. It will not do to conclude that an organism is a strict aerobe until it has 
been tested anaerobically in the presence of a variety of carbon foods with uni- 
formly negative results. One who has had some experience may often give a shrewd 
guess as to behavior in fermentation-tubes by carefully noting the growth of buried 
and surface colonies in ordinary media. 
(2) Fermentation-tubes.—The fluids may be Uschinsky’s solution (without the 
glycerin unless this is the carbon compound to be tested); peptone water (2 per 
cent Witte’s peptone with 0.5 per cent sodium chloride); and filtered tap water, or 
sugar-free beef bouillon with addition of 1 per cent Witte’s peptone (preferably for 
most purposes this latter fluid). ‘The substances to be tested (which should be 
chemically pure or as nearly so as possible) are grape-sugar, fruit-sugar, cane-sugar, 
milk-sugar, galactose, maltose, dextrin,* mannit, dulcit, raffinose, glycerin, ethyl 
alcohol,} methyl] alcohol, acetone, ammonium lactate, ammonium tartrate, asparagin, 
sodium asparaginate, urea, etc. One to 5 per cent of the various sugars, etc.; may 
be used ; 2 per cent is a good quantity. 
Fig. 46. 
Observe’ carefully what substances induce clouding in the closed end and 
whether any gas is produced. ‘Test from time to time for acids. ‘The relative vigor 
of growth in the open end should also be noted. Does growth stop in the U with 
a sharp line of demarcation? Does the addition of calcium carbonate reduce or 
prevent the formation of gas or favor growth in any way? Is the reaction in the 
closed end, as the result of growth, different from that in the open end? Pipette 
out all the fluid from the open end, determine its reaction to litmus, and then test 
the reaction of that which remains. How is the difference, if any, accounted for? 
If growth finally ensues in the closed end, is there any reason for thinking it due 
to absorbed air? How can this be determined ? 
It should be remembered that often, after a time, air is absorbed into the closed 
end of fermentation-tubes and may lead to confusing results. For this reason, 
if they have stood on the shelf any length of time after sterilization, they should 
be re-steamed and the bubble of air tilted out before they are inoculated. They 
*The dextrin should be freely soluble in cold water and should not give any red reaction with 
iodine—i, e., should be free from amylo-dextrin (erythro-dextrin). Such dextrin is hard to procure. 
tThis and the next four should be added, after sterilization, by means of sterile pipettes. ‘The 
ammonium. salts may be obtained in a sterile condition without loss of ammonia by dissolving 
10 grams in 200 cc. of water and forcing this through a Chamberland filter into a sterile flask, 
from which the proper quantity may be pipetted into the culture medium after sterilization. 
¢ Fic. 46—Wooden carrier for fermentation-tubes, the flanging base being held under the 
grooves. Much reduced. 
