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 i 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. 464 



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 t. 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. 



\ FIG. 46. 'Wooden carrier for fermentation-tubes, the flanging base being held under the 

 grooves. Muoh reduced. 



