36 



BACTERIA IN RELATION TO PLANT DISEASES. 



cultures. Too uiau}' plate cultures can scarceh' be made. Dishes with flat and very 

 thin bottoms (0.3 mm.) are desirable for some purposes, but are difficult to procure. 

 For quantitative work, plates with flat bottoms are necessar)-, and when photographs 

 are likely to be wanted plates nmst be selected which do not have rings, wavy places 

 or other flaws in the glass on the bottom. There is room for much improvement 

 in the qualitv of the Petri dishes now on the market. 



The student is advised to use agar media for all general laboratory work. When 

 he has learned the beha\'ior of an organism on nutrient agar, he may then try gelatin. 

 Do any of the organisms under observation soften or liquefy the medium ? 



z\gar roll cultures may be made in 

 test tubes readih' if the amount ot 

 fluid agar is reduced to one-half cubic 

 centimeter. 



When colonies are to be counted, 

 special pains must be taken to dis- 

 tribute the gelatin or agar uniformly 

 over the bottom of the dish. 



Various persons — Pake, Jeffer, 

 Weiss, Mace, et al. — have devised 

 ruled plates for counting the number 

 of colonies of bacteria in Petri-dish 

 poured plates. The writer prefers to 

 coimt by square centimeters or frac- 

 tions thereof When the plate is sown 

 thin enough, tlie entire number of 

 colonies should be counted. When 

 it is very dense, the average may be 

 taken of ten square centimeters se- 

 lected with care, provided the bottom 

 is flat, otherwise the whole plate must 

 be counted. If the counting plate 

 is to be placed under the dish, it maj' 

 be opaque, /. e.^ a black surface with 

 white lines, not the reverse. If it is 

 to be placed on top of the dish, the latter preferably bottom up, then it should be 

 of glass or some other transparent substance. The spaces may then be nded on 

 with a diamond, or drawu on in very fine black lines with India ink. The gelatin 

 film of an unexposed, fixed ]>hotographic di'j'-plate is a \'ery good surface for holding 

 the ink. P'or counting colonies on very denseb' sown plates, the writer has found 

 convenient a rectangle 20 mm. b)' 5 mm. divided into tenths. 



Silicate; Jsllv. 



In recent years, in the hands of Wiuogradsk)- and his students, silicate jelly has 

 pla}'ed an important ]3art in the isolation of various organisms, which do not take 



Fig. 34.-' 



*Fic., 34. — I'oliJed frlter papers mafle by Sclik-iolier & Sciliul: 



