36 BACTERIA IN RELATION TO PLANT DISEASES. 
cultures. ‘Too many plate cultures can scarcely 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 necessary, and when photographs 
are likely to be wanted plates must 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 quality 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 behavior of an organism on nutrient agar, he may then try gelatin. 
Do any of the organisms under observation soften or liquefy the medium ? 
Agar roll cultures may be made in 
test tubes readily if the amount of 
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, Macé, et al.—have devised 
ruled plates for. counting the number 
of colonies of bacteria in Petri-dish 
poured plates. The writer prefers to 
count by square centimeters or frac- 
tions thereof. When the plate is sown 
thin enough, the 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 may 
be opaque, z. é., 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 ruled on 
with a diamond, or drawn on in very fine black lines with India ink. ‘The gelatin 
film of an unexposed, fixed photographic dry-plate is a very good surface for holding 
the ink. For counting colonies on very densely sown plates, the writer has found 
convenient a rectangle-20 mm. by 5 mm. divided into tenths. 
Fig. 34.* 
SILICATE JELLY. 
In recent years, in the hands of Winogradsky and his students, silicate jelly has 
played an important part in the isolation of various organisms, which do not take 
*Fic. 34.—Folded filter papers made by Schleicher & Schiill, 
