102 Proceedings of Indiana Academy of Science. 
THE LENGTH OF TIME TO INCUBATE PETRI PLATES. 
H. A. Noyes, EDWIN VoicT and J. D. LucKETT, Purdue University. 
Investigations of the steps entering into the plate method for the 
enumeration of the number of bacteria present in soil are few. So little 
agreement was observed in the procedures followed in different soil bac- 
teriology laboratories that investigations were undertaken in this station 
to develop a reasonably accurate technic for the bacteriological exam- 
ination of soils.. The present paper gives data in support of the ten-day 
period of incubation at 20° C. for soil plates. The work was done jointly 
with that on methods of sampling soil for bacteriological analysis’? and 
the number of colonies satisfactory for a petri plate.’ Among the soil 
factors considered in connection with the length of time to incubate 
plates were the kind of soil, the nature of its flora, temperature when 
sampled, the uniformity of sampling, the moisture content, and the con- 
dition of aeration. 
It was early decided that probably the chief reason why confidence 
is lacking in the significance of plate counts is because the organisms 
have not usually been given the proper chance to develop into colonies. 
Table I has been prepared to show how differently organisms develop 
into colonies under different periods of incubation. The technic followed 
was that previously described,’ and the figures are based on the average 
of three plates in each case. 
1 Noyes, H. A., and Voigt, Edwin, in Proceedings of Indiana Academy of Science, 
1916, pp. 272-301. 
* Noyes, H. A., in Journ. Amer. Soc. of Agron., No. 5, 1915, pp. 239-249. 
® Noyes, H. A., and Grounds, G. L., in Proceedings of Indiana Academy of Science, 
1918. 
