(Vou. 7 
272 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
table v, produced in the same period of growth. Probably 
this increased growth explains the more rapid utilization of 
the dextrose than was calculated. The maximum growth was 
attained at 168 hours—3,750,000,000 bacteria per cc. Thus 
this culture showed the least inhibition of any of the experi- 
mental cultures and serves as a standard for comparison with 
the others. 
To study more in detail the effect of small amounts of acid, 
a series of cultures was observed in which the only individual 
variation was in the initial amount of dextrose. To 250 cc. 
of plain bouillon in each of five 500-cc. flasks were added re- 
spectively .05, .1, .15, .2, and .3 per cent of dextrose, and all 
were inoculated from the same culture tube of Bacillus coli. 
The growth and changes in Py, are presented in table vu. The 
cultures are numbered, as indicated in the table, from 1 to 5 
in order of increasing amounts of dextrose. Cultures 2, 4, and 
5 are plotted in fig. 7 as representative of the series. The Py 
eurves show that acid was produced in each culture and that 
the amount of acid formed corresponded to the amount of dex- 
trose provided. "The cultures formed a regular series of increas- 
ing acidities. Following the acid production there was a rever- 
sion of the reaction toward alkalinity which was quite rapid 
in the first four cultures but slower in Culture 5, where a Py 
of 5.1 was maintained from 24 to 96 hours. Comparing the 
growth curves of the five cultures during the period from inocu- 
lation to 48 hours, it is seen that Culture 2 makes the best growth 
and that Cultures 3, 4, and 5 follow in order. It would appear, 
then, that .1 per cent of dextrose or less is stimulative in effect 
and that there is no acid injury from a short exposure to Py 5.9 
(Culture 2). There is, however, some acid inhibition from a 
Pn of 5.5 (Cultures 3 and 4) and quite a marked inhibition— 
sufficient to cause some decrease in numbers—from 3 days' 
exposure to P4 5.1 (Culture 5). In each case the growth curve 
ascended as the Pa curve descended toward the alkaline side. 
The maximum growth was approximately the same for all the 
cultures—1,400,000,000 to 1,800,000,000 bacteria per ec.—and 
was reached when the hydrogen ion concentration fell in a zone 
between Py 7.0 and 7.6. At the point of maximum growth 
the hydrogen ion concentrations for the cultures in order were 
