Fungi on Culture Media . 119 
It is interesting to compare the growth curve of Fusarium or Sphaero- 
psis on potato agar with that on potato gelatine (same potato extract 
in each case) in the light of the results shown in Table V. The differ¬ 
ence between the growth curves on potato agar and potato gelatine con¬ 
sists in this—that, other conditions being the same, staling is shown sooner 
on potato agar, but subsequently it becomes more intense on ,,the gelatine 
medium. This result is explicable on the grounds of two factors: 
(1) the greater evolution of ammonia from the gelatine cultures as growth 
proceeds ; (2) the greater capacity of gelatine media to absorb free ammonia 
without becoming alkaline. The first of these factors is illustrated in the 
table just given ; the second has already been illustrated in the experi¬ 
ments dealing with the staling - of fresh media when exposed to fungal 
cultures (Table IV). A more instructive illustration is given in Table VI. 
To equal volumes of potato agar and gelatine equal quantities of a series 
of dilutions of ammonia were added, and the Ph of each determined 
colorimetrically. The concentrations of ammonia added were in the order 
25 , 5 > !• 
Table VI. 
Medium. 
A mmonia 
added. 
Ph^ 
Potato gelatine (io % gelatine) 
0-0 
5-4 
>> yy >5 
1-0 
5-4 
yy yy yy 
5 -o 
6-3 
yy yy yy 
25.0 
9.2 
Potato agar (1-5 % agar) 
0-0 
6.4 
yy yy yy 
1-0 
S.i-8-2 
yy yy y' 
5 -o 
9-5 
yy yy yy 
25.0 
too high to measure 
This table shows that the addition of a small amount of ammonia 
to potato agar has a comparatively greater influence in diminishing its 
H-ion concentration than is the case with potato gelatine. The earlier 
incidence of staling in the cultures on potato agar is readily understandable 
in the light of this result. 
Incidentally one may note that agar media are preferable to gela¬ 
tine media for the purpose of keeping stock cultures, at least in the case 
of certain organisms. The large evolution of ammonia that may occur on 
gelatine media may be injurious or even fatal to the organism. 1 
The evolution of an alkaline gas can be shown simply by placing a 
piece of litmus paper in the lid of the culture. Extended observation has 
shown that the more rapidly a particular culture changes the colour of red 
litmus paper placed in the lid, the more vigorously does it produce staling in 
a plate of fresh medium exposed to it, and in particular where no alkaline 
gas can be demonstrated by means of litmus, no staling effect on a plate 
1 For an illustration of an effect of this nature see Boas, Ber. d. deut. bot. Gesell., xxxvii, 
p. 63, 1919. 
