122 Brown.—Experiments on the Growth of 
All cultures were examined every two or three days, and at each time 
of measurement each culture was started anew in ordinary air. By follow¬ 
ing the growth at intervals it was seen that in the early stages all the 
cultures were growing at the same rate, but by and by staling set in in 
the closed dishes, whereas it was later in appearing in the open ones. Cul¬ 
tures of Sphaeropsis on deep layers of potato agar do not stale appreci¬ 
ably for several days. Thus differences of the type recorded above are 
not striking in the shorter experiments on deep medium. With shallow 
layers of medium staling sets in sooner, and thus it is found that more 
striking differences are obtained with shallow plates, as the above table 
shows. 
In accordance with the results here recorded it is possible to take 
a culture (especially of Sphaeropsis ) which has been kept in a closed Petri 
dish until it shows obvious marginal staling, and then, by removing the lid 
and placing in a large container in the usual way, to effect a very marked 
recovery of the rate of growth at the margin. The effect is still more 
marked if the plates are transferred to an atmosphere containing a moderate 
concentration (5 or 10 per cent.) of carbon dioxide. 
Attempts were made to obtain results similar to those recorded in 
Table VIII by comparing the growth in closed Petri dishes with that 
in dishes which simply had the lids raised so that free air exchange could 
take place, the use of containers being dispensed with. The majority of 
these experiments failed through contamination at the edges, but even 
where trustworthy readings could be made the same result was not obtained. 
The result was unexpected, and an explanation of the apparent discrepancy 
was sought for in the fact that the cultures thus exposed to laboratory air 
show a considerable amount of drying up of the medium. Control ex¬ 
periments in which the cultures were inverted over desiccating agents 
showed that in fact exposure to drying of a plate on which a fungus was 
growing resulted in slowing down the rate of growth. Plates of various 
media which had been inoculated with Sphaeropsis were inverted over 
Petri dishes of the same size containing 15 c.c. of various concentrations 
of sulphuric acid. The following table shows a typical set of results. The 
figures represent the growth in four days of Sphaeropsis on potato agar. 
Table IX. 
Closed Petri Dishes. 
Petri Dishes inverted over 
5~54 
' 25 % h 2 so 4 . 15 % h 2 so 4 . 10 % h 2 so 4 . 
4*55 5 - 12 5- 2 9 
There was a gradual reduction of growth-rate with increasing desic¬ 
cation. Exactly similar results were obtained with other media, e^g. plum 
gelatine. In this connexion a somewhat striking result was met with, in 
that while desiccation during growth slows down the rate of growth, a 
