116 Brown.—Experiments on the Growth of 
growing at almost the same rate. Now the Petri dishes are of approxi¬ 
mately the same volume, and if limitation of oxygen is the factor concerned 
in staling, one would anticipate that staling would appear more strongly in 
the closed Petri dishes with deep layer of medium than in the correspond¬ 
ing ones with shallow layer of medium, first because there is greater free 
air-space in the latter, and secondly because there is throughout greater 
fungal growth in the former and consequently greater demands on oxygen 
supply. This, however, is not the case, whence it follows that the supply of 
oxygen is not the factor responsible. This conclusion is in agreement with 
former work, where it was shown that the amount of growth made by 
a fungus is independent of oxygen pressure within very wide limits. 1 The 
second alternative is therefore the one indicated, i.e. the differences observed 
are due to the action of gaseous or volatile products of the metabolism of 
the fungus. 
The remaining portion of the experimental part of this paper'will be 
devoted to an account of the nature of this action and how it can be demon¬ 
strated. It will aid to clearness to state here the main conclusions arrived 
at. The two gaseous products concerned in this differential action are carbon 
dioxide and ammonia. The former arises of course in the ordinary process 
of respiration; the latter is a product of the break-down of certain nitro¬ 
genous compounds (proteins, peptones, amides) which are present in most 
media, e.g. in potato and in all media containing gelatine. According to 
the extent to which these products accumulate or are disposed of, the amount 
of staling is affected. The ammonia, which is produced on some media in 
an amount greater than the fungus can take up, dissolves in part in the 
medium and renders it stale, or, more correctly, makes it more stale than it 
would be if this product were removed. The staling action of ammonia is 
partly counteracted by the carbon dioxide, and in accordance with the 
experimental conditions which allow the one or the other to accumulate there 
will be greater or less staling. From the physico-chemical point of view 
the principles involved are better stated as follows: When carbon dioxide is 
in excess, the ammonia goes into solution in the medium in the form of the 
bicarbonate; when ammonia is in excess, as ammonium carbonate and free 
ammonia. The latter is the more actively staling combination, probably 
on account of its higher hydroxyl concentration, but whether the greater 
activity is entirely due to this is not certain. 
Two main lines of experiment were adopted. The first of these con¬ 
sisted in exposing plates of fresh medium to the gases given off by fungal 
cultures, after which the staling action of the exposed plates was tested 
by inoculating them with various fungi. The second method consisted 
in growing fungal colonies in atmospheres which were controlled in respect 
of their carbon dioxide and ammonia content. 
The method of experiment in the first case is very simple. A batch of 
1 Loc. cit., p. 266. 
