106 Brown.—Experiments on the Growth of 
problems of fungal physiology. One of these is the problem of strains— 
i.e. accepting the existence of several strains of the same organism, what 
physiological differences can one establish between them, and can one 
correlate with these any of the distinct appearances of the particular strains ? 
Such an investigation is much required in these days, when numerous 
strains are being discovered in many organisms. If in these cases, as is 
generally conceded, the morphological criteria are insufficient, and therefore 
recourse has to be had to physiological means of differentiation, it is 
obviously important to gain some idea as to what physiological characters 
will be of value in this connexion, i. e. what physiological characters are 
sufficiently constant and determinable with sufficient accuracy to be of use 
from the systematic standpoint. 
The second problem may be described as that of the ‘ suitability ’ of 
a particular nutrient for a particular fungus. On this basis one may 
attempt to interpret the different growth appearances of an organism on one 
medium as compared with those on another, and in particular to explain the 
fact of an organism growing well on one medium and not at all on another. 
This latter problem is of the greatest interest in pathology, as its solution 
would form a vantage ground for the study of immunity in so far as the 
latter is based on nutritional factors. 
The present paper deals with the so-called ‘ staling * of fungal cultures. 
The term ‘ stale ’ and its derivatives are in common laboratory use, and as 
some such term will be required in the present account, if much circumlocu¬ 
tion is to be avoided, it is proposed to adopt the term throughout. By 
a ‘ stale culture ’ is understood one which has ceased or practically ceased 
growing; by a ‘ stale medium ’ one understands a medium which, through 
the growth in it of an organism, has been made useless, or nearly so, for 
further growth of the same or other organism. By ‘ staling substances or 
products ’ one means those metabolic products of the organism which are 
responsible for slowing down or stopping its growth. The use of this last term 
tacitly assumes that the cause of the slowing down of growth in a culture 
is the presence of products due to the organism and which are deleterious 
to growth, and that the staling is not to be ascribed merely to the removal 
from the medium of certain food substances: for many fungi, at any rate, 
this assumption appears to be fully justified. It is interesting to note that 
the phrase commonly used in German literature in this connexion is 
‘gebrauchte Nahrlosung ’, a phrase which refers simply to the history of the 
cultural solution and not to any properties which it may have developed in 
consequence of that history. 
In defining a particular word for use in this connexion one must take 
into account the present state of knowledge on the subject. Without going 
into details, one may state it as follows: The earlier workers, e. g. Duclaux , 1 
1 Duclaux : Traite de Microbiologie, 1900. Ref. in Lafar’s Handbuch, vol. i, p. 504, 1907. 
