107 
Fungi on Culture Media. 
believed that a ‘ used solution ’ is necessarily a stale solution—that is, that 
the growth in a medium of a particular organism rendered the medium less 
fit for further growth, at any rate of the same organism. Even to-day this 
rule will probably be found to cover the great majority of cases. But 
exceptions have been shown to exist, e. g. in a more recent work of Niki- 
tinsky , 1 according to which a certain amount of growth of a fungus in 
a medium may render the latter more suitable for the further growth of the 
same fungus. A special and very important illustration of the same thing 
is the so-called ‘ Bios ’ effect described by Wildiers . 2 Here it was found 
that the growth of an organism in a fresh cultural solution was enormously 
accelerated by the transference with the inoculum of some of the metabolic 
products from the older culture, and in some cases this transference was in¬ 
dispensable. Thus it is clear that a ‘ used solution ’ is not necessarily a stale 
solution in the common acceptation of the term stale. It is of course possible 
that the accelerating substances which must be postulated to explain 
Nikitinsky’s and Wildiers’s results are the same as the staling substances, the 
difference in effect being due simply to difference in concentration. In that 
case the extension of the term { staling’ to include the effects of stimulation 
could be defended on grounds of analogy with, for example, toxic sub¬ 
stances. Thus it is usual and justifiable to describe copper sulphate as toxic 
to fungi even though it has been demonstrated that it stimulates fungal 
growth when present in very minute quantity. However, the identity of 
the active growth-affecting substances in the case of fungi has not been 
demonstrated, and any extension of the term ‘ stale ’ along the lines indB 
cated above is not at present justified. Further, any definition of staling 
cannot, in the present state of our knowledge, be based on the * staling 
substances ’ themselves, which are in the main substances of unknown 
chemical nature, the existence of which is postulated to explain observed 
effects. The definition must be made on the basis of the observed effects. 
The question now remains as to what are the effects observed. These 
will appear in the sequel, but it may be said here that the general rule is 
that the rate of spread of a fungal colony increases to a maximum at which 
it remains steady or from which it subsequently declines. Evidence will 
be brought forward to show that this decline from the maximum rate of 
growth can be controlled and modified in a variety of ways, all of which 
indicate that certain products of the metabolism of the fungus are affecting 
the medium at the growing margin in such a way as to cause a retardation 
of growth, and further, that if these products could be removed or otherwise 
rendered inoperative, the colony would continue growing at the maximum 
rate. A colony, therefore, which, though kept under the same external 
conditions, is not growing in diameter at the same rate as formerly, will be 
1 Nikitinsky: Jahrb. f. wiss. Hot., 1904, xl. 1. 
2 Wildiers: Koch’s Jahresber., 1901, xii. 133. 
