Fungi on Culture Media . 113 
all cases the rate of growth at the seventh day is much reduced from what 
it was on the second and third days. 
One can obtain all gradations from those fungi which would, as far as 
can be seen, grow on indefinitely at the same rate to those which form small 
colonies and then cease growing entirely. Fungi of the latter type are 
sometimes referred to as showing ‘ limited growth but there are little 
grounds for thinking that there is anything absolute in their limitation. 
They are simply to be looked upon as fungi which have very strong ‘ staling 
tendencies’, and one could anticipate that, if means were devised for removing 
or reducing in intensity the staling products, the amount of growth would 
cease to be limited. 
A simple and effective way of demonstrating that the slowing down of 
the marginal rate of growth is not due to anything inherent in the growing 
Fig. 3 Fusarium sp. on potato agar. 
hyphae, but is caused by staling products diffusing outwards from the 
mycelial mass of the colony, is to cut away day by day the central portion 
of the colony, leaving only a fine fringe of marginal mycelium. Experiments 
of this type were carried out with the present and with another more 
strongly staling Fusarium. It was seen that the difference due to cutting 
out the central region of the colony was imperceptible when the fungi were 
grown on a medium on which no marginal staling took place (e. g. a dilute 
potato extract with agar), whereas when the medium was one which gave 
staling (e. g. potato agar of standard strength), the effect of cutting out the 
central portion of the colony was to postpone the incidence of staling. An 
illustration of the latter result is given in Table I. 
Table I. 
Growth during 
r 
First 
yd 
4 th 
5 th 
6 th 
7 th 
8 th 
2 days. 
day. 
day. 
day. 
day. 
day. 
day. 
Uncut 
0-85 
0-85 
0.9 
°-7 
o -5 
0.3 
0.3 
Cut 
• 
0-85 
0.85 
0-9 
0.78 
0.65 
0.63 
0-68 
1 
