Brown.—Studies in the Physiotogy of Parasitism. I. 319 
lumps. The subsequent treatment is as usual. Plate cultures are incubated 
at 20 0 C. (at 15 0 the spore yield is very much diminished). Spore forma¬ 
tion begins in about 4 days; in 10 days to a fortnight the plates are ready 
for the subsequent treatment. 
The problem now is to obtain the spores from the plate cultures. 
For this purpose each plate is covered with a layer of distilled water; then 
by gentle rubbing with the finger, beginning at the centre and working to 
the margin, it is possible to expel the entangled air without at the same 
time unduly contaminating the atmosphere of the room with spores, though 
this is unavoidable to some extent. The whole aerial portion of the culture— 
mycelium and spores—is now rubbed off by gentle scraping with a blunt 
knife. With a little practice it is quite easy to perform this operation 
without disturbing the underlying solid medium, so that, apart from the 
fungus itself, only liquid substances are removed from the plate. The 
fungal debris, &c., is now filtered through a fine clean muslin cloth (20 
threads to the cm.). The spores and finer particles pass through, whereas 
the general mycelium, apart from occasional very short pieces of hyphae, is 
completely held back. The spore suspension is now centrifuged at a moderate 
speed. By this means it is possible to separate the heavy spores which are 
readily thrown down from the finer debris which remains in suspension. 
The * wet ’ volume of the centrifuged spores is noted for purposes of the 
following treatment. 
The centrifuged spores thus obtained in a practically pure form are now 
suspended in definite proportion in a nutrient liquid and spread over a glass 
plate in order to germinate. The following general considerations may be 
noted : 
1. Length of Period allowed for Germination. 
As it was proposed to examine the physiological conditions prevailing 
within the c infection drop ’ 1 with the object of throwing some light on the 
manner in which the fungus first actually enters the host plant, an attempt 
was made to obtain an extract from the fungus at a stage comparable with that 
at which it actually penetrates the host. The period elapsing from sowing 
to penetration varies somewhat with different hosts and under different 
conditions, but generally speaking it may be set down as lying between 
12 and 24 hours. Throughout this work the spores were allowed as nearly 
as might be 23 hours’ germination, this being considered sufficiently close to 
the ideal time, as well as offering conveniences for the systematic day-by-day 
repetition of the process which the method entails. 
1 By this is meant the drop of fluid in which sowings of the fungal spores are made on the 
surface of the host plant. With Fungi of this type, this represents the usual procedure in artificial 
infection. 
