Brown . — Studies in the Physiology of Parasitism. IX. 287 
after each experiment and allowed to dry out completely in the open air 
of the laboratory. This was to ensure that all traces of volatile substances 
from the preceding experiments had become dissipated before the dishes 
were used again. Latterly the method was adopted of strongly heating the 
dishes in a dry-air oven. 
In the course of the investigation it was found that the moist filter- or 
blotting-paper, which it is usual in experiments of this kind to insert in the 
lids of Petri dishes for the purpose of preserving a moist atmosphere, pro- 
duces a distinct effect on the germination of spores in the Petri dish. This 
effect will be described in the latter portion of this paper, but here it may 
be stated that filter- or blotting-paper which has lain moist in the lid of 
a Petri dish for some days gives off a volatile substance which reduces the 
amount of fungal germination taking place in pure water or in very dilute 
nutrient. Throughout the greater portion of this work parallel series were 
run, the one of Petri dishes set up with moist filter- or blotting-paper in the 
usual way, the other in which no such paper was present. In the latter case 
drying up of the drops was prevented by liberal moistening of the inner 
surface of the Petri dish. 
It may be added that this paper effect proved to be of value from the 
experimental point of view, as it afforded a means of greatly reducing or 
actually suppressing the amount of germination which takes place in pure 
water in the case of some fungal spores. The stimulating effect arising 
from plant tissues becomes then more striking, as will appear from the 
tables given below. 
In each experiment at least two, and usually four, Petri dishes were 
employed for each particular treatment. In each Petri dish two to four 
slides, each with two drops of the same spore suspension, were placed. 
Thus in each case abundant provision was made for accidental variations. 
The amount of germination was determined by measuring the length of 
germ -tube of a large number of spores (usually twenty-five from each drop) 
chosen at random from the central region of the drop, and dividing the 
total length of the germ-tubes by the number of spores counted. The 
restriction as to measuring germ-tubes from the central region of the drop 
is rendered necessary from the fact that under conditions of feeble nutrition 
germination is always somewhat better at the margins than in the centre of 
the droff. This point is also noted by Duggar in the paper referred to. 
In practice it was found impossible to measure quantitatively the 
germination in all the drops which it was customary to set up. The 
number of Petri dishes used in any one experiment usually ranged from 
twenty to fifty, each containing four to eight drops, so that it was manifestly 
impossible to measure satisfactorily the amount of germination in each 
particular one. The large number of tests enabled the experimenter to 
arrive at a general impression of the result, and then as many measurements 
Y 2 
