320 Brown.—Studies in the Physiology of Parasitism . /. 
2 . Uniformity and Completeness of Germination. 
For this purpose, the factors of importance are— 
(a) The spores must be uniformly distributed throughout the nutrient 
fluid ; 
{b) The quantity of spores per unit volume of nutrient should not exceed 
a certain limit; 
(c) The film of nutrient on the plate should be of uniform depth 
throughout. 
The details of the method, as given below, were only gradually evolved, 
and being based on numerous subsidiary experiments may be taken as 
representing the best set of conditions from the point of view on the one 
hand of effective and uniform germination, and on the other of economy of 
space and labour. 
The method of dealing with 0-5 c.c. of centrifuged spores, which is about 
the amount obtainable from two, fairly good, 8 cm. diam., Petri dish 
cultures, may be stated. This quantity of spores is suspended in 50 c.c. 
of the nutrient fluid. The plates on which the spore suspension is sown 
are flat, circular, of 8 in. diameter. Each plate is supported on three 
wedge-shaped corks in the bottom of a large Petri dish, the atmosphere of 
the latter being kept moist by the usual devices. Each plate is accurately 
levelled by means of a spirit-level previous to sowing, this being effected 
by manipulation of the supporting corks. Immediately before each plate 
is sown, the spore suspension must be agitated. This can readily be 
effected by blowing in air through the pipette which is employed for 
measuring out the allowance for each plate. The spore suspension is now 
spread over the plates up to in. from the margin at the rate of 5 c.c. to 
each, spreading being most readily effected by means of the finger. With 
practice it is quite easy to sow as many as 20 plates in half an hour. The 
spores are now left to germinate at the ordinary laboratory temperature 
(about 20° C.). 
With the plates employed and with the given suspension density of 
spores (o*i c.c. spores to 10 c.c. nutrient) it has been found that any reduction 
from the amount given above (viz. 5 c.c.) for each plate has resulted in 
a diminution of yield together with increased tendency to lack of uniformity 
in germination. Any reduction in the amount of liquid should be accom¬ 
panied by a corresponding reduction in the density of suspension. 
In the earlier stages of this investigation, the nutrient fluid employed 
was a commercial grape preparation ( Welch's Grape Juice), and for 
purposes of the manufacture of spore material this medium proved quite 
satisfactory. This preparation, however, when laid in the form of drops on 
the surface of plant tissues (leaves, &c.) has a very strong plasmolysing and 
killing effect; and in view of the fact that one object of the research was to 
