340 BOTANICAL GAZETTE [NOVEMBER 
for its growth reduces to a minimum the dangers both of contami- 
nation and of too extensive a diffusion of the substances tested; and 
the spores do not cling together in large clumps as in Penicillium. 
For the experimental work, mica was selected on account of 
its impermeable nature. Small plates of this were cut from large 
sheets and perforated with a sharp needle. The area perforated 
measured 24X24 mm., while at two opposite sides of this square 
piece small winglike projections were left, in order to rest the prep- 
aration on supports. At first, the perforations were made 2 mm. 
apart, 121 holes thus being punched in each plate; but after experi- 
mental chemical tests demonstrated a comparatively rapid diffusion 
through such plates, the number was changed to 16, the holes then 
being spaced 6 mm. apart. After each experiment the mica plates 
were boiled, first in alkali, then in acid, and, after being rinsed and 
boiled in 2 or 3 changes of distilled water, they were dried finally 
in a hot air oven. 
The spores were grown in thin layers or films of medium approxi- 
mately o.5 mm. thick, placed on these mica plates by a method 
to be described presently. For the medium, agar was used invari- 
ably, both because compared with gelatin it afforded less nourish- 
ment when used without addition to the substances to be tested, 
and because in the manipulation of the films, as explained later, 
it was found impracticable to use gelatin. A 1.5 per cent agar was 
used for the plain or “‘non-nutrient”’ medium; while for the nutrient 
a double strength solution of the required substance was mixe 
with an equal volume of 3 per cent agar to make a medium of desired 
strength containing 1.5 per cent agar. 
The method finally adopted for making the films and sowing 
the spores was as follows. As already indicated, each preparation 
consisted of a perforated mica plate bearing a thin film of medium 
on each of its surfaces. The film on one side might contain the 
spores, while that on the other side would contain the chemical 
substances to be investigated; but various combinations were 
tried, as will be shown later. 
For various reasons it was found desirable to make all films of 
approximately uniform area and thickness. An area approximately 
2424 mm., the size of the perforated area in the mica plates, was 
