166 BOTANICAL GAZETTE |SEPTEMBER 
below. A single stack, therefore, can be used in testing each 
of two races by contrasts with a plus and a minus tester. The 
stacks of culture dishes are dry sterilized, and when cool filled with 
sterilized nutrient agar slightly above the melting point. If the 
agar is too warm, moisture may condense on the covers and later 
drop on to the surface of the hardened agar. The process of pour- 
ing agar into the dishes is carried on in a special culture room to 
minimize danger of contamination. The cultures are ready for 
inoculation as soon as the agar has begun to harden. Pouring 
agar into sterile dishes rather than sterilizing the dishes after they 
have been filled not only saves considerable time in the process, 
but avoids the spattering of nutrient on the edges of the dishes, 
which is likely to ensue when they are autoclaved, and is a ready 
source of infection. It has not been found necessary to use cleared 
agar, but the sediment at the bottom of the flask from which the 
stacks are poured has usually been discarded. This sediment may 
be conveniently anchored to the bottom by cooling the flask when 
full in a shallow pan of water. 
InocuLaTions.—Since forty or more individual races in test 
tubes may be used in inoculating a given series, it is obvious that 
some precautions are necessary to avoid contamination from spores 
falling from these tubes, or from the weft of spore material taken 
from them for inoculation. Exposed Petri cultures have shown 
that this danger is great if not guarded against. In making inocula- 
tions, great care was taken never to expose to the air in the inoculat- 
ing chamber any spore material in a dry condition. A fairly 
large piece of rather soft agar was transferred with a flamed plati- 
num needle or spatula to the tube from which it was wished to 
obtain a transfer. The piece of agar was cautiously pushed against 
the spore material, and thoroughly mixed with it, care being 
observed that no dry filaments adhered to the needle or to the 
mixture of agar and spore material to be used as the inoculum. 
Moreover, the cotton plug was not removed suddenly from the 
test tube, since if this is done spores are likely to be shaken into 
the air, sometimes in visible clouds. After the inoculation the 
needle was not at once flamed, as the heated inoculum may sputter 
and scatter bits containing viable spores. The needle, therefore, 
was left in a tube of about 80 per cent alcohol while the label was 
