FUNGI 
211 
to pick it off; and on liquid media, the growth is abnormal. Mrs. 
Alice A. Bailey has devised a method which is ideal for securing ma¬ 
terial. She puts about the usual amount of a potato dextrose 
agar in a Petri dish and pours over it a potato decoction about 2 mm. 
deep. The + and — strains are then added. The potato decoction 
is made as follows: Use 300 g. of Irish potatoes and 1000 c.c. distilled 
water. Peel and slice the potatoes and boil for 1 hour in the distilled 
water. Strain off the liquid through cheese-cloth and make up to 
the original quantity by 
adding distilled water. 
Flask and sterilize. 
The potato dextrose 
agar is made as follows: 
to 1,000 c.c. of potato 
decoction, add 20 g. of 
dextrose and 30 g. of 
agar. Boil | hour in 
a double boiler. Strain, 
flask, and sterilize. 
The potato dextrose 
agar is an excellent 
medium and the thin 
layer of potato decoction 
keeps the material from 
sticking to the agar so 
that it can be lifted off 
intact. Binse it under 
the tap and fix in 
the chromo-acetic-osmic 
solution. 
Zygospores may begin 
to form within 3 days, 
and mature zygospore xso. 
may appear within 4 or 
5 days. Watch the cultures and fix so as to secure a series of stages. 
(Fig. 53). 
Paraffin sections should not be thicker than 5 n, and 3 n is better 
for nuclear detail. Iron-alum haematoxylin is best for nuclei, but 
safranin, gentian-violet, orange will give beautiful preparations. 
Fia. 53 .—Rhizopus nigricans: various stages in the 
development of zygospores from a culture on bread; prepara¬ 
tion stained in eosin and mounted in Venetian turpentine. 
