GROWING BACTERIOLOGICAL!/!' STERILE POTATO PLANTS 13 
The glue and varnish could not be subjected to steam sterilization, 
as the volatile parts would have been boiled away. They were pas- 
teurized, and testing discovered no organisms in them. The brushes 
used were sterilized by steam and varnish. 
No contamination of the cans could be traced to the tapes, adhe- 
sives, or varnishes used in sealing the cans. 
The lamp chimneys were prepared with cotton plugs in the upper 
ends, tied in strong individual paper sacks, and sterilized at 20 to 30 
pounds pressure in the kettle for half an hour. 
The tapes, utensils, and other pieces of apparatus or supplies which 
were sterilized in quantities were sterilized in the kettle. 
PREPARATION OF CULTURE MEDIA 
A potato agar was much used in testing the potato seed pieces for 
sterility. It was made by steaming sliced peeled potatoes in water, 
in the proportion of 200 grams of potatoes to 1 liter of water, decant- 
ing the liquor, restoring it to volume, and adding 1 per cent powdered 
agar. The medium was again steamed for an hour and sterilized at 
a pressure of 15 pounds for 20 minutes. This agar was usually made 
in 10-gallon quantities, the steaming and sterilizing being done in the 
kettle. 
Other media used were made under laboratory conditions. 
PREPARATION OF POTATO SEED PIECES 
The true seed of any plant can usually be sterilized with ease and 
safety. This is not possible with the potato seed piece. Attempted 
sterilization at the surface with mercuric chloride or other solutions 
never resulted in complete sterilization. A number of methods, solu- 
tions, and chemicals were tried, to no avail. Upon planting the seed 
piece on potato agar, bacteria and mold developed. 
The only successful method consisted in paring away the surface 
of the tuber, leaving the raw sterile tissue uncontaminated. The 
work was done in a closed culture room. The potatoes were thor- 
oughly scrubbed with a stiff brush and treated for 1 hour in l-to-1,000 
mercuric-chloride solution. A single eye was selected as the center 
of operations. The tuber was firmly impaled on a common fork, and 
the paring process begun. The knives were flamed before using and 
only one stroke of the knife was made before flaming it. A knife 
could be used for only five or six strokes, as the potato juice burned 
on the blade in the flaming and made it impossible to use it after a 
few strokes. As much tuber flesh as possible was left with the se- 
lected eye, the remainder being cut away toward the fork. The piece 
was finally cut across the eye. This left a square block of potato on 
the fork with an eye in the center of the upper side. Then a second 
round was made, cutting away a thin slice on each side. The top 
side was cut again, a very thin slice being cut away in the hope of 
leaving the germ of the eye. Finally nothing remained to be done but 
to cut off the seed piece above the fork and drop it into a container. 
The containers were pint preserving jars with screw tops. An 
inch or more of potato agar was in the bottom of each jar. The 
mouth was plugged with cotton. The jars were sterile. The agar 
was softened by heat so that the potato seed piece sank into it. 
Usually a few cubic centimeters of potato agar was dashed over the 
