GROWING BACTERIOLOGICALLY STERILE POTATO PLANTS 15 
introduction of the thermograph or its presence. The wire was well 
secured on the outside, so that there was no motion which would 
break the seal. 
PREPARATION OF STERILE WATER 
After being planted the cans were set in the open and left for the 
-seed piece to sprout and for the plant to develop. During this inter- 
val the cans steadily lost weight, sometimes in fairly large amounts. 
As they were thoroughly sealed except for the devious passage through 
the cotton plug of the chimney and the vent of the cap on the water 
pipe, the only possible loss must have been in the form of water vapor 
representing a direct loss of moisture from the soil. It was necessary 
to restore this moisture from time to time. 
Before water could be introduced into the watering machine, how- 
ever, the machine itself had to be rendered perfectly sterile. This 
was done b} T attaching a steam line at the valve F (fig. 3) and forcing 
live steam under a pressure of 30 pounds through all parts of the 
machine for half an hour. This included all pipes, valves, cylinders 
and their parts, and the flexible pipes. As some water of condensa- 
tion would accumulate in the bottom of the water tank, the valve E 
(fig. 3) was opened in sterilizing the cylinders, in order to use steam 
free from water. No greater pressure than 30 pounds could be used, 
lest the brass-covered pith float T (fig. 4) should collapse. 
After the kettle and its parts were sterile, the valve E (fig. 3) and 
valve O (fig. 4) w^ere closed. A hose was attached at the open end 
above valve D and clean water allowed to flow into the tank until it 
reached the level of the pet cock N. The water hose was removed 
and the steam again supplied through the valve F, this time under 
high pressure. The water soon reached the boiling point, and the 
tank was boiled by live steam at a pressure of not less than 50 pounds, 
as shown by the gauge M, for two hours. The acquired water of con- 
densation completely filled the tank. The machine and its contained 
water were now sterile. All valves were tightly closed, the steam line 
disconnected, and the machine allowed to cool overnight. It was 
then ready for use. 
SUPPLYING THE CANS WITH STERILE WATER 
The operation of watering the cans, though simple, was attended 
with many precautions. The air tank of the machine (fig. 3) was 
filled with air under considerable pressure. The cylinder G filled 
with asbestos wool, the check valve C above it, and the pipe into 
the tank were flamed with a gasoline torch until red hot. The torch 
was usually trained on the cylinder G for an hour or more. 
The air was slowly admitted through the valve B and the pressure 
in the water tank maintained at about 15 pounds, as shown on the 
gauge M. The air, leaving the pipe H, forced the water up through 
the pipe J to the valve of Figure 4. The vent cap and pipe of the 
can cover, shown in diagram in Figures 1 and 2 and also in Plates 4 
and 5, were thoroughly flamed with the gasoline torch. The cap 
was taken off and the opening flamed. The end of the flexible pipe 
of the machine with the cuff attached and covered by the mitten 
was thoroughly flamed. The mitten was taken off and the opening 
flamed before it was inserted in the water pipe of the cover. The 
valve Y remained closed in the left-hand cylinder and was opened 
