1913] REED & COOLEY—TRANSPIRATION 425 
than two apple leaves could be used in an experiment with a moder- 
ate rate of aspiration; if more were used, water would sometimes 
collect in the jar A. A thermometer was inserted in the jar with 
the twig, or suspended close to it. The previously weighed U-tube 
was connected on one side with tube 2 and on the other with the 
aspirator C, which contained 19 liters of water. Care was taken 
that the temperature of the water used in the aspirator should be 
very close to that of the air. When the siphon was started, the 
stoppers (3, 3) were turned to allow air to flow through the appara- 
tus. The flow of water could be regulated by means of the screw 
clamp (4) after a little experience, so that the time required to | 
draw out 19 liters should be close to an hour. 
The first apparatus was set up with healthy leaves in the jar A; . 
another experiment was similarly set up with leaves infected with 
cedar rust; a third experiment was set up but with the omission of 
the jar A. These three, each with its own aspirator, were started 
as nearly simultaneously as possible, and the temperature kept 
uniform. The purpose of the third set of apparatus was to deter- 
mine the amount of moisture in 19 liters of air. 
The criticism might justly be made that the evaporating power 
of the atmosphere was not taken into strict account by this sort 
of an experiment. If the purpose had been to study the conditions 
or amount of transpiration, such determinations should have been 
made, but the purpose was to study the comparative transpiration 
of the healthy and diseased apple leaves, and as such it is believed 
to fulfil its purpose. Another possibility of error might be found 
in the vapor pressure from the water in C, which would carry back 
some moisture into the tube B and cause the results to be too large. 
This source of error, however, is obviated by the use of the blank, 
whose increase in weight was subtracted from each of the accom- 
panying tests. 
As soon as the aspirator ceased running, the glass stoppers of 
the U-tubes were closed and the time noted. The leaves were 
plucked from the twig, placed in a labeled envelope, and taken 
to the laboratory along with the set of U-tubes. After weighing 
the tubes and computing the gain in weight, the necessary correc- 
tion was made for the moisture absorbed from sources other than 
the leaves as registered by the increase in weight of the blank. 
