210 
BOTANICAL GAZETTE [MaRcH 
of water; the water-level is then noted on the stem, which is cut at this 
point under water, the shoot being later, when dried, placed upright in the 
chamber. (It hardly 
PIGS: 
shows in the figure because of irregular reflections 
from the surface of the chamber.) 
Taking advantage of the fact that 
the shoot will carry on photosynthe- 
.sis for a time in an atmosphere con- 
taining carbon dioxid in demon- 
strably large amount, even up to 10 
per cent. or more, we add some 
selected percentage of that gas to 
the apparatus in this way. The 
measuring tube, with stop - cock 
closed, is inverted and filled with 
water of room-temperature, up to a 
figure of the graduation expressing 
the selected percentage, for the tube 
is graduated in cubic centimeters, 
which are, of course, percentages of 
the total gas capacity of the appara- 
tus.. The stopper is then placed on 
the tube, and its stop-cock closed; 
its hollow is filled with water and 
inverted in a pneumatic trough (or 
equivalent dish of water), which has 
been standing in the laboratory long 
enough to take the temperature of 
the air. The lower stop-cock is 
then opened and carbon dioxid 
from a generator is allowed to enter 
the tube, either from below or, as is 
most convenient, through the top of 
the tube. The admission of the 
gas may be perfectly controlled by 
cautious manipulation of the upper 
stop-cock, which is closed at the 
moment when the water has been 
wholly driven out to the bottom of the bore of the lower stop-cock, which 
point is held exactly at the water-level. 
The lower stop-cock is now closed, 
and the combination, which now contains exactly the desired percentage 
