102 PLANT PHYSIOLOGY 
(m) Place a number of fresh leaves or a short shoot with 
leaves in the large end of a retort with a little water and place 
the small end under a surface of mercury to prevent the 
entrance of gases. Keep in a dark moderately warm place for 
from twelve to twenty-four hours. Note that the volume of 
the gas does not seem to be changed. Carefully without allow- 
ing any air to enter run a pipette full of strong KOH solution 
into the small end of the retort or introduce a small piece of 
stick potash (KOH) with a few drops of water, these rising to 
the surface of the mercury. As the COz is absorbed the 
mercury rises. When the ascent ceases (i.e. all the CO: has 
been absorbed) introduce a strong solution of pyrogallic acid. 
This has the property when mixed with alkaline solutions of 
absorbing oxygen. Note whether the mercury rises any 
further. If it does so it shows that some oxygen was present. 
Repeat the experiment using a retort without any leaves in it. 
It will be found that about as much CO2 was produced by the 
leaves (as shown by the height to which mercury rose with the 
KOH alone) as oxygen was present (as shown in the control 
experiment by the distance the mercury rose with the KOH 
and pyrogallic acid). If this can be done with graduated cylin- 
ders the amounts can be measured more accurately. 
(n) That COs is given off by a living plant may be demon- 
strated in the following way also. Place a potted plant under 
a bell jar with a dish of Ba(OH).2 solution or (less preferably) 
Ca(OH). solution, Put in a dark place. The CO: given off 
forms a crust of BaCO; (or CaCO;) on the surface of the liquid 
while in a control experiment with no plant under the bell jar 
the amount of CO:z in the air (3 parts in 10,000) produces only 
a very small precipitate. 
(0) Soak some peas over night and then place them in a 
tall glass jar filling it about half full, and cover with a vase- 
lined glass plate. After a few hours remove the plate and 
lower a burning taper into the cylinder. It is extinguished 
by the CO2 which has replaced the oxygen. If the air is 
very still it is more striking to place a small lighted taper in 
the bottom of another jar and to pour the COz from the jar of 
peas into this jar, extinguishing the light. 
(p) Soak some peas over night. Fill a test tube with mer- 
cury and invert over a dish of mercury. Force three or four 
peas under the mercury so that they come under the edge of the 
