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 C0 2 is absorbed the 

 mercury rises. When the ascent ceases (i.e. all the C0 2 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 C0 2 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 CO 2 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) 2 solution, Put in a dark place. The C0 2 given off 

 forms a crust of BaCOo (or CaC0 3 ) on the surface of the liquid 

 while in a control experiment with no plant under the bell jar 

 the amount of C0 2 in the air (3 parts in 10,000) produces only 

 a very small precipitate. 



(o) 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 C0 2 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 C0 2 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 



