A DEMONSTRATION OF PHOTOSYNTHESIS 
III 
In addition a water seal may be provided by tying a piece of oil- 
cloth around the neck of the bottle so as to project in the form of a 
tube above it. This tube should be filled with water until all the 
joints are submerged. Or the whole apparatus may be placed in a 
large jar filled with water. These precautions will not be necessary 
if the joints are made secure by heating the glass before the rubber is 
slipped on, so as to make a permanent union on cooling. 
It is important to have a control (containing no plants) which is 
kept beside the apparatus containing the plant. It is also desirable 
to have a control containing plants which is kept in the dark. 
Respiration may be studied in precisely the same way as photo- 
synthesis (it is not necessary in this case to add CO2 at the start). 
The method here outlined permits us to follow the progress of 
photosynthesis by making analyses at frequent intervals. In this 
way a time curve of photosynthesis may be plotted in order to study 
the dynamics of the process. 
The influence of reagents (anesthetics, etc.) on respiration and 
photosynthesis may be studied advantageously by this method. It 
may be added that it has likewise been useful in studying the respira- 
tion of animals. 
Summary 
# 
An apparatus for the demonstration of photosynthesis is described 
which permits: 
1. Removal at intervals of satisfactory samples of the gases by 
which the leaf is surrounded. 
2. Stirring and mixing of the gases when necessary. 
3. Analysis of the gases by a simple method which is sufficiently 
accurate for ordinary purposes. 
Harvard University, 
Laboratory of Plant Physiology 
