FUNCTIONS OF LEAVES 187 
EXPERIMENT XVII 
Oxygen-Making in Sunlight. — Place a green aquatic plant in a 
glass jar full of water at a temperature of about 70° Fah. in front 
of a sunny window.! On the surfaces of the plants watch for minute 
silvery-looking bubbles. These may be partly air but some are 
oxygen. Remove to a dark closet for a few minutes and examine by 
lamplight to see whether the rise of bubbles still continues. 
This gas may be shown to be oxygen by collecting some 
of it in a small inverted test-tube filled with water and 
thrusting the glowing coal of a match just blown out into 
the gas. It is not, however, very easy to do this satisfac- 
torily before the class. 
Repeat the experiment, using water which has been well boiled 
and then quickly cooled. Boiling removes all the dissolved gases 
from water, and they are not redissolved in any considerable quantity 
for many hours. 
Ordinary air containing a known per cent of carbon dioxide, 
if passed very slowly over the foliage of a plant covered with a pell- 
glass and placed in full sunlight, will, if tested chemically, on com- 
ing out of the bell-glass, be found to have lost a little of its carbon 
dioxide. The pot in which the plant grows must be covered with a 
lid, closely sealed on, to prevent air charged with carbon dioxide (as 
the air of the soil is apt to be) from rising into the bell-glass. 
163. Disposition made of the Absorbed Carbon Dioxide. — 
It would lead the student too far into the chemistry of 
botany to ask him to follow out in detail the changes by 
which carbon dioxide lets go at least part of its oxygen 
and gives its remaining portions, namely the carbon and 
perhaps part of its oxygen, to build up the substance of 
1 Elodea, Myriophyllum, Chrysosplenium, Potamogeton, Fontinalis, any of 
the green aquatic flowering plants, or even the common confervaceous plants, 
known as pond-scum or ‘‘ frog-spit,’’ will do for this experiment. 
