FOOD OBTAINED FROM THE AIR 33 
VII.—FOOD OBTAINED FROM THE ATR. 
WORK OF LEAVES. 
Absorption of Garbon-dioxide (CO,). A descrip- 
tion of the composition of air is given on the appendix page. 
Plants absorb carbon-dioxide through their leaves. Fit 
a leaf into an air-tight cork through which a small piece of 
bent glass tubing is inserted as shown, 
and allow the end of the leaf-stalk to 
dip into the water in the jar. Apply- 
ing the lips to the tubing suck out 
the air over the water—immediately 
bubbles of air are seen to come from 
the cut end of the leaf-stalk and rise 
through the water. We can continue 
this experiment for some time and so , 
prove that the air which bubbles from 
the leaf stalk is not merely air im- 
prisoned in the leaf, but has come from 
the surrounding atmosphere. When 
the air is drawn out through the tube, 
the pressure inside the jar and hence 
on the leaf-stalk is so reduced that the 
ordinary atmospheric pressure on the 
blade is able to force the air into the 
leaf and through the tissues until it reaches the cut 
surface. 
Each surface of a leaf is covered with a thin transparent 
skin. On the upper surface of most plants this skin is 
made up of closely fitting cells—whilst on the lower 
surface it is perforated by an immense number of minute 
openings or pores called stomata. It is through these 
pores that the carbon dioxide of the air enters. 
C 
