THE RELATION OF THE PLAN? TO THE AIR 125° 
CHAPIEBR XX 
THE RELATION OF THE PLANT TO THE AIR ~ 
Arr is a mixture of certain gases, chiefly nitrogen (and argon) 
and oxygen, with small quantities of carbon dioxide and a 
varying amount of water-vapour. The relation of plants to 
the air is threefold : | 
1. They take in oxygen and give out carbon dioxide. This 
is respiration. 
2. They take in carbon dioxide and give out oxygen in 
connection with the manufacture of food. This is assimila- 
tion. = 
3. They give out water-vapour. This has already been 
discussed under transpiration. | 
Experiments show that a plant is not capable of using the 
free nitrogen of the air except indirectly, therefore that con- 
stituent may be disregarded. 
During this process, oxygen is taken in and 
carbon dioxide given out. The tests that may 
be applied to detect. the absence of oxygen and the presence 
of carbon dioxide are the following: A taper will not burn 
in air from which the oxygen has been exhausted, and lime- 
water is turned milky by the presence of carbon dicxide. 
These tests will be applied in the experiments on respiration 
of plants. 
Respiration. 
Take an empty jar (i.¢., one containing only 
air), which we will call L, and another jar IL, in 
which germinating seeds have been kept tightly corked for 
twenty-four hours. Plunge a lighted taper into each. 
Result. The taper burns in [,, and will not burn in II. 
are Oxygen is present in I., but little or no oxygen 
in II., therefore the germinating seeds have used 
up the oxygen in II. | 
Experiment I. 
