8 A TEXTBOOK OF GENERAL BOTANY 
we cannot detect its presence by sight, taste, or smell. It is, 
however, easy to demonstrate that water exists in the air. When 
a vessel containing a cold liquid, such as ice water, is left in a 
room where the air is considerably warmer than the sides of 
the vessel, we find that water collects on the outside of the 
vessel. This is due to the fact that the low temperature of the 
vessel causes water vapor to condense to water. 
Gases of the atmosphere. The principal gases of the atmos- 
phere are nitrogen, oxygen, carbon dioxide, and water vapor. 
' Nitrogen composes about four fifths of the air, and oxygen about 
one fifth. Carbon dioxide is always present in small quantities, 
while water vapor occurs in varying quantities, depending on 
the circumstances. 
In respiration (breathing), plants and animals take in oxygen ; 
without it they could not carry on respiration and so would 
soon die. Respiration is performed in all the living parts of 
plants and is just as important for plants as for animals. We 
can see this readily in the case of seeds, which, if deprived of — 
oxygen, will not germinate. 
Carbon dioxide, as we have seen, is used by plants for the 
manufacture of sugar. 
The water vapor in the air is also an important factor in the 
environment of plants. The principal constituent of the living 
parts of plants is water, and plants expose moist surfaces to the 
air. When water or a moist surface is exposed to the air, some 
of the water is changed to water vapor and mixes with the air. 
This process is known as evaporation. If a small amount of 
water is left in an uncovered glass, we know that the water 
evaporates and leaves the glass dry. We also see the result of 
evaporation when we dry wet clothes. The rate of evaporation 
depends in part on the amount of water vapor in the air. We 
are familiar with the fact that when the air is wet it takes much 
longer for anything to dry than it does when the air is dry. In 
other words, as the amount of water vapor in the air increases 
the rate of evaporation decreases, because moist air will take up 
less water than will dry air. The moist surfaces of plants, like 
a 
