THE PLANT AND ITS ENVIRONMENT 9 
any other moist surfaces, lose water by evaporation. The evap- 
oration of water from plants is known as transpiration and is 
a very important process from the standpoint of the plant, and 
one that we shall examine in considerable detail in a later chapter. 
The rate of transpiration will naturally be influenced by the 
amount of water vapor in the air, the rate decreasing as the 
quantity of water vapor in the atmosphere increases. If a plant 
continues to lose water faster by transpiration than it can obtain 
it from the soil, the plant will wilt and finally die. It is for this 
reason that many delicate plants cannot be grown in places 
where the air is very dry, as they would dry up and die from 
excessive loss of water. The amount of water vapor in the air 
is not the only factor which affects the rate of evaporation, as 
the rate is increased by wind, heat, and light. 
Composition of the gases. We have considered briefly the 
principal gases of the air and their importance. We may now con- 
sider their composition. AJ] the matter on the earth is composed 
of ninety-two elementary substances, known as elements. Com- 
mon examples of elements are iron, lead, copper, sulphur, and 
mercury. The gases oxygen and nitrogen are elements. 
Two or more elements can be united into a single substance 
known as a compound. If we expose iron to the action of the 
atmosphere, it will combine with oxygen and form iron rust. 
This rust is a compound, because it is composed of iron and — 
oxygen and can be separated into these two elements. Rust has 
properties which are very different from those of either iron or 
oxygen. - 
W ater is a compound which is composed of oxygen and another 
gas known as hydrogen. Hydrogen is the lightest of gases and 
is colorless, tasteless, and odorless. It is an inflammable gas 
and, owing to its tendency to combine with other elements, 
exists uncombined in only very small quantities. Carbon dioxide 
also is a compound and is formed by the union of the elements 
carbon and oxygen. At ordinary temperatures carbon is a solid. 
It is familiar as graphite and, in almost pure form, as charcoal. 
Soot is a very pure form of carbon. 
