THE PLANT AND ITS ENVIRONMENT 7 
production of offspring. The success of the plant, if we look 
at it from the standpoint of the plant itself, depends not on its 
size or beauty but on its power to reproduce its kind and occupy 
territory. The leaves contribute toward this end by manufac- 
turing food used in the seed; the stems, by giving support to 
the leaves and reproductive structures ; the roots, by absorbing 
water and mineral matter and anchoring the plant in place; 
and the flowers, by producing the seed. | 
Environment. We have seen that a typical higher plant is 
an organism with leaves, stems, flowers, and fruits, all of which 
are in the air, and roots, which usually grow in the soil. In 
discussing the environment of a plant we have, therefore, to 
consider the component parts of the air and of the soil. 
The air is a mixture of gases. The principal gases in this 
mixture are colorless, tasteless, and odorless, so that we cannot 
see, taste, or smell them. In still air even the sense of touch 
does not reveal their presence, and the space occupied by air 
may seem empty. That this is not the case is shown by winds, 
which cause us to feel the force of the movement of the gases. 
The phenomena of atmospheric pressure show that air has a 
very decided weight. The fact that air can hold up flying birds 
and airplanes likewise demonstrates that air is a real substance, 
and that the stems and leaves of plants do not grow in empty 
space. Not all gases resemble the principal ones in the atmos- 
phere in being colorless, tasteless, and odorless. The gas chlo- 
rine, for example, is yellowish green, has a disagreeable taste 
and odor, and is very poisonous. 
Phases of matter. In general the same substance may exist 
in three forms, or phases, — namely, solid, liquid, and gas, — de- 
pending on temperature and pressure. Water affords a good 
example of the three forms of a single substance. At tempera- 
tures above 0° C. and below 100° C. water exists as a liquid. 
When it is cooled below 0° C., it becomes a solid which we call 
ice. If it is heated to 100°C. or more, it changes into a gas 
known as water vapor. This gas is colorless, tasteless, and odor- 
less, so that when it is mixed with the gases of the atmosphere 
