have very little effect upon the life out on the plains.

Animals and plants brought to this region from a moister climate
must drink continually to make up for the rapid evaporation of
moisture from their bodies; a day without water may result in death.
And yet the living things that have homes in the desert can resist
the dry air for many months without a renewal of their moisture.
There are areas where the average rainfall is less than three inches,
and sometimes two years may pass without a drop of rain. It will
certainly be worth our while to find out something about these
desert plants and the way in which Nature enables them to get along
with so little water.

Go where we will, from the moist heat of the tropics or the dry
heat of the deserts to the icy north, we find that everywhere the
plants and animals are suited to the climate of the particular
place in which they live. Therefore we might conclude that they
thrive better in those places than they would anywhere else, but
that is not always true.

A struggle is going on continually among plants for a footing in
the soil and for a share of the sunshine. The weaker plants are
generally killed, while those hardy enough to survive have to adapt
themselves to new conditions of life, becoming stunted and deformed
upon barren slopes; but they have plenty of room there because
fewer plants are striving for the same place.

It is not likely that the deserts of the southwest have always been
as dry as they are now. As the amount of rainfall slowly lessened
through thousands of years, the animals could migrate when it became
too dry; but the plants, fixed in one place, had either to give up
and die, or change their characters and habits to suit the demands
of the changing climate. The fact that these extremely dry deserts
are filled with plant life to-day is without doubt due to this
ability to change.

In a moist, warm climate plants are luxuriant; they take up a large
amount of water through their roots and evaporate it through the
leaves. If placed in a desert, such plants would immediately wither
and die. To avoid too rapid evaporation the bodies of the desert
plants have become smaller, and their leaves have either shrunk
greatly or wholly disappeared. Strong-smelling, resinous juices
exude from the remaining leaves and stems, and form a surface varnish
through which water passes with difficulty.

Some forms of plant life, such as the prickly-pear, are prov