There are few plants so well armed as the Cactus, the evident design of
which is to conserve its moisture. This is accomplished in several ways.
Of course, the very shapes of the plants are all in their favor. Being
either round, globular, or cylindrical, they offer a limited surface to
the dry air inconceivably less than a plant of the same size bearing a
quantity of leaves. The thick skins, too, play a big part in keeping in
the moisture, and many kinds of cacti, such as that known as Old Man’s
Beard, are covered with dense masses of hair.
Many of these succulent desert plants grow to a great size. Thus the
Giant Cactus sends up a tall column, often with only a very few
branches, which may be eighty or even one hundred feet in height.
Curiously enough, some cacti produce the most beautiful flowers,
blossoms without rival in the whole world. The various kinds bear
flowers of every conceivable shade except blue, and the blooms are often
of an immense size. It is not unusual for the blossoms to measure
eighteen inches, or even two feet, across.
Living as they do in arid regions, cacti are peculiarly liable to be
attacked by thirsty animals. Now, a common mode of defense is the
covering of the plant with sharp spines. These spines are so arranged
that they completely shield the juicy stem from any possibility of
attack, it is said that on occasion Mexican ponies will try to knock a
cactus to pieces with their heels when they are thirsty. More often than
not the animals suffer cruelly for their temerity by being severely
pricked.
In much the same way the Aloes and Agaves are protected, so that a hedge
of these plants when placed round a field, is better than the most
perfect barbed wire fence.