if the ground habitually becomes
dry for a considerable period during which the weather is warm enough
for growth. Desert vegetation, on the other hand, which consists
primarily of bushes with small, drought-resistant leaves, needs only a
few irregular and infrequent showers in order to endure long periods
of heat and drought. Discontinuity of moisture is the cause of deserts,
just as continuity is the necessary condition of forest growth. Grasses
prevail where the climatic conditions are intermediate between those of
the forest and the desert. Their primary requisite is a short period of
fairly abundant moisture with warmth enough to ripen their seeds. Unlike
the trees of the forests, they thrive even though the wet period be only
a fraction of the entire time that is warm enough for growth. Unlike
the bushes of the desert, they rarely thrive unless the ground is
well soaked for at least a few weeks. Most people think of forests as
offering far more variety than either deserts or grass-lands. To them
grass is just grass, while trees seem to possess individuality. In
reality, however, the short turfy grass of the far north differs from
the four-foot fronds of the bunchy saccaton grass of Arizona, and from
the far taller tufts of the plumed pampas grass, much more than the pine
tree differs from the palm. Deserts vary even more than either forests
or grass-lands. The traveler in the Arizona desert, for example, has
been jogging across a gravelly plain studded at intervals of a few yards
with little bushes a foot high. The scenery is so monotonous and the
noon sunshine so warm that he almost falls asleep. When he wakes from
his daydream, so weird are his surroundings that he thinks he must be in
one of the places to which Sindbad was carried by the roc. The trail has
entered an open forest of joshuas, as the big tree yuccas are called in
Arizona. Their shaggy trunks and uncouth branches are rendered doubly
unkempt by swordlike, ashy-yellow dead leaves that double back on the
trunk but refuse to fall to the ground. At a height of from twelve to
twenty feet each arm of the many-branched candelabrum ends in a stiff
rosette of gray-green spiky leaves as tough as hemp. Equally bizarre and
much more imposing is a desert "stand" of giant suhuaros, great fluted
tree-cacti thirty feet or more high. In spite of their size the suhuaros
are desert types as truly as is sagebrush.

In America the most widespread type of forest is the ev