Many people think of a desert as an area of shifting sand dunes without
vegetation except in areas where springs provide moisture. This is by no
means true of our Southwestern deserts which are characterized by a rich
and diversified plant cover. However, the majority of true desert plants
are equipped by Nature to meet conditions of high temperatures and
deficient and uncertain precipitation. The way in which desert plants,
closely related to common species found growing under normal temperature
and moisture conditions, have adapted themselves to meet the severe
requirements of desert life is truly remarkable and forms an absorbing
and fascinating study.
Shreve groups desert plants into three categories based on the manner in
which they have contrived to conquer the hazards of desert life.
These are:
1. Drought-escaping plants;
2. Drought-evading plants;
3. Drought-resisting plants.
_Drought-escaping_ plants are the “desert quickies,” or ephemerals.
Taking advantage of the two seasons of rainfall on the desert (midsummer
showers and midwinter soakers) they develop rapidly, blossom, and mature
their seeds which lie dormant in the soil during the rest of the year,
thus escaping the season of heat and drought. There are two groups of
these “quickies,” the summer ephemerals and the winter ephemerals. The
former are hot-weather plants; the latter are species that thrive during
the cool, moist weather of winter and early spring. These “quickies”
present their spectacular floral displays only following seasons of
above-average precipitation.
_Drought-evading_ plants (in common with the deciduous plants of
northern and colder climes which remain dormant while below-freezing
temperatures prevail), meet the heat and drought by reducing the bodily
processes to maintain life only, dropping their leaves, and remaining in
a state of dormancy until temperature and moisture conditions, suitable
to renewed activity, again prevail.
The _drought-resisting_ plants are the bold spirits which take the worst
that the desert has to offer without flinching, or resorting to evasive
tactics. Chief among these are the cacti which store moisture in their
spongy stem or root tissues during periods of rainfall, using it
sparingly during drought. To reduce moisture loss to a minimum, they
have done away with their leaves, the green skin of their stems taking
over the function of foliage. Other plants, such as the Mesquite,
develop deep or widespread root systems that extract every drop of
moisture from a huge area of soil. The majority of the drought-resisters
either cut down their leaf surface to an irreducible minimum, or coat
the leaves with wax or varnish, thus restricting the loss of moisture.
Methods, techniques, devices, or body modifications which desert plants
have developed or evolved to enable them to withstand the rigors of
long-continued drought and heat are legion. Many of them are known and
understood, but it is probable that there are many others which
scientists have not yet discovered.