The environmental conditions encountered by plants in
the arid regions are widely different from those of moister
regions. Precipitation is not only slight, but it shows an enormous
range in variation from year to year; the rate of evaporation is
high; the temperature of the air and soil varies widely both during
the day and with the seasons; the light is of great intensity,
and the soil is low in humus content and may contain an excess
of salts. These, the most striking physical factors of deserts,
are present in different combinations with resultant differences
among deserts, and an arid region may be so large as to include
such variation within its borders. Also, a desert may be so far
from the ocean, or it may include such diversity of topography,
as to show great variation in biological features as well as in
its surface phenomena.
In the flora of any arid region, the mutual relations of the
constituents of the flora and its general and detailed relation to
the physical environment are quite different in the main from these
features in the flora of the more humid regions. Thus, the leading
relation is the relation to water, and on the response of the plants
to this relation much of the phenomena associated with plant life in
the desert directly depends. For example, in the extreme deserts it
is probable that the elements of competition between the perennials,
which is an important factor in the survival of a species in the
moister regions, is wholly lacking. It should be noted, however,
that in the less extreme deserts, as in the vicinity of the Desert
Laboratory, competition exists between plants, although this is not
at first apparent. In this case the competition is not for room,
but for water, and is not manifest by palpable crowding, but by
invisible competition of the roots. Thus the reactions are with
the physical environment and are exhibited in a variety of ways,
some of which concern the plants themselves in an intimate manner,
being morphological and physiological, some being concerned with
the flora as a whole. The environmental responses are often obscure
and complex, but in other instances they are less obscure and
apparently direct.
As is well known, a desert flora is in part perennial, lasting
with but little outward change from season to season, and in part
ephemeral, consisting of short-lived species which appear with the
rains and which disappear with the return of the dry season. The
ephemeral flora differs in no essential regard from annuals of the
moister regions; also, the environment to which they are exposed
closely resembles the environment of the annuals of such regions. But
the perennial desert flora, on the other hand, offers very striking
departures from the corresponding flora of the moister regions,
just as the environment to which they are exposed for the most of
the year is also different. It will be sufficient, for the purpose
of bringing out the point of view, to notice a few of the leading
characteristics of desert plants and of their physical environment
and, in a few instances, to observe possible relations between
the two.
The most obvious features of desert plants are associated,
in whatever way, with the subaerial portions. Leaves are usually
greatly reduced or wanting, during the dry seasons at least. Spines
are frequently present and the exposed parts are often well covered
with hairs. The stomata are sometimes deeply sunken, the cuticle
often very heavy, and a waxy substance may cover leaves or stems. The
chlorophyll-bearing cells are arranged with the long axis at right
angles to the leaf or stem surface. All or most of these characters
are associated with the low humidity of the air. In certain deserts
plants are also to be found with greatly enlarged stems and branches
which serve as water-storage organs. It should be noted, however,
that plants which do not have a constant surface undergo many marked
changes with a betterment of the water relations, particularly if
this comes when the temperatures are favorable. For example, many
cacti organize leaves which are unsuited in structure for periods
of extreme drought, and which consequently fall away soon after
the close of the rainy season. These leaves enormously increase
the rate of transpiration at a time when this is not injurious.
It is not in the subaerial parts alone, however, that the plants
of one desert are different from those of another, that plants
are unlike in the same desert, or that plants of a desert are
different from those of the more humid regions. The root-habits
also exhibit not a little diversity and show marked reactions
to the pressure of their environment. For instance, the desert
shrubs of the region surrounding the Desert Laboratory at Tucson
have well-marked root-systems, apparently constant under natural
conditions, which may roughly be designated as the tap-root type,
the superficial type, and the generalized type. Other conditions
being equal, species with characteristic root-types have also
characteristic distribution, or exhibit in other regards consistent
reactions. Thus, the widely extending and superficial type of roots
is confined, among independent plants, to such as have water-storage
capacity. The relation of this root-type to the distribution will be
mentioned later. Plants with a dominating tap-root are confined to
areas where the soil is relatively or actually deep, while species
having generalized roots have a local distribution which may be
considered the maximum.
The relation of the superficial type of root-system to the
distribution of the species is not so apparent in this relation
in plants as with the other types of roots, but is undoubtedly
close and possibly definitive. The absorption roots of plants with
water-balance mostly lie less than 10 cm. beneath the surface and
are thus subject to extreme desiccation for the maximum time, or
in other words they are exposed to favorable moisture conditions
the minimum time. How long the optimum water-absorption time
may be for such species is not known, but that it is longer than
might be supposed (from the period certain species can survive
without water) is highly probable from the facts concerning their
distribution. In brief, the best development of the fleshy species
in question occurs where the rainfall is periodic, occurring twice
each year, and they are wanting or sparse where the rainfall is
uncertain or occurs but once annually. Had these plants a deeply
penetrating type of root-system the local as well as the general
distribution would be very different from what it now is. Owing
to the unfavorable character of the rainfall in southern Algeria,
plants with a water-balance are wanting there, just as they are
wanting in portions of southwestern United States where the amount
or the character of the precipitation is likewise unfavorable.
Besides the characteristics of the root-systems of the desert
perennials whose significance has been sketched, there are other
features of importance. For example, the roots lying near the
surface bear tufts of delicate roots, which are formed during
periods of active growth and perish when such seasons cease. By
the organization of deciduous rootlets the absorption area of the
species is enormously increased, and quickly, without at the same
time increasing the distance of water transport.
There need be mentioned here only one additional feature of the
desert plants. It is now known, in brief, that the non-fleshy
perennials of the desert, not halophytes, may possess a very dense
cell-sap. This fact has been demonstrated in the subaerial parts
of several species, and is assumed to hold for the roots also of
the same species. As suggested, not all of the desert plants,
however, are capable of developing dense juices. Thus, certain
fleshy species, and such mesophytes of the desert as have been
studied in this connection, do not have more highly concentrated
cell-sap than the ordinary plants of the humid regions. Further,
it appears that desert species which, under natural conditions
and during the dry seasons, form extremely dense juices, lose this
capacity when grown under humid conditions. So far as is now known,
species capable of developing a cell-sap with high osmotic power have
generalized roots, although this may be of no especial significance;
but the relation of this capacity to survival in an arid substratum
is apparent and vital.