Owing to the small amount of soil, close observation of the
root-systems of the plants growing on the mountains and the plain
was difficult; examination of the roots in the field was therefore
carried on mostly in the valley, although enough was seen of the
roots of plants in the other habitats to permit a characterization
of them. The roots of several species growing on the hamada close to
the valley were examined with the following as the leading results:
_Deverra scoparia_ was found to have a main root running directly
downward 20 cm. without giving off large laterals; at that depth it
forked, the resultant branches running thereafter in a horizontal
direction. (See fig. 33.) _Teucrium pseudo-chamæpitys_, _Centaurea
pubescens_, and _Salvia ægyptica_, all from the hamada to the north
of the M’Zab Valley, have pronounced main roots. A similar type of
root was seen in _Zollikoferia resedifolia_ and _Fagonia bruguieri_
from the plain to the south of the valley. Grasses growing on the
plain had roots which, as usual with grasses, showed more diversity,
but on the whole penetrated rather deeply. The root-system of
_Haloxylon articulatum_ is of the modified generalized type,
penetrating deeply also, and will be best described as an inhabitant
of the valley.
From these observations it would appear that the roots of most of
the plants growing on the plain have a well-developed main root,
and that few, if any, perennial roots lie near the surface of
the ground. This condition is rather different from that seen
at Laghouat, where the root-system of typical inhabitants of a
similar habitat is of the strictly generalized type and may point
to a difference in some character of the habitat—for instance,
the precipitation at Ghardaia.[24]
In certain portions of the floor of the M’Zab Valley the soil
conditions favor full and normal development of all forms of
root-systems, but in other portions, because of the presence of an
impervious hardpan, such development is not possible. Where there
is hardpan the presence of species with obligate tap-roots is
precluded, while such as have a more flexible root-system (e.g.,
_Peganum harmala_) can to a degree accommodate themselves to
the unfavorable soil conditions. However, one instance was seen,
which will be reported below, where a plant with a tap-root was
found growing on a hardpan stratum so hard as to be excavated with
the greatest difficulty. The character of the root is very greatly
modified by the presence of the hardpan. Following is a sketch of
the root-systems of a few plants growing naturally in the valley.
_Peganum harmala_ is one of the most characteristic plants of
the floor of the M’Zab Valley; so far as my observations go, it
usually occurs where there is much soil and where the water relations
are the most favorable. Several studies of it were carried out on
plants growing in different parts of the valley. In order to make
the leading study of its roots, a typical habitat was selected east
of Ghardaia and nearly in the midst of the valley. Here the upper
soil, to a depth of about 20 cm., is a fine sand carrying water-worn
pebbles and coarse stones. Below the sand is a denser stratum a few
centimeters in thickness, but not the hardpan described in another
place. Below the harder stratum, to an undetermined depth, are
mingled sand and gravel coarser in texture than the superficial sand.
The first specimen of _Peganum_ studied in this habitat was about
30 cm. high and in full leaf. Its roots were fibrous, that is, they
were not fleshy. The root-system consisted of a main root, which ran
directly downward 17 cm., and several laterals. Probably owing to
the influence of the hard stratum, the main root at length turned
abruptly and maintained a horizontal course for 1.5 meters. It was
1.35 cm. in diameter at the crown and 2 mm. in diameter where left,
at a depth approximating 27 cm. The main root gave off a lateral
5.5 cm. from the surface of the ground, which also branched; the
ultimate branches followed a fairly level course for 45 cm. At the
sharp bend of the main root another branch arose and this branched
in turn, the daughter branches going somewhat downward for over
43 cm. Branches from the latter roots descended to within 8 cm. of
the surface. (See fig. 35.)
Differing in details, the roots of other individuals of the same
species in the same habitat have on the whole a root-system
essentially like the one just sketched. Even in relatively or
actually deep soil the roots of this species do not penetrate deeply,
but reach rather widely. They are of the generalized type,[25]
similar in many ways to the root-system of _Covillea tridentata_
of the Tucson region. When growing in a habitat where hardpan comes
close to the surface, the main root is not so well developed, but
there is a better development of laterals than in the specimen more
favorably situated as regards soil, in the habitat above alluded to.
Growing in or near the habitat of _Peganum harmala_, whose
root-system has been sketched, were other species whose roots
were also examined. Among these were _Haloxylon articulatum_ and
_Euphorbia guyoniana_; the former is one of the native plants most
eagerly sought by animals for food, for which reason it was not
found possible to secure for study specimens whose shoots were
entirely normal; but the plants finally chosen were the least
damaged of any found outside of the protected areas. The shoot
of the specimen of _Haloxylon_ examined was about 50 cm. high;
its gnarled base showed that it had been subject to intermittent
attacks by animals. It was found to have a main root which ran
directly downward more than 30 cm.; at a depth of 10 to 15 cm. a few
small laterals took their origin. (Fig. 36.) The other specimens
studied showed the same type of root-system, so that _Haloxylon_
has here a well-developed main root.[26]
Growing not far from the two species whose root-systems have
just been sketched, and under similar soil conditions, were
several specimens of _Euphorbia guyoniana_, whose roots were also
examined. This plant has a habit of growing in groups whose members
are more or less widely separated. The first specimen studied was
one of a colony of a half-dozen whose habit of growth is shown in
fig. 38. Its shoot was about 15 cm. high and bore several narrow,
smooth leaves, and was 2.5 mm. in diameter at the base. The shoot
was found to go directly downward until it joined a horizontally
placed fleshy root, from which the other individuals of the colony
were seen to take their origin. This, apparently a root-stock,
was 8 mm. in diameter and gave off two branches, 3 and 5 mm. in
cross-section, which were also horizontally placed.
On the southern side of the oued, where the soil is cemented into
a sort of hardpan, _E. guyoniana_ grows in greater abundance,
frequently in groups but also singly. Other observations confirm
those above reported, namely, that the species reproduces largely
vegetatively, new plants springing from old roots, which at
Ghardaia are essentially water- and food-storage organs. Only two
other species were seen to have water-storage capacity, namely,
_Citrullus colocynthis_, to be described directly, and _Phelypæa
violacea_, which was seen at Biskra and will be described later.
In the habitat of _Peganum_, and a few meters distant, was found
a specimen of _Henophyton deserti_, which had been little injured
by flocks. The shoot of the plant was about 30 cm. high, and,
in spite of the long dry season, was in full leaf. It possessed
a long tap-root, of which 75 cm. was recovered. No laterals were
given off along the portion of the root seen. Other specimens of
the same species were also examined, and in every instance a similar
type of root was found.
Somewhat nearer the side of the valley, but growing where there was
considerable depth of sand, were several specimens of _Tamarix_,
the roots of one of which were partly exposed. The plant studied
was over 3 meters high and had not been injured by animals. Its
root was of the tap-root type, since the main root went unbranched
directly downward.
There occur in the valley a few specimens of _Citrullus colocynthis_
described by Schimper, in “Plant Geography,” as follows:
A cucurbitaceous plant resembling our cultivated pumpkin, and its
long, juicy, relatively thick-foliaged and large-leaved shoots remain
green throughout the summer, producing fruits as large as a child’s
head. It presents, therefore, the appearance of being protected in
an unusual manner against the loss of water. As a matter of fact,
however, severed shoots dry up in a few minutes. The extraordinary
length of the roots of colocynth alone renders its existence possible
in the desert.
While the description of Schimper is a good one, it conveys the
idea of much greater luxuriance of growth than was observed for
the species at Ghardaia. As a matter of fact, the leaves of this
plant are small and much dissected, and the fruit is the size of an
orange. How deeply the roots penetrate the ground, or their length,
was not learned, but a very striking characteristic of the root is
its fleshiness, which is shown by fig. 37, and the ability of the
species to successfully withstand drought may lie in the fact that it
is thus provided with a very well-developed and very well-protected
water-storage organ, as much as in having a great length of
root. _Citrullus_ occurs typically where there is considerable
depth of earth, especially close to oueds, although it is rarely
to be found on the plain and only where the soil is deepest. In
the latter habitat the possibility of a permanent connection with
a perennial water-supply is without question excluded. The species
is rather to be considered one of the forms, rare to the Sahara,
which possess a water-balance, and which has the power of storing
sufficient water during the widely separated rainy seasons to last
it during the period of drought. It can be pointed out here that
plants having hypogeous water-storage organs have a very different
relation to the climate of the desert than such as have such organs
above ground. The inclosing soil is a protection, nearly perfect,
against drying. Where the soil is removed the protected parts
quickly become dry.[27] Given two species, both having water-balance,
but one with the storage epigeous and the other hypogeous, other
conditions being equal, the latter should survive under more arid
conditions than the former. It is rather surprising, therefore,
that there are not more of the latter type in the Sahara.
A few measurements of the root of a typical _Citrullus_ from the
M’Zab Valley will give a concrete idea of the water-storage
capacity of the species. A specimen growing in the oued near
Ben Isguen was selected for examination (see fig. 37); it had a
luxurious shoot and several fruits. The shoot was found to arise
from a large and aged root 26 cm. in diameter at the crown, but
tapering rapidly, it was only 12 cm. in circumference at a point
14 cm. beneath the surface, where it forked, giving off one branch
which was 1.7 cm. and another 1.8 cm. in diameter; several smaller
ones took their origin close to the surface of the ground. In its
general appearance the main root of _Citrullus_ is very like that
of _Cucurbita digitata_, which grows on the domain of the Desert
Laboratory and in a similar habitat.
Up to this point the plants whose roots have been described were
found growing where the soil was favorable to a fairly normal
development, but the soil of the valley is not all of this character,
and where hardpan is present the substratum is often extremely
hard and, one would think from inspection, impermeable to water
as well as impenetrable to roots. In such habitats the number of
individuals and also of species is very naturally limited.
About 2 kilometers west of Ghardaia is an area where the hardpan
reaches nearly or quite to the surface. Here _Peganum harmala_
is to be found, since its generalized type of root-system is
capable of not a little variation, adjusting the species to a
variety of soil conditions not otherwise possible. Associated with
_Peganum_ was a single specimen, of dwarfed and badly eaten form,
growing out of the hardpan itself. The position of the plant was
so unusual that its root-system was in part excavated. The form,
an undetermined chenepod, was found to have the exceptional form of
root, thus proving the rule just suggested, for it had a well-marked
tap-root. A gnarled main root was found to take a zigzag course
through 27 cm. of hardpan to the softer stratum beneath. The root
did not follow a crack, but struck boldly downward through soil so
hard that it was removed by the use of a sharp iron instrument and
only with great difficulty.