Botanical features of the Algerian Sahara

by William Austin Cannon

The sentiments of the residents of Ghardaia, which led to the establishment and protection of the cemeteries, made a close botanical study of them injudicious. Enough was seen, however, to establish several points, the most important being, at least for the areas considered, that there is growing in them, without irrigation, a somewhat rich flora composed of relatively large perennials. And from this fact it seems probable, if also protected against the predatory attacks of animals, that other areas under the present rainfall and other physical environmental conditions now obtaining would support a much heavier vegetation than is generally the case. How far the presence of man and of his flocks has otherwise modified the flora, especially as regards its composition, is another question, but it certainly has not been without its effect. In the portions of the M’Zab region, except certain areas rather remote from the towns, where there is no protection, the only forms which are at all abundant, or at least conspicuous, are such as are armed, poisonous or distasteful, or too small for use as fuel. Among these are Peganum harmala, Zilla macroptera, Tamarix, and others seen later. However, Haloxylon articulatum, although eaten by all animals so as never to develop in a normal manner, is surprisingly abundant, although by no means conspicuous.

ROOT-HABITS IN THE GHARDAIA REGION.

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.