Part 13

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. 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.

LEAF-HABITS IN THE GHARDAIA REGION.

Observations of the leaf characters of the desert perennials, as shown in November, offer some points of interest, especially since there had been no rain for a year previous to the visit. As would be expected, the leaf-habit is various, ranging from scale-like or none to fair size. Several species appear to be evergreen, including Artemisia herba-alba, Capparis spinosa, Echium humilis, Fagonia bruguieri, Gymnocarpon sp., Haloxylon articulatum, Helianthemum sessiliflorum, Henophyton deserti, Herniaria fruticosa, and Salsola. Certain plants probably, as Henophyton, are facultative evergreens, retaining the leaves if the season is moist and dropping them if it is excessively dry; several of this genus were seen without leaves, but with green stems. (See, also, figs. 39, 40, and 41.)

Some idea of the variation in size of the leaves of a single species, as well as the average size of the leaves, will be had from the results of a few measurements. The leaves of Henophyton deserti range in length from 2 to 3.1 cm., and in breadth from 0.2 to 0.5 cm. The average length of 12 leaves from a single branch 7 inches in length was found to be 2.82 cm.; the average breadth was 0.35 cm. On another branch, 23 cm. long, were 35 mature leaves which averaged almost exactly the size of those just given. Since these leaves are rather numerous and of good size, it will appear at once that the species has a relatively large leaf-surface, even if the area can not be stated more definitely. (Fig. 30.)

The evergreen shrub Capparis spinosa has the largest leaves of any plant native to the Ghardaia region, and probably of southern Algeria. Its leaves are bilateral and nearly round and are placed upright on the branches; a series of 29 leaves from one branch 33 cm. long varied from 2 to 3.2 cm. in length and slightly less in width, the average being, length 2.9 cm., breadth 2.2 cm.

Euphorbia guyoniana has a deciduous leaf-habit. Its leaves are rather small but numerous. A shoot 22 cm. long bore 40 leaves varying in length from 1.5 to 2.1 cm. and in breadth from 1.2 to 2.0 mm., with the average length and average breadth 1.7 cm. and 1.6 mm. respectively.

It is worth noting that the surface of the leaves, as shown by inspection, is, possibly, most often smooth, as, for example, in Capparis, Henophyton, and Haloxylon. Dense hairiness is an exception among the species seen, Lithospermum callosum being the only plant striking in this regard.

GROWTH AND FLOWERING HABITS IN THE GHARDAIA REGION.

Upon arriving at Ghardaia in November, it was surprising to find several species of perennials, under strictly desert conditions, putting on new growth, forming fresh leaves, or coming into flower. Especially was this unexpected in view of the fact that no rain had fallen in the region for twelve months. A study of the roots in relation to the depth to water showed also that most of the native plants, during the dry season, could not have penetrated to a depth anywhere near that of the water-table of the valley, and the water relations of the plants growing on the plain above the M’Zab Valley were even more severe. Further, most of the species do not possess water-storage faculty. It should be noted that the only apparent difference in the water relation between November and in early autumn, or summer, was that of raising the relative humidity of the air through the lower temperature. In a preceding section it has been stated that little or no evaporation took place at night in November at Ghardaia, and possibly less in the daytime than would have been supposed. The leading environmental changes were, of course, the lower daily temperature and the really cool nights.

It is not uncommon in the Arizona desert for a species to form flowers or leaves, following a change in temperature, the moisture conditions being not otherwise changed, but, so far as I know, the temperature changes bringing about this result are always from a cooler to a warmer condition and not the reverse. It would not be expected, consequently, that in the present instance a renewal of vegetative activities would follow as a direct result of a lower temperature, although analogous changes are necessary before certain species, after rest, will start development. Whatever may be the immediate cause of the renewal of activity on the part of the plants at Ghardaia as noted, the following species were seen to have formed new leaves: Henophyton deserti and Zilla macroptera; also the following fresh flowers with or without shoot growth: Fagonia bruguieri, Haloxylon articulatum, Henophyton deserti, Ononis polyclada, and Zollikoferia resedifolia, and three other species not determined.

=GHARDAIA TO TOUGGOURT.=

From Ghardaia the route, consisting of camel trails only, pursued a course south of east to Ouargla, and from Ouargla a direction east of north to Touggourt, over 400 kilometers, Ouargla being about half-way. There are no villages between Ghardaia and Ouargla and none between this place and a point 20 kilometers south of Touggourt, so that in accounts of the vegetation or the topography, lacking convenient points around which to center descriptions, the device will be adopted of using distance estimations to or from the three chief towns.

The trail followed the valley of the Oued M’Zab, or kept close to it, for 63 to 73 kilometers before finally leaving it. It passed the sister towns of Ghardaia and crossed short intervals of plain, descending occasionally to the oued. The walls of the M’Zab Valley, 60 meters more or less at Ghardaia, become lower and less precipitous as one goes down the drainage, until at length they become little more than rounded banks. The low, flat-topped mountains, which are a feature of the topography about Ghardaia, were soon left behind, and nothing similar was encountered until the vicinity of Ouargla was reached. Between Ghardaia and El Ateuf are small dunes in the valley and sand is drifted along the base of the walls at various places. Near and immediately east of this town the sand is especially abundant and, being shifted by the winds, constitutes an ever-present menace to the small gardens belonging to the inhabitants of the place; in order to control its drifting, fences of palm leaves are made or the sand is removed when it becomes too abundant. Often the gardens are abandoned, leaving the palm fences to mark their sites after the sand has gone beyond. Where the air-currents are most powerful or most consistent, or the walls are broken down, the sand may be carried in small quantities onto the plain, where it constitutes a mulch, influencing in a striking manner the character of the vegetation (fig. 42).

Finally leaving the valley of the M’Zab about 73 kilometers from Ghardaia, the trails wound upwards through low rounded hills to the hamada. This is the northern edge of the region of the Gantara, 100 by 150 kilometers or more in extent, reaching from the valley of the M’Zab on the north to the region of the dunes to the south. It slopes towards the Oued Igharghar, or the drainage depression connected with this great oued. The Gantara has a few chotts and is crossed by three oueds in the southern portion. It is probably the most arid part of southern Algeria. About 60 kilometers of the plain were crossed and here it was gently rolling and stretched without a break to the horizon. The surface resembles that of the hamada at Ghardaia, that is, stones of various sizes, usually small, lie on its surface, but never forming a continuous cover, as in some portions of the Arizona desert. The soil is brown, of fine grain, and with little or no addition of sand. In the innumerable little hollows the soil is deeper than on the slight rounded ridges. Wind is apparently the most potent erosive agent.

Two chotts were encountered between Ghardaia and Ouargla, one unimportant, the other large and with many features of interest. The latter, the Chott Mellala, is about 10 by 15 kilometers in size. The trail descends from the plain, winding through a zone of rounded, cone-shaped hills or mamelons, to the floor of the chott, which lies about 60 meters lower than the general level of the plain. The chott was quite dry in November when we visited it, but at rare intervals water is said to flood the central portion. Toward the outer edges the floor is thrown into waves, where the heavy incrustation of salts is broken. In the center the salt crust forms an unbroken and level surface. Gypsum (calcium sulphate) is the predominant salt. On the eastern side a long and high ridge of sand rears its uneven summits. The height of this ridge was estimated to be 250 meters, and was said by Massart to be the largest seen by him in the Sahara. This dune we had seen lying on the eastern horizon for one or two days before reaching the chott.

The relation of the Chott Mellala to the country to the north or the south was not seen, but between it and Ouargla there lies a succession of smaller and more irregular chotts, which together form a fairly well-connected chain. These chotts are separated by low passes and flat-topped hills whose summits are on a level with the neighboring plain. Many of the hills are cone-shaped and in other topographic features the region shows the eroding action of wind. About 7 kilometers from Ouargla an opening in the mamelons gives a view of a plain extending on a lower level to the horizon. This is the reg, or fluvial desert. The Ouargla plain, or reg, is connected with the drainage of the great Oued Ighaghar and has a character which in many ways is different from the Gantara, over which we had just passed.

With the descent to the reg desert a more monotonous region is encountered. In the vicinity of Ouargla and for some kilometers to the north the topography is quite flat and gives the impression of a flood-plain. To the east it stretches unbroken to the horizon, but to the west it is bounded by a fairly abrupt wall, the Gantara escarpment. At intervals of several kilometers low sand ridges cross the route over the plain, and on the second day somewhat higher ground was traversed and a sand ridge about 4 kilometers across was encountered. The country then becomes somewhat more broken and presents the appearance of being the remains of an ancient and more elevated plain. About 56 kilometers from Ouargla are the largest dunes crossed; where traversed, these were 10 kilometers from north to south and extended beyond our vision both to the east and the west. This is apparently the edge of extensive dune regions which lie mainly east of Touggourt and of the Oued Rirh. For possibly the last 30 kilometers of the journey to Touggourt there are dunes and chotts in alternation.

A word should be said regarding the hydrography of the region whose surface features have been sketched above. Between the cities of Beni M’Zab and Ouargla two wells were passed, although a route could have been taken which could have included three wells. The wells are 125 meters or less in depth and are maintained by the government for the benefit of the caravans, as well as to provide water for the large number of goats and sheep pastured in the neighborhood. The situations of the wells are always in depressions, either along the Oued M’Zab or in similar although smaller drainage areas, and none are on the Gantara. At Ouargla and on the reg desert to the north of the town the water lies very close to the surface of the ground. It can be dipped with buckets and the roots of the palms reach to the water-table. The water from the shallow wells is strongly impregnated with salts. Before reaching Touggourt standing water was seen where the trail crossed certain chotts. Numerous artesian wells have been made by the government which penetrate the ground several hundred meters and give a large and continuous supply of sweet water.

GHARDAIA TO OUARGLA—VEGETATION.

The plants seen during the first day’s march from Ghardaia were such as have already been observed to be characteristic of the valley of the Oued M’Zab or of the neighboring hamada. The vegetation of the hamada, usually very sparse, was noticeably more abundant wherever the sand had been drifted over it from the dunes of the valley, even if the thickness of the sand was so slight as to be little more than a mulch. Here low grasses, much eaten, were the prevailing forms. On the dunes in the valley of the M’Zab, 20 kilometers from Ghardaia, the number of species and individuals is relatively large, the most abundant species being drinn (Aristida pungens), although Deverra scoparia is also fairly numerous. Somewhat farther on the route, and in a sandy flat, besides these two species, there is much Ephedra alata. On the slopes leading from this flat and on the plain above, there is an almost pure stand of Rhantherium adpressum.