Ghardaia
The Ghardaia region can be characterized as a vast plain, broken to the north by low, irregularly disposed mountains, and stretching for a great distance to the south and southeast with a fairly monotonous surface, diversified only by oueds, chotts, or occasional dunes, which may be the size of small mountains. Thus on the one hand one finds the fairly diversified and stony Chebka and on the other the hamada, which has been aptly described by Brunhes18 as “le désert par excellence, la vrai désert... les grandes plaques pierreuses indéfinies des hamadas!”
The leading oued of this region is the M’Zab, which extends for about 270 kilometers in a direction south of east across the southern part of the Chebka. It takes its origin about 80 kilometers west of Ghardaia and extends to the vicinity of Ouargla, where it debouches on the Ouargla plain. At Ghardaia the oued lies in a valley, with abrupt sides, which is sunk about 60 meters below the surrounding plain and which at this place is about 3 kilometers in width. (See fig. 16.) There are four main tributaries of this oued, all of which join it from the north or the Chebka side. The valley of the M’Zab becomes more and more shallow as one proceeds eastward and at last lies but little below the general level of the country. Like the other deeper valleys of the Chebka, the M’Zab Valley represents the work of erosion by water at an earlier geological epoch, when the great terraces were formed. The filling of the eroded valleys has perhaps taken place during the long arid period since that time and has probably proceeded very slowly.
It appears to be uncertain how long the M’Zab has been inhabited by man, or, more accurately, by the race now dwelling there;19 but it has probably been not less than nine centuries.20
At Ghardaia are several well-defined plant habitats, which may or may not be distinct topographical areas, and which differ from one another in exposure, soil conditions, and water relations. These are the plain of the Chebka (hamada), the low and flat-topped mountains resting on the plain of the Chebka, the walls of the M’Zab Valley, and the valley floor with its gardens, cemeteries, and waste lands.
The soil conditions of the areas mentioned are very diverse. On the valley walls and the mountains there are bold rock outcrops with soil in the interstices only, and here the most intensely arid conditions prevail. The soil on the hamada also is exceedingly meager. Rocks of various sizes strew the surface. It is only between them, as well as in the washes of gentle gradient, that the best soil conditions of the plain are to be found. Here a cursory examination shows a large admixture of small stones to the fine clay, the prevailing soil type, and that at a depth less than 50 cm. A white hardpan, similar in appearance to the caliche of the southwestern United States, may usually be encountered. In the drainage depressions the soil is relatively more coarse than on the more level portions of the hamada. There is also great variation in the character of the soils of the valley. Above the upper palm gardens, which are about 2 kilometers above the town of Ghardaia, will be found much sand and fairly large stationary dunes, while smaller dunes, shifted by the winds, are to be found at various places in the valley. About 10 kilometers down the valley, toward the east, the sand is blown against the valley sides, and in certain places where the walls are low it has been sifted in a thin layer over the plain itself. At the sister city of El Ateuf the drifting sand is a continuous menace to the gardens.
Between Ghardaia and the upper palm gardens, and also between this city and Beni Isguen and Melika, are bare areas, free from sand or clay, where the soil is so hard as to be used for threshing floors and where the small amount of grain grown in the valley is threshed and winnowed. The hardpan is similar in appearance to the caliche of the southwestern United States and may be essentially the same. It is of wide extent in the valley and probably underlies the largest portion of it. Near the threshing floors the upper portion of the hardpan stratum is from 2 to 3 meters above the bed of the oued M’Zab; the stratum is about 30 centimeters in thickness and is of fairly uniform structure throughout. Beneath this is another stratum, less well defined perhaps, of approximately the same thickness, and with nearly the same character, but carrying a noticeably large percentage of sand. The lower stratum is less hard than the upper one. Underneath the second stratum is soil, largely sand, containing rocks of various sizes. Where erosion of the oued banks has occurred the soft lower hardpan stratum and the yet more soft underlying soil are both removed, leaving the upper stratum projecting as a shelf, sometimes as large as 2 by 4 meters in extent. When the shelving banks break they remain practically intact, partly buried by the sandy floor of the oued. (See fig. 17.)
Along the sides of the valley, and at a distance more remote from the oued, there are occasional washed-out areas, really box cañons, where the banks show a slightly different condition of the hardpan from that just described. Here there may be three strata of hardpan. The uppermost is of the same stratum as the top stratum by the oued, and the second stratum also resembles the lower one just described. There is also a third hardpan stratum of a much different character, in that it has a very large admixture of sand and gravel, and large as well as small stones, making it more easily eroded than either of the upper strata. The soil in which the roots studied were found varied from a fine sand, with waterworn pebbles, near the oued, to a clay mixed with sand nearer the sides of the valley. In places the sand is cemented so as to resemble one of the hardpan strata above described, but it is less hard and apparently is penetrated by water without great difficulty.
The Oued M’Zab, whose channel is 15 meters more or less in width, is dry most of the year, containing water for only a few hours following the rare storms. Wells are very numerous in the valley and furnish a good supply of water. At the time of my visit to Ghardaia the water lay from 10 to 25 meters from the surface, depending on the position of the wells. The depth to water in a single well is said to vary from 1 or 2 meters to 15 meters; in other words, the water-table of the valley varies 13 to 15 meters between the dry and the moist seasons. No analysis of the water is available, but it is reputed to be noticeably saline near and below the town of Ghardaia, while above the town this quality is not apparent to the taste. The water relations of the plain are much less favorable for plants than those of the M’Zab Valley. In addition to the fact that the soil of this area receives only such water as falls directly on it, there is so little depth that the water escapes shortly after it falls, leaving only the most favorably situated soils, for example, those beneath shallowly placed rocks, or between rocks, or in deeply penetrating cracks or the depressions, with sufficient moisture for long use by plants. The depth to water on the hamada is so great that successful wells have never been dug.
The Oasis of Ghardaia
Each of the cities of the M’Zab has its palm gardens as well as gardens in which grains of various sorts and vegetables are grown. Intensive gardening is practised and the fruits of the soil, although won with great labor, are nevertheless not inconsiderable.21
Perhaps the most palms are to be found about 2 kilometers above Ghardaia, where they are so abundant as to form a small forest. Here, in the most thickly planted portions, one finds a veritable jungle in which the desert glare is softened by the spreading tops of the palms and by the close canopy of grape-vines which reach from one palm-stem to another. There is a second story of apricots, peaches, almonds, and figs, and on the floor one finds a variety of vegetables. Outside of the palm gardens, and adjacent to them, are the plots in which cereals are grown. These gardens are divided into diminutive fields, frequently not larger than 1 by 1.5 meters, which are separated by small irrigating ditches and smaller laterals, from which they are given water (fig. 19). Here barley, oats, and wheat are raised, and often with them are planted carrots, turnips, or other vegetables. The main ditches are rendered impermeable by heavy coats of plaster, making it possible to use with the least waste the difficultly acquired water.
The Plain (Hamada) of Ghardaia
A superficial view of the plain (hamada) which lies both to the north and to the south of the M’Zab Valley does not suggest any vegetation whatever, at least during the dry season. The desert is quite as barren in appearance, as, for example, portions of the Libyan Desert are in reality. In every direction one sees grayish-brown stones and bowlders, with little earth, and in some places blackened stones, blackened by “fires from heaven” the Arabs believe, but nothing to indicate the presence of plants. Tristram has described the plain as “one mass of naked rock, rough stone, and coarse débris, from the neighboring mountain, but without a scrap of earth or a vestige of the minutest vegetation.” But close study of the plain makes out a better case than this; in fact, where the soil has accumulated in pockets, where there is a slight drainage depression, or where spaces between the rocks are filled with soil, careful examination shows the remains of annuals and not a few living perennials; but like other intense deserts, plants, even when relatively numerous, are not present in sufficient numbers and not of sufficient size to give character to the landscape or to hide the surface of the ground.
The plain on both sides of the valley was studied and a few areas carefully examined with results which are summarized in the following paragraphs.
It has already been mentioned that the walls of the M’Zab Valley at Ghardaia are precipitous, rising between 60 and 100 meters from the valley floor, their summits being the general level of the plain. Both to the north and to the south of the valley there are short but steep tributary gulches. In these gulches, and especially at the heads of the gulches, are pockets filled with earth, and here may be found some perennial vegetation. For example, at the head of such a ravine, 3 kilometers north of the valley, 10 undetermined living species were found, of which 6 were perennials and the balance were long-lived annuals or biennials. (See fig. 20.) In an analogous situation, but on the plain to the south of the valley, the aspect being similar, a larger number of plants were found, including, among other species, Aristida sp., Centaurea pubescens, Deverra scoparia, Fagonia bruguieri, Peganum harmala, and Teucrium pseudo-chamæpitys. A census of plants was taken, where the individuals were most numerous, with the following result: On an area 16 by 16 meters there were 330 living specimens. The three dominant species were Aristida sp., Deverra scoparia, and Helianthemum sessiliflorum.
On the level portions of the plain one sees almost no perennials and only the dried remains of annuals, although here and there may be found an isolated specimen of Peganum harmala or even of Citrullus colocynthis, the latter strangely out of its proper surroundings. In one place, also, a small date palm was found surviving the extremely arid conditions. But on the hamada it is only in relatively favorable situations that plants are to be found. One such was given above and another was found on the open plain, but near the base of a low mountain, where there was a slight depression and where some water was received from the mountain run-off. The area alluded to is 10 kilometers north of the north valley wall; the south base of the nearest mountain is 100 meters to the north of the area. The ground inclines gently to the south, and rises slightly both to the east and to the west. The surface is thickly strewn with stones and the soil is clay mixed with sand, the latter predominating in the center of the depression, where there is also a relatively large proportion of small pebbles. The area studied, 16 meters square, was so selected that the depression crossed the middle portion, leaving the two sides as representing the larger part of the plain. (Fig. 21.) In a country where the conditions of plant life are so severe it is of interest to observe how slight advantages of whatever kind, such as in the square under consideration, work for the betterment of the vegetation. The dominating species was a bunch-grass, probably Aristida sp., but there was also present Haloxylon sp. (eaten to the surface of the ground by the passing flocks) with other undetermined forms. On the area given 414 living perennials were found with numerous dead annuals. All of the plants were growing in the depression, there being, in fact, none on the adjacent but somewhat higher parts of the hamada. The character of the soil of this square and a discussion of the root characters of plants growing in it are given in another place.
How far the paucity of plants on the plain is owing to the arid conditions obtaining there, apparently a sufficient explanation in itself, and how far to the fact that herbivorous animals, wild as well as domestic, for several centuries have been gaining their food from the plain, can not at present be well told. Observations given below, however, indicate that if areas are protected against the depredations of animals, the plants are noticeably more numerous and of a larger size than when there is no protection. This conclusion applies to portions of the plain as well as to the other habitats under discussion.
The Mountains About Ghardaia
The vegetation of the low mountains and of the rocky valley walls is extremely meager, mainly on account of the small amount or total absence of soil. In certain places (for example, near Melika) the plants of the hamada descend the rocky gulch nearly to the floor of the valley, and a similar condition has already been noted at the heads of two of the larger gulches. In such places we find species of grass and Haloxylon articulatum mainly, but these species are not typical of this habitat. Only two forms appear to occur on the walls of the valley or on the mountains, and nowhere else. These are the “kabar,” Capparis spinosa, and one or more crustaceous lichens. (See figs. 22 and 23.) The kabar is a large shrub, 1 to 2 meters high, which bears persistent and fairly large leaves. The shrub is uneaten by animals, owing to some disagreeable flavor,22 but is provided with small spines. The species is poorly represented, there being but few individuals, and it does not exhibit exposure preference, but grows in crevices between rocks, sometimes at the base of the walls, or wherever it can attain a foothold. I have seen lichens on the flat and horizontal upper surfaces only of a ridge of low mountains about 4 kilometers north of the valley of the M’Zab; search failed to reveal any on the north surface of the mountains or on any rocks vertically placed; their position would thus subject them to the greatest temperature ranges and to the most intense aridity (fig. 26).
The Valley of the M’Zab
The bottom lands, as already has been shown, are relatively favorable for plant life; here the soil is the deepest and the water relations are the best. Accordingly we should expect to find in the valley of the M’Zab more plants than we have seen on the plain, and we will not be disappointed in these expectations; but it is almost certain that in primitive times the vegetation of the valley was even richer than at present. In fact we now find in the unprotected places only such plants as are too small for fuel or are not good for food, and the useful sorts are largely wanting. In other and similar valleys, which have not been so much disturbed by man as the M’Zab and where primitive conditions still largely obtain, there is a surprising wealth of vegetation. Such conditions were seen between Ghardaia and Ouargla and will be specially noted on another page.
At present no trees occur naturally in the valley in the vicinity of Ghardaia or any of the sister cities. The French portion of Ghardaia contains ornamental or shade trees, such as the ash, sycamore, and betoum. The largest native shrubs are a species of Tamarix, growing by the oued, and a few specimens of Zizyphus lotus, the latter confined to the side gulches and numbering half a dozen specimens. Among the most generally distributed plants in the valley are Peganum harmala (figs. 27 and 28) and Haloxylon articulatum; the former is a half-shrub of wide distribution in southern Algeria, occurring from Biskra on the north, and although not strictly confined to the flood-plains of the oued is most abundant where the soil is relatively deep. The leaves are rather large and do not appear to have unusual protection against drought.23 Like its relative in the southwestern portion of the United States, the creosote bush (Covillea tridentata), it is not eaten by any animals, although not armed and not poisonous. This species, therefore, is one of the few which to-day probably retains essentially the same distribution and appearance it had before the country was inhabited. It is interesting to note that Peganum is generally distributed through the M’Zab Valley, being especially abundant between Ben Isguen and Melika. Here in November Peganum, except where trodden under foot by the flocks and caravans, retained much of its foliage, although rain was said not to have fallen for twelve months. In the protected areas also, as will be mentioned below, this species was found to be fresh green, showing little or no indication of the long drought. Of other species found in unprotected places in the valley, Haloxylon articulatum and Henophyton deserti, although eaten by animals so as to be recognized only with difficulty, were also fairly abundant. There were found also Euphorbia guyoniana, called “le bain” by our French-speaking Arab helper, because it is used by the natives as a soap, and Nolletia chrysocomoides, Æluropus sp., and others.
Protected Areas Near Ghardaia
Of the plant habitats whose leading characteristics have been briefly given above, only the oasis and its gardens are secure against the inroads of animals. However, owing to the long settlement of the region, rather large tracts of land are at present in what must nearly approach their primitive condition. The areas referred to are the cemeteries, which, for the reason suggested, possess special interest to the botanist, showing briefly what plants might be expected to occur in the region naturally. The typical M’Zab cemetery is of varying size and surrounded by a stone wall. No plants are introduced to decorate it and no irrigation is practised within its walls. The only disturbance of the natural condition of the land is in the use for which it is set aside. The interments are so conducted that the ground appears to be always used progressively, that is, there is always an older portion and always a newer portion, and the part once used is never afterwards made use of again. From this fact, as well as others which need not be entered into, the more ancient portion of the cemeteries, after a lapse of several centuries, or even several decades, without disturbance, must be in essentially the same condition as regards the soil and water relations, which would be most affected by the fact of interment, that they were in pre-M’Zabite times.
Several cemeteries near Ghardaia and the other M’Zab towns vary greatly in their position as well as exposure; some are on the valley floor below the town of Ghardaia, and others are in side cañons; one is on the south wall of the valley with a northern exposure, and another is on the opposite wall and hence with a southern facing; one cemetery is on the edge of the plain itself. So far, therefore, as the flora of the older portions of these areas represent the ancient vegetation of the same areas, we have in them at present a means of learning something of the kinds as well as the abundance and the habits of the plants which formerly occurred here, and (by inference) of the plants which were in the region in primitive times.
Below and not far from Ghardaia, in the valley floor, is a very ancient cemetery, or rather a cemetery with a very ancient part. In the old portion the drifting sand has obliterated all traces of graves, which have long since been forgotten by the citizens of the town. In the newer portion, farther from the edge of the oued and on higher ground, the sand gives place to clay. In the older portion of the cemetery may be found a fairly rich flora and rather large plants—a striking contrast to the vegetation of the unprotected area immediately without the wall. Here one finds Haloxylon articulatum and Henophyton deserti, both species eagerly eaten by animals, as well as Deverra scoparia, Lithospermum callosum, Zilla macroptera, and Helianthemum sessiliflorum; also grasses and other plants which I did not know. Something of the abundance and the large size of the plants is indicated in figs. 29, 31, and 32.
In one of the cemeteries situated against the south wall of the valley, but not including the wall, the conditions are somewhat different from those just sketched; the soil is a sandy loam, with rocks of various sizes in abundance, and here may be found a fairly rich flora. In the ancient portion of this cemetery the most numerous species is perhaps Haloxylon articulatum, also Fagonia glutinosa, Fagonia bruguieri, Cleome arabica, Echinopsilon muricatus, Helianthemum sessiliflorum, Zollikoferia resediflora, Salsola sp., and others. The plants are relatively abundant and of fairly large size.
In a cemetery on the north wall of the valley, reaching from the floor to the plain above, the wall is less precipitous than at other places and there is a small amount of earth. The number of species here is very limited, being confined almost wholly to Haloxylon articulatum, which is fairly abundant; but in the upper portion of the cemetery are also found Peganum harmala and Capparis spinosa.
The cemetery situated wholly on the edge of the plain has an unexpectedly large number of plants, almost all of them Haloxylon articulatum, which is of good size. Outside this protected area the species is neither large nor abundant, since it is eagerly sought after by camels, sheep, and goats, and a shoot no sooner appears than it is eaten to the base.
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.
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.
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.