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.
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.Citrullusoccurs 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 periodof 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 typicalCitrullusfrom 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 ofCitrullusis very like that ofCucurbita 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. HerePeganum harmalais 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 withPeganumwas 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. ofhardpan 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.
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, includingArtemisia herba-alba,Capparis spinosa,Echium humilis,Fagonia bruguieri,Gymnocarponsp.,Haloxylon articulatum,Helianthemum sessiliflorum,Henophyton deserti,Herniaria fruticosa, andSalsola. Certain plants probably, asHenophyton, 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,and41.)
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 ofHenophyton desertirange 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 shrubCapparis spinosahas 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 guyonianahas 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, inCapparis,Henophyton, andHaloxylon. Dense hairiness is an exception among the species seen,Lithospermum callosumbeing the only plant striking in this regard.
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 thisunexpected 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 adirectresult 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 desertiandZilla macroptera; also the following fresh flowers with or without shoot growth:Fagonia bruguieri,Haloxylon articulatum,Henophyton deserti,Ononis polyclada, andZollikoferia resedifolia, and three other species not determined.
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. Thelow, 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.
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), althoughDeverra scopariais also fairly numerous. Somewhat farther on the route, and in a sandy flat, besides these two species, there is muchEphedra alata. On the slopes leading from this flat and on the plain above, there is an almost pure stand ofRhantherium adpressum.
Crossing relatively small portions of the plain, in place of always following the bends of the Oued M’Zab, about noon of the second day we reached the bordj Zolfana, in the valley of the Oued amidst low and narrow dunes, which are moving slowly across the flats. (Fig.44.) No vegetation appears on the dunes, but on the fixed sand between them, or on the stationary dunes at the border of the flats, some plants are to be found. Among these the most abundant, but really not numerous, areEuphorbia guyonianaand retam (Retama retam), which was often seen later along the line of march as well as in the vicinity of Biskra. Retam superficially resemblesEphedra alatain having rudimentary leaves and green, reed-like branches; it is carefully avoided by animals, althoughGenista saharæ, a very similar plant, which grows in like situations between Ghardaia and Ouargla, is said by Massart (loc. cit.) to be eaten greedily by them. (Fig.43.)
A short distance beyond the bordj the bottoms suddenly widened, the dunes disappeared, and for the remainder of the day’s march we passed through the richest vegetation we had seen since reaching the M’Zab region. (Figs.45,46,and47.) Here the shrubs were of fair size and of sufficient abundance to give character to the landscape. About the 63 kilometers camp the leading species areRetama retam,Ephedra alata, andHaloxylon schmidtianum. On the hamada adjoining the flat are several species, includingAristida ciliata,Artemisia herba-alba,Farsetia ægyptiaca,Farsetia linearis,Gymnocarpon fruticosum,Helianthemum eremophilum,Henophyton deserti,Marrubium deserti,Salsola vermiculata,Teucrium polium,Thymelæa microphylla, andZollikoferia mucronata. On the hamada just adjoining the place of our camp, however, there appeared to beHaloxylon schmidtianum, to the exclusion of other species.
After crossing the bottoms of the oued the trail climbed up to the hamada and we did not see the M’Zab Valley afterward. The vegetation of the hamada soon becomes very sparse and as far as the eye can reach the appearance is that of entire bareness. But, as was found to be the case nearGhardaia, close examination revealed the presence of many living perennials as well as the remains of the previous annual flora. This plain, the Gantara, of wide extent, is the most arid region seen in southern Algeria. Massart estimates that there is an average precipitation of 15 cm. on the desert, but, from data previously cited, it would appear that this amount is rarely attained; indeed, several months, or even two or more years, may pass without any rainfall whatever. When one searches the hollows he finds a few small perennials; plants are almost wholly absent on the low ridges. But in some areas on the plain, where superficial examination does not show any plants, a surprising number were found. For example, on the level hamada, 96 kilometers from Ghardaia, on an area 16 meters square, 389 living plants were found, but only 24 were so large as to be seen from a distance; all were either eaten badly or trampled to the ground, so that it could not be learned what the flora of the area might have been had no animals interfered with its full development.
After leaving the 96 kilometer station the perennials were seen to diminish rapidly in numbers and to decrease in size, until the ridges of the low undulations were absolutely without plants, and there were but few in the hollows. There was no apparent change in the character of the hamada or in that of the soil. This sterile condition persisted for 26 kilometers, when vegetation similar in character to that previously seen was again encountered. The zone of better vegetation lasted for 3 kilometers, when the country became barren once more, which condition lasted for 10 kilometers. The presence of barren belts on the Gantara, where the plants at the best are insignificant in size as well as number and without change in topography or soil, points to an especially arid belt. This conclusion is further strengthened by the observation that the plants found were relatively of very small size.[28]
The flora was also sparse upon the route followed by Massart across the Gantara, which was apparently somewhat farther to the north than the one now being described. He mentions having foundArgyrolobium uniflorum,Asteriscus graveolens,Fagonia microphylla,Deverra chlorantha,Fagonia glutinosa,Halogeton alopecuroides,Helianthemumsp., andHerniaria fruticosa. He says thatDeverrais one of the rare glabrous forms on the hamada. It is said to have the odor of parsley, and the Arabs have a belief that camels which eat it become blind, but Massart’s camels were not injured by eating the plant.
As we drew near the Chott Mellala, on the eastern edge of the barren zone mentioned in the preceding paragraph, we suddenly encountered a belt ofEphedra alenda, stretching to the north and to the south as far ascould be seen (fig.49). On one side was the barren zone, on the other the immense plantation. Upon examination it was found that theEphedrawas the only species of perennial. It had several points of interest, but the short time at our disposal precluded more than a superficial examination. Usually keen about the desert species, it is curious to note that the Arabs appear not to recognize this, called by them “alenda,” as being related to the larger species ofEphedrawith which they are well acquainted. It is a small species, growing from 30 to 40 cm. high. It does not occur singly, but has the habit of growing in groups of a half dozen or more. Between theEphedracolonies were only the dried remains of the annual flora of the last rainy season. An examination of the root-system of the species showed that, like other species of the genus, it has a well-developed tap-root (fig.50).
Alenda, however, has a root-habit which, although not peculiar to it, is at least very striking and of great importance to its survival. From the root-crown a stolon arises which extends away from the parent for a distance of about a meter. From this stolon there arise shoots which develop into daughter plants. Through this method of reproduction the small colonies of the species are formed and possibly the species mainly multiplied. From the stolons small roots arise, giving aid to the mother root in providing the offspring with moisture. This habit is very like that ofKœberlinia spinosaof the Arizona desert, which has a similar type of root-system and which reproduces vegetatively in a similar manner.Alendacontinued to be the dominant species until near the edge of the big chott (Mellala), a few kilometers east of where it was first seen.
Practically no plants were seen when descending through the eroded portions of the hamada to the floor of the chott (fig.52), but on reaching the bottom of the great chott a surprisingly large number was observed, includingAnabasis articulata,Aristida pungens,Ephedra alata,Limoniastrum guyonianum,Retama retam, andTraganum nudatum(fig.51). In the wide central part of the chott, where the salts are perhaps most dense, there are no plants; but on the eastern side areEuphorbia guyoniana,Anabasis articulata, andZygophyllumsp. These small species were growing far apart and were badly injured, either by being trodden under foot or by being eaten by the passing animals. To the south of where we crossed the chott and also to the north the sand mountains arise. After leaving Chott Mellala we ascended gradually to go over a low pass separating it from a small chott to the east. Here we obtained a backward view of the sand mountains, low as seen from the pass, with sharp, wind-made ridges, and bearing a few specimens ofAristida pungens. Descending somewhat, another but smaller chott was crossed. Between the latter chott and the Ouargla plain the plants were very few and confined to the slopes and the higher ground, avoiding almost wholly the depressions. Of those recognized,Traganum nudatumwas the most abundant. This is the“vamran” of the Arabs and is an inconspicuous shrub frequently seen later on the way to Touggourt.
Ouargla is an ancient, rambling town, somewhat in decay, set in the midst of extensive palm gardens. There are said to be 500,000 date palms at the oasis. It was founded in the tenth century by the M’Zabites and later taken possession of by the Arabs. The town is peculiar in its situation and its gardens, lying, as before described, in the flood-plain of an ancient river. The plants cultivated in the town as ornamentals are fewer than at Ghardaia, but of the same kinds, and (besides dates) the gardens contain fewer fruits and apparently fewer kinds of vegetables. Between the gardens one findsTamarixin some abundance.
Leaving Ouargla by the western gate and turning north, we soon passed through the crooked streets and reached the reg desert to the north. There is little vegetation near Ouargla, but on some low dunes extending over the reg we foundAristida pungensandPhragmitessp., growing in a hollow. On the flood-plain there were a large number of individuals and probably a large number of species. Among the most conspicuous of the shrubs wereRetama retamandLimoniastrum guyonianum, the “zaita” of the Arabs. (See figs.56and57.) Zaita is a handsome shrub with cylindrical leaves often covered with a fairly heavy salt incrustation. So abundant is the exudation that in localities where the species is especially abundant, as at our camp 32 kilometers south of Touggourt, the plants have the appearance of being covered with snow. Although, like many other species between Touggourt and Ouargla, zaita can live where there are salts in excess, it appears not to be an extreme type of halophyte and does not occur where the salts are most dense. It was seen both on the dunes and on the low lands. Other common halophytes are “souid” (Salsola tetragona), “belbel” (Anabasissp.), andHalocnemon strobilaceum, which appears especially resistant. (See figs.59,60,61,and62.)
Other forms appear on the higher ground and on the plain about 50 kilometers from Ouargla, whereEphedra alataespecially is common. There are no dunes on the plain, but some sand swept across it from the fluvial desert to the west passes eastward and augments the dunes farther east. The larger specimens ofEphedraare rather effective sand-binders and bring about the formation of diminutive dunes. The effect on the growth of the species by the piling sand is peculiar. As the sand accumulates it covers the lower branches of the shoot; these are stimulated to unusual growth and new branches may spring from them. Thus the effect is similar to that habitually occurring inE. alenda, but inE. alatathe habit is not a fixed one. As an instance of the length which such submerged branches may attain, it may be mentioned that one, 2 to 3 cm. in diameter, was over 4 meters long.
About 75 kilometers from Ouargla, and on a plain similar to that just referred to, “dhamran” (Traganum nudatum) was the most common species, but was by no means abundant. The census of an area 16 by 16 meters, taken in a locality where dhamran was dominant, resulted in finding 31 living and a few dead plants, probably allTraganum. It is to be understood that the vegetation of the reg desert not far distant is much richer, not only in species but especially in individuals, and also that the plants are much larger.
From 56 kilometers to about 71 kilometers from Ouargla the route lay over more or less continuous dunes, where the leading species seen wereAristidasp.,Ephedrasp.,Euphorbia guyoniana,Limoniastrum guyonianum, andTraganum nudatum.
In or about the edges of the chotts, which are the leading topographical features of the region immediately south of Touggourt, the most common species areHaloxylonsp.,Salsola tetragona,Limoniastrum guyonianum, andArabis aphyla. Besides these forms, about 28 kilometers south of Touggourt fine specimens ofHalocnemon strobilaceumwere seen growing in an extremely salty situation, to the total exclusion of other species.Tamarixalso is to be found in and about salt spots near Touggourt. (See fig.64.)
Both Massart and Doumet-Adanson (Bull. Soc. Bot. France, 39, 1892) have discussed the flora of the Touggourt-Ouargla region. The following exhaustive list is given by the latter author as having been collected between the two desert towns:
From Ouargla to Touggourt we have found that the country gradually descends, the former place being 124 meters and the latter 77 meters above the level of the sea. From Touggourt, also, for a distance of about 120 kilometers, the descent along the route continues until at the Chott Merouan a level of 6 meters is attained. The lowest places in every case are of course the chotts and the connecting oueds. The most important chott of this series is Melrirh, 11 meters or more below sea-level. This drainage system is the northern culmination of the vast one of which the Oued Igharghar is the most important part. In an earlier age water came north in the oued from the highlands of central Sahara and poured into the Chott Melrirh, having passed successively through the lesser chotts farther south. At that time, also, the Chott Melrirh probably was connected with the Gulf of Gabes.
From Touggourt to Chott Merouan, the lowest portion of the route to Biskra, the topography is that of a region of chotts; that is, there are salt spots surrounded on every side by higher ground, which in many cases is of sand. From Chott Merouan the route passes over a higher desert of a different character, which in part bears a remote resemblance to the Gantara and in part to the Ouargla plain. It is a vast plain, with little topographical diversity, which rises to meet the Atlas Mountains to the west and north. In the eastern portion it is somewhat rolling, stones are strewn plentifully on its surface (hamada), and there has been considerable erosion, so that gullies are formed. In the portion nearer Biskra the surface is more level, the soil is fine (reg), and there has been comparatively little erosion.
The soil of the chott region is largely of sand; on the hamada there is much clay, while on the reg it is fine and easily blown by the wind, and this in spite of the fact that the most vegetation seen in the Sahara was in this region. A slight breeze picks up the dust and carries it long distances in dark clouds. It fills the throat, nose, and eyes of the traveler and makes crossing the reg exceedingly disagreeable.
The plant life as seen along the portion of the route through the chott region consists almost wholly of halophytes, as would be expected, in addition to which, where for short distances the hamada or dunes were crossed, there were forms characteristic of such areas. TheTamarixis especially common in the region andLimoniastrum guyonianumis also often met.
The date gardens of the Oued Rirh are justly famous. One passes numberless plantations where the date is cultivated, and in the neighborhood of each group of gardens one sees squalid Arab villages. Over 19,000 tons of dates are said to be carried each year by camel from the Oued Rirh to Biskra, whence they go to the markets of the world. One day we passed 700 camels laden with dates going to Biskra.
As soon as the chott region is left and the higher ground is reached a different as well as a richer flora is encountered. Here diversity of topography favors diversity of plant life. On the reg near Biskra vegetation is especiallyabundant. Here, in fact, one passes through large thickets ofTamarixand other species, and sees that the desert is much less intense than that in the south, especially in the region of the M’Zab, where in many respects the topography is similar. The kinds of plants also, as the list below will indicate, are different in the main from those farther to the south. Where the surface is most rolling we findTamarixsp. on the heights,Zizyphus lotusin the hollows, and the following grasses:Stipa tortilis,Hordeum maritimum, andPhalaris minor. A salsolaceous shrub (Arthrocnemon macrostachyum) may be found in washes,NitrariaandLimoniastrum guyonianumon sandy places, andOdontospermum pygmæumandAnastatica hierochunticaoccur between rocks. On clay flats one findsHalocnemon strobilaceumandSuæda vermiculata, indicating the presence of salts.
The preceding notes are in part from Massart. The following species are given by Doumet-Adanson as having been collected by him between Biskra and Touggourt:
Biskra lies immediately south of the Atlas Mountains, in the Department of Constantine, 220 kilometers from the Mediterranean and about 400 kilometers from Ouargla. To the northeast of the oasis lie the Aurés and to the west the beginning of the Saharan Atlas, which run south of west across Algeria into Morocco. Just north of the place are small hills and low, jagged mountains—detached spurs from the main ranges. These are the Djebel Bou Rhezal, running nearly northeast and southwest. The highest of them, directly west of Biskra and about 8 kilometers distant, has an altitude of 463 meters. The Bou Rhezal Mountains have a precipitous southern face, but fall away more gradually to the north, where the slope joins a wide and undulating plain. The latter extends to the base of the main Atlas ranges. Southwest of the oasis, and about 2 kilometers distant, a range of rocky hills extends for a distance of about 6 kilometers, or until they join the mountain range of the Saharan Atlas. These hillsare called Ed Delouatt. To the south of Biskra, as has already been stated, there extends a vast plain, the reg, which dips gently to the south and drains into the Chott Melrirh, 50 kilometers or more distant. The situation of Biskra relative to the mountains on the one side and the desert on the other, together with its altitude, governs the climate of the place. Except Laghouat no vicinity in the desert proper was seen with so great an amount of precipitation (about 200 mm.) as Biskra, which is indicated by the relatively rich flora. One who has seen only Biskra can not draw conclusions regarding the vegetation or the conditions of plant life of those portions of the Sahara that lie farther to the south, where much more intense conditions of aridity obtain.
The soils of the vicinity of Biskra are various. That of the low hills between the town and the Djebel Bou Rhezal is only a few centimeters in thickness, but in the washes from the hills it is a meter or more. Here the soil is a sandy loam with an admixture of small stones and pebbles. On the flat ground to the north and to the south of these hills it is of a finer texture, approaching the adobe of the southwestern United States. On the reg to the south of the oasis the soil is also fine, and in some places, if not underlying the plain as a whole, there are strata of gravel at varying depths beneath the surface. This soil in places carries considerable salts. It dries to a powder during the long dry seasons and is easily blown by the winds. Owing to outcropping rock, the south face of the Bou Rhezal Mountains has but scant soil, but that of the northern side resembles the soil of the low hills to the south, which has already been characterized. Near the town are dunes of good size. Especially to the southwest the sand banks against Ed Delouatt hills is in large amount. In the opposite direction, but farther from the oasis, the dunes are fairly extensive.
The Oued Biskra is of great importance to the oasis, since it carries water for several weeks of the year and furnishes water for irrigation. Its channel lies about 3 meters, or possibly more, below the general level of the oasis, and possibly in earlier times may not have been so well defined as at present, spreading its waters over its flood-plain during high water. The oued is made up of several tributary oueds which cross the plain north of Djebel Bou Rhezal, unite where there is a pass in these mountains, and finally debouch on the reg to the south of the town, where the channel becomes continually less well defined. Another oued takes its origin in the Bou Rhezal Mountains in several independent branches which unite at a pass in Ed Delouatt hills and extend for a distance of 15 kilometers or more into the reg. One of the feeders of this oued is from hot springs, Hamman es Salahine, about 8 kilometers northwest of Biskra.
From the preceding sketch of the leading topographical conditions of the vicinity of Biskra it will be seen that the plant habitats are more diverse than at any other place visited. For the present purpose the habitats maybe distinguished as follows: The alluvial desert (reg), which lies on every side of the oasis save the north; low hills adjoining the oasis on the north; Ed Delouatt hills, southwest; the oueds and their flood-plains; the Dj. Bou Rhezal; the hamada (?) lying both to the north and to the south of the latter.
A glance over the list of plants which grow naturally in the vicinity of Biskra[30]shows that many are the same as occur farther south, with many unlike these, having affinities outside of the desert proper; also, the number of plants as well as species is greater at Biskra than farther south. This would be expected from the greater rainfall and more diverse topography.
The flora of the Biskra oasis, according to the authors referred to above, consists of 175 or more species. Of cultivated plants there are 25 or more species, the most conspicuous being the date. The other species are mostly the same as have already been noted at other Algerian oases, except that both the peach and the apricot are wanting at Biskra, although cultivated at Laghouat, Ghardaia, etc. On the outskirts of the town the fairly extensive flood-plain is given over mainly to the cultivation of grain, barley predominating.
The hills and mountains, and the bajada at their base, as well as certain oueds with water relations, exposure, and soils different from those of the oasis, have also a very different flora, which, for the most part, is desertic in character. For purposes of comparison, some of the leading characteristics of the plants growing in a half-dozen localities will be given.
As stated above, to the southwest of Biskra there runs a range of hills, Ed Delouatt, to the south and the north of which may be found interesting plants and plant conditions. On the south side the slope (bajada) descends gradually to the great reg, and near the base of the range, in the vicinity of the place where tradition says a Roman town formerly existed, there is a wide, sandy plain, reaching from the Oued Melah, which pierces Ed Delouatt hills, nearly or quite to the western extension of the oasis. There are no large dunes here, but sand billows about a meter in height and sand hillocks diminutive in size. Between these the plain is fairly level. Over this whole tract there seemed to be only one species (Euphorbia guyoniana), but this was fairly abundant. (Fig.65.) As at Ghardaia and elsewhere, this species grows in small colonies because of its suckering habit, and acts to a small degree as a sand-binder, each group being situated on a sandy hillock. It will be remembered that this species at Ghardaia, as well as at a certain bordj east of that place, had roots which were somewhat fleshy as well as roots which were fibrous, on one and the same plant. I was interested to learn whether similar conditions should obtain at Biskra, since it had been learned, in the case of two species ofOpuntiain the Tucson region,that a different habit followed certain differences in habitat. Several specimens ofEuphorbiawere removed with care from the soil and in no case was it found that the roots were fleshy, but in every instance they were entirely fibrous. It is supposed that the reason for the variation in behavior may possibly be traced to differences in the water relation, as was found to be the case of the variation in cactus roots cited, but no experiments have been made onEuphorbiato prove this.[31]
There is a variety of habitats on the opposite (north) side of the hills, ranging from the large dune (which reaches the summit to the west of the pass through which the oued passes and which must be 100 to 150 meters above the oued) to the flood-plain, with water very near the surface, during rainy season at least. There are rocky slopes, also, and sandy slopes apart from the dunes referred to, as well as a fine clay with sand admixture on the flood-plain. In all of these habitats, except the large dune, the vegetation is actually or relatively abundant. (See fig.67.) Among the rocks are small shrubs or half-shrubs, and also on the plain below. Here one may find, among other species,Echiochilon fruticosum,Helianthemumsp.,Atractylis serratuloides,Gymnocarpos fruticosum,Thymelæa microphylla,Nitraria tridentata, andAcanthyllis tragacanthoides. On the sandy slope was growing a very numerous population of liliaceous forms, mainly, perhaps wholly,Asphodelus fistulosus(fig.72).
It has been noted that the oued which pierces Ed Delouatt is made up of the confluence of all of the small oueds lying between the Bou Rhezal Mountains to the north of Biskra and the hills lying directly north as well as those (Ed Delouatt) to the southwest. The united oueds reach the base of the hills nearly 1 kilometer east of the pass, and turning abruptly follow the base the remainder of the distance. At the place where the oued touches the range the soil is moist nearly to the surface of the plain (reg) and for several meters back from the oued. Here, then, the water relations of the plants are such as to favor, probably most of the year, the growth of mesophytes (?) or even of hydrophytes, but for some reason there is not much vegetation there; whether large species have been removed as for fuel, have been destroyed by animals, or never existed, was not learned. The most interesting plant found was the well-known parasitePhelypæa violacea, which grows, according to Mobius, on a salsolaceous host.[32]Only a fewspecimens were seen at the time of the second visit to Biskra, in March, and they were just appearing above the ground. (See figs.69,70,and71.) When removed from the soil the longest specimen was found to have penetrated over 59 cm.; it was 6.5 cm. in diameter at a point 50 cm. from the tip. The plants were exceedingly heavy, being gorged with sap, and appeared to be able to absorb moisture from the wet soil through the white and delicate epidermis of the entire portion submerged. If this is the case the parasitical relation is of especial interest, as the species is wholly dependent on its host for organized foods, but not for water—a condition opposite that found in such semi-parasites as the mistletoes, which obtain water and unorganized foods in solution from the host, but which, save for the fact of attachment, are otherwise independent of it.
To the northeast of Ed Delouatt hills and north of Biskra, but immediately adjoining the town, is an irregular group of low, rounded hills, mostly very arid, which support a scant vegetation of typical desert plants. These hills, without a name, are eroded to a degree and have shallow washes leading from them in every direction. The soil appears to be thin, except in the washes, where it has accumulated to the depth of a meter or more. On the rounded summits it is rather fine, but in the washes there is much gravel and larger stones. Here one finds the greatest range in exposure, and probably also an accompanying difference in the temperature of the air and soil.
In considering the vegetation of the hills it must be remembered that no plants suitable for forage or large enough for fuel would be left untouched, as the biotic factor is quite as much in evidence in modifying the Biskra flora as that of the other regions visited. As the plants are at present, however, and for whatever definitive causes, there is a considerable variation in numbers and apparently also in kinds. An examination of different parts of the hills by which various exposures as well as other conditions are seen bears out the hypothesis. On the southern slopes, particularly the upper portions, the plant covering is especially poor. The population of an area 16 by 16 meters in the upper south slope included 134 perennials and numerous annuals. Of the species,Haloxylon scopariumwas the most numerous; there were alsoDæmia cordata,Thymelæa microphylla, andFagonia sinaica(figs.78and80). All of these species were small, so that from superficial examination the area appeared fairly barren (fig.73).
When one crosses the hills to the north, so that northern and northwestern exposures are seen, a striking difference is immediately apparent. The northern slopes are much richer in perennials than the opposite facing. The population of an area of the size before taken shows this observation to be correct. On a square 16 by 16 meters, with north exposure, 536 living perennials and numerous annuals were found, withHaloxylon scopariumagain the dominant species. The species second in number wasFerula vesceritensis, which is to be found chiefly on the north exposure (figs.74,75,and77). There were also a few specimens ofAcanthyllis numidica, and many annuals, which, unlike those on the south facing, were not in flower.
On the lower slopes, in the small and open gulches and the lower portions of the washes and their flood-plains, the plants were most numerous and include a variety of forms, withPeganum harmalathe most conspicuous and most numerous.
The roots of several species growing on the hills were studied with the following as perhaps the leading results: The root-system ofHaloxylon scoparium, as has already been seen to be the case with an allied species in the country of the M’Zab, may be said to be a modification of the generalized type. One specimen, partly exposed by erosion in a wash, had a tap-root over 113 cm. in length (fig.76). As this root was 4 mm. in diameter where left, and was 8 mm. in diameter at the crown, it may have penetrated much beyond the point where it was left, provided the soil conditions continued favorable. The main root gave off two large laterals, of which one left the parent root 8 cm. and the other 18 cm. beneath the surface of the ground, but there were also numerous filamentous roots about 2 cm. in length, which were borne in tufts. (See fig.79.) These resembled the deciduous rootlets found on many perennials in the Tucson region and doubtless function quite as they, namely, they are organized at the beginning of the rainy season, operate to increase the water-absorbing area of the plant quickly and greatly, and die as soon as unbearably arid conditions set in.
In the same wash, where the soil was deep with an admixture of small stones and pebbles, the roots of other species were also examined. One of these wasPeganum harmala, whose roots were examined at Ghardaia. The root-systems of the plant in the two regions were similar in being generalized. A main root was found extending downward over 61 cm. and it gave off three good-sized laterals, arising from 15 to 27 cm. beneath the surface. The uppermost lateral took a horizontal course. Figure82gives a fairly good idea of the general character of the root-system of the species.
There were many specimens ofPlantago albicansgrowing in the same habitat asHaloxylonandPeganum. Its root-system was also studied with the following results: The tap-root is strongly developed. In one instance the slender main root was found to go straight down over 71 cm. Numerous laterals were borne between 8 and 20 cm. beneath the surface of the ground. The species has an interesting habit of propagating vegetatively by means of fleshy stolons. An examination of the stolons showed them to be of very unequal age, some having been lately formed, while others had been organized in previous years and were no longer living. Many specimens were examined to learn the probable service of the fleshiness of its stolons, and the conclusion was that this factor probably enables the species to pass over periods of excessive drought.
Running across country are the Bou Rhezal Mountains, of which the north and south faces were examined in one or two places, on each side somewhat in detail. The south face is precipitous and has little vegetation, but there are many plants on the opposite face, especially near the base, where the soil conditions are relatively favorable (fig.84). Among the perennials were found a few specimens ofRhus oxyacanthaandZizyphus lotus, which, although dwarfed, were the largest plants seen away from the oasis, being about 1.5 meters high. In another gulch were found the following species:Acanthyllis tragacanthoides,Dæmia cordata,Ferula vesceritensis,Haloxylon scoparium, and other plants unknown. Toward the upper portions of the gulchesHaloxylonwas seen to be especially abundant, but in the bed of the washes and on their flood-plainsPeganumgrows in large numbers (fig.83). In the most favorable places on the northern slope in March the annuals were the most abundant observed in the vicinity of Biskra, although even there they failed to completely conceal the ground. Where the situation was less favorable as regards soil and water conditions (for example, on the side of the gulches, on the summit of ridges, and the like) there were almost no annuals and those present were relatively small.
In this brief account of some of the most striking features of the Biskra flora it should be noted that several habitats are omitted as not being pertinent to the points in view. Especially, nothing has been said regarding the flora of the salt flats along the Oued el Hamman or that of the dunes, since the influences here are largely edaphic, while the present interest lies mainly in the relation between plants and climate; and the flora of the oasis has been largely neglected for similar reasons.
Not only is there in certain regards a larger number of plants in the Biskra region than had been previously seen in southern Algeria, but there are certain types, mostly new, which point to more favorable conditions of plant life. These are such forms as have a water-storage habit, likeAsphodelus,Ferula,Plantago, andPhelypæa, which, although not wholly absent farther south, appear to be much more numerous near Biskra. The presence of bulbous plants is well known as being one of the floral characteristics of the High Plateau, and it is also known that similar forms are not to be found where the arid conditions are the most severe, which probably accounts for the facts noted. It may be pointed out also that plants at Biskra exhibit exposure preference where soil conditions appear to be parallel. This condition is not so marked farther in the desert as at Ghardaia, for example, where, provided there is sufficient depth of soil, apparently any species may be found on any exposure. In other words, exposure preference implies a certain amount of water as well as sufficient soil.
So far as shown by observations of the root-systems carried out on similar species growing in the Biskra region and at Ghardaia, the essential root characters of plants growing in the two regions are the same. The single exception to this so far noted is that of the roots ofEuphorbia guyoniana,which, contrary to the expectation, are apparently wholly fibrous at Biskra, although they are in part fleshy farther south, where the soil conditions are surely more arid. At Biskra, also, the only plant observed with typical generalized root-systems (Peganum) does not grow where the soil is shallowest, but where it is relatively deep. So far as seen, also, plants growing where the soil is shallow have either a generalized root-system or a root-system approaching this type, even if the branching is only relatively deep.
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-rootare 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.