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, with Haloxylon scoparium again the dominant species. The species second in number was Ferula vesceritensis, which is to be found chiefly on the north exposure (figs. [74,] [75,] and [77]). There were also a few specimens of Acanthyllis 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, with Peganum harmala the 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 of Haloxylon 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 was Peganum 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. Figure [82] gives a fairly good idea of the general character of the root-system of the species.
There were many specimens of Plantago albicans growing in the same habitat as Haloxylon and Peganum. 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 of Rhus oxyacantha and Zizyphus 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 gulches Haloxylon was seen to be especially abundant, but in the bed of the washes and on their flood-plains Peganum grows 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, like Asphodelus, Ferula, Plantago, and Phelypæa, 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 of Euphorbia 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.