THE BISKRA REGION. 65 



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 follow- 

 ing species: Acanthyllis tragacanthoides, Dcsmia cordata, Fenda vesciritensis , 

 Haloxylon scoparium, and other plants unknown. Toward the upper por- 

 tions of the gulches Haloxylon was seen to be especially abundant, but in 

 the bed of the washes and on their flood-plains Pegamim grows in large 

 numbers (fig. S^). 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 regard- 

 ing 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 in- 

 terest 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, Planiago, and Phelypcea, 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 charac- 

 teristics of the High Plateau, and it is also known that similar fonns are 

 not to be found where the arid conditions are the most severe, which prob- 

 ably 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 Ghar- 

 daia, for example, where, provided there is siiSicient depth of soil, appar- 

 ently any species may be found on any exposure. In other words, exposure 

 preference implies a certain amount of water as well as suflficient 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, 



