28 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



be recalled that the species has a top consisting of several shoots, spring- 

 ing from the root-crown at the level of the ground, and a root-system in 

 which the laterals and the main root are alike well developed. In order 

 that a plant, already established, can maintain this general relation during 

 the heaping up of the sand, the main root must grow at the crown as fast 

 as the sand encroaches. This is exactly what happens, so that, when the 

 dune has passed on, the shoot of the species is elevated for a space equaling 

 the depth of the sand, which is half a meter or more. From the probability 

 that Acanihyllis is of very slow growth and that the dunes are low, it follows 

 that the rate of dune movement must be slow. 



THE MOUNTAINS ABOUT LAGHOUAT. 



The southern spurs of the Saharan Atlas lie to the north of Laghouat, 

 within 3 miles of the oasis. This range (the Nomad Mountains) is not 

 over 266 meters in height. Higher mountains extend both to the west and 

 to the east, but only the Nomads are visited. There are also two high, 

 rocky hills on opposite edges of the oasis, the town lying between, and it 

 is perhaps because of their presence that the water is forced to the surface, 

 or near the surface, and the oasis is formed. The hill to the west of the 

 town is called the Rocher des Chiens. On the south side of the Nomad 

 Mountains is a long outcropping of gypsum; the south face of these moun- 

 tains is also abrupt, carrying little soil ; but on the northern face the slope 

 is gradual and there is much soil, although rocks are abundant. 



Two localities only in the Nomad Mountains were seen. One was in a 

 pass through which caravans travel en route to Laghouat, from the High 

 Plateau or the more distant Tell; the second to the east of the pass 

 is much less often visited by flocks. The physical plant conditions appeared 

 to be the same in both locations. As one approaches the mountains from 

 the south the number of plants grows less until on the south face they 

 quite disappear, but on attaining the crest they quickly increase and form 

 a noticeable element in the landscape. In fact, the flora of the northern 

 gentle slope, where the soil conditions are relatively favorable, is much 

 richer than on the plain or the dunes. Had the shrubs been of good size 

 they would have been very conspicuous.* 



A certain area on the north slope of the Nomad Mountains, not far 

 from the pass through which the Oued Mzi goes, was selected for making 



* The most striking change in the general character of the vegetation which the 

 traveler notices as he goes from the less arid to the more arid portiohpf southern Algeria 

 is its decrease in amount. This occurs through dwarfing effects of whatever cause and 

 through a decrease in the number of individuals. Within certain limits the results 

 observed are to be attributed mainly to the first of these, since there is often a sur- 

 prisingly large number of perennials on any given area. But in other and more intensely 

 arid regions (as portions of the Arabian-Egyptian desert, and indeed a limited area on 

 the hamada between Ghardaia and Ouargla) plants are wholly wanting. Whether such 

 is generally the case on the reg or the hamada farther south in southern Algeria is not 

 known. 



