76 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



if the sand is only a few centimeters in thickness. This acts as an effectual 

 mulch, increasing the retentive capacity of the soil, and very strikingly 

 changes the character of the vegetation. The final habitat to be mentioned 

 is the salt spot, or chott, where gypsum constitutes an important salt. 



As regards their relative importance the habitats in southern Algeria can 

 probably be grouped in the following sequence, a relation which very pos- 

 sibly holds good for the Sahara taken as a whole: Hamada, dune, oued, 

 flood-plain, reg, daya.* 



The habitats are unlike as regards the relation to the rainfall and its 

 effects. This is in part due to the differences in soils, or their depth, and 

 in part to topographical differences. By the latter the low-lying areas 

 receive a relatively large amount of water; and since their soil is relatively 

 deep the water is retained longer than on the hamada, for instance, where 

 the soil is shallow. 



The habitat preferences of the plants of southern Algeria are marked, 

 as would be expected from the striking differences in the habitats. On 

 the dunes, for example, we find drinn (Aristida pungens) as possibly the 

 most commonly occurring and the most widely distributed sand-plant. One 

 finds on sandy areas also Tamarix sp., Euphorbia guyoniana, Ephedra sp., 

 Retama retam, Limoniastrum guyonianum (zaita), and other forms in smaller 

 numbers. On the oued banks there are Tamarix, Nerium oleander (Lag- 

 houat), and, near the oases, date palm and other introduced plants. On 

 the flood-plains will be found a large number of species, among the most 

 typical of which are Peganum harmala, Retama retam, Ephedra, Genista 

 sp., and Haloxylon sp. The typical plants of the dayas are Pistacia atlantica 

 and Zizyphus lotus, the latter occurring on flood-plains and the reg as well. 

 On the chotts we find mainly such halophytes as Anabasis articulata, Hal- 

 ocnemon strobilaceum, Salsola sp., Limoniastrum guyonianum, and others. 

 The flora of the reg, so far as my observations show the conditions obtain- 

 ing, is essentially like that of the flood-plains, which would be expected 

 from the relation of the two habitats. South of Biskra, however, where 

 the reg is probably not of fluvial origin, one finds a forest of Zizyphus lotus, 

 and much Ephedra sp. among the most striking forms. Finally, the flora 

 of the hamada, which has a peculiar stamp, can be briefly characterized. 



On the hamada are to be found the fewest species and the smallest indi- 

 viduals. Probably most perennials of the hamada are under 30 cm. in 

 height. Among the species characteristic of the hamada are Artemisia, 

 Teucrium, Deverra, Centaurea, Acanthyllis, Thymelcea, Echinops, Henophy- 

 ton, and Haloxylon. The last named is possibly the most widely distributed, 

 occurring in other habitats as well. 



* The mountains have been disregarded, since in southern Algeria they are nearly- 

 barren. The only exception to this that I saw was that of the crustaceous lichens in small 

 numbers at Ghardaia. In the central Sahara, however, where the mountains are of 

 great elevation, the mountain climate brings about favorable conditions for plant life. 



