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_ (Laghouat), 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_, _Halocnemon strobilaceum_, _Salsola_ sp., _Limoniastrum guyonianum_, and others. The flora of the reg, so far as my observations show the conditions obtaining, 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 individuals. 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_, _Thymelæa_, _Echinops_, _Henophyton_, and _Haloxylon_. The last named is possibly the most widely distributed, occurring in other habitats as well. ROOT-CHARACTERS AND SPECIES DISTRIBUTION IN SOUTHERN ALGERIA.