When viewed somewhat more closely, one finds other features in which the flora of southern Arizona and of southern Algeria are unlike. Travelers, botanical as well as non-botanical, have described the armed condition of the Saharan plants until the impression is general that such plants as persist from season to season are usually well provided with spines. What may be the proportion of armed to unarmed plants in the northern Sahara I do not know, but to a person familiar with the plants of southern Arizona, where spinose forms are very numerous, the Algerian plants do not appear especially well protected. As this appears to be a general condition, it is scarcely an accident that the spines of the American species of the genus _Zizyphus_, for example, are much better developed than are those of the Algerian representative of the same genus. From the circumstance that grazing by wild as well as by domestic animals is very destructive in Algeria, apparently more so than in Arizona, where the results of grazing are scarcely to be noted, the general facts regarding spininess in plants, as given above, suggest the really small influence such animals play in shaping such a character in desert plants. Finally, it need only be remarked that plants with a water-balance are wanting in southern Algeria, and that they constitute one of the striking features of the flora of the southwestern United States. [Plate 2: Illustration: Fig. 4. Shoot-habit of Acanthyllis tragacanthoides. Laghouat. Illustration: Fig. 5. Shoot-habit of Zollikoferia spinosa. Laghouat. Illustration: Fig. 6. Acanthyllis tragacanthoides on sandy plain. Laghouat. Illustration: Fig. 7. Zollikoferia spinosa in habitat, plain (hamada). Laghouat.] [Plate 3: Illustration: Fig. 8. Detail of north slope of Nomad Mountains where Zollikoferia spinosa is the dominant species. Laghouat. Illustration: Fig. 9. Vegetation of plain (hamada) at Tilrempt. The conspicuous shrub is Haloxylon articulatum. Illustration: Fig. 10. The Daya of Tilrempt from the plain, showing the character of the depression.] [Plate 4: Illustration: Fig. 11. Near view of the Daya of Tilrempt. The fortified stage station, bordj, and a nomad camp are to be seen. The flattened, level, lower surface of the trees is the effect of grazing, mainly by camels. Illustration: Fig. 12. Jujube (Zizyphus) shrubs and betoum (Pistacia) at the Daya of Tilrempt.] [Plate 5: Illustration: Fig. 13. A young specimen of betoum in the midst of a protecting jujube, at the Daya of Tilrempt. Illustration: Fig. 14. Leaf and shoot habit of the jujube (Zizyphus lotus). Daya of Tilrempt. Illustration: Fig. 15. Leaves of the betoum (Pistacia atlantica), from the Daya of Tilrempt.] [Plate 6: Illustration: Fig. 16. South wall of the valley of the Oued M’Zab at Ghardaia.