Illustration: Fig. 65. Biskra. Habitat of Euphorbia guyoniana, looking toward the Djebel Maouya, with the Chaine de Sfa in the background.] [Plate 29: Illustration: Fig. 66. Characteristic vegetation on the north slope of the Djebel Bou Rhezal, Biskra. Haloxylon scoparium is a prominent species. Illustration: Fig. 67. North base of Ed Delouatt hills, west of Biskra, showing the low-facing dunes. To the right is a glimpse of an oued which pierces the hills in the middle distance.] [Plate 30: Illustration: Fig. 68. Flood-plain of the Oued Hamman es Salahin, Biskra. Halophytes of various species occupy the foreground. Illustration: Fig. 69. Habitat of Phelypæa violacea shown in figure 70.] [Plate 31: Illustration: Fig. 70. Young shoots of Phelypæa violacea, at north base of Ed Delouatt hills, Biskra. Illustration: Fig. 71. Habit of young specimens of Phelypæa. Except for about 15cm., plants were buried by sand. Biskra. Illustration: Fig. 72. Asphodelus sp. at north base of Ed Delouatt hills, Biskra. Photographed in March.] [Plate 32: Illustration: Fig. 73. Detail of square No. 1, on low hills north of Biskra. Illustration: Fig. 74. Vegetation of north slope of the low hills which are north of Biskra. Ferula vesceritensis is the perennial appearing in the figure.] [Plate 33: Illustration: Fig. 75. Detail of square No. 2, neat area shown in fig. 74. Ferula and Haloxylon are the leading species. Illustration: Fig. 76. Root-habit of Haloxylon scoparium, from a wash near Biskra. Illustration: Fig. 77. Young shoots and mature roots of Ferula vesceritensis. North of Biskra.] [Plate 34: Illustration: Fig. 78. Root-habit of Fagonia growing on edge of a wash. Biskra. Illustration: Fig. 79. Large lateral root of Haloxylon, with numerous deciduous rootlets, no longer functional. Illustration: Fig. 80. Shoot-habit, taken from above, of Fagonia from the flood-plain of a small oued near Biskra.] [Plate 35: Illustration: Fig. 81. Spring annuals, March 17th, on north slope of the Djebel Bou Rhezal, Biskra. Illustration: Fig. 82. Root and shoot habit of Peganum harmala, Biskra. The main root is especially well developed although the species has a generalized root-system. (See text.)] [Plate 36: Illustration: Fig. 83. Annuals growing with Peganum, near Biskra. Illustration: Fig. 84. General view of the north face of the Djebel Bou Rhezal, Biskra. Apparently barren, plants are rather numerous in the rock crevices and small washes of the mountains.] FOOTNOTES: [Footnote 1: Kearney and Means, Agricultural explorations in Algeria, Bul. No. 80, Bur. Plant Ind., U. S. Dept. Agric., 1905.] [Footnote 2: A. Engler, Die Vegetation der Erde IX. Die Pflanzenwelt Afrikas. 1 Bd., 1910, page 902.] [Footnote 3: The seasonal distribution of rain (by percentages) is somewhat different from that given by Engler, which is given in the accompanying table.