[Footnote 31: Briefly the case is as follows (see The root systems of desert plants, _loc. cit._): _Opuntia arbuscula_ growing near Tucson develops fleshy roots, but what is probably the same species growing about 100 miles distant has fibrous roots. Also, seedling opuntias have fleshy roots. _Opuntia vivipara_, which occurs naturally in the bottom of an arroyo (oued), may or may not have fleshy roots. By preliminary series of experiments it was learned that all opuntias tested which had an abundant water-supply developed fleshy roots, and it is assumed from this that the differences in this character as observed in nature had also such a physiological basis.] [Footnote 32: Eine botanische Exkursion nach Algier and Tunis, Bericht der Senckenbergischen Naturforschenden Gesellschaft in Frankfort a. M., p. 76, 1910.] [Footnote 33: Plants with subterranean water-storage organs—bulbous plants—are said to be a feature of the High Plateau.] [Footnote 34: Through Timbuctu and across the Great Sahara, 1912, p. 266.] [Footnote 35: Soil samples were taken from the plain about 3 kilometers north of Ghardaia from an area where the vegetation is relatively good (see p. 40). The surface of the soil at the place is slightly depressed. Soil samples from the depressed area and samples from portions of the plain adjacent to it were settled under water, with the result that the proportion of fine soil was found to be less in the depressed area. Samples of the soil from the lower area were placed in air-tight cans and the moisture content determined subsequently. The soil was found to contain 0.8 per cent water. Through the kindness of the Bureau of Soils, U. S. Department of Agriculture, the critical moisture-content of the same soil was determined, which was 5 per cent. The critical moisture-content of mesa soil, taken from the creosote-bush slope at the foot of Tumamoc Hill, Desert Laboratory, as determined by the Bureau of Soils, is 10.5 per cent, which forms an interesting comparison of nearly similar situations.] [Footnote 36: 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.] [Footnote 37: Die Wasserversorgung und die osmotischen Druckverhältnisse der Wüstenpflanzen. Zeitschr. f. Botanik, 4, 1911.] [Footnote 38: Hayward, _loc. cit._, p. 320, says that at In Salah camels are driven 200 kilometers before finding suitable grazing-grounds.] [Footnote 39: It should be understood that such densely populated areas are separated by wide stretches where are few or no plants.] Transcriber's note: pg 15 Added period after: vegetation of the country pg 17 Changed: _Quercus bollota_ to: _ballota_ pg 24 Changed: _Rhus oxycantha_ to: _oxyacantha_