62 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



that a different habit followed certain differences in habitat. Several speci- 

 mens of Euphorbia were removed with care from the soil and in no case was 

 it found that the roots were fleshy, but in every instance they were entirely 

 fibrous. It is supposed that the reason for the variation in behavior may 

 possibly be traced to differences in the water relation, as was found to be 

 the case of the variation in cactus roots cited, but no experiments have 

 been made on Euphorbia to prove this.* 



There is a variety of habitats on the opposite (north) side of the hills, 

 ranging from the large dune (which reaches the summit to the west of the 

 pass through which the oued passes and which must be ioo to 150 meters 

 above the oued) to the flood-plain, with water very near the surface, during 

 rainy season at least. There are rocky slopes, also, and sandy slopes apart 

 from the dunes referred to, as well as a fine clay with sand admixture on 

 the flood-plain. In all of these habitats, except the large dune, the vege- 

 tation is actually or relatively abundant. (See fig. 67 .) Among the rocks are 

 small shrubs or half-shrubs, and also on the plain below. Here one may find, 

 among other species, Echiochilon fruticosum, Helianthemum sp., Atractylis 

 serratuloides, Gymnocarpos fruticosum, Thymelcea microphylla, Nitraria tri- 

 dentata, and Acanthyllis tragacanthoides. On the sandy slope was growing a 

 very numerous population of liliaceous forms, mainly, perhaps wholly, 

 Asphodelus fistulosus (fig. 72). 



It has been noted that the oued which pierces Ed Delouatt is made up 

 of the confluence of all of the small oueds lying between the Bou Rhezal 

 Mountains to the north of Biskra and the hills lying directly north as well 

 as those (Ed Delouatt) to the southwest. The united oueds reach the base 

 of the hills nearly 1 kilometer east of the pass, and turning abruptly follow 

 the base the remainder of the distance. At the place where the oued touches 

 the range the soil is moist nearly to the surface of the plain (reg) and for 

 several meters back from the oued. Here, then, the water relations of the 

 plants are such as to favor, probably most of the year, the growth of meso- 

 phytes ( ?) or even of hydrophytes, but for some reason there is not much 

 vegetation there ; whether large species have been removed as for fuel, have 

 been destroyed by animals, or never existed, was not learned. The most 

 interesting plant found was the well-known parasite Phelypcea violacea, 

 which grows, according to Mobius, on a salsolaceous host.f Only a few 



* 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. 



t Eine botanische Exkursion nach Algier and Tunis, Bericht der Senckenbergischen 

 Naturforschenden Gesellschaft in Frankfort a. M., p. 76, 1910. 



