THE BISKRA REGION. 63 



specimens were seen at the time of the second visit to Biskra, in March, 

 and they were just appearing above the ground. (See figs. 69, 70, and 71.) 

 When removed from the soil the longest specimen was found to have 

 penetrated over 59 cm.; it was 6.5 cm. in diameter at a point 50 cm. from 

 the tip. The plants were exceedingly heavy, being gorged with sap, and 

 appeared to be able to absorb moisture from the wet soil through the white 

 and delicate epidermis of the entire portion submerged. If this is the 

 case the parasitical relation is of especial interest, as the species is wholly 

 dependent on its host for organized foods, but not for water — a condition 

 opposite that found in such semi-parasites as the mistletoes, which obtain 

 water and unorganized foods in solution from the host, but which, save 

 for the fact of attachment, are otherwise independent of it. 



To the northeast of Ed Delouatt hills and north of Biskra, but immedi- 

 ately adjoining the town, is an irregular group of low, rounded hills, mostly 

 very arid, which support a scant vegetation of typical desert plants. These 

 hills, without a name, are eroded to a degree and have shallow washes lead- 

 ing from them in every direction. The soil appears to be thin, except in 

 the washes, where it has accumulated to the depth of a meter or more. 

 On the rounded summits it is rather fine, but in the washes there is much 

 gravel and larger stones. Here one finds the greatest range in exposure, 

 and probably also an accompanying difference in the temperature of the 

 air and soil. 



In considering the vegetation of the hills it must be remembered that no 

 plants suitable for forage or large enough for fuel would be left untouched, 

 as the biotic factor is quite as much in evidence in modifying the Biskra 

 flora as that of the other regions visited. As the plants are at present, 

 however, and for whatever definitive causes, there is a considerable varia- 

 tion in numbers and apparently also in kinds. An examination of different 

 parts of the hills by which various exposures as well as other conditions 

 are seen bears out the hypothesis. On the southern slopes, particularly the 

 upper portions, the plant covering is especially poor. The population of an 

 area 16 by 16 meters in the upper south slope included 134 perennials 

 and numerous annuals. Of the species, Haloxylon scoparium was the most 

 numerous; there were also Damia cordata, Thymelcza microphylla, and 

 Fagonia sinaica (figs. 78 and 80). All of these species were small, so that 

 from superficial examination the area appeared fairly barren (fig. 73). 



When one crosses the hills to the north, so that northern and north- 

 western exposures are seen, a striking difference is immediately apparent. 

 The northern slopes are much richer in perennials than the opposite facing. 

 The population of an area of the size before taken shows this observation 

 to be correct. On a square 16 by 16 meters, with north exposure, 536 living 

 perennials and numerous annuals were found, with Haloxylon scoparium 

 again the dominant species. The species second in number was Ferula 

 visciriiensis, which is to be found chiefly on the north exposure (figs. 74, 



