32 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



of western America. On the plain in the northern portion of the daya region 



the following may be observed : * 



Echinops spinosus. Citrullus colocynthis. Teucrium polium. 



Acanthyllis tragacanthoides. Artemisia herba-alba. Aristida obtusa. 



Thymelaea microphylla. Artemisia campestris. Stipa gigantea. 



Peganum harmala. Haloxylon articulata. Noasa spinosissima. 



Euphorbia guyoniana. Anabasis articulata. Astericus pygmaeus. 



In the dayas one sees Zilla macroptera, Peganum harmala, but chiefly 

 Zizyphus lotus and the betoum {Pistacia atlantica), which is perhaps the 

 only species of tree outside of the oasis in this portion of the Sahara, 

 Massart mentions not seeing any tree away from oases from the time he 

 left Biskra until he reached the region north of Berrian, the northern por- 

 tion of the Chebka. As the betoum is so conspicuous among the Saharan 

 plants, and also from the very remarkable relationship existing between it 

 and the jujube, the species is of very great interest. The relationship will 

 be described under an account of one of the largest dayas of the region, 

 that of Tilrempt. 



DAYA OF TILREMPT. 



The daya of Tilrempt is one of the largest (about 103 hectares) and is 

 the most southerly of all. It lies near the southern margin of the daya 

 region, and is surrounded by a gently undulating plain (hamada), whose 

 stirface is strewn with stones and pebbles, with apparently an luiderlying 

 strattmi of impervious material, since such is to be seen wherever erosions 

 have laid it bare. The floor of the daya is free from stones, being composed 

 of soils of a fine texture which have been washed or blown by the wind 

 from the surrounding higher country, and is apparently not underlaid by a 

 hardpan. Judging from the depth of the two wells at the daya (which 

 were dug, not bored), the deeper of which is said to be 95 meters, there is 

 an abundance of earth for the roots of the plants. Besides the wells there 

 is a cistern, stmk below the general level of the daya floor, which receives 

 and stores up flood- water; it is rarely filled, but occasionally contains con- 

 siderable water, possibly up to one-fourth of its capacity; its filling is very 

 uncertain, depending on the rare and scant rains. According to Massart, 

 no rain had fallen dvuing the two years previous to his visit, and the cis- 

 tern was empty when we were there. These observations are given to show 

 under what intensely arid conditions such a tree as the betoum (mature 

 specimens of which carry an immense evaporating area) can become of 

 large size, giving but slight evidence of a severe struggle against such ad- 

 verse conditions. The altitude of the daya is about 600 meters. 



As one crosses the plains in the vicinity of Tilrempt he notes the scanti- 

 ness of the vegetation. (See fig. 9.) Here and there in the depressions are a 

 few betoums, often only one specimen, but usually more, and in the erosion 

 channels leading to the dayas is a sparse population of low, gray shrubs. 



* Massart, loc. cit. 



