FROM LAGHOUAT TO GHARDAIA. 3 1 



for example, the betoum (Pistacia atlantica) probably formerly either grew 

 in the oasis of Laghouat or near by, since it was formerly in common 

 use as a fuel. Tristram mentions the betoum, and no other species, as a 

 source of fuel, but inquiry failed to show that at present this species is in 

 general use for this purpose; but the betoum is a familiar sight in the 

 region of the dayas south of Laghouat, where it is the only arboreal 

 species of the region. 



From what has been said regarding the large numbers of domestic ani- 

 mals that gain their entire living on the desert, it follows that of plants 

 growing within the range of the flocks only such as are poisonous, distaste- 

 ful, armed, or otherwise protected, escape partial or complete consumption. 

 Only such species as are too small for fuel or can not be eaten by animals 

 attain to the usual development year by year. In the vicinity of Laghouat 

 the most prominent of the immune plants is the quedad (Acanthyllis tra- 

 gacanthoides), which is well protected by its stout spines; and even this 

 species is not wholly undisturbed. The jujube, also, although not common 

 here, is provided with short spines. It is stated by Massart* that the 

 camels used by him, not having eaten for five days, consumed the branches 

 of the jujube in spite of the spines, and that Anvillea radiata, a composite 

 with acrid juice, was passed by. Perhaps the plants most frequently eaten 

 are of the genus Haloxylon, generally distributed from the High Plateau 

 southward into the region of the M'Zab. That this genus is not extermi- 

 nated is interesting, since it is rarely permitted to come into flower and fruit, 

 and it appears not to reproduce to any extent, if at all, in a vegetative way. 



FROM LAGHOUAT TO GHARDAIA. 



REGION OF THE DAYAS. 



From Laghouat the way lay through a gently rolling country, ever dip- 

 ping to the south and southeast. No mountains relieved the monotonous 

 horizon. "A hard stony desert alternated with rolling sandhills," followed 

 by a "vast level plain dotted with dayas," to quote again from Tristram's 

 narrative. Somewhat to the south of Laghouat depressions are met here 

 and there, separated from each other by the low ridges of the plain, which 

 receive the drainage each of a limited territory. (See figs, i o and 1 1 .) These 

 are dayas, and are in fact oases with an uncertain water-supply but with 

 favorable soil conditions, so that such rain as falls on them, or is conducted 

 to them from higher ground, sinks deeply and creates relatively favorable 

 conditions for plants. In that the day a is the center of a drainage system, 

 and has no visible outlet, it is comparable to the chott or salt-spot, but it 

 is to be distinguished from the chott by the absence of salts in excessive 

 amounts. That salts are not present in the dayas in quantity is probably 

 due to subterranean drainage, the daya being in fact similar to the bolsons 



* Un voyage botanique au Sahara, Bull. Soc. Bot. Belg., 1898. 



