294 



D. T. MacDOUGAL 



though taken by Schimper to be very important in some parts of 

 northern Africa. 



The Cahuilla basin in southern Cahfornia shows an annual 

 evaporation of about 116 inches, but the measured evaporation 

 from locahties near our point of departure is estimated at 150 

 inches yearly and is probably greater to the westward. It is 

 evident therefore that it is the ground water that is to be taken 

 into consideration in a study of the vegetation. The oases are 

 mostly in the lowermost parts of great basins or hollows in the 

 sandstone and limestone where seeps, flowing wells and springs 

 occur. Such places are fully occupied and cultivated by man, 

 and in addition to the economic plants grown there are to be 

 found the usual number of weeds and introduced plants, which 

 have only a minor interest to the student of arid regions. 



At many other places however, the ground w^ter comes to the 

 surface layers in such diffusion that it will not permit of agricul- 

 ture. Here and in sand dunes and crevices of rocks are the habi- 

 tats in which the undisturbed native plants are to be sought. 

 One hundred and seventy-two species of plants were collected in 

 the.f ew days devoted to the eastern desert with its diversified topog- 

 raphj^, varied habitat conditions and definitely recurring rainfall, 

 a state of affairs duplicated in the arid regions of southwestern 

 America. But thirteen species were found in the month in which 

 we were wandering over the Libyan desert, which were not affected 

 by irrigation or agricultural operations, and one of these was a 

 parasite living on the roots of one or more of the other twelve. A 

 brief characterization of this notable dozen will afford the basis of 

 some profitable considerations: 



Alhagi Maurorum (Leguminosae) or aqul is an erect, much 

 branched spinose shrub reaching a height of three to five feet, 

 which is widespread in sandy and gravelly places over northern 

 Africa. It forms a very valuable food for camels, and these ani- 

 mals do not seem to be deterred from grazing on it by the sharp 

 and pungently odorous spines, which are fairly rigid and about 

 an inch in length. The plant is said to yield a manna which is 

 used locally and exported from some places in Arabia (see figure 

 of leaves and branches of Alhagi, Schimper's Plant Geography, 

 p. 613, 1904). 



