DEPARTMENT OF BOTANICAL RESEARCH. 57 



Biskra a wide monotonous plain, probably "reg," separates the great Chott 

 Melghir from Biskra and the southeastern extension of the Atlas. 



Gypsum occurs in large quantity in the immediate vicinity of Laghouat. 

 The plains throughout southern Algeria are rather shallowly covered with a 

 dark adobe-like soil with an admixture of pebbles and large stones. In the 

 slight drainage depressions the proportion of fine soil is less than on the 

 higher areas. Comparatively little sand was encountered, although at a 

 large chott west of Ouargla dunes 150 meters in height were seen. On the 

 flood-plains and in the floors of the dayas the soil is of a depth sufficient for 

 extensive root development, but on the plains this is not the case. A deter- 

 mination of the water-content of soil taken at a depth of 10 to 15 cm. on 

 the plain near Ghardaia showed 0.8 per cent water. Through the kindness 

 of the Bureau of Soils, U. S. Department of Agriculture, the critical mois- 

 ture-content of the same soil was determined to be about 5 per cent. As a 

 comparison it may be stated that the critical moisture-content of mesa soil 

 taken from the domain of the Desert Laboratory was 10.5 per cent. 



The vegetation of southern Algeria reflects the extreme aridity of the 

 region. Plants with a water-balance are almost wanting, and there are no 

 trees outside of the oases and dayas. The flood-plains of the oueds support 

 a comparatively fairly abundant vegetation, which is characterized by great 

 reduction of leaves. Plants are rare on the plains, and certain portions of 

 the plains (gantara) between Ghardaia and Ouargla are quite barren. 



Although the plains appear wholly void of vegetation, careful examination 

 often reveals from 300 to 400 on an area 16 by 15 meters, a paradoxical 

 condition due to the dwarfing effects of the arid climate. 



The vegetation is subject to the ravages of herbivorous animals, wild as 

 well as domestic, to a degree possibly not equaled in any other region, al- 

 though probably no species have become extinct from this cause. Many 

 plants are left untouched because they are distasteful, poisonous, or well 

 armed, and one example of total dependence of an unarmed species on a 

 species which was armed was observed. 



The delicate response of perennials to slight environmental changes was 

 seen at Ghardaia, where many perennials were forming new shoots at a time 

 when relatively better moisture conditions were brought about by a lower 

 temperature; no rain had fallen for over 12 months. 



The root-systems of the plants of southern Algeria are not so diverse as 

 in the Arizona forms. There are but two well-marked types, namely, those 

 with a well-developed tap-root and those with roots that are generalized. 

 The former is the type of plants which are confined to the flood-plains of 

 the oueds, or where the soil is deep. Plants with roots which may extend 

 far horizontally and which penetrate deeply are found on the plain and also 

 in the oueds. The superficially placed type of root-system, which is typical 

 of fleshy plants, especially of the cacti, is not represented. Should the cacti, 

 for example, have a deeply penetrating type of root-system, their range into 

 arid regions would probably be greatly extended. 



