40 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



ilar, a larger number of plants were found, including, among other species, 

 Aristida sp., Centaur ea pubescens, Deverra scoparia, Fagonia bruguieri, Pega- 

 num harmala, and Teucrium pseudo-ckamcepitys. A census of plants was 

 taken, where the individuals were most numerous, with the following result : 

 On an area 16 by 16 meters there were 330 living specimens. The three 

 dominant species were Aristida sp., Deverra scoparia, and Helianthemum 

 sessiliflorum. 



On the level portions of the plain one sees almost no perennials and only 

 the dried remains of annuals, although here and there may be found an 

 isolated specimen of Peganum harmala or even of Citrullus colocynthis, the 

 latter strangely out of its proper surroundings. In one place, also, a small 

 date palm was found surviving the extremely arid conditions. But on the 

 hamada it is only in relatively favorable situations that plants are to be 

 found. One such was given above and another was found on the open 

 plain, but near the base of a low mountain, where there was a slight de- 

 pression and where some water was received from the mountain run-off. 

 The area alluded to is 10 kilometers north of the north valley wall; the 

 south base of the nearest mountain is 100 meters to the north of the area. 

 The ground inclines gently to the south, and rises slightly both to the east 

 and to the west. The surface is thickly strewn with stones and the soil 

 is clay mixed with sand, the latter predominating in the center of the de- 

 pression, where there is also a relatively large proportion of small pebbles. 

 The area studied, 16 meters square, was so selected that the depression 

 crossed the middle portion, leaving the two sides as representing the larger 

 part of the plain. (Fig. 21.) In a country where the conditions of plant life 

 are so severe it is of interest to observe how slight advantages of whatever 

 kind, such as in the square under consideration, work for the betterment of 

 the vegetation. The dominating species was a bunch-grass, probably Aris- 

 tida sp., but there was also present Haloxylon sp. (eaten to the surface of 

 the ground by the passing flocks) with other undetermined forms. On the 

 area given 414 living perennials were found with numerous dead annuals. 

 All of the plants were growing in the depression, there being, in fact, none 

 on the adjacent but somewhat higher parts of the hamada. The character 

 of the soil of this square and a discussion of the root characters of plants 

 growing in it are given in another place. 



How far the paucity of plants on the plain is owing to the arid conditions 

 obtaining there, apparently a sufficient explanation in itself, and how far 

 to the fact that herbivorous animals, wild as well as domestic, for several 

 centuries have been gaining their food from the plain, can not at present 

 be well told. Observations given below, however, indicate that if areas 

 are protected against the depredations of animals, the plants are noticeably 

 more numerous and of a larger size than when there is no protection. This 

 conclusion applies to portions of the plain as well as to the other habitats 

 under discussion, 



