80 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



COMPARISON OF SOME GENERAL FEATURES OF THE VEGETATION OF SOUTHERN 

 ALGERIA AND OF SOUTHERN ARIZONA. 



When we compare the most striking characteristics of the vegetation of 

 the Algerian Sahara with that of the Tucson region where the Desert Lab- 

 oratory is situated, we find some interesting differences, which may be 

 summed up in the terms "desert" and "semi-desert," as applied to the 

 two widely separated regions. What is meant by these terms will be 

 apparent from the following short characterization : 



Passing into the Sahara from the Saharan Atlas, over the route which 

 I followed, one encounters a great variety of topography, of which the 

 most extensive may for the moment be classed as plains. The plains are 

 divided into three well-marked regions, that of the dayas, the Chebka, and 

 the Gantara (hamada). The topography is further diversified by oueds 

 and their flood-plains and by low, flat-topped mountains. On the northern 

 portion of the plains one encounters a sparse population of low perennials, 

 and as Ghardaia is approached the plains vegetation becomes continuously 

 poorer until at Ghardaia there appears to be none. On the hamada be- 

 tween Ghardaia and Ouargla areas are to be crossed, several kilometers in 

 width, where perennials are wholly lacking. The decreasing plant popula- 

 tion of the plains, until it entirely disappears, is entirely due to the increase 

 in aridity as one goes from the mountains to and across the Ghardaia- 

 Ouargla country. The low mountains are almost entirely barren. The 

 flood-plains of the oueds, however, support a surprisingly luxuriant popu- 

 lation of perennials. 



Should we contrast the topography and vegetation of the Algerian Sahara 

 with that of southern Arizona we would find little that is similar and much 

 that is different. The wide-stretching plains (bajada) of southern Arizona 

 are well covered with perennials of good size. The water-courses are fringed 

 with trees, and often an open forest is to be found on the flood-plains. The 

 low mountains have a fairly dense plant population, partly of trees, and the 

 lower mountain slopes are often covered with a mixed flora of shrubs and 

 trees. It may be said that there is probably no large area in southern 

 Arizona, where the soil conditions are favorable for plants, where the water 

 conditions are too meager to support a perennial flora of some sort. The 

 greater aridity of the northern portion of the Sahara is evident, therefore, 

 from the great contrast in its flora. 



In crossing the plains of southern Algeria one is likely to call a region 

 barren when close inspection will show that this is not the condition. In 

 fact, it was found that areas on the plain, 16 by 16 meters in extent, car- 

 ried as many as 330 or more perennials, although a casual glance did not 

 reveal the presence of any conspicuous vegetation.* Both of these condi- 



*It should be understood that such densely populated areas are separated by wide 

 stretches where are few or no plants. 



