56 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Botanical Features of the Algerian Sahara, by W. A. Cannon. 



The observations of Dr. Cannon on the plants and conditions of plant 

 life in southern Algeria, which were made in 1910-11, have been compiled, 

 and the leading results may be summarized as follows : 



Algeria is divided into three climatic provinces which correspond to the 

 main topographical divisions. These are the Tell, the High Plateau, and 

 the desert. The climate of the former is coastal and that of the latter conti- 

 nental, while the High Plateau, a semi-arid region, has a climate partaking 

 of the characteristics of both of the other regions. The rainfall of Algeria 

 decreases as the distance from the coast increases, until from a maximum of 

 about I meter on the littoral it becomes 100 mm. and less, and on the desert 

 it frequently happens that no precipitation whatever occurs for many months. 

 The relative humidity with a mean of 85 per cent at Fort National, near the 

 coast, sinks to a mean of 42.6 per cent at In Salah, 600 miles from the Medi- 

 terranean. At Algiers the evaporation is 1,654 mm. and at Ghardaia, 400 

 miles from the coast, it is 5,309 mm. At the latter place the average evap- 

 oration is 46.5 times the average precipitation. The great aridity at Ghar- 

 daia is the more apparent when it is considered that the evaporation-rainfall 

 ratio for Yuma, Arizona, is 35.2. 



The annual and diurnal range in temperature on the desert is great, al- 

 though the records consulted possibly do not give the maximum range. The 

 greatest annual range given for southern Algeria is 57.0° C. (Ghardaia). A 

 diurnal range of 24.7° C. was observed between 3 p. m. and 6 a. m. in the 

 open desert in November. The maximum yearly temperature given in the 

 records consulted was 52° C. at Ouargla. Freezing temperatures are ex- 

 perienced probably every winter on the desert. 



The portion of the Algerian Sahara visited has great topographical di- 

 versity. Beginning at Laghouat, at the southern base of the Saharan Atlas, 

 the main regions visited were as follows : The region of the Daya is rolling 

 and contains many small basins (dayas without surface drainage). Here 

 the surface of the ground is covered with pebbles and small rocks, and the 

 soil is shallow except in the "dayas" themselves, where it is very deep. A 

 hardpan (caliche) extends nearly to the surface on the plain of the daya 

 region. The plains are covered with a sparse vegetation, including some 

 trees. South of the region of the daya is that of the Chebka, where the 

 low, flat-topped mountains cross the country in a confused manner, but 

 where watercourses ("oueds") occur. As the southern portion of the 

 Chekba is reached, the plains between the mountains broaden rapidly until 

 they constitute the main topographical feature. Ghardaia is on the edge of 

 the Chebka, and the important Oued M'Zab separated this region from the 

 Gantara, which is the "hamada," or stony desert. Towards the eastern side 

 of the Gantara are extensive "chotts," or saline areas. At Ouargla the 

 flood-plain of the Oued Igharghar, over 750 miles in length, is reached ; this 

 is the alluvial or "reg" desert. Between Ouargla and Touggourt and to the 

 north of Touggourt there is much sand (the "areg" desert). South of 



