The temperatures of the air vary greatly for the different regions, and usually the variation is to be directly related to the positions of the stations as regards the coast and the altitude. Along the coast, however, the temperatures vary even if the stations are at approximately the same elevation. The mean annual temperature at Oran is 16.9° C. and at La Calle 17.7° C., while a study of the January temperatures shows that a similar relation holds good for the entire south coast of the Mediterranean, between Oran and Alexandria. Algiers furnishes one exception to this statement, in that the mean annual temperature is 18.1° C. On the High Plateau the mean temperature falls to 12.7° C. (Batna) and 13.5° (Setif). No records appear to have been made for the high mountains of eastern Algeria, although the fact that snow may remain in sheltered places as late as the latter part of July[7] would indicate that the mean temperature at 2,000 meters elevation is much lower than any above given. On the Algerian Sahara the mean annual temperature is usually higher than at any point nearer the coast, but even here there is considerable variation, depending, among other factors, on the altitude of the stations and their relation to the Atlas. The mean temperature is given by Engler as 20.5° C. for Biskra, 21.0° C. for Ghardaia, and 22.2° C. for El Golea. As increased temperature means increased evaporation, we have in this one factor a powerful agent making for aridity, and when this is associated with decreasing rainfall, as one leaves the coast region, its influence as a determinative factor in the environment of plants is thus seen to be of great importance. The annual variations in the temperature of the air are very considerable throughout the colony and are especially great on the desert and the High Plateau. At Algiers the variation is 40.7° C., at Setif it is 48.2° C., and at Ghardaia it is 47.9° C. An annual absolute variation of 50° C. is not uncommon on the desert, and Engler cites a range of 57.0° C. at Ghardaia.