The physical environment of the plants of southern Algeria is, in a few broad features, similar to that of the southwestern portion of the United States. These regions have about the same latitude, both are separated from a large sea by mountains, and the range in altitude is similar. There are other features, however, particularly as regards the amount and the distribution of the precipitation, in which the two widely separated regions are very unlike, and a correlated difference in the habit and composition of the floras of the two regions is apparent. The Algerian climate as a whole is a mild, temperate one, but very diverse. The latitude and topography taken in connection with the presence of large seas to the north and west, and a large continent leading away to the south are its chief determinants. The climate, therefore, of the northern portion is coastal; that of the southern portion continental. Probably the most important of the secondary factors which modify the climate of Algeria is its highly varied topography. An important mountain system, the Atlas, made up of many more or less detached groups and secondary systems, a plateau or steppe lying 3,000 feet more or less above the sea, and finally the northern edge of the Sahara, which has a very diverse topography of its own—such is the surface of Algeria. Algeria is divided into three climatic provinces corresponding to the leading topographical differences: the Tell, including the littoral, or portion between the maritime Atlas and the Mediterranean; the High Plateau, or steppe, which lies between the Tellian Atlas and the Saharan Atlas; and the desert. These provinces have marked individual differences in rainfall, temperature, and other climatic features. In the Tell and the High Plateau the winds from the sea deposit most of their moisture. Along the coast as much as 700 mm. of rain is recorded, while in other parts of the Tell it is about 570 mm. On the High Plateau the yearly precipitation sinks to 310 mm. In the desert south of the Saharan Atlas, however, where the altitude is lower and the temperature greater than in either of the other provinces, the yearly rainfall is 200 mm. and less. In some years, in fact, no precipitation whatever is reported in the desert.