The Himalaya system intersects various climatic belts along the approximately 1900 miles of its length. To the south-east, in Assam and Bengal as far as Bhutan, the lowlands are covered by tropical rain-forests, further west, in Sikkim and Nepal, by humid monsoon forests (Sal-forest), from west Nepal to Sutlej by drier, semihumid monsoon forests, in Pandjab by subtropical, semia rid shrub steppes. In the extreme north-west, in Ladak, Karakorum, Gilgit and Chusistan, the alpine heights rise from the floors of desert-like valleys. Thus, the arid belt of Central Asia extends over the Himalayan rangein the north-west into the southern forelands, whereas in the extreme south-east, in the Tsangpo region, the humid forests cross the mountain region to penetrate south Tibet. In the longest, central section from the Chenab Valley as far as Subansiri, the main chain of the range forms a fairly sharp dividing-line between the humid region to the south which lies under the influence of the summer monsoon and the northern flank in Tibet. In accordance with thesehorizontal climatic zones, the entire Himalayasystem falls into the following natural divi sions: the Tsangpo Himalaya, the Assam Himalaya, the Sikkim-east Nepal Himalaya, west Nepal-Garhwal Himalaya, Pandjab Himalaya, Indus Himalaya, and the Tibetan Himalaya, to which may be added the so-called Trans-Himalaya situated north of the great trough formed by the Indus and the Tsangpo valleys (cf. map).