214 FLORA INDICA. 



arid, but not quit;e Tibetan, Pinus Gerardiana being very 

 common. Its flora is, however, scarcely known. 



Tibet. 



Tibet includes the mountain valleys of the Indus and Yaru 

 (or Brahmaputra), together with the whole axis of the Hima- 

 laya and the heads of many of the valleys which descend on 

 the Indian side, and which are situated beyond the mass of 

 snow throughout a great extent of the chain. Beyond the 

 Indus and Yaru are the southern slopes of the Kouenlun, 

 which according to our definition do not form a part of the 

 Himalaya, but of Tibet. Politically its boundary is an irre- 

 gular one, accidental circumstances having regulated the line 

 of separation between the Indian and Tibetan states. Botani- 

 cally, the boundary of Tibet is best drawn at the place where 

 the climate becomes too arid to support such a vegetation 

 as flourishes at equal elevations on the Indian watershed, 

 and especially where there is a total absence of forests below 

 13,000 feet. The flanks of all the great Himalayan rivers, 

 when above 13,000 feet, are, owing to the elevation, devoid of 

 trees, whether the climate be humid or arid; but when their 

 course is oblique, as is the case with the Satlcj and the Aran, 

 there are no trees at far lower elevations than this, and a con- 

 siderable part of their upper course is through a Tibetan cli- 

 mate. Thus, in the valley of the Satlej the climate is too dry 

 for trees at the junction of the Piti river, elevated 9000 feet ; 

 and the whole of Piti, as well as the upper course of the Satlej 

 itself, forms part of Tibet. In the valleys of the Ganges and 

 Jumna, on the other hand, whose course is perpendicular to 

 the plains, trees ascend to 10,000 feet, and only the alpine 

 zone is arid and hence belongs to the Tibetan Himalaya, in 

 contradistinction to " Himalaya interior. 



Tibet may be divided into two parts, one to the westward 

 (the basins of the Indus and Satlej), the other to the eastward 

 (those of the Yaru and Aran, and perhaps of the Monas, Su- 

 bansiri, and other rivers). From the position of the Hima- 



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