1915-16.] BOTANICAL SOCIETY OF EDINBURGH 3 



southwards into Burma ; and the Indo-Malay flora which 

 has travelled northwards into the Burmese hinterland 

 and into China, mingling with the descending stream. 

 Naturally, to distinguish and disentangle these conflicting 

 streams of vegetation over so large and diversified an area 

 where several floral regions (i.e. regions characterised by 

 endemic species) meet is a difficult matter, with all the 

 fascination of hunting and tracking. This paper, however, 

 has but an indirect bearing on the larger question of 

 origins. The monsoon climate extends, in modified form, 

 as far east as the Sal ween valley, beyond which every- 

 thing — fauna, flora, people — changes. Up to a certain 

 point, however, about latitude 28° N.i the Mekong-Salween 

 divide too receives a copious summer rainfall from the 

 west, and is covered with mixed forest and dense under- 

 growth, including many giant herbs. This may be called 

 temperate or mixed rain forest, and it is the dominant 

 formation along the entire length of the range from 

 latitude 28° southwards. 



The Mekong-Salween divide, however, to a great extent 

 masks the Mekong- Yangtze divide, acting as a rain screen, 

 so that the latter range, though only a few miles to the 

 east, receives a reduced rainfall, and the dominant forma- 

 tion is no longer rain forest, but coniferous forest and 

 scrub, chiefly oak. 



Above the mixed rain forest of the Mekong-Salween 

 divide come several specialised incidental formations, of 

 which the most important are (a) Abies forest, chiefly 

 confined to the sheer valley walls, well above the stream, 

 but in places disputing the lower ground with (b) bamboo 

 brake. Though disapjoearing from the valley bottoms 

 sooner than Salix or Betula — which towards the tree limit 

 are much flattened out and stunted by wind, — this Abies 

 grows on the precipices, protected from wind, at yet higher 

 altitudes. Where the valley is broad and flat, its floor is 

 occupied chiefly by (c) alpine meadow, a growth of tall 

 herbaceous plants with conspicuous flowers, scattered 

 amongst which are alder and birch trees, or in some places 



^ North of this point coniferous forest with a scanty undergrowth 

 predominates, and the flora of the Mekong-Salween divide resembles 

 that of the Mekong- Yangtze divide both in its nature and actual species. 



