16 TRANSACTIONS OF THE [Sess. lxxx 



knowledge a vast amount of exploration, some portion of 

 which I hope may yet fall to my share, is still necessar3^ 

 Nevertheless, inadequate as are the facts so far collected, 

 and though much revision, addition, and correction will be 

 needed as knowledge increases, it seems to me that some 

 useful purpose may be served by the following attempted 

 explanation. 



I will begin with a brief description of the frontier ^ 

 region and the distribution of plants tliere according to 

 climate. 



Geogrcqihy and Climate. 



A glance at the map of Asia will show that in the region 

 of longitude 98°-99' E. and between the 27th and 30th 

 parallels of latitude several big rivers break through from 

 Tibet and flow for some distance due south, parallel to one 

 another and close together, being separated by high, narrow 

 ranges of mountains. Further east, and again in Upper 

 Burma, the trend of the mountains is the same, the peaks 

 D-rowino" lower as we tro south : however, we need not for 

 the present concern ourselves with these minor ranges, con- 

 centrating our attention on the three principal ones : namely, 

 the Irrawaddy-Salween, Mekong-Salween, and Mekong- 

 Yangtze divides, the first-named being the most westerly. 

 Beyond the Salween-Irrawaddy divide come the mountains 

 of the Burmese hinterland, the valleys between which are 

 filled with monsoon jungle, which also clothes the mountains 

 to at least 8000 feet. The monsoon climate in fact, char- 

 acterised by hot, wet summers and a dry season of greater 

 or less extent (which becomes also a cold season in the 

 north and at high altitudes), extends a little further east, 

 into the Sal ween valley itself, where in the gullies, even as 

 far north as latitude 28°, I have found a monsoon flora 

 with such plants as Aspleniuni Nidus, Linn., the banana, 

 Asdepiadaceae, numerous epiphytic ferns and orchids, 

 climbing Aroids, etc. When w^e reach the Mekong-Salween 

 divide we find that great range also clothed with luxuriant 

 forests and meadows, the former lacking many of the 

 characteristic arborescent monsoon genera, but neverthe- 



' The frontier between Tibet and Yunnan in the north, Burma and 

 Yunnan in the south, spoken of throughout this paper as the Burma- 

 Yunnan area. 



