INTRODUCTORY ESSAY. 99 



Of the tropical and subtropical plants that accompany this 

 high summer temperature and withstand the cold of consider- 

 able elevations, are many of those mentioned towards the 

 commencement of this section as natives of dry tropical forests 

 with contrasted seasons, at the level of the sea or on plains 

 raised but little above it. Populus Euphratica, a Cynanchum, 

 Chloris harhata, and Cyperus arisfatus, all of which ascend 

 to 11,000 feet in Ladak, are other remarkable instances, as is 

 Pegamum Harmala, which attains 9000 feet. 



In the Himalaya the truly temperate vegetation supersedes 

 the subtropical above 4000-6000 feet; and the elevation at 

 which this change takes place corresponds roughly with that 

 at which the winter is marked by an annual fall of snow. 

 This phenomenon varies extremely with the latitude, longi- 

 tude, humidity, and many local circumstances. In Ceylon 

 and the Madras Peninsula, whose mountains attain 9000 feet, 

 and where considerable tracts are elevated above 6-"8000 feet, 

 snow has never been known to faU. On the Khasia moun- 

 tains, which attain 7000 feet, and where a great extent of 

 surface is above 5000, snow seems to be unknown. In Sik- 

 kim snow annually falls at about 6000 feet elevation, in Nipal 

 at 5000 feet, in Kumaon and Garhwal at 4000, and in the 

 extreme West Himalaya lower still. 



It is hence only on the Himalaya and Mishmi mountains 

 that a purely temperate flora prevails, to the exclusion of all 

 tropical forms ; though in Ceylon, the Nilghiri mountains, and 

 Khasia, the temperate forms are very numerous, and so pre- 

 valent on the highest summits as to render it very desirable 

 that these heights should be subjected to a very close botani- 

 cal examination. Local circumstances, again, seem to bring 

 the temperate forms lower upon the Khasia and Nilghiri moun- 

 tains than upon the Himalaya, which are further north ; and 

 of these causes the fact that the exposed flat or undulated 

 surfaces of the Khasia are swept by violent winds, is one of 

 the most powerful. The contrast in this respect between the 

 Khasia and the Sikkim-Himalaya is very remarkable, many 



