VEGETATION OF THE RAIN-FOREST. 27 



transpiration and growth. Indeed, the temperature conditions on the 

 windward slope between the altitudes mentioned are made more uni- 

 form than on the leeward slope by the fact that much of the dynamic 

 cooling of the air driven up from the near-by coast goes into the con- 

 densation of moisture. The differences of altitude that exist within 

 our area are accompanied by negligible differences in rainfall and cloudi- 

 ness. The leeward slopes, however, get a somewhat higher rainfall just 

 below the Alain Ridge than at lower altitudes, which is true no matter 

 at what altitude on the Main Ridge, and is merely due to precipitation 

 from clouds which are carried beyond the crest of the ridge by wind. 



The sets of factors indicated do not operate independently, neither 

 do the different habitats fail to shade into one another in the character 

 of their vegetation. Deep ravines on the leeward slope resemble in 

 many respects less deep ones on the windward side; peaks and ridges 

 at lower altitudes resemble those at higher altitudes; ridges which are 

 at the same time gaps resemble ravines more than they do the more 

 exposed ridges. The ravines vary in width and depth, according to 

 their age; when followed upward they broaden and emerge into the 

 upper slope of the valley to which they are tributary. 



The ravines and valley bottoms and their adjacent slopes will be 

 shown to be the most hygrophilous habitats in the rain-forest; particu- 

 larly on the windward slope they show a wealth and luxuriance which 

 rival that of the lowland forests, together with the predominance of 

 bryophytes and pteridophytes, which is the strongest characteristic of 

 the region. 



The following sections embrace a brief descriptive account of the 

 vegetation of the Blue Mountain region. The habitats under which 

 the descriptions are grouped have been distinguished in accordance 

 with the conditions just discussed. The most important distinction 

 within the region is that between the two slopes of the range, which 

 are designated the Windward and Leeward rather than the Northern 

 and Southern, in order to emphasize the fact that it is the climatic 

 difference between them due to the trade wind and not the chance fact 

 of their geographical orientation which is critical. Second in import- 

 ance as a distinguishing factor is the topography, which leads to a sub- 

 division of the two main slopes into ravines, slopes, and ridges. The 

 fact that the differences between the ridges of the Windward and 

 Leeward slopes are negligible has led to their combined treatment. 

 The extreme summits of Blue Mountain and Sir John Peaks are treated 

 separately, and the epiphytes have also been given special treatment, 

 because their occurrence and distribution are more dependent upon 

 vertical differences of conditions within the rain-forest than on the 

 horizontal differences between the habitats recognized. 



The Windward Ravines exhibit to the most striking degree the char- 

 acteristics of the rain-forest, and the other types have been treated 



