VEGETATION OF THE RAIN-FOREST. 29 



potent factors in determining the character of the vegetation of the 

 ravines, as well as in differentiating them from other habitats. Caused 

 primarily by the abundant and well-distributed rainfall, as well as the 

 prevalent fog, the humidity is maintained through the immense evapo- 

 rating surface provided by the litter on the ground, the wet foliage, 

 and the sponge-like masses of hepatics and mosses. Sheltered by the 

 winds which sweep over the ridges and peaks, the Ravines are pro- 

 tected also from the mid-day rise of temperature, both through the 

 uppermost layers of foliage and through the fogginess, by virtue of 

 which conditions the constancy of the high humidity is almost unbroken. 

 Influences which tend to lower the humidity, and which operate through 

 only a few hours, are offset by an increased rate of evaporation from 

 the wet surfaces. Continued prevalence of such conditions through 

 many days, however, serves to lower the humidity at the forest floor, 

 with results fatal to many of the terrestrial herbaceous plants and the 

 more hygrophilous epiphytes, as I had opportunity to observe in April 

 1903, after three months with a rainfall of 3.45 inches (8.7 cm.), in 

 which the normal fall is 16.32 inches (41.5 cm.). Coupled with the 

 high humidity are temperature conditions of great constancy, the daily 

 range varying from 5.8 to 7.6 F. 



The top of the Ravine forest, as seen from the adjacent slopes, 

 presents an irregularity of surface much greater than that of the Slope 

 and Ridge forest; the largest trees standing well apart from each other, 

 bearing crowded masses of epiphytes, and festooned with pendant 

 mosses, while between them the canopy is formed by the crowns of 

 smaller melastomaceous or rubiaceous under-trees or groups of tree- 

 ferns. This irregularity of the canopy is due to the downfall through 

 erosion of some of the largest trees and the slowness of the growth of 

 the younger trees by which they will be replaced ultimately. The 

 largest of the trees found only in Ravines are Solanum punctulatum, 

 Guarea swartzii, Hedyosmum arborescens, and Turpinia occidentalis, 

 while together with them grow trees more frequent on the slopes, such 

 as Hcemocharis hcematoxylon, Alchornea latifolia, Meriania purpurea, 

 Ilex montana var. occidentalis, Lyonia jamaicensis, and Clethra occi- 

 dentalis. The under-trees of the ravine forest are species which never 

 reach the size of those just mentioned, and grow either in their shade 

 or else themselves form the canopy of the forest. The commonest of 

 them are Mecranium purpurascens, Bcehmeria caudata, Palicourea crocea, 

 Psychotria corymbosa, Eugenia biflora var. wallennii, Cestrum hirtum, 

 and Miconia rubens. With these grow the tree-ferns, the commonest 

 of which are Cyathea pubescens, Cyathea tussaccii, Cyathea furfuracea, 

 and Cyathea insignis. A number of smaller under-trees and shrubs are 

 equally characteristic of the lower layers of the ravine forest, notably 

 Piper geniculatum, Piper fadyenii, Tournefortia cymosa, Datura suaveo- 

 lens, Acalypha virgata, Besleria lutea, and Senecio swartzii. 



