108 A MONTANE RAIN-FOREST. 



activity, but is slow in the majority of common trees, including those 

 which are completely defoliated in the winter months. Extremely 

 slow rates of growth prevail among the trees which possess the most 

 sclerophyllous types of foliage, and also among the herbaceous flowering 

 plants of the forest floor. 



The normal daily course of weather conditions in the rain-forest region 

 is such that the total daily water loss of all plants is extremely low. 

 The trees and shrubs are capable of relatively high rates of transpiration 

 in full sunshine, but there are few days in which these rates are 

 maintained for more than three or four hours in the early morning and 

 perhaps an additional hour or two in the afternoon. The hygroph- 

 ilous plants of the floor of the Windward Ravine forest are incapable 

 of withstanding insolation for more than one or two hours, even at 

 high humidities, without wilting. When brought into the climate of 

 the Windward Slopes these plants lose from 3 to 3| times as much 

 water per unit area as do the herbaceous plants of the least hygrophilous 

 habitat, the Ridge Forest. When placed in the moist atmosphere of 

 their own habitat the Windward Ravine plants lose only 2 to 2| times 

 as much water as the plants of the Ridge Forest. The open mesophyll 

 and thin epidermis of the hygrophilous ferns enables them to maintain 

 surprisingly high rates of transpiration in the shade, in an atmosphere 

 of very high humidity; the rates of water loss per unit area are only 

 half as great in the herbaceous flowering plants of the Ravines and 

 Slopes, and from one-third to one-fourth as great in the plants of 

 Ridges and in the epiphytic orchids. 



The prevailing conditions of the interior of the rain-forest are inhibi- 

 tory to transpiration and also to photosynthesis. The constant high 

 humidities and the dull light which prevails may well be responsible, 

 through these functions, for the prevailing low rates of growth. The 

 lowness of the temperature within the forest, and possibly also its 

 equable character, are also connected intimately with the slow opera- 

 tion of the individual functions of the plant and with the cumulative 

 effect upon growth. 



When the transpiration rates of rain-forest plants are converted into 

 rates of relative transpiration, and thereby correlated with the pre- 

 vailing atmospheric conditions which are the determinants of the rate 

 of evaporation and are the chief external factors determining trans- 

 piration rate, they are then found not to be low. The rates of relative 

 transpiration in Jamaican rain-forest plants and in plants of the Arizona 

 desert are found to be of the same general order of magnitude. This is 

 merely saying that the rates of transpiration in the two regions are 

 proportional to the rates of evaporation which prevail in them. While 

 the plants of the rain-forest are capable of losing much more water per 

 unit area than are the plants of the desert if the two kinds of plants 

 are brought under the same conditions, it is nevertheless true that as 



