TRANSPIRATION BEHAVIOR OF RAIN-FOREST PLANTS. 59 



of growth exhibited by rain-forest plants are occasioned by low rates - 

 of transpiration and adverse conditions for photosynthesis, the former 

 being due chiefly to the prevailing high humidities and the latter to the 

 high percentages of cloud and fog. The fact that growth is slower in the 

 montane than in the lowland regions of the tropics is not surprising, 

 since, in addition to the factors mentioned, temperature differences also 

 enter the complex in favor of more rapid growth in the lowlands. 



TRANSPIRATION BEHAVIOR OF RAIN-FOREST PLANTS. 



METHODS AND MATERIAL. 



The work reported in the succeeding pages was directed to an investi- 

 gation of the amounts and behavior of transpiration in characteristic 

 montane rain-forest plants. The object kept in mind in planning the 

 experiments was to secure results that would at once contribute to a 

 precise knowledge of transpiration in the plants of an extremely moist 

 region, and at the same time elucidate some of the local features of 

 plant distribution as related to the physical characteristics of the habi- 

 tats which had already been under investigation. 



Through the use of atmometric observations I have been able to 

 institute a strict comparison between series of transpiration readings 

 taken at different times and between the conditions of the field and 

 the laboratory. The securing of simultaneous readings of transpiration ^ 

 and evaporation makes possible also the comparison of transpiration 

 amounts and behaviors in plants of widely separated locahties, with a 

 basis of accuracy which removes this subject from the limbo of con- 

 troversy into which botanical literature has sometimes seen it descend. 



The work on transpiration comprised the determining of (a) the daily 

 march of the rate of water loss under the natural conditions of a mon- 

 tane tropical region, (6) the effect of high humidities and of darkness 

 on the rate, (c) the comparative amounts of stomatal and cuticular 

 transpiration in the slightly cuticularized and thin-walled leaves of 

 rain-forest plants, (d) the behavior of stomata as affecting the rate 

 of transpiration, (e) the comparative transpiration rate and transpira- 

 tion behavior of different types of rain-forest plants as simultaneously 

 measured, and (/) the daily march of the relative transpiration^ rate. 



The plants used in these experiments were Alchornea latifolia and 

 Clethra occidentalis, two of the commonest trees in the rain-forest; v- 

 Dodoncea angustifolia, one of the commonest shrubs on the Leeward 

 Slopes of the Blue Mountains; Peperomia basellcefoUa, a thick-leaved 

 herbaceous plant of the open Ridge forests; Pilea nigrescens and Pep- 

 eromia turfosa, characteristic herbaceous plants of the floor of the 

 Windward Slopes, and Diplazium celtidifoUum and Asplenium alatum, 



^The term "relative transpiration" is used in the sense in which it was employed by Livingston 

 Carnegie Inst. Wash. Pub. 50) to denote the ratio of transpiration to evaporation. 



