102 



A MONTANE RAIN-FOEEST. 



maximum of plants in light, and that such a degree of openness is 

 probably maintained throughout the 24 hours, is in accordance with 

 the high rates of relative transpiration already stated as occurring in 

 plants placed in darkness. Livingston has reported^ a higher rate of 

 relative transpiration by night than by day for certain species of cacti, 

 a phenomenon in which stomatal behavior is probably not concerned. 

 Although stomatal behavior has been shown in a preceding section not 

 to be the controUing factor in the diurnal fluctuations of transpiration 

 in the rain-forest plants which I have investigated, it does show an 

 increasing tendency toward such control in the later hours of the day, 

 and the results just given indicate that the wide openness of stomata 

 in prolonged darkness is responsible for the high rates of relative trans- 

 piration in darkness, I have no evidence calculated to explain the 

 aberrant behavior of Peperomia turfosa, in which the relative rate is 

 lowered in the darkness. 



INFLUENCE OF HIGH HUMIDITY ON TRANSPIRATION. 



The retarding influence of high percentages of humidity on the rate 

 of absolute transpiration is well known both upon experimental and 

 theoretical grounds. I have taken the opportunity to investigate the 

 rates of absolute and relative transpiration under conditions of high 

 humidity in the five species already mentioned as used in other experi- 

 mental work. The plants were placed in the moist chamber which 

 has been described, and the humidity was kept above 90 per cent and 

 usually above 95 per cent, the percentage being determined by means 

 of a Lambrecht polymeter, calibrated for high humidities by use of a 

 sling psychrometer. The results as respects absolute transpiration are 

 what was expected — there is a decided cutting down of the rate. The 



Table 41. — Transpiration of Pilea and Peperomia at high humidities. 

 Series run in moist chamber in diffuse light of laboratory. 



'Livingston, B. E. Kelative Transpiration in Cacti. Plant World, 10: 110-114, 1907. 



