TRANSPIRATION BEHAVIOR OF RAIN-FOREST PLANTS. 91 



and closing of stomata in other words that the principal regulatory 

 functions reside within the leaf itself and are perhaps active, perhaps 

 passive agents in determining the rate of water loss through the 

 stomata, whatever may be the state of their openness. 



The evidence of the curves of relative stomatal and cuticular trans- 

 piration depends for its value on the normal stomatal behavior of the 

 plant in which the upper surfaces of the leaves were coated, a matter 

 which could not be investigated during the transpiration weighings, 

 by any available method. 



Pilea nigrescens was used in the second experiment, the detailed 

 results of which are not given. In this test the average increase of 

 top alone plus bottom alone over the uncoated leaf was 77 per cent, 

 and the average percentage of the actual stomatal transpiration to the 

 total transpiration of the uncoated leaf was 41 per cent. The latter 

 percentage indicates that the ratio between the stomatal transpiration 

 and the actual total cuticular transpiration is of the same order of 

 magnitude in Pilea and in Diplazium. The matter of the number of 

 stomata per unit area, which I have not determined, is an important 

 factor in affecting this ratio, as also is the amount of cutinization and 

 thickening of the epidermis. 



STOMATAL BEHAVIOR. 



The possession of relative transpiration data greatly clarifies the 

 investigation of the influence of fluctuations of stomatal movement on 

 transpiration. The effects of wind, temperature, and humidity are 

 eliminated by their use, and it is possible to compare stomatal condition 

 with the fluctuations of transpiration which are due to internal factors. 

 Such internal factors, whether active or passive in their agency, are 

 alone responsible for the departures of the relative transpiration curve 

 from a straight line parallel to the axis of abscissas. 



My purpose in securing readings of stomatal aperture concurrently 

 with transpiration weighings was to learn in how far the changes of 

 stomatal openness might be correlated with the fluctuations of relative 

 transpiration rate. The existence of a positive 1 correlation might be taken 

 as proof of the control of relative transpiration by stomatal movement . 

 or as proof that stomatal movement and the fluctuations of the relative 

 transpiration are both governed by more deep-seated internal factors. 



The methods by which I measured transpiration and secured stomatal 

 readings were such that I necessarily obtained my epidermis for the 

 latter purpose from other individuals than those in which the transpira- 

 tion was being measured. This is an extremely unfortunate limitation 

 to the combined use of the weighing method oi determining trans- 

 piration and Lloyd's method for stomatal measurement. I secured 

 epidermis from potted plants which had had the same history as those 

 that were being weighed, which looked just like them in general char- 



