TRANSPIRATION BEHAVIOR OF RAIN-FOREST PLANTS. 77 



in the sun, as would be the case under the natural conditions of the 

 open. The figures for relative transpiration not only serve as an index 

 of the changing physiological conditions of the plant (fluctuations of 

 stomatal aperture, of water content of leaf, of vascular transfer of 

 water, conditions of soil moisture, etc.), but they also make possible 

 a strict comparison of the behavior of a species when investigated on 

 different days. 



The usual daily course of the relative transpiration in all of the 

 species investigated shows an early morning rise to a maximum which 

 is earlier than the maximum of evaporation or that of transpiration 

 and is usually the maximum of the relative rate for the entire day. 

 In case the evaporation runs on to its maximum at a later hour than 

 the maximum transpiration, or in case the two maxima coincide, it 

 quite commonly happens that the relative rate reaches its maximum 

 at an earlier hour than either. The fact that the rates of increase in 

 evaporation and transpiration preceding their maximal points have 

 been such that the rate of rise was greater for evaporation than the 

 transpiration, causes a fall in the relative rate. Such fall is quite 

 commonly followed in a few hours by a recovery, due to a pronounced 

 fall in evaporation rate, accompanied by a less fall, of perhaps a rise, 

 in the transpiration rate. The relative transpiration fluctuates during 

 the mid-day and early afternoon in an irregular manner, sometimes 

 reaching its daily maximum after the noon hour, but more commonly 

 fluctuating below its morning maximum and finally falling in the late 

 afternoon. The behavior of the rates for Alchornea and Clethra (table 

 16, fig. 2) is typical for a large number of cases investigated on normal 

 days. The curves for Pilea nigrescens and Peperomia turfosa (table 

 18, fig. 4) show an even greater amount of mid-day fluctuation, and 

 at 2 p. m. the former plant exhibits a maximum well above its early 

 maximal point at 7 a. m. 



Figures 8, 9, and 10 have been drawn to show the character of the 

 daily relative transpiration curves in several experiments with Pilea 

 nigrescens, Peperomia turfosa, and Diplazium celtidifolium respectively. 

 Each individual plant used in more than one series is designated by 

 the same letter throughout. The actual rates upon which these curves 

 are drawn may be found in tables 23, 24, and 25. The relative rates 

 of all three of these characteristic rain-forest herbaceous plants are 

 characterized by their uniformity, indicating a weak operation of the 

 physiological regulations to which the inconstancy of the relative rate 

 must be attributed. The maximum and minimum relative rates of 

 these three species are shown in table 29. 



