Some Observations on Transpiration 
CARLTON C, CURTIS 
Variations in the rate of transpiration have been observed almost 
from the beginning of physiological research. These irregularities 
present a most puzzling problem since they appear in the results 
obtained by any method that may be employed to demonstrate 
the phenomena of transpiration and notwithstanding the fact that 
the external conditions may be apparently uniform and constant, 
I have been interested to take note of the records that have been 
made from year to year, when this subject comes up for experi- 
mentation in the laboratory with a view to determine whether there 
is any rhythm to be found in the fluctuations, and if so, whether 
this is related to the tension of fluids in the stem. In other words, 
to find out in what degree transpiration might be connected with 
the vital phenomena of the plant. 
The table on the following page illustrates the character of the 
fluctuations and these examples are presented since they were ob- 
tained under as constant conditions as could be expected with 
natural light. 
The amount of transpiration is expressed in milligrams ob- 
tained by hourly weighings beginning in the morning and running 
through the day, and below each series is given the changes of 
temperature and humidity while on the right hand of the table 1s 
noted the character of the day. 
The intensity of the light may account in a certain degree for 
the maxima of the curves coming near the middle of the day al- 
though there was no apparent variation in the amount of illum 
nation at that time of day. However, there are minor fluctuations 
in the curves quite independent of the slight variations of the cli- 
matic conditions and the light intensities and they are as likely 2 
be at variance with any perceptible changes as in keeping with 
them. These irregularities may result in the rise or fall of the 
rate for a short period or again they may extend over longer 
periods. They are quite as appearent in the consecutive readings of 
potometers and especially in the device of Darwin's where a shoot 
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