Igtt] LIVINGSTON—LIGHT INTENSITY AND TRANSPIRATION 427 
allow air circulation. When a reading was to be taken, this sheath 
was removed far enough to expose the scale, and a reading of the 
shade temperature was taken. Then the sheath was completely 
removed and the rise of temperature which took place in a single 
minute was noted. The photographic instruments were operated 
by exposure in the hand, close to the plants, a stop watch being 
used to determine the length of time needed to produce darkening 
of the sensitive paper to the degree of the standard color, a bit of 
which was attached to the case of the instrument, directly adjoining 
the exposure opening. All instruments and plants had been in 
full sunshine for an hour or more at the beginning of an experiment. 
Wind velocity was taken, and shade temperatures, but since these 
data show no relation to the resulting transpiration ratios, they need 
not be reproduced here. Throughout the tests there was always 
some air motion and never a high wind, the velocity varying from 
©.2 or 0.3 to 2.0 or 3.0 miles per hour. The temperature varied 
from about 30 to 35°C. The plants used were Physalis angulata 
L. var. Linkiana Gray, Xanthium commune Britton, and Mar- 
tynia louisiana Mill. They will be referred to merely by the 
' generic names. , 
Results 
The first series of observations extended from 8:00 A.M. to 1:00 
P.M., August 9, r910. A plant of Physalis, one of Xanthium, and 
the three porous cup atmometers (brown, black, and white) made 
up the series of objects. During the second hour the shade used 
was of white “‘8-ounce cotton duck” or tent canvas. During the 
fourth hour the shade was of a single thickness of ‘‘cheesecloth.” 
The data from this series are given in table I. 
To study, in a general way, the comparative effects of shade on 
the rates of water loss of the different objects, it is expedient to 
reduce each series of figures to relative values. We may take the 
datum for period 4 as unity in each case, and form the new series 
by dividing this datum into each of the remaining data. Graphs 
of these derived quantities are given in fig. 1, all of them passing 
through the common point (unity) at period 4. These graphs are 
thus directly comparable as to the relative heights of their ordinates 
