18 



BURTON EDWARD LIVINGSTON 



at any time, excepting for a few minutes about 11 hours 25 min- 

 utes, when the approach of the storm already mentioned was 

 accompanied by several strong gusts. With the coming of rain, 

 at 11 hours 35 minutes, the instruments and plants were taken 

 into the laboratory. 



The general agreement of the three graphs bearing upon tran- 

 spiring power is such that if all three were smoothed so as to elimi- 

 nate irregularities in the lines, all would be in very satisfactory 

 agreement. As should be expected, the relative transpiration 

 graph by the white atmometer is uniformly higher than that by the 

 brown instrument during the hours of sunshine, the absorption 

 of radiant energy causing the rate of water loss from the brown 

 cup to exceed the rate from the white one. Since sunshine 

 must alwaj's accelerate transpiration, because of the absorption of 

 some radiant energy by the leaves, the brown instrument is 

 theoretically better suited to the determination of transpiring 

 power and its variations than is the white,^" and in this connection 

 it is clear that the graph from the dark instrument is more nearly 

 in agreement with that from the paper test than is the other. That 

 the relative transpiration graphs exhibit marked accelerations 

 and retardations that are only slightlj^, or not at all, indicated 

 by the graph from the direct method seems to be explained by the 

 supposition that the atmometers are much more sensitive to 

 changes in the siu-rounding conditions than are the plants. There 

 is available, from other sources, much evidence in favor of this 

 supposition, but the time for detailed discussions of these minor 

 questions has not yet arrived. 



Experiment II. This is a series of observations similar to that 

 of experiment I, carried out with the same purposes in view. The 

 plants used were small specimens of Datura stramonium L. Two 

 series of hourly weighings, for the absolute transpiration rates, 

 were carried out, also the white atmometer and the brown radio- 

 atmometer were operated with hourly readings. The cobalt 

 chloride test was apphed hourly to the upper and lower surfaces 

 of leaves of medium age, four plants being used, two at each deter- 



" Livingston, B. E. Light intensitj' and transpiration. Loc. cit. 



