EVAPORATION AND CAUSAL FACTORS. 63 



stems in determining the normal hourly transpiration from rooted 

 plants. The data from the various series were used to calculate the 

 theoretical water-loss of the white-cylinder atmometers according 

 to the method employed by Johnston (1919), following the precaution 

 of averaging the vapor-pressure deficit for the beginning and end of 

 each hour as prescribed. The results calculated in this manner for 

 series 33 with those actually obtained with the atmometer are shown 

 graphically in figure 33. As the curves show, the relation is not at 

 all close. But when the wind velocity is divided by a factor to 

 reduce its effect upon the results, and a constant added to represent 

 the replacement by diffusion of still air around the evaporating 

 surface, the calculated results are brought into much closer accord 

 with the observed rate of evaporation. Even then there are discrep- 

 ancies that can not be explained fully by radiant energy, as sug- 

 gested by Johnston. The method may be of value, however, as a 

 first step toward an analysis of the part played by each factor in 

 producing water-loss from the atmometer, and thus lead the way to 

 a new attack upon the problem of transpiration. 



In all the series made in the Great Salt Lake region, low relative 

 humidity and strong sunlight were found associated, and usually 

 higher temperatures as well. For this reason it was not easy to 

 separate the action of each, but a study of the precise effect of each 

 was beyond the scope of the investigation. The general effect of 

 increased evaporation as determined by these three factors is shown 

 for alfalfa in figure 9. This represents essentially what occurs in most 

 of the mesophytes studied, low relative humidity and intense sun- 

 light producing day closure. On the other hand, no closure was found 

 taking place during the day if the humidity was high. If the water- 

 content had become critical, no opening occurred at all, even under 

 a condition of high humidity, but if the plant could obtain sufficient 

 moisture to produce opening, the stomata would show no mid-day 

 closure. The increased irregularity in the rate of movement following 

 increased evaporation is shown in figure 9. 



Each factor concerned plays at times a r61e independent of evapora- 

 tion in producing changes of stomatal movement. Thus, light which 

 acts through radiant energy in producing water-loss from the plant 

 has been shown to act independently of air-temperature. The latter, 

 which is a very important factor in evaporation, also plays an 

 important part in the speed at which opening occurs. Humidity is 

 more closely related to evaporation, but, when high, precipitation 

 occurs in the form of dew or rain, and this causes changes in stomatal 

 movement. Wind increases the effect of low humidity, but also 

 carries dust, and when high probably also causes currents of air to 

 flow through the leaf. 



The effect produced upon stomatal movement by the wetting of 

 the leaves by dew or rain is most clearly shown in series 35, begun 



