74 THE RELATION OF DESERT PLANTS TO 



Experimental data are presented upon the effect of air currents in 

 increasing the rates of evaporation and transpiration, the relative 

 humidity of the air remaining 1 constant. This effect is so marked that 

 methods of transpiration measurements involving the placing of plants 

 in closed chambers, while valuable in studying the physiological condi- 

 tion of the transpiring tissues, must be regarded as giving no clue to 

 the actual amount of transpiration occurring in the open air. 



Transpiration studies showed that the rate of water loss per unit 

 of leaf surface is relatively low in the most xerophytic forms studied 

 and somewhat higher in the semimesophytic forms which appear only 

 in the rainy season. A comparison was made between the rate of trans- 

 piration and the rate of evaporation from a water surface, with the 

 result that a physiological regulation of the former rate was unquestion- 

 ably shown to exist. By means of a newly devised form of evapori- 

 meter the hourly rate of evaporation from unit water surface was ob- 

 tained simultaneously with the hourly rate of transpiration from several 

 different plant forms, for different periods throughout the day and 

 night, and curves were constructed showing the variations in the ratio 

 of transpiration rate to evaporation rate. This ratio has been termed 

 the rate of relative transpiration, and denotes the number of square 

 centimeters of -leaf surface necessary to exhibit as great a water loss as 

 was observed, for the same time and place, from a single square centi- 

 meter of free water surface. 



From the curves constructed for Euphorbia, Tribulus, Allionia, and 

 Boerhavia, relative transpiration was found to vary from a minimum 

 occurring about 8 p. m. to a maximum between 6 h 30 ra a. m. and 1 p. m. 

 The highest relative transpiration observed in the experiments was 

 0.785 and the lowest 0.008. The physiological regulation which this 

 variation shows to exist is not mainly related to nyctitropic movements 

 of the leaves, although these movements may have some auxiliary 

 effect in those forms in which the leaves are nyctitropic. There is 

 slight evidence that the regulatory response is related to evaporation 

 rate, and no evidence at all that the checking of transpiration occurs 

 with diminished intensity of illumination, as is commonly supposed. It 

 is barely possible to explain the phenomenon observed on the sup- 

 position that the checking of the transpiration begins when increasing 

 light intensity reaches a certain point and that the check is removed 

 with the removal of light altogether in the early evening; but this sup- 

 position is highly improbable and the data at hand are not sufficient to 

 test the question. The supposition that the variation in relative trans- 

 piration is due to some chronometric rhythm in the protoplasmic activ- 

 ities of the plant receives absolutely no support from the evidence at hand. 



