64 THE RELATION OF DESERT PLANTS TO 



trusted, the temperature of the surrounding air seems to be the control- 

 ling condition which governs this regulative response. It seems that 

 when the temperature reaches a certain point in its daily rise the checking 

 of transpiration begins to be effective, and that the check is removed 

 when the air temperature has passed its daily maximum and again 

 decreased to a certain point. The latter point seems, in most cases, to 

 lie somewhat below the point at which the checking response begins. 

 The physiological maximum, at which transpiration begins to be checked, 

 lies, for the forms studied, between 79 and 90 F., and the corresponding 

 minimum, at which the check is removed, occurs between 75 and 80 F. 

 There seems to be no evidence from these experiments for supposing 

 light intensity to be the controlling condition for this regulation, as it 

 is commonly taken to be for most plants,* for the checking of transpi- 

 ration begins to be noticed too early in the day to be due to diminished 

 light intensity. It is of course possible that with high intensity of illu- 

 mination the checking of water loss occurs and that this check is 

 removed with the coming on of the nocturnal darkness, but this suppo- 

 sition is the direct opposite of the prevalent idea regarding this regula- 

 tion. More data are necessary for a test of this point. 



There is some evidence that intensity of evaporation is the con- 

 trolling factor, in some cases at least, but this is not as consistent as 

 the evidence for air temperature. There is practically no evidence from 

 these experiments that the response is due to some chronometric rhythm 

 within the plant. The data at hand do not bear upon the question as to 

 whether this regulation is in any way connected with photosynthesis. 



Table XIV presents in tabular form the data obtained as to the rela- 

 tive efficiency of the regulative response. In the first two columns are 

 given the number and subject of the experiment, in the next two the 

 maximum and minimum rate of relative transpiration, as nearly as 

 these can be ascertained. The fifth column gives the efficiency of the 

 regulation of transpiration, being denoted by the ratio of the minimum 

 to the maximum, the former being considered as unity and the latter 

 expressed in round numbers. Thus, in the case of Experiment I, the 

 symbol 1/9 means that the minimum of relative transpiration is approxi- 

 mately one-ninth of the maximum. In the last column the external 

 conditions which apparently control the response are stated, T referring 

 to air temperature and E to intensity of evaporation. It appears from 



*The experimental evidence in regard to stomatal movements and their cause is 

 not very conclusive. For a presentation of the whole subject of the effect of light, 

 temperature, wind, etc., upon the absolute transpiration rate, the reader is referred 

 to Burgerstein (1904). The preliminary character of the present results renders a 

 thorough discussion of the literature unnecessary. 



