RESISTANCE BY LEAVES TO WATER LOSS 



33 



power of the leaves of Physalis and of Datura (experiments I 

 and II), grown in pots as already described, exhibits a diurnal 

 march or fluctuation, from a low value in the night hours to a 

 much higher one in the day. A similar condition of affairs is 

 suggested in the case of the two observations upon Ruellia (experi- 

 ment V). Furthermore, the diurnal fluctuation in foliar tran- 

 spiring power of the three forms just mentioned is the resultant 

 of very different fluctuations in the separate transpiring powers 

 of the two leaf surfaces. Variations in this feature are not at all 

 the same for the two leaf surfaces as for the leaf as a whole; the 

 two surfaces may alter in this respect either more or less rapidly 

 than does the entire leaf surface, or one of them may actually vary 

 in the opposite direction. WTiile there is evidence that stomatal 

 distribution and movements may account for these fluctuations 

 in many cases, there is evidence also that some other condition is 

 frequently of prime importance. There appears no logical ground 

 for doubting that the phenomenon recently called incipient drying 

 hy Livingston and Brown (loc. cit.) plays an important role in 

 the determination of the fluctuations here considered, but the 

 time is not ripe for a thorough treatment of this problem. 



As to the actual magnitudes of the indices of foliar transpiring 

 power given in the present paper, it will be instructive to sum- 

 marize these in table 7. The data are rounded off with the third 

 place of decimals. The indices corresponding to the minimum 

 and maximum transpiring powers of the entire leaf surface are 

 given for the first two plants. The lowest index in the table is 

 the indeterminate one for the upper leaf surface of Rhus aromatica 

 var. mollis, and the lowest determinate one is 0.009, for the lower 

 surface of Physalis at 22 hours. Following the indices for the 

 separate leaf surfaces are placed numbers in parentheses, these 

 numbers indicating the order of magnitude of the index, in the 

 series given in the table. The indices for the entire leaf surface 

 are not considered in this numbered series. 



WTiile there are a number of cases in which the two indices for 

 a given observation occur in the same region of the numbered 

 series, e.g., Martynia (28, 29), Zea (13, 12), Sphaeralcea, Oracle 

 (25, 27), etc., yet there are also cases wherein the two simultane- 



THE PLANT WORLD, VOL. 16, NO. 1, JANUARY, 1913 



