TRANSPIRATION STUDIES. 35 



DISCUSSION OF TRANSPIRATION EXPERIMENTS. 



Livingston* found a similar early maximum for relative transpiration in 

 the case of potted plants of four species, but in general he did not find an 

 early maximum in the actual transpiration, nor does he mention the rise 

 succeeding the drop. It is possible that his readings were taken at too 

 long intervals to bring out the phenomena. In the above experiments, 

 Nos. I and II are mentioned as exceptions, but in these cases readings were 

 taken at too great intervals for the drop and rise to appear if it existed. It 

 may be noted that their afternoon curves for relative transpiration approxi- 

 mate straight lines. Although the drop and rise do not appear in plants 

 Nos. 1 and 2 used in experiment IX, there is a corresponding decrease in 

 the angle of the slopes of both the actual and the relative transpiration- 

 curves which is followed by an abrupt angular increase. Plant No. 3 of 

 this experiment shows the drop in relative transpiration, but no rise up to 

 the time the experiment had to be discontinued, while No. 4 shows both 

 drop and rise. In the case of experiment VI the closing of the leaves took 

 place at such a time as to interfere with conclusions regarding the drop and 

 rise. The closing of the leaves undoubtedly cuts down transpiration from 

 the dorsal surfaces, since the air in the narrow space between the leaves 

 would quickly rise in humidity. The second hour during which the leaves 

 were closed showed a slight rise in actual transpiration over the first hour, 

 but this made a very small change in the angle of the relative transpiration 

 curve and might easily be due to an increase in transpiration from the 

 ventral surfaces resulting from an increased vapor pressure within the leaf 

 which would be caused by the lessening of evaporation from the dorsal 

 surface. Thus it seems necessary to omit experiment VI altogether from 

 the discussion of the occurrence of the drop and rise mentioned. In all the 

 other experiments the closure of leaves took place too early in the day 

 to influence the drop and rise. It will be noted that in the cases of trees, 

 where the drop and rise is most pronounced, the leaves were closed by 5 a. m. 



There is, then, in all of the above experiments, no clear negative evidence 

 for the existence of a drop and rise ; and from the experiments the generali- 

 zation may be made that in trees, leafless and in leaf, a distinct drop and 

 subsequent rise occur in both the actual and relative transpiration, in the 

 morning before the time of maximum evaporation for the day; and that 

 potted plants show a tendency towards the same phenomena, but in some 

 cases the drop appears only as a straight-line "jog" in the relative trans- 

 piration curve. 



An inspection of the values summarized in table 14 shows the following. 

 The relative transpiration maxima which occur immediately i^receding the 

 drop in the relative transpiration curves vary so much that it can not be 

 said that any relation exists between the amount of relative transpiration 

 and the occurrence of the drop. However, the values for the actual trans- 

 piration at this period do show a tendency towards a constant value for 

 the different forms used, so that it ma.v l>e said that approximately 0.0044 



Livingston, B. E., Cam. Inst. Wash. Pub. 50, p. 63. 



