1919] CRIBBS—TILIA AMERICANA 271 



ity of the air, and in some cases upon growth water. In recording 

 the daily transpiration march in Tilia this saturation deficit was 

 found to be very characteristic, especially at times when the rela- 

 tive humidity is low. It is also induced more readily when the 

 growth water is near the zero point. 



There is another condition, however, under which a very 

 similar behavior in the graph is noted and may very easily be 

 confused with the preceding. This would be especially liable to 

 happen when one is employing the porometef and cobalt chloride 

 paper only, as was done in the work of Trelease and Livingston 

 (16). This cause lies in a sudden change in relative humidity. 

 Thus if there is a sudden increase in relative humidity the effect 

 on the transpiration index is to depress it. Such depressions 

 may be detected from saturation deficits by an accompanying 

 lowering in the curve of evaporation, for when it is due to a deficit 

 the evaporation curve will be high. Inasmuch as a depression 

 in transpiration caused by an increase in relative humidity is 

 not accompanied by a closure of stomata, according to Lloyd 

 (11), the porometer of course would continue to record the relative 

 stomatal opening, and a divergence in the graphs would result. 

 Instances of this appeared a number of times during the experi- 

 ments, and were very marked on the occurrence of transient 

 showers. 



The sudden depression at 8:00 a.m. in graphs of station C 

 (fig. 7) is due to this influence. It will be noticed that there is 

 a corresponding increase in relative humidity, with a lowering of 

 temperature and evaporation power. The behavior at this time 

 is definitely attributed to the passing of a thundershower some 

 distance to the south. It is interesting to note that the influence 

 is more pronounced at station C than at A or B, especially as 

 regards transpiration and evaporation. The drop in evaporation 

 at C is attributed mostly to a sudden period of calm which had 

 less effect at .4 and B because of their more sheltered position. 

 The greater effect in the relative transpiration curve at C is prob- 

 ably to be interpreted as the result of a greater susceptibility of 

 the leaf to a change in relative humidity when transpiring at a 

 high rate. 



