iqiq] CRIBBS—TILIA AMERICANA 273 



den shower which began at 7:40 and lasted until 8:20. If the 

 stomatal behavior is similar in Tilia to that found by Lloyd 

 (11) to exist in Fouquieria, Verbena, and Ampelopsis, a porometer 

 curve, if such had been made, would probably have continued to 

 rise from 8:00 to 10:00 a.m. and would have given the type of 

 curve that is obtained when a true deficit occurs. The evapora- 

 tion curve alone shows that this was not a deficit depression, 

 and one could safely infer the same from the much higher maxi- 

 mum that immediately follows; but if such a drop had occurred 

 near midday, the second higher mode may not have occurred and 

 a fall identical with that of the saturation deficit may have 

 resulted. It will be noted that, notwithstanding the low evapora- 

 tion rate, the transpiration index is quite high. This was found to 

 be true for positions on the open sand, although under the same 

 atmospheric conditions on humus the index was always much 

 lower. This is probably largely due to the greater light intensity in 

 the former position, and to a certain degree to higher temperatures. 



It may be interesting to note the following atmospheric con- 

 ditions in their general relation to the transpiration indices for 

 this particular day: 5:00-7:20 a.m., partly cloudy; 7:20-7:40, 

 cloudy; 7:40-8:20, rain; 8:20-9:30, cloudy; 9:30-10:00, clear- 

 ing; 10:00-12:30 P.M., sun; 12:30-3:00 P.M., cloudy; 3:00-6:00 

 P.M., clear, but station shaded. From these data it will be 

 seen that the drop from i : 00 to 3 : 00 p.m. was probably due to 

 the sudden cloudiness of that period, which may have caused 

 sufficient closing of the stomata to effect a depression. There is 

 also a small drop in temperature and relative humidity at this 

 time which would also have their influence, although not recorded 

 by the atmometer. It should be said, however, that the white 

 cup is less influenced by Hght changes than is the leaf (Livingston 

 6), and this was found to be most noticeable on the open sand 

 when the index of transpiration was high. 



One of the most striking relations that appeared in these 

 comparative studies was found in the readings taken at the sta- 

 tions located on humus and those on the open sand during the 

 latter part of the summer. As previously stated, stations A and 

 B are in a forested complex which has a well developed humus 



