1921] '  CRIBBS—TILIA AMERICANA 203 
transpiration index as recorded in forest habitats more frequently 
showed a slow steady rise, beginning with the morning opening 
of the stomata and continuing to a maximum which usually occurred 
about midday. The maximum was approximately three hours later 
in the forest than on the open dune sands. It was also much 
lower relatively than was found to be true for the sand environ- 
ment. In only one instance was the coefficient observed to reach 
©:5 on the forested clay, while in the former case it not infrequently 
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. 5.—Data for stations F and Gin prairie flood plain forest near Chicago; both 
seliiees for August 18; stronger saturation deficit recorded for station G than for F. 
attained 0:9. Following the morning maximum, a feature of the 
transpiration curve, as pointed out by various investigators, is 
the depression which occurs, causing a divergence from the curves 
representing temperature, relative humidity, and «evaporation. 
This has been thought to be due to the creation of a deficit of water 
in the mesophyll cells. Such depressions are strongly evident in 
the dune graphs, but were inconspicuous throughout the work on 
Tilia in clay habitats. Fig. 5 shows a deficit depression in the 
graph representing station G. It is less evident at station / for 
the same day. Figs. 2 and 3 show a slight divergence from the 
