1921] CRIBBS—TILIA AMERICANA 307 
the attainment of maximum absorption is possible because of the 
reserve water which is available in excess of that supplied by the 
translocation stream. 
Curve no. 3, which indicates the transpiration for August 26, 
shows some features conspicuously unlike those recorded for the 
two preceding readings. The maximum is lower in this instance 
than was the minimum index for either of the preceding days. 
The low amount of growth water, which was only 0.480 per cent 
at 2.5 dm., was a prominent cause for the lower transpiring power. 
At 10:00 A.M. visible wilting occurred, and turgidity was not restored 
within the mesophyll cells of the leaves until about 5:00 P.M. The 
development of absciss tissue may be considered as contributing 
to the reduced rate, being earlier at this station than on the open 
sand, owing to the more frequent production of a wilting coefficient. 
Evaporation and relative humidity show curves which lie much 
higher than do those of July 16 and 21. The extremely high evapo- 
_Yation from about 1:30 to 4:30 P.M. was due to high wind velocity. 
It will be seen that on different days readings from the same 
leaves may give widely different results. The three days’ readings 
shown in this instance present different combinations of potent 
factors, one being a cloudy day with plenty of available water, 
one a clear day with sufficient growth water, and the third a clear 
day with low humidity and soil water reduced to the wilting coeffi- 
cient. Thus the environmental factors which initiate variation in 
the daily transpiration rate at any given station were found to be 
the same as those which led to different averages for the various 
stations. This daily variation is more marked on the sand series 
than on the clays, although the same phenomena were characteristic 
of the latter situations. There was found to be more variation in 
transpiring power between cloudy and clear days than was regis- 
tered for days when the atmospheric conditions were similar, 
whether cloudy or clear. 
Transpiration on prairie 
The transpiration index as recorded for Tilia on the prairie 
indicates that there the species is subjected to a physiological stress 
in most respects very similar to that found on the dunes (fig. 10). 
