2 74 BOTANICAL GAZETTE [October 



and an abundant herbaceous undergrowth, and C is on the edge 

 of the same complex, where it is being destroyed by a blowout. 

 These stations, when compared with D and E, stand out in con- 

 trast by their greater mesophytism. This is true in regard to 

 the texture of their foHage and its richness; but during the months 

 of August and September the Tilia complex rapidly undergoes a 

 change which is very noticeable in the vegetation and is conspic- 

 uous in the relative transpiration indices. This change is ini- 

 tiated by an early reduction of the soil moisture to the wilting 

 coefficient, evidently because of the heavy vegetation the sands 

 are supporting and the excessive transpiration rates caused during 

 this period by the highly desiccating atmospheric conditions. 

 Stations D and E show, during this same period, a higher growth 

 water content on the exposed dunes; and the abscission which 

 is carried on rapidly in the Tilia complex is entirely unnoticed 

 here until much later. The greater soil moisture on the open 

 sand as compared with the pine and oak dune stages has been 

 pointed out in the work of Fuller (5), and is seen to be similar 

 for the Tilia complex. Fig. 9 emphasizes this relationship. The 

 data for stations A, B, and C were taken on August 26, and that 

 for station E on August 11. Although the 2 sets of readings 

 were not taken at the same time, the atmospheric conditions on 

 the 2 days were practically identical. Both days were sunny 

 thr6ughout, and the general paralleHsm of temperature, evapora- 

 tion, and relative humidity was rather unusual. Curves for these 

 factors were plotted for stations A and E only. 



The transpiration indices of A, B, and C were all very low, 

 that of C being slightly higher than A or B. They rose slowly 

 in the morning from a low point and reached a low maximum at 

 8:00-8:30 A.M. A deficit occurred then because the soil mois- 

 ture had reached the wilting coefficient, and the indices remained 

 low throughout the day, rising slowly in the evening as the leaves 

 regained their turgor. Visible wilting occurred about 10:30 a.m. 

 at these stations, and was sufficient to cause stomatal closure. 

 The temperature, evaporation, and relative humidity are seen to 

 have remained high, reaching a maximum about 3:00 p.m., after 

 which they decHned rapidly. 



