284 BOTANICAL GAZETTE [October 



5. In the forested complex the curve representing relative 

 transpiration tends to develop a single mode about midday, and 

 this maximum tends to coincide with the maxima of temperature, 

 relative humidity, and evaporating power of the air. The greater 

 the mesophytism and relative humidity the more striking becomes 

 this tendency. 



6. In the most exposed situations on the open sand the rela- 

 tive transpiration maximum usually appea/s about 10:00 a.m., 

 while the maximum temperature, relative humidity, and evaporat- 

 ing power occur from 2 : 00 to 4:00 p.m. This divergence from par- 

 allelism is due to the development of a saturation deficit, which 

 appears successively earlier as the exposure of the habitat in- 

 creases. The more mesophytic the habitat the less noticeable 

 becomes this deficit, until it disappears entirely, especially on 

 humid days. 



7. The foliar transpiration index is influenced less by wind 

 currents than is the porous cup atmometer. 



8. Transpiration curves showing a saturation deficit depres- 

 sion usually develop a second mode about 4:00 p.m., which is, 

 so far as noticed, always lower than the mode preceding the deficit 

 depression. 



9. Bimodal transpiration curves have been found to be due 

 either to a saturation deficit or to a sudden increase in relative 

 humidity, although lesser depressions may result from fluctuating 

 temperature or intensity of light. 



10. No evidence of visible wilting occurred in Tilia on the 

 open sand at any time during the summer, although the so-called 

 " incipient drying" was a common feature of the stations through- 

 out this period. On the forested complex, however, visible wilting 

 occurred during the first week in August because the vegetation 

 was so dense that the water content of the soil was reduced to 

 the wilting coefficient quite early. 



1 1 . The average mesophytism on the Tilia complex is consider- 

 ably greater than on the open sand, the growth water averages 

 being 2.5 and 1 .25 per cent respectively; but the open positions 

 are practically constant in their water relations, while the forested 

 complex represents a decreasing mesophytism as the summer 



