1919] CRIBBS—TILIA AMERICANA 283 



Summary 



1. Cobalt chloride standardized paper was found well suited 

 for comparative studies in the relative transpiring power of leaves 

 in the field. 



2. The daily march of transpiration in Tilia was found to 

 vary greatly for the same leaf on different days. This variation 

 was found to be influenced by relative humidity, temperature, 

 light intensity, soil moisture, and presumably by soil temperature. 



Fig. 13. — Typical graph illustrating low transpiration indices occurring in mo- 

 rainic mesophytic forest where growth water is always high; note low transpiration 

 for a day with so low a relative humidity. 



3. The foliar transpiring power of Tilia was found to increase 

 in dune environments from an index of 0.15 on the forested Tilia 

 complex to 0.55 in the most exposed situations on the open sand. 

 In habitats between these extremes the transpiration power was 

 found to be directly proportional to the relative exposure. 



4. The morning rise in the daily march is more rapid on the 

 open sand, where it reaches a maximum 1 to 2 hours earlier than 

 in forested situations. 



