282 BOTANICAL GAZETTE [October 



At station F there is considerable difference in the environ- 

 ment, and the factors accompanying it, from that of the dune 

 series. It has already been noted from the introductory descrip- 

 tion that the undergrowth is composed of a more shade-requiring 

 assemblage than even the most mesophytic positions found on the 

 established dune complex. The humus is sKghtly more developed, 

 but the soil underlying it is very different, and unlike in the for- 

 ested dune habitats the growth water was always adequate to 

 support an abundant and diversified vegetation, and never reached 

 the wilting coefficient. The average available water at 2 dm. was 

 19 .40 per cent and the minimum 12 .45 per cent. Thus if growth 

 water is inducive of a high transpiration, one would expect to 

 find it here, but the average transpiration at this station for 5 

 complete days' readings was only 0.16, an index practically iden- 

 tical with that of station A of the Tilia forested complex where 

 the growth water averaged 2 . 5 per cent. 



The same tendency displayed by the dune graphs to show 

 curves with a single mode recurs here where the mesophytism 

 is greater. The maximum power of transpiration comes later in 

 the day, commonly from 12 : 00 to 2 :oo p.m., and is more frequently 

 coincident with the maxima of temperature, evaporation, and rela- 

 tive humidity. This relation is shown in fig. 13, which is a graph 

 of station F on June 14. At this time there was a growth water 

 content of 26.74 per cent and a relatively low humidity. The 

 temperature was low, while the evaporation was high when com- 

 pared with the transpiration, higher than commonly found in 

 dune environments. The morning rise of the transpiration index 

 was very slow and the maximum reached was not very high. 

 On this particular day it was clear until about 2:00 p.m. with 

 increasing cloudiness through the afternoon, followed by showers 

 at 8:00 P.M. Although it was clear during the forenoon the 

 station was shaded thr.oughout the period. 



The fact that direct sun upon the leaves in a habitat of this 

 sort almost always leads to a rapid increase in the transpiration 

 rate would suggest that the low fight intensity of the densely 

 shaded forest is one of the chief factors leading to the low average 

 commonly found there. 



