1921] CRIBBS—TILIA AMERICANA 309 
leads to a high transpiration rate and to the development of mor- 
phological features in the leaves which are very similar to those 
recorded at station E on the dunes. 
Transpiration in relation to mesophytism 
Water content of the soil has long been recognized as a factor 
which directly influences the climax type of flora that may develop 
in any given region. The relatively high content as shown for the 
mesophytic forest has quite generally been ascribed as the reason 
for the development of the larger thin type of leaf characteristic of 
that region, in contrast with the smaller more thickened type 
characterizing open associations. Correlated with this concept 
frequently has been the idea that transpiration in the mesophytic 
forest is very high. Observations on the foliar transpiring power of 
Tilia throughout its range of habitats, from the most to the least 
mesophytic conditions under which it develops, have led to a con- 
clusion in regard to this species which is quite different from that 
just stated. The greatest transpiring ability was exhibited in the 
most open situations where the available growth water was least, 
and the transpiring ability was found to be least in the dense shade 
of the mesophytic forest and moist ravines where the growth water 
was relatively very high. It may be asked why the water loss is 
less in the forest. The question is answered by pointing out that 
the environmental factors for high transpiration are less potent in 
the forest than in more exposed positions. Relative humidity is 
greater, evaporation is less, temperature is lower, light is less 
intense, soil temperature is lower, and winds are less effective. 
All of these physical factors combine to give a lower rate of tran- 
spiration than occurs in the exposed open situations. 
Summary and conclusions 
1. The morning rise in the foliar transpiration index for 
Tilia is commonly much slower for the clay series of environments 
than for the sand dunes, and the maximum attained is com- 
paratively much lower. 
2. The time of maximum transpiration is usually from 12:00 
to 1:00 P.M. on clay soils, whereas on the sands it occurred from 9:00 
