278 BOTANICAL GAZETTE [October 



5 cc. per unit; transpiration o.i; relative humidity 15 per cent; 

 soil temperature f C; atmospheric temperature 10 per cent; 

 and growth water 1 per cent per unit. From these it will be seen 

 that the conditions existing at stations A and B are closely similar; 

 but from B to E the stations represent a graded series of habitats 

 as regards these factors, just as clearly as they do when we con- 

 sider their comparative positions in the vegetative cycle. This 

 graded variation found in the dune environments is much more 

 pronounced than that for different situations on clays, which will 



■3 31 «fl si zr 



A 

 B 



C 

 D 



Trx nspiration 



Evaporation — 



Re I. Humidity—— 

 Growth Water ^^— 



Soil. Temp. 



Air.i os. Temp. 



e ^^nar^r^T^ 



Fig. 12.— Comparative graph illustrating conditions of transpiration, etc., for 

 the 5 dune environments; close gradation of factors evident here; increase in tran- 

 spiration accompanying decrease in growth water is characteristic of dune habitats. 



be mentioned only briefly in this paper. The increase in relative 

 transpiring power that occurs with an increase in exposure of 

 habitat is very evident. 



Concerning evaporation, the only variation occurs at station 

 C. Here the water loss is considerably more than at D. This 

 is because of the greater access of wind at C, which is located 

 on the edge of a blowout, while D is on a protected lee slope where 

 only south and southeast winds have access. 



The average relative humidity decreases rapidly from A to 

 E, and is almost parallel with the increase in the transpiration 

 index. Variation in humidity has been considered the most poten- 



