Jan., 1916] Evaporation and Plant Zones 99 



The Scirpiis zone, with its maximum exposure to wind 

 and Hght, shows the highest rate by far — 413%. It seems 

 not improbable that to survive in such an environment the 

 Scirpus must possess unique characters beyond its undoubted 

 abihty to withstand the heavy beating of surf, which Jennings^ 

 mentions. 



The zone characterized by Castalia and Potamogeton ranks 

 second, with an evaporation percentage of 343. While not 

 directly exposed to severe winds this zone is undoubtedly one 

 of rather free air movement, and certainly one of continual 

 and merciless exposure to the sun. The relatively high rate 

 of evaporation here may well be the factor that is prohibitive 

 of emersed forms, other than stray plants of Scirpus, although 

 the factor of water depth cannot be ignored. 



That this habitat is a rigorous one for plant life is further 

 shown by the fatal effects of a day's exposure of the under side 

 of a Castalia leaf to the air of its habitat, whether by a con- 

 tinued light breeze from the proper quarter, or by accident or 

 experiment. This phenomenon, which it is interesting to 

 compare with "wind-burning" as noted by Gates, ^"^ was fre- 

 quently observed during the course of the work here described. 



Dianthera is a plant characteristic of washed sand bottoms,^ 

 a condition implying more or less exposure, and in this marsh 

 it is found under conditions of evaporation distinctly com- 

 parable with those obtaining in the windswept tops of Cal- 

 amagrostis, at an elevation of five feet (1.5 metres). 



The remaining figures are chiefly valuable as showing rather 

 strikingly the modifying power of vegetation on evaporation. 

 Especially marked is the difference between the nearby instru- 

 ments in the Spirodela and Typlia zones, respectively, at the 

 same level. Likewise the waterless is much greater at the same 

 level in the case of low-growing than of high-growing vegetation. 

 Moreover the instrument standing in the compact and sheltered 

 lower layers of the Calamagrostis lost decidedly less than the 

 one in the leafless and wide spaced lower layers of Phragmites. 



Finally the differences between upper and lower stations 

 in Calamagrostis, Phragmites and Typha respectively, are 

 ample enough to confirm Yapp's observation^ that different 

 strata of marsh vegetation afford vastly different habitat 

 conditions, as regards the evaporational factors. This is 

 not the less true because none of the three zones mentioned 

 happen to show any distinct stratification. 



