128 CLARK— ON WATER CONTENT 



a species difference. The water content for Populus tremuloides was 

 always greater under hydrophytic clear and cloudy than under xerophy- 

 tic clear and cloudy situations (5.3% and 9.3% respectively.) This 

 difference may be accounted for on the grounds that under xerophytic 

 conditions there is probably a scarcity of available soil moisture. The 

 amount of moisture lost in transpiration would exceed that taken in by 

 absorption, and the water content would decrease. 



Average transpiration was \.2\%, .83%o, and .43% for Fagus ameri- 

 cana, Hamamelis virginiana, and Quercus alba respectively. These 

 results agree with those of von Hohnel/* who stated that transpira- 

 tion in beech leaves was greater than in oak. Rate of transpiration 

 in all three species was highest during the developing season in May, 

 and early June. This confirms Holtermann's^^ work on Paradenyia. 

 Wiesner^'' found that falling leaves transpired more rapidly than those 

 that fell later. Fig. XXXIII shows that in Fagus americana, Hamame- 

 lis virginiana, and Quercus alba, on the contrary, transpiration decreases 

 steadily from September 1st to leaf-fall. Water content is in no way 

 influenced by the factors of transpiration, temperature, or relative 

 humidity. 



Average water content for the season was 59.9%, 63.1%, and 59.2% 

 for Fagus americana, Hamamelis virginiana, and Quercus alba respec- 

 tively. Fig. XXXIII for seasonal variation shows that for Fagus ameri- 

 cana, Hamamelis virginiana, and Quercus alba, during the developing 

 period in May and early June, there is a steady decrease in water con- 

 tent. With fluctuations the water content is constant through June. 

 During July and August there is a slight gradual decrease, and from 

 September 1st to leaf-fall, there is a gradual rise in water content. 

 Benedict^'' finds that in Vitis vulpina L. and certain other plants senile 

 changes are evident. Seasonal variation in water content in Fagus 

 americana, Hamamelis virginiana, and Quercus alba is probably due to 



i^ow Hdknel, F. v., tJoer das Welken abgeschnittener Sprosse." (Wissensch. 

 prakt. Unters. auf dem Gebiete des Pflanzenbaues, herausg. v. Fr. Haberlandt, Bd. 

 II, Wien 1877, p. 120.) 



" HoUermann, K., Anatomisch- physiologische Untersuchungen in den Tropen. 

 Die Transpiration der Pflanzen in den Tropen. (Sitzb. d. kgl. preuss. Akad. d. 

 Wissensch. Berlin, Bd. XXX, 1902, p. 656.) 



^^Wiesner, J., Untersuchungen uber die herbsthche Entlaubung der Holzge- 

 wachse. (Sitzb. d. K. Akad. der Wissensch. Wien, Bd. LXIV, 1871, p. 461.) 



" Benedict, Senile Changes in Leaves of Vitis Vulpina L. and Certain Other 

 Plants. Cornell University Agricultural Experiment Station. 1915. p. 281. 



