106 CLARK— ON WATER CONTENT 



I. The Problem 



The problem is, to determine the water content and transpiration 

 in the leaves of Fagus americana, Hamamelis virginiana, and Quercus 

 alba during successive seasonal changes. Fagus americana, Hamamelis 

 virginiana, and Quercus alba are all native species of our Pennsylvania 

 hardwoods, and in this connection, have been studied, as far as possible, 

 under normal conditions in their natural habitat. 



By "water content" is meant, the percentage of the whole fresh 

 leaf that is water; it includes both the water within the walls of the 

 leaf cells and that held in the intercellular spaces. "Transpiration" 

 is used in the sense of Burgerstein^ to mean the amount of moisture 

 given off in the vapor form from the surface of the leaf ; it includes both 

 epidermal and stomatal evaporation. The work started with the 

 earliest leaf development in the Spring of 1915, extended through the 

 Summer, and ended with the leaf -fall in the Autumn months. 



II. Work Previously Done 



Much work has been done along the lines of transpiration. It has 

 been studied extensively from morphological, physiological, ecological, 

 and purely physical standpoints, and detailed research has been carried 

 on showing the relation between transpiration and sap flow. While 

 Miinsch^ has shown that water content in plants is a determining fac- 

 tor in the appearance of fungous diseases, little connected work has 

 ever been done heretofore along the lines of transpiration and the water 

 content of leaves. 



III. Available Methods 

 Transpiration 



As pointed out by Burgerstein^ any of the following methods are 

 .available for determining the amount of water lost in transpiration: 

 a. Physical methods. 



1. direct weighing. This consists of weighing specimen and 

 apparatus both before and after a certain transpiration 

 period. 



2. collecting and weighing water vapor in a closed room. 



^ Burgerstein, "Die Transpiration der Pflanzen," 1904, p. 3. 



^Munsch, " Untersuchungen iiber Immunitat und Krankheitempfanglichkeit 

 »der Holzpflanzen. " Naturwiss. Zeitschr. f. Forst. u. Landw. 1909, 7:54-75, 87-114, 

 129-160. 



^ Burgerstein, "Transpiration der Pflanzen," 1904, p. 4-28. 



