AND TRANSPIRATION OF TREES 127 



For daily readings the maxima and minima values of water content 

 occur at no definite periods. As a general rule the two minima for 

 transpiration occur early and late in the day, and the maximum at 

 1 p. M. The minimum temperature is at 8 a. m. and the maximum at 

 2-3 p. M. Maximum percentage for relative humidity is at 8 A. m. and 

 minimum at 2-3 p. m. Weather conditions are on the whole regular 

 and constant. While daily variations in transpiration, temperature, 

 and relative humidity are fairly constant, we find no connection between 

 these and water content. Water content therefore must be indepen- 

 dent of the influence of transpiration, temperature and relative humidity. 



According to Dixon^' the loss of water due to evaporation "can only 

 be made good by drawing in water from adjacent tracheae, and this 

 pull acting on the upper ends of the cohering columns of sap is propo- 

 gated downward through the tree. We may then regard secretion or 

 evaporation as the force which actually exerts the tension on the sap, 

 and this tension is transmitted through the leaf cells to the sap in the 

 conducting tracts. " Also the same author states that " the amount of 

 transpiration falls off as water is exhausted from the soil." It would 

 seem therefore that so long as the available soil moisture remained 

 constant, there would be no change in the water content of the leaves. 



Because of the presence of a stream flowing on two sides of the plot 

 and because of unusually great seasonal rainfall, there could have been 

 no scarcity of available soil moisture, and daily water content should 

 have been constant. Reference to Table VI shows that the average 

 variation from the average water content for each day lies between 

 .4 of 1% for Fagus americana (May 9) and 4.87% for Hamamelis vir- 

 giniana (May 2). For the whole season the average variation from the 

 average water content was 1.5% for Fagus americana, 2.4% for Hamame- 

 lis virginiana, and 1.5% for Quercus alba. These differences run so 

 low that on the grounds of probable error they may be disregarded. 

 We may consider that from 8 a. m. to 5 p. m. the water content for 

 Fagus americana, Hamamelis virginiana, and Quercus alba is constant. 



In the final results of the preliminary experiment it was found (Table 

 rV) that the difference in average water content between the species 

 of Fopulus under various conditions was generally negligible. Under 

 hydrophytic conditions, both cloudy and clear, the water content for 

 Fopulus tremuloides was always greater than for Fopulus grandidentata 

 (5.3% and 3.7% respectively.) Here there is probably the presence of 



'^^ Dixon, H. Transpiration and the Ascent of Sap in Plants, 1914, p. 118-140. 



