112 



a plant under given conditions are found at a given level, and if we 

 change these conditions as to moisture, temperature, etc., we shall 

 change the distance from the surface down to this level. (Wollny's 

 Forschungen, Vol. XIV, p. 132.) 



TRANSPIRATION. 



The quantity of water transpired by trees and plants depends upon 

 the amount of water at their disposal, as well as on the temperature 

 and dryness of the air, the velocity of the wind, the intensity of 

 sunlight, the stage of development of the plant, the amount of its 

 foliage, and the nature of its leaf. The following are some of the 

 results of measurements at European experiment stations. (See 

 Fernow, Report, 1889, p. 314.) 



F. B. Hoehner found that the transpiration per day per 100 grams 

 of dry weight of leaves is for conifers 4.778 to 4.990 grams, but for 

 deciduous trees about ten times as much, 44.472 to 49.553. During 

 the whole period of vegetation a unit weight of dry leaves corre- 

 sponded to a total weight of evaporated water, as shown by the fol- 

 lowing table, for three diflferent years. 



Transpiration of tratcr corresponding to c/rontJi of unit weight of dry leaves. 



The variability of transpiration is shown by the action of a birch 

 in the open air, which transpired on a hot summer day from 700 to 

 900 pounds, while on other days it probably transpired not more 

 than 18 to 20 pounds. A beech about 60 years old had 35,000 leaves, 

 whose total dry weight was 9.86 pounds; hence its transpiration, at 

 the rate of 400 j^ounds of water per pound of leaves, would be 22 

 pounds daily. An acre containing 500 trees would, during the total 

 period of vegetation, transpire nearly 2,000,000 pounds of water, or 

 about 50 pounds to the square foot. 



A younger beech, thirty-fi^'e years old, with 3,000 leaves and a dry 

 weight of 0.79 pounds, would, under the same conditions, transpire 

 470 pounds per pound or 2| jiounds per day from June to November. 

 An acre containing 1,600 such trees would transpire about 600,000 

 pounds per acre or 15 pounds to the square foot from June to 

 November. 



Of the entire mass of wood and foliage on an acre of forest from 56 

 to 60 per cent of the weight is water and 44 to 40 per cent dry sub- 



