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.) 
KF. 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 different years. 
Transpiration of water corresponding to growth of unit weight of dry leaves. 
Plant. 1878. | 1879. | 1880. || Plant. 1878. | 1879. | 1880. 
| | | 
Bireh and linden .::-3.<|7,.650)|'7'1,000)|| , 90), Oaks;<4 ees Aes 250 400 | 59 
ING ees See 550 700 101 | Spruce and Scotch pine. 60 | 150 | 13 
IBCCCH Banas eee ees 475 600 £2) Ul Vis) ota bea eo eS 3D | 100 9 
Maple; ie oh. 2. eee 425| 550|  70|| Black pine __........-..- 35 75 4 7 
| | 
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 pounds 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-five 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 24 pounds 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- 
