106 
Those who desire further information on the subject are recommended to read 
the chapter " Bainfall in, above, and near Forests," by M. W. Harrington, at page 106 
of the work " Forest Influences," already quoted. 
7. PHYSIOLOGICAL ACTION OF TREES TRANSPIRATION. 
Another phase of the conservation of water by trees is the question of 
transpiration. This is the technical word for what may be described as the 
perspiration of plants. The tree absorbs moisture by its roots, which is utilised to 
continue the functions of the plant, and a portion of it is exhaled in the form of 
vapour by each leaf, and passes into the atmosphere. The effect of a single tree is 
a very large multiple of that of a single leaf, and that of a forest is similarly greater 
than that of a single tree. This emission of vapour by plants is more or less fully 
dealt with in all works on vegetable physiology. In this way a forest has an 
appreciable effect on the humidity of the atmosphere, and this is one of the reasons 
why, on the ground of transpiration alone, the atmosphere of a forest is moister than 
that immediately above the surrounding land, and it is desirable to conserve the 
forest growth on that account. 
Botanists have made many measurements of its amount, and their results are 
extremely varied, due partly to the fact that this function varies much naturally, 
and still more to the fact that experiments are generally made under conditions 
which are not natural to the plant. Sachs says that it is no rarity for a tolerably 
vigorous tobacco-plant at the time of flowering, or a sunflower the height of a man, 
or a gourd-plant with from fifteen to twenty lai'ge leaves, to transpire from 1 to 2 
pints of water on a warm summer day ; and so far as may be judged by the use of 
branches with the cut end in water, it may be believed that large fruit-trees, oaks 
'or poplars, absorb, transport through their stems, and transpire through their leaves, 
10 to 20 or more gallons of water daily. It is not generally practicable to compare 
the transpiration with known meteorological phenomena, such as evaporation from 
a water surface, or from the soil, or the precipitation, but some such comparisons 
have been made. For instance, comparing the leaf surface to an equivalent water 
surface, linger makes transpiration from the former 0'33 of the.evaporation from the 
latter ; Sachs, for white poplar, 0'36, the sunflower 0'42. Comparisons have also 
been made between the transpiration from plants and from the evaporation from the 
surface over which the plants stand. Schlciden thought that the transpiration from 
the forest was three times that of the water surface equal to the territory covered by 
the forest ; Schubler thought it only a quarter ; and Pfaff, who studied a solitary oak 
in a garden, found that it varied from O'ST to 1'oS. Comparing the transpiration 
of plants with the evaporation of ths bare soil which would be covered by them, 
Hartig thought the transpiration cf a forest less, Schiibler found it 0'6 for the forest 
and 3'0 to 5'0 for the sod. Marie-Davy found it for firs 1'18, for beeches 1 32, for 
sod 1-80. 
