76 PLANT PHYSIOLOGY 
advantage of a plant to have transpiration taking place 
but just the contrary. 
113. The amount of water given off by transpira- 
tion is very large. The water loss from a Birch tree, 
standing alone and estimated to have 200,000 leaves was 
calculated by von Héhnel at about 500 liters on a very 
hot dry day and about 60 to 70 liters on average days. 
An acre of hops will evaporate three million to four 
million liters of water in a season. Dietrich estimates 
that for every gram of dry substance found in a plant, 
from 250 to 400 grams of water have been evaporated. 
In twelve hours, a grape leaf evaporates as much water as 
would form a film 0.13 mm. deep over the whole leaf, 
while for cabbage and apple leaves in the same length of 
time the figures are respectively 0.31 and 0.25 mm. 
In one season, an oak tree, during the time it holds its 
foliage, evaporates an amount equivalent to 33 mm. over 
all its leaves. An open water surface would evaporate, 
in the same time, 500 to 600 mm., showing that the 
evaporation (transpiration) is far less from the leaves 
than from a free surface. 
114. It has been shown that an impermeable surface 
with very numerous openings, as for example, the 
epidermis with its numerous stomata, evaporates nearly 
as much water as if it were a free water surface. The 
stomata, however, are capable of closing and thus almost 
wholly preventing water loss for such periods of time as 
they may remain closed. At night they are nearly 
closed. When the plant begins to wilt, it has been 
shown that they also close automatically through re- 
duced turgor of the guard cells thus preventing too great 
a loss of water. All physical phenomena which increase 
evaporation also increase the water loss from the leaves 
as long as the stomata remain open, e.g. increased 
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