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 Hohnel 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 



