TRANSPIRATION UNDER NORMAL CONDITIONS 251 



9 sq. m. (lof sq. yds.) lost 0-85 kilogrammes (i pint) of water during a single 

 dry day. During a very hot day large trees may lose more than 400 

 kilogrammes, but during a rainy day at times not more than a few kilo- 

 grammes. V. Hohnel has calculated that a birch-tree with about 200,000 

 leaves transpired 300 to 400 kilogrammes of water during a single hot day, 

 while for a ii5-year-old beech he estimated the average amount of water 

 transpired daily between June i and Sept. i to be 75 kilogrammes. The 

 amount of water given off from 400 to 600 similar trees occupying an 

 area of one hectare (2^ acres) from June i to Dec. i is, according to 

 v. Hohnel, from 2-4 to 3-5 million kilogrammes. 



In making these calculations, v. Hohnel attempted to allow for the 

 fact that the conditions are not equally favourable for the different parts 

 of the plant, and that a tree surrounded by others will naturally transpire 

 less actively than when by itself in the open. The estimations made by 

 F. Haberlandt are apparently somewhat too high, because these factors 

 were not sufficiently taken into consideration. Haberlandt found that 

 oat-plants covering one hectare transpired 2,277,760 kgs. during a single 

 vegetative period, while barley plants covering a similar area exhaled 

 1,236,710 kgs. of water. He also calculated that the amount of water 

 transpired by a single plant during its development was, in the case of 

 maize 14 kgs., hemp 27 kgs., sunflower 27 kgs., the periods of development 

 being 173 days, 140 days, and 140 days respectively. These amounts of 

 water correspond to a rainfall of 227-8 to 123-7 millimetres. 



The annual amount of rainfall, which in Germany is about 600 mm. 

 or 23^ inches, and in England is about 28 inches, is thus seen to be greater 

 than the amount of water which can be lost by transpiration, especially 

 since during winter the latter is reduced to a minimum. Some such 

 relationship must naturally exist when the whole of a country is taken 

 into consideration, although the direct evaporation from the soil is by no 

 means inconsiderable. In the neighbourhood of rivers, or where the 

 ground-water is abundant, more water may be transpired over a small 

 area with very luxuriant vegetation than the annual rainfall furnishes. 

 These are all matters which are of the utmost importance in the economy 

 of nature, but they come more within the scope of a textbook of agri- 

 culture, and hence no comparison need be made here between the 

 amounts of water-vapour given off by soil when bare and when covered 

 with vegetation 1 . 



1 See for example Sachsse, Agriculturchemie, 1888, p. 427 ; Wollny, Forsch. a. d. Geb. d. Agri- 

 culturphysik, 1881, Bd. IV, p. 85 ; Alessandri, Jahresb. d. Bot., 1888, I, p. 74. 



