172 BOTANY. 



from the leaves of the White Poplar to be about one third as 

 rapid as from water. Unger places the evaporation from 

 most leaves at about one third that from equal areas of 

 water ; in some cases, however, running as low as one fifth 

 and one sixth. * 



224. — PfafE calculated the amount of water evaporated 

 from an isolated oak tree during the growing season. The 

 tree selected was a close- topped one 6f metres (20 ft. ) high, 

 bearing about 700,000 leaves. The results were as follows : 



May (14 days) 883 kilograms = ( 1,944 lbs.) 



June 26,033 " = (57,250 " 



July 28,757 " = (63,365 " 



August 21,745 " =(47,839" 



September 17,674 " =(38,883" 



October 17,028 " =(87,450" 



The evaporation from each leaf was for the season of five 

 and a half months (one hundred and sixty-seven days) .16 

 kilograms (.35 lbs.) ; allowing forty-eight square centimetres 

 of surface to each leaf, this amounted to a layer of water 

 3.33 centimetres (1.31 in.) deep over the whole evaporating 

 surface, f 



225.— The Movement of Water in the Plant. It is clear, 

 from what has been said, that in polycellular plants there 

 must be a considerable movement of water in some parts, to 

 supply the loss by evaporation. Thus in trees there must be 

 a movement of water through the roots, stems, and branches 

 to the leaves, to replace the loss in the latter. This is so 

 evident that it scarcely needs demonstration ; it can, how- 

 ever, be shown by cutting off a leafy shoot at a time when 



* The three last statements and ihe following are given on the 

 authority of Ducliartre (" Elements de Botanique," second edition, 1877, 

 pp. 844 and 846). 



f Pfaff found that the water evaporated during the season, when con- 

 sidered with reference to the area of ground covered by the tree top, 

 was equal to a layer 5.39 metres high (213 inclies). Observation had 

 shown the annual rain-fall to be .65 metres (25.6 inches) ; so that the 

 water evaporated from the tree was eight times the amount which fell 

 upon the earth under it. The evaporation is very much less in dense 

 forests than in isolated trees, but with every allowance it is suflBcient 

 in dry, hot seasons to quickly exhaust the moisture of the soil. 



