240 THE MOVEMENTS OF WATER 



to tranr piration alone, for other influences may frequently come into play, 

 while the peculiarities observed may be due to the action of a combination 

 of different factors l . 



The external conditions act as a stimulus which causes the plant to 

 accommodate itself to them as far as its specific powers allow, and in the 

 majority of cases its possible range of accommodation is not sufficient to 

 allow it to develop indifferently either on land or submerged in water, or 

 in both dry and wet habitats. During its development the plant adapts 

 itself as far as possible to the conditions under which it is growing, 

 but nevertheless even when adult a certain amount of accommodation is 

 still possible, though the permanent changes which can be induced are 

 comparatively trifling in extent. Transient and rapid changes, such as the 

 movements of the stomata, &c., serve to modify the transpiration according 

 to the conditions existing at the moment, and thus to exercise a certain 

 regulatory control. The vital activity of the plant is therefore of decisive 

 importance in determining the activity of transpiration 2 , while the warmth 

 which the plant itself produces may also have some influence upon this 

 function. The actual evaporation of water is a purely physical phenomenon, 

 dependent in a plant, as in a dead body, upon the physical properties of the 

 body in question, and upon the external conditions. These properties, 

 however, alter when the plant dies, and the evaporation of water is then 

 usually accelerated, as is indicated by the more rapid drying of dead parts, 

 and as direct experiments have proved 3 . Moreover, it must not be for- 

 gotten that the permeability of the cork and cuticle may be altered when 

 a plant is killed, especially if heat is employed for that purpose. 



Water evaporates more rapidly from a free surface, or from wet 

 filter-paper, than from a leaf. Unger found from numerous researches that 

 under similar conditions 1*4 to 6-9 times more water evaporated from 

 a free surface of water than from a similar area of ordinary green 

 leaves 4 . Plants such as Cacti probably often exhale a hundred times 

 less vapour than would a similar area of water. In ordinary leaves the 

 amount of internal free surface is very great, but the intercellular spaces 

 are always filled with damp air. Diffusion takes place only slowly through 

 the narrow stomatal pores, and hence the most markedly developed 



1 On light and shade leaves, Stabl, Kinfluss d. sonnigen u. schattigen Standorts a. d. Laubblatter, 

 1883, p. 19; Eberdt, Her. d. Bot. Ges., 1888, p. 371 ; Stenstrom, Flora, 1895, p. 131 ; Genean de 

 Lamarliere, Rev. ge"n. d. Bot., 1893, T. iv, p. 481, and the literature here quoted. 



[Only in this sense can transpiration be regarded as a vital process. Cf. H. Dixon, Proc. R. 

 Irish Soc., 1898, Vol. I, pp. 618, 627.] 



3 Mohl, Bot. Zeitung, 1847, p. 323 ; Nageli, Sitzungsb. d. Bair. Akad., 1861, Bd. I, p. 262 ; 

 Just, Cohn's Beitrage z. Biol., 1875, Bd. i, p. 24. 



* Cf. Knop, Versuchsst., 1864, Bd. vi, p. 350 ; Baranetzky, Bot. Zeitung, 1872, p. 62, footnote. 

 On the evaporation of water from the soil see Sachsse, Agriculturchemie, 1888, p. 203; Alessandri, 

 Bot. Jahresb., 1888, p. 74. 



