THE MECHANISM OF WATER-TRANSPORT 227 



out under pressure (Sect. 42), and owing to the saturation of the tracheal 

 elements with water, the conditions are not quite the same as when transpiration 

 is active. In the former case the chain of air and water becomes more and more 

 reduced, and may ultimately consist of water only, so that the water-current has less 

 resistance offered to it, and is hence accelerated, as may be directly shown when 

 a trachea is opened at both ends, and the chain of air and water driven out by 

 a continuous current of fluid. 



Observations made on filtration under pressure can only be extended to the 

 transpiration-current with extreme caution. Water niters but slowly through 

 living cells under pressure, so that any water forced into a piece of stem passes 

 almost entirely through the wood, and especially the tracheae, to appear on the 

 other cut surface. Water filters through the wood more slowly across the 

 vascular elements, and the power of filtration diminishes still more in the 

 duramen (Sect. 35) \ The amount of water filtering through a cut surface 

 gradually diminishes, owing to the occlusion of the channels by suspended 

 particles and by slimy growths, which may be derived partly from bacteria and 

 partly from the escaped contents of opened cells, while a formation of tyloses may 

 also aid in blocking the tracheal channels. The occlusion is, however, mainly at 

 the cut surface, for the removal of a thin section markedly increases the rate 

 at which water is absorbed 2 . It is, perhaps, owing to the fact that no solid 

 particles enter them, that intact tracheal elements are able to serve as good 

 conducting channels for a long time, provided that they undergo no special 

 modifications. 



The rule of transmission of water by simple filtration under pressure is rapid, 

 and thus when a piece of stem from a plant saturated with water is removed and 

 suspended vertically, a certain, and under appropriate conditions a considerable, 

 amount of water exudes from the lower cut surface 3 . Water continues to be exuded 

 until the menisci formed in the tracheae on the upper surface are able to support 

 the columns of water suspended from them. If a drop of water is placed on the 

 upper surface, the equilibrium is disturbed, and a corresponding drop appears on 

 the under surface. This phenomenon, which has been frequently misunderstood, 

 was first correctly explained by Godlewski 4 . 



1 See Sachs, Arb. d. But. Inst. in \Yiirzburg, 1879, Bd. II, p. 285; Strasburger, Bau u. 

 Verricht. d. Leitungsbahnen, 1891, p. 819; Nageli u. Schwendener, Mikroskop, 1877, 2 - Aufl., 

 P- 3 8 5 ! Unger, Sitzungsb. d. Wien. Akad., 1868, Bd. LVIII, Abth. i. 



2 Hales, Statics, 1748, p. 71 ; Briicke, Ann. d. Physik. n. Chemie, 1844, Bd. LXHI, p. 187; 

 Sachs, Arb. d. Bot. Inst. in Wiirzbnrg, 1879, Bd. II, p. 300; v. Hohnel, Bot. Zeitung, 1879, 

 p. 302 ; Strasburger, 1. c., 1891 , p. 564 ; Wieler, Cohn's Beitrage zur Biologic, 1893, Bd. VI, p. 149 ; 

 Biol. Centralbl., 1893, Bd. xin, p. 584. 



3 Observations on various plants have been made by Gaudichaud, Portenu, and Schimper. Cf. 

 the literature given by Strasburger, 1891, 1. c., p. 823. 



4 Godlewski, Jahrb. f. wiss. Bot., 1884, Bd. xv, p. 588; Schwendener, Sitzungsb. d. Berl. 

 Akad., 1886, Bd. xxxiv, p. 582. 



Q 2 



