226 THE MOVEMENTS OF WATER 



Hales proved that the water travels in the wood, and that the transference of 

 water takes place in transpiring plants by means of an upward sucking action, and not 

 by a vis a tergo. The general principles regulating the movements of water were 

 thus early established, and since then repeated attempts have been made to explain 

 the mechanism as well as the causes of the transpiration-current 1 . Some authors 

 saw a sufficient explanation in capillary or osmotic forces, both of which were 

 regarded as insufficient by others, while explanations were not wanting in which 

 a vital action was postulated. The problem is still unsolved, in spite of all the 

 advances made and the deeper insight which has been gained' 2 . 



Sachs (1878) regarded the water as being transferred by imbibition through the 

 tracheal walls, without considering any vital action necessary, but this ' imbibition 

 theory' is now untenable (p. 219). Godlewski, on the other hand, considered 

 the pumping action of living cells to be necessary, but was unable to prove this 

 hypothesis 3 . Hence no discussion of his theoretical conclusions is necessary, 

 nor of those of Janse and Westermaier 4 , as to the way in which living cells may 

 act in raising water. Schwendener did not support any particular theory, but 

 apparently was inclined on theoretical grounds to regard a vital action as necessary. 

 By the clearness of his physical explanations he has, however, aided much towards 

 a precise definition of the problem at issue. That Hartig's so-called 'gas-pressure' 

 theory did not suffice to explain the raising of water in trees had already been 

 shown by Godlewski ''. 



We owe various important experiments to B6hm fi , who was, however, less 

 fortunate in his interpretations of the facts observed, and who finally came to 

 regard a simple capillary ascent as a sufficient explanation. Bohm showed that 

 water may be transferred through a dead piece of stem for a considerable 

 distance. Strasburger extended these experiments, and obtained many important 

 results, without however being able to give a satisfactory explanation of the 

 problem. The why and wherefore is not explained by the conclusion that 

 the distribution of the air, water, &c. is such as to produce a condition of 

 equilibrium, any disturbance of which causes a corresponding flow of water. 

 Nor do the important researches of Dixon and Joly, or those of Askensasy, 

 afford a completely satisfactory explanation. 



Filtration under pressure. In the process of normal bleeding, water is forced 



Treviranns, Physiol., 1835, Bd. i, p. 300. [Nehemiah Grew, Anatomy of Plants, 1683, pp. 79, 

 "30 



1 Cf. Mohl, Grundz. d. Anal. u. Physiol., 1851, p. 70; Meyen, Pflanzenphysiol., 1838, Bd. n, 

 PP- 5. 55, &c. ; Treviranus, Physiol., 1835, Bd. i, p. 284 ; de Candolle, Physiol., 1833, T. I, p. 79. 



2 Cf. also the literature references in the Annals of Botany, 1896, Vol. X, p. 630. 



3 Godlewski, Jahrb. f. wiss. Bot., i88.j, Bd. xv, p. 602. [A similar explanation was given 

 two hundred years ago by Nehemiah Giew, 1. c., p. 126.] 



* Janse, Jahrb. f. wiss. Bot., 1887, Bd. xvni, p. 68 ; Westermaier, Ber. d. Bot. Ges., 1883, 

 p. 371, and Sitzungsb. d. Berl. Akad., 1885, Bd. XLVIII, p. 1105. 



* Godlewski, 1. c., p. 583. There also (p. 627) the untenable hypotheses of Scheit are dealt 

 with. R. Hartig, Die Gasdrucktheorie, 1883 ; Schwendener, Sitzungsb. d. Berl. Akad., 1886, 1893. 



6 Bohm, Ber. d. Bot. Ges., 1889, Generalvers., p. 46 ; 1892, p. 692 ; 1893, p. 203, &c. 

 A critical discussion of Bohm's views is given by Godlewski, 1. c., p. 571 ; Schwendener, Sitzungsb. 

 d. Berl. Akad., 1892, p. 936. 



