IV.] VARIOUS THEORIES, &c. 63 



towards the modern ideas of imbibition. Sachs in his 

 Experimental-Physiologies took both views into ac- 

 count, and thought that capillarity as well as imbibi- 

 tion came into play. In 1868 Unger^ concluded that 

 the water does not ascend in the lumina of the vessels, 

 &c., but that it passes up as water of imbibition in 

 the substance of the cell-walls ; and Sachs, in the 

 fourth edition of his Lehrbiich^ definitely threw over 

 the capillary theory, and assumed that the water 

 moves either entirely as water of imbibition in the 

 substance of the walls, or (as Quincke had suggested) 

 as a thin film of water on the inside of these walls. 



Meanwhile observers had begun ranging themselves 

 more or less into two groups as it were. Boehm ^ in 

 1864 suggested that the elasticity of the epidermal 

 cell-walls would come into play, and affect the 

 pressures on and in the air-bubbles in the cavities of 

 the elements : and Theod. Hartig ^ had insisted upon 

 the alternate expansions and contractions of these 

 air-bubbles as important factors in causing water to 

 move from cell to cell. 



From other points of view Von Hohnel showed^ 



^ Sitziingiber, d. Akad d Wissensck. zu WieUj Ivu. abth. i. 1868. 

 ^ Sitzungsber, d, kaiser i. Akad, der JFzss, zu Wzen^h. 50 i. (1864). 

 2 Bot. Zeitg,^ 1 861, p. 18. 

 ^ yah7'b,fur Wiss. B&Lt 1879, b. 12, p, 77. 



