412 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



bore and closed at. the lower end, is filled with a solution of a dye, 

 e. g., fiichsin, and set upright. A small funnel containing a denser 

 salt solution is attached to its upper end. The heavy solution imme- 

 diately begins to gravitate downward, and in doing so displaces an 

 equal volume of the lighter fluid upward. The rise may be noted 

 by the passage of the colored fluid upward in the funnel. 



There is no doubt that this mechanism could work in uninjured 

 plants whose roots continued to pass comparatively pure water into 

 the conducting tracts, provided there were an arrangement to pre- 

 vent the mixing of the descending and ascending fluids. In the 

 plant, we may suppose, the column is not supported below as in the 

 model, but is held up by the capillary forces of the imbibed cell walls. 

 This would explain the presence of reduced air pressure in the cavi- 

 ties of some of the wood tracheae, which would be impossible if the 

 water surrounding them were in compression. But, however prom- 

 ising for a time, the theory had to be given up. The mingling of 

 the dilute ascending solutions with, the concentrated descending 

 fluids which inevitably takes place in narrow tubes, would certainly 

 destroy this gravitational action in the tracheae of plants, and there 

 is no evidence whatever of isolated upward and downward currents. 



Quincke's theory^ (which suggested itself independently to us), 

 viz, that the water is drawn up in a tensile state over the surfaces 

 of the walls of the conducting tracheaj in the form of a thin film, 

 had also to be laid aside. Not, however, by reason of Sachs's ob- 

 jection, who rejected it because there are not continuous tubes in 

 plants. In reality this objection is quite invalid, since the water 

 films may be regarded as continuous through the imbibed material 

 of the transverse and oblique walls. Nevertheless the theory had to 

 be abandoned, since, as we shall see later, such a film of water un- 

 supported on one side, if exposed to tension, infallibly draws out 

 thinner and thinner until it breaks across and leaves no water on the 

 surface. 



A modification of this theory, combining it with the Unger- Sachs 

 imbibition theory, then suggested itself. In order to escape the in- 

 evitable thinning out of the unsupported water films, we assumed 

 with Sachs, that the moving water is located in the substance of 

 the walls, and that the surface-tension forces developed at the sur- 

 face of the fine-textured substance of the wall prevent the water 

 from drawing out thinner and thinner. Thus the tension generated 

 at the leaves is transmitted downward through the imbibed water in 

 the walls. This theory has undoubted advantages over the imbibi- 

 tion hypothesis. It replaces the diffusion flow by a movement 

 under great tensions, and so the rate of transmission may be in- 



^ J. von Sachs, Lectures on the Physiology of Plants. Trans, by H. Marshall Ward, 

 Oxford, 1887, p. 238. 



