32 Henry H. Dixon. 



fluid upwards. The rise may be noted by the passage of the coloured 

 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 prevent 

 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 cavities 

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

 surrounding them were in compression. But however promising 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 tracheae in the form of a thin film, 

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

 objection, 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 

 unsupported 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 

 inevitable 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 surface 

 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 downwards through the imbibed water in the 

 walls. This theory has undoubted advantages over the imbibition 

 hypothesis. It replaces the difi'usion flow by a movement under 

 great tensions, and so the rate of transmission may be increased 

 proportionately to the increased tension. But it is open to many of 

 the objections which overthrew the imbibition hypothesis, viz., the 



1) J. vonSachs, Lectures on the Physiology of Plants. Trans, by H. M a r s h a II 

 War;d, Oxford 1887, p. 238. 



