82 TRANSPIRATION AND ASCENT OF SAP ch. 



by the addition of carbo-hydrates. The denser fluid 

 thus produced and passed into the tracheids would settle 

 downwards. As it passed down it would displace upwards 

 the less concentrated solutions entering at the root. An 

 accumulation of the denser material in the lower part of 

 the tree may be supposed to be prevented by the abstrac- 

 tion of materials from the concentrated sap all the way 

 down. In this way it is secured that the ascending " raw ' 

 sap is just overbalanced by the denser descending column, 

 and the very dilute solutions brought into the root might 

 in this way be raised to any height. A model illustrating 

 the hypothesis is easily set up. A tube say 1 mm. bore 

 and closed at the lower end is filled with a solution of a 

 dye, e.g., fuchsin, and set upright. A small funnel con- 

 taining a denser salt-solution is attached to its upper end. 

 The heavy solution immediately begins to gravitate down- 

 wards, and in doing so displaces an equal volume of the 

 lighter fluid upwards. The rise may be noted by the pas- 

 sage of the coloured fluid upward into the funnel. 



There is no doubt that this mechanism could work in 

 uninjured plants the roots of which continued to pass 

 comparatively pure water into the conducting tracts, pro- 

 vided 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 trachea?, 

 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 liquids 

 which inevitably takes place in narrow tubes, would cer- 

 tainly destroy this gravitational action in the trachea? of 

 plants, and there is no evidence whatever of isolated 

 upward and downward currents. 



