iv COHESION THEORY OF ASCENT OF SAP 81 







Electrical theory. An electrical theory also presented 

 itself. It is well known that when a colloid dispersed 

 through water is exposed to an electric field the colloid 

 tends to move to one pole or the other, depending upon 

 its electric sign. If the colloid is held stationary, the 

 water will be translated in the opposite direction. Con- 

 sequently, if there is a difference of electric potential at 

 the upper and lower extremities of a stem, we would expect 

 a tendency to motion upwards or downwards according to 

 the sign of the colloidal walls of the conducting tracts. 

 It seemed possible that atmospheric electricity, which main- 

 tains a potential gradient usually amounting to 50-150 

 volts per metre elevation from the ground, might produce 

 the necessary field. 



It was disappointing to find that no experimental evi- 

 dence could be obtained in support of this hypothesis. 

 Thus, when leads coming from the terminals of a Wims- 

 hurst electrical machine were introduced into small reser- 

 voirs fixed to the opposite ends of a piece of stem about 

 2 m. long, actuation of the machine brought about no 

 observable motion of water from one reservoir to the other, 

 whether the branch and reservoirs were filled with water 

 or with a dilute solution. 



Tensile film theory. Quincke's theory (which sug- 

 gested 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. Our reason for discarding it was not 

 that which led Sachs to oppose it ; for he objected to it 

 on the grounds that 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 



G 2 



