356 COMPARATIVE ELECTRO-PHYSIOLOGY 



manner, this primary movement being aided by accessory 

 factors. 



Among the physical theories which have been pro- 

 pounded for the explanation of the ascent of sap those of 

 atmospheric pressure and of capillarity are admitted to be 

 inadequate. But that of osmosis and transpiration, put 

 forward by Dixon, Joly, and Askenasy, is of much greater 

 weight. According to this, the ascent is brought about by 

 transpiration from leaves. The fluid in the mesophyll cells 

 of the leaves becomes concentrated by evaporation ; thus 

 osmotic attraction is set up by the leaves, and the suction 

 thereby exerted is supposed to be transmitted backwards as 

 far as the roots, through cohering columns of water. The 

 difficulties in the way of this theory lie (i) in explaining how 

 a slow osmotic action could produce so rapid a water-current ; 

 and (2), in the absence of any conclusive proof that, under 

 actual conditions within the plant, the water-column could 

 have sufficient tensile strength. Even apart from these 

 objections, however, the fact remains that energetic water- 

 movements take place in the plant in the entire absence of 

 transpiration. For example, sap exudes from the cut end of 

 a tree which may exert a pressure as great as that of a 

 column of liquid 1 3 metres in height 



It is thus seen that there is an independent activity 01 

 some kind which maintains the movement of water through 

 the plant. That this activity, moreover, is not resident in the 

 root merely is seen from the fact that exudation of water 

 takes place from the tips of grass-blades when their cut stems 

 have been placed in water. Criticising the theory of trans- 

 piration, Strasburger rightly remarks that transpiration only 

 makes a place for inflowing water, but cannot furnish the 

 force necessary to convey a large volume of fluid rapidly for 

 a considerable distance through wood. From the considera- 

 tion of these and other facts, Pfeffer, in his summary, was 

 led to the conclusion which he states as follows : * A satis- 

 factory explanation of the means by which the transpiration- 

 current is maintained has not yet been brought forward. If 



