300 PLANT RESPONSE 



The theory of osmotic action cannot be considered satis- 

 factory, since such action is too slow ; besides which, there is 

 no fixed distribution of osmotic substances, such as would 

 account for the necessary transportation-current. 



The theory of root-pressure, again, is open to the objection 

 that it cannot possibly account for the maintenance of a 

 sufficient force during the process of active transpiration, when 

 root-pressure is found to be negative. Moreover, this root- 

 pressure itself requires an explanation. 



There is, however, another theory, due to Dixon, Joly, 

 and Askenasy, which has apparently more to support it than 

 any of those yet mentioned. According to this, the ascent is 

 brought about by transpiration from the leaves. The fluid 

 in the mesophyl 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 ; (2) in the absence of any conclusive proof 

 that, under actual conditions within the plant, the water- 

 column could have sufficient tensile strength ; and, lastly, 

 (3) in the fact which I shall demonstrate, that, when evapo- 

 ration is not taking place in the leaves, the transport of water 

 is still very considerable, and that, besides, other related 

 phenomena, like exudation pressure, continue to take place even 

 in the complete absence of evaporative activity in the leaves. 

 I shall, moreover, be able to show that the movement of water 

 often takes place in the plant in a direction opposite to that 

 which would be the case if osmotic action were alone involved. 



* Thus,' to quote Pfeffer, ' a satisfactory explanation of 

 the means by which the transpiration-current is maintained 

 has not yet been brought forward. If no vital actions take 

 part in it, then it is obvious that we have only an incomplete 

 knowledge of the causes at work, and of the relationship of 

 the different factors concerned.' ' 



' Pfefter, Physiology of liaiits, English Iranslalion, 1903, p. 224. 



