378 



PROTOPLASM 



+ +++-H-+ 



+*-►++♦+ 



when the pH vahie is on the alkaline side and toward the anode 

 when on the acid side of the reversal (isoelectric) point. 



The one-way diffusion of water through the membranes of 

 tissues, as in the case of the inner wall of the intestines, is a 

 well-known but not fully understood phenomenon. Electro- 

 endosmosis is a possible interpretation of it. The membrane is 

 a mass of capillaries, each presumably lined by a Helmholtz 

 double layer (Fig. 163). If there is a potential 

 gradient across the membrane, water will flow in 

 one direction only. 



Electroendosmosis has also been resorted to as an 

 explanation of gland secretion. Glands exhibit 

 anomalous flow in that substances in solution move 

 from a region of low concentration in the gland to 

 one of high concentration in the surrounding tissue. 

 For example, the nectary of a flower continues to 

 secrete nectar even when its own tissue actually 

 contains less nectar than the solution outside the 

 gland in the nectary. Possibly, electroendosmosis 

 is the force responsible, for then, under the influence 

 of an electric potential, a solution will flow contrary 

 to simple diffusion laws. 



N. Marinesco has applied electrokinetic forces to 

 E f e^c t^r r~ ^^^ problem of the ascent of sap in plants, a problem 

 charges lining that has long interested botanists and never been 

 mrmbrane°^ ^ conclusively Settled. He has submerged a bundle of 

 xylem vessels (the sap-conducting capillaries of 

 woody plants) in an electrolytic solution and applied an 

 external electromotive force to the extremities of the tubes, 

 thus provoking a displacement of the liquid which traverses the 

 tubes. If a difference in potential exists between the stem and 

 root of a plant, it is possible that the rise of sap may be an 

 electroendosmotic phenomenon. That such a potential differ- 

 ence exists Marinesco has shown to be true, as have others. He 

 finds a potential difference of 400 mv. between the stem and root 

 of Fuchsia. This electromotive force may be produced by the 

 upward flow of the sap {i.e., it is a stream potential). Obviously, 

 if this is true, then the electromotive force cannot be the cause 

 of the rise of the sap. Which is cause and which effect it is 

 impossible to say. However, as the two phenomena are quite 



+++++++ 



tt+±t+i 



