ABSORPTION AND TRANSMISSION OF WATER 105 



necessary that the concentration of the cell sap at the point 

 of entrance be higher than at the point of exit. If such a 

 condition could be maintained, a slight movement of water 

 would undoubtedly take place, though in the writer's 

 judgment it would be very inadequate in amount. But the 

 insurmountable difficulty in this explanation is the continu- 

 ous maintenance of this difference of concentration in differ- 

 ent parts of the same cell. This could only be accomplished 

 by the active absorption and precipitation or fixation of the 

 active osmotic substances in one region of the protoplast, 

 and their secretion and solution in another part. Such a 

 supposition in its simplest form involves the carrying back 

 of these substances by the protoplasm, for example, from one 

 end of the cell to the other, with as great rapidity as they can 

 diffuse in the opposite direction through the cell sap. This 

 could only occur with enormous expenditure of energy on 

 the part of the protoplasm, and it is difficult to imagine any 

 adequate source for this energy. 



Still another proposed explanation of the passage of water 

 through a cell in any given direction has been offered by 

 Pfeffer. 1 He points out that, if the membrane where the water 

 enters be less permeable to solutes than that where it escapes, 

 a continuous flow will take place. This is undoubtedly true, 

 but the flow would be still more marked if the second mem- 

 brane were removed altogether; for it can act only as an 

 obstruction to the upward expansion of the inclosed liquid as 

 water is taken in through the semi-permeable membrane 

 below. The resulting system is such as would be obtained if 

 the thistle tube osmometer used for illustration of osmotic 

 pressure were to have its stem plugged with cotton. The cot- 

 ton would hinder the rise of the liquid column, but would not 

 stop it altogether. Copeland 2 has actually constructed a 



i W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877, p. 225. 

 2E. B. COPELAND, "An Artificial Endodermis Cell," Bot. Gaz.,Vol. XXIX (1900), 

 pp. 437-9. 



