ABSORPTION OF WATER 141 



were, a single cell. When immersed with its lower branches in a 

 moist substratum, this cell imbibes water and expels it from its 

 upper parts. By means of plasmolysis, Lepeschkin has demon- 

 strated that the protoplasm of the sporangiophore actually shows a 

 somewhat greater permeability than that of the absorbing hyphae. 

 On the basis of this observation, he has constructed a theory of 

 pressure in one direction. In places with less permeable protoplasm, 

 the pressure on the plasmatic membrane by a solution of the same 

 concentration will be greater than at points with a more permeable 

 protoplasm. In the latter instance, part of the pressure is lost, 

 due to the diffusion of some of the dissolved substance through the 

 protoplasm, since, according to the laws of hydrostatics, the 

 elastic counterpressure exerted on the protoplasm by the distended 

 membrane is everywhere the same, and on the whole equals the 

 average magnitude of the pressure of the solution on the mem- 

 brane. The result will be that in places of less permeability the 

 water is imbibed owing to the excess of the inner pressure over the 

 outer while in places showing greater permeability, the water is 

 pressed out through the protoplasmic membrane. Thus, an unin- 

 terrupted water current proceeds in one direction through the 

 cell. 



The fluid exuding from the cell is not pure water, but a solu- 

 tion somewhat more dilute than the cell sap. The whole mechanism 

 depends upon and works at the expense of substances leached out 

 from the cell. This explains the necessity of supplying the cell 

 with nutritive substances from other parts of the plant. A physi- 

 cal model of such a pumping cell may be constructed in the follow- 

 ing way : A glass cylinder, open at both ends, is filled with a sugar 

 solution. One end of the tube is covered with a more permeable 

 membrane than the other, for instance, animal bladder and 

 vegetable parchment. To the more permeable end a rubber cap is 

 fastened, ending in a glass tube in which the exuding sap accumu- 

 lates (Fig. 57). If this model of a cell is immersed in water, the 

 same pressure in one direction will be observed, as is exhibited 

 by Pilobolus. 



In addition to that of Lepeschkin, another idea, tending to 

 explain the flow of water in one direction, has been proposed 

 chiefly by Priestley and recently has gained wide recognition. 

 This scheme is based on the consideration that the water-absorb- 

 ing cell, being saturated, develops no suction by itself. This is in 



