436 



THE AMERICAN NATURALIST [Vol. XL 



of the water with reference to the membrane of the cell. To ob- 

 tain pressure externally by osmotic action it is necessary to assume 

 a fow of water through the cell. But if the two reservoirs of supply 

 and excretion are confluent then there" will be a flow through the 

 cell, in at one point and out at another and back again outside to 

 the starting point, thus forming a circle of flow; and there would 

 be no external pressure. The reservoir of supply to the cell must 

 be distinct from the reservoir of excretion. Pressure will then be 

 produced in the latter reservoir while at the same time, in the 

 former a tendency toward suction will occur. 



Flow through a cell will occur as Pfelfer ^ has already shown 

 (a) if the solute passes through the membrane more easily at one 

 end than at the other. Osmotic tension will here be less and the 

 water will be forced out by the tendency to greater pressure at the 

 other end. Flow will be in at the side of less permeability and out 

 at that of greater permeability. Such a condition has been 

 demonstrated experimentally in an artificial cell by Copeland.^ 

 Or (h) a flow will occur if the solute is more concentrated at one 

 end of the cell. Water would enter in this case at the region of 

 greatest concentration (greatest osmotic pressure), and pass out 

 at the region of least concentration; and would continue to flow 

 as long as the solute remained thus distributed. The difficulty 

 in this case would lie in the maintenance of unequal concentration 

 of the solute within the same cell. Since diffusion would soon 

 equalize any such irregularity it could be accomplished only by 

 the constant production of more solute at a certain point. 



If pressure is due to the unequal permeability of the membrane 

 to solute then there must always be a secretion of solute along with 

 the sap into the chamber showing the increased pressure. Sap in 

 this reservoir cannot be pure water, or even nearly pure water, 

 unless the plant possesses some means of ridding the sap of such 

 solute after its excretion either by its immediate change to solid 

 form or its use in metabolism. In case pressure is due to an 

 unequal distribution of solute within the cell then no excretion 



' Pfeffer. Pflanzenphysiologie, ed. 2. 



' Copeland, E. B. " Physiological Notes II, an Artificial Endodermis Cell." 

 Bot. Gaz., vol. 29, p. 437, 1900. 



