No. 474] 



SAP FLOW IN MAPLE 



435 



is, therefore, one known to occur in plant tissues and is apparently 

 much more widespread than was formerly supposed. Research 

 is tending to show that bleeding occurs among cells of widely 

 different tissues, and is probably to be considered a normal and 

 very general phenomenon in plants.^ 



The exudation can be conceived to be produced in either of two 

 ways : either by change in permeability of the diffusion membrane 

 allowing water to pass with less friction, or by a change in osmotic 

 tension. In regard to the first method it may be said that although 

 diffusion membranes are considered to be freely permeable to water 

 they really are not quite so. A force is required to press water 

 through such a membrane as is shown by the fact that a bladder 

 may be filled with water and suspended in air without the water 

 escaping immediately. It is conceivable, therefore, that a portion 

 of the cell membrane might become quite freely permeable to the 

 solvent while the remainder continued dense. But so far as we 

 know the resistance to the passage of water is very slight and plays 

 no great part in the determination of pressure in osmotically 

 active cells. So far as our knowledge goes, osmotic pressures are 

 the same, no matter what membranes are used, providing that the 

 solute is of the same nature and density, and that the membrane is 

 permeable to it in the same degree, and also permeable to water. 

 The osmotic pressure of water has been demonstrated in con- 

 nection with some artificial membranes, but was always found to 

 be slight. So far as we know at present all pressures of any 

 moment in connection with semipermeable membranes are pro- 

 duced directly or indirectly by the action of the solute, and are 

 proportional to the quantity of the latter present. 



We have remaining the alternative of a change in osmotic ten- 



in itself. Water might he excreted from the cells by a simple 

 change in pernieabilitv of this sort, but the production of pressure 

 in the surrounding tissue would be impossible, for as the water 

 passed out from the cell, the latter would decrease a like amount 

 in volume and no pressure would ensue, simply a change of location 



» Wieler, A. " Das Bluten der Pflanzen." Cohn's Beitrdge, vol. 6, p. 1, 1892. 

 See also Pfeffer, Pflanzenphysiologie. 



