PASSAGE OF WATER FROM PLANT CELL. I I / 



of molecular structure under certain conditions, as is now often 

 assumed in the water-adhesion theory of the ascent of sap. Xow, 

 if a molecule is taken from this body of water at the crystal surface 

 we may conceive that the whole structure will be pulled along one 

 molecule nearer the crystal, and so on. This is similar to ])ulling 

 on the end of a string, thereby moving all the molecules of the 

 string along a similar distance. The greatest objection to this 

 idea that the crystal pulls the water out lies in the doubt as to 

 whether water of imbibition possesses a rigidity similar to that 

 of a column of great diameter. 



Either of these views will explain the formation of ice-masses 

 where the water passes to the crystals from a distance much 

 greater than the diameter of the cell, as it does sometimes for a 

 distance of many cells. There may be either a general equaliza- 

 tion of imbibition or a pulling on the various strings of water as 

 above indicated. 



Either view will also explain the formation of ice-crusts and 

 ice-masses in connection with dead tissue, for the conditions iiT 

 dead material as regards surface film and water of imbibition are 

 essentially the same as in living tissue. 



In the case of living cells, to the force of imbibition in the 

 sap and protoplasm must also be added the osmotic force of the 

 cell. The ice crystal in abstracting water from these two struc- 

 tures must overcome both sets of forces. Since, however, as com- 

 pared with the force of imbibition in moderately dry substances, 

 the osmotic pressure is never very great, this last factor is of 

 importance only at the very inception of freezing. In tender 

 tissue killed bv freezing it would disappear very soon. 



Normal tissue usually contains a large quantity of water and 

 the series of structures concerned in freezing are nearly saturated 

 with water of imbibition. Hence when freezing first begins very 

 little force is required to abstract the water, but as the process 

 goes on the imbibition-force regularly but rapidly increases until 

 it becomes very great, as we have already seen. \\^ith the force 

 of crystallization the increase is not regular. Before freezing 

 begins the force is absent. With the first crvstal-formation it 



