PASSAGE OF WATER FROM PLANT CELL. I 1 5 



only very indirectly. The molecular force drawing the fluid 

 particles to the crystal becomes more intense in proportion as the 

 temperature falls or pressure increases. It follows also that any 

 force acting in opposition to this molecular force will tend to 

 retard the crystallization. Such a force acts when a liquid is a 

 solvent in a solution, or is water of imbibition in a swollen sub- 

 stance. In either case the molecules of the solute or substance, 

 as the case may be, in one sense exhibit an attraction for those 

 of the liquid, tending to prevent the latter from passing to the 

 crystal. Consequently, crystallization will take place with more 

 ease in pure liquids free from solute or from imbibition. This 

 really means that crystallization will take place at a higher tem- 

 perature and lower pressure in pure liquids. 



We are now in position to determine what will occur when the 

 temperature falls below the zero point centigrade in succulent 

 plants provided with intercellular spaces as are all leaves, stems, 

 tubers, roots, etc. Of the system outlined in a previous paragraph, 

 w^ater film, cell wall, protoplasm and cell sap, the water is held 

 most firmly by the cell wall, less so by the more porous protoplasm, 

 still less so by the cell sap of varying concentration, and least 

 firmly by the surface film of nearly pure water on the outside 

 of the wall and lining the intercellular space. In this layer, as 

 it exists normally adjacent to the free space, only the inner mole- 

 cules are held firmly by the wall particles. The outer ones are 

 held less and less firmly until the outermost are barely able to 

 maintain their position. 



Therefore as the temperature gradually falls crystals will begin 

 to form in this surface film rather than in any other part of the 

 ■cell. This explains the occurrence of ice only in the intercellular 

 spaces. 



The passage of water from the cell to these crystals may be 

 conceived of as taking place in either of two ways : 



(a) The formation of the crystal takes water from the surface 

 film with considerable force, thus leaving the surface of the wall 

 Avithout its quota of water of imbibition, and that force will then 

 rapidly increase at that point. Following ordinary physical laws 



