ICE IN PLANT TISSUE. 35 



pure. If several of these masses of ice from a frozen beet are 

 placed in a watch glass and allowed to melt and evaporate no 

 residue will remain, at least very little ; they are almost pure ice. 

 This accords well with the generally established observations in 

 physics that when a solution freezes the water separates from 

 the solute to form almost pure water crystals. It is not always 

 entirely pure, in many cases some molecules of foreign substance 

 being seemingly caught in the ice. The results of experiments 

 by Aliiller-Thurgau showed only very slight, almost undetectible, 

 residue after evaporation of 21.08 grams of ice from the ice 

 clumps in the inside of a large beet. Ice formed on the surface 

 of a piece of beet did not seem so pure, and this was to be ex- 

 pected, because, unlike the former case, the water was not filtered 

 through protoplasm and cell wall, but might contain fragments of 

 either of these structures from the cut outer cells in contact with 

 the ice. 



Sachs found that the ice crust of prismatic crystals formed 

 on the cut surfaces of beets, etc., when protected from evapora- 

 tion, was not pure ice but contained an acid in sufficient quantity 

 to strongly redden blue litmus paper on which it was allowed to 

 melt. 



In the case of solutions and water-soaked materials of other 

 kinds the separation of the water from the solid substance has 

 been studied in detail by Molisch. The substances studied by 

 him were as follows : ( i ) Colloidal substances — gelatine, starch 

 paste, gum tragacanth, gum arable, egg albumen, Gleocapsa gela- 

 tine ; (2) emulsions — milk of Ficits clastica, aqueous carmine 

 solution, indigo solution, caoutchouc emulsion; (3) dye stuffs — 

 solutions of anthocyanin, red-beet sap, nigrosine, methyl blue; (4) 

 salt solutions — potassium nitrate, magnesium sulfate, potassium 

 monophosphate, cobalt chlorid. From all of these nearly pure 

 water separated to form the ice crystals distributed through the 

 substance. 



Except in woody structures the cells of frozen tissues are 

 always in a more or less collapsed condition. Since the water is 

 abstracted from the cell when ice formation occurs, collapse must 

 necessarily follow, especially in soft-walled tissue. 



