36 THE PLANT WORLD. 



Frozen tissue when sectioned shows between the ice masses 

 dense areas composed of the collapsed cell-walls packed closely 

 together. Frozen buds and bark of hardy trees show this con- 

 dition markedl}' and appear entirely disorganized, but on thawing 

 the cells again expand and become normal. So is it also with 

 evergreen leaves. This extreme shrivelled, shrunken or collapsed 

 appearance of the cells is one of the most marked charactristics 

 of frozen tissue. 



In solutions, especially of substances having considerable 

 " affinity " for water, the force with which the water and solute 

 are held together is often considerable. This is shown by the 

 slowness with which certain solutions of colloidal substances, if 

 exposed to evaporation, dry out after they reach a certain concen- 

 tration. The more concentrated the solution becomes the greater 

 is the force with which the remaining molecules of water are 

 held. Considerable water is held in the protoplasm and cell 

 walls by a still stronger force, that of imbibition ( molecular capil- 

 larity). Here again the retaining force increases as the water 

 is withdrawn. In freezing these forces must be overcome by 

 the ice-forming forces before ice will be produced. The force 

 of crystallization, although strong at the start, increases in pro- 

 portion to the fall in temperature after the crystal begins. 

 Consequently ( i ) the largest quantity of ice will be formed near 

 the freezing point since then the freezing force is relatively 

 strone: ; but soon sufficient water is removed from the tissue so 

 that the two sets of forces come into equilibrium, after which 

 more ice is formed only as the temperature falls. (2) The 

 amount of ice formed per degree, as the temperature falls, con- 

 sequentlv becomes less and less. In succulent tissue most of 

 the water is frozen out at temperatures but slightly below the 

 freezing point, while in drier tissues, with most of the water in 

 the protoplasm and walls, a smaller percentage separates out at 

 these higher temperatures. An apple which Miiller-Thurgau in- 

 vestigated contained at — 4.5° C. 53 per cent, of ice by weight, 

 which was equal to 63.8 per cent, of the water content. At 

 - — 15.2° C. onh' 79.2 per cent, of the water had frozen. There still 

 remained a considerable quantity to freeze at still lower tem- 

 peratures. 



