THE FORMATION OF ICE IN PLANTS 241 



bring about the same result by influencing antagonistic tissue-strains 

 unequally. 



As in the case of a partially frozen saline solution, a condition of 

 equilibrium corresponding to the temperature is reached in a partly frozen 

 plant. A further fall of water causes more water to be frozen, a rise of 

 temperature causes the amount of ice to decrease. This was observed by 

 Muller-Thurgau on transferring frozen plants from a temperature of 10 to 

 2C. Since, however, the percentage of salts in the cell-sap is only 

 sufficient to produce a slight lowering of the freezing-point 1 , usually more 

 than half the water in a plant is converted into ice at 3 to 8 C. Muller- 

 Thurgau 2 found that an apple frozen at 4-5 C. contained 53 per cent, by 

 weight of ice, so that 63-8 per cent, of the water normally present was frozen 

 at this temperature, and 79-3 at 15-2 C. At very low temperatures there- 

 fore of from 50 to iooC. the cell no longer contains any perceptible 

 fluid contents, although the more firmly retained portions of the water of 

 imbibition may still remain unfrozen. 



The same general considerations apply when the ice is formed inside 

 the cell 3 . Molisch observed that the wholly internal ice-formation in the 

 sporangiophore of Phycomyces nitens takes place at 17 C., but at 4 C. in 

 epidermal cells of Tradescantia lying in water. An internal formation of 

 ice must always ultimately occur if the temperature is lowered sufficiently, 

 unless the withdrawal of water by the external ice- formation keeps increasing 

 the concentration of the cell-sap so rapidly as to prevent freezing until the 

 cell is dry internally, and freezing is therefore no longer possible. Since 

 both the external ice-formation and the degree of sub-cooling are factors of 

 importance, it is not surprising that an intracellular formation of ice is not 

 observed in all cases and under all circumstances. It is probably favoured 

 by rapid cooling, although even here the water may be very rapidly 

 withdrawn from the cells during the extracellular formation of ice. 



The plasmodia of Myxomycetes 4 and also of Amoebae 5 are killed by 

 freezing, but it does not follow that all protoplasm is killed when ice- 

 crystals are formed in it. Whether all plants resistant to freezing are also 

 indifferent to an intracellular formation of ice has still to be determined, for 

 the protoplasm and cell-contents might gradually dry up as extracellular 

 freezing occurred. For this reason it is uncertain whether any ice is formed 

 within bacterium cells when they are frozen in a turgid condition. 



1 A i-oi per cent, solution of KNO 3 freezes nt 0.308 C., a 10-1 per cent, solution at 

 3-08 C., and the same applies to solutions of equivalent osmotic concentration. 



a Muller-Thurgau, Landw. Jahrb., 1886, Bd. xv, p. 472. In the case of the potato 77-2 per 

 cent, of its water is frozen at 5 C. This is determined by calorimetric comparison of equal 

 weights of frozen and unfrozen potatoes. 



3 Goppert, Warmeentwickelung i. d. Pflanze, 1830, p. 26; Regel's Gartenflora, 1879, p. 260; 

 Muller-Thurgau, 1. c., 1880, Bd. ix, p. 184; Molisch, Das Erfrieren d. Pflanzen, 1897, p. 16. 



* Klihne, Unters. ii. d. Protoplasma, 1864, p. 88. 5 Molisch, 1. c., p. 17. 



PFEFFER. II 



