510 



die, therefore, as a result of tlic dying of the protoplasts, but of a cooling 

 down below the specific minimum." 



We find in this a confirmation of our earlier standpoint, viz., no simple 

 process of crystallizing out the water is caused by the action of the cold but 

 a material disassociation. This action of the cold makes the life functions 

 impossible. Besides these essentially mechanical processes, however, chem- 

 ical decomposition often plays a part. This will be initiated sometimes by 

 too great cooling, sometimes without it. Not every plant needs to be first 

 cooled down in order to freeze, but it probably freezes more rapidly, i. e., 

 is cooled down to a sub-minimum temperature, if the freezing occurs in 

 association with supercooling. At least this is shown by Mez's experi- 

 ments with pieces from the stem of Impaticns parviflora. We learn from 

 these experiments how very much the supercooling depends upon the con- 

 stitution of the cell sap. Gases, dissolved air, hinder or decrease super- 

 cooling just as do emulsified oil, gum or plant mucilage. It is also found 

 that pruned plant parts, cooled down in water, always freeze without any 

 further reduction of temperature or, at least, without an essential one. It 

 happens that plant stems, standing partially in water, are found to be frozen 

 as far back as they extend into the air. Molisch tested the question experi- 

 mentally by letting branches of Tradcscantia zehrina lie half in water. 

 During the night a temperature of 5 degrees C. below zero acted upon them. 

 After a slow thawing in a cool room, the half of the sprouts which had been 

 left in the air were found to be frozen, while the lower half, sticking in the 

 ice, remained uninjured. The upper half, surrounded by air, will have 

 been cooled down rapidly by supercooling and is thereby frozen. On the 

 other hand, as far as the plants stood in water, the cooling down takes place 

 slowly on account of the high specific warmth of the water, and the super- 

 cooling will be hindered by the freezing water about the stem as well as by 

 the ice in the tissues above the water, which have been frozen. 



An observation made by Miiller-Thurgau, that in a heap of beets, the 

 outer frozen roots protect the inner ones from freezing, calls attention to 

 the specially favorable influence of the formation of ice. This point is 

 emphasized by Mez, since he says in general that the transformation of the 

 cell sap into a solid aggregate condition forthwith protects from too rapid 

 radiation the energ}^ still retained in the plant. The conducting of heat is 

 very^ much lower in ice than in water in which the warmth is distributed by 

 currents. 



The danger of freezing, i. e., the lowering of the temperature to the 

 specific death-dealing minimum, can in part be promoted by secondary cir- 

 cumstances and in part hindered by them. The decrease lies in the use of 

 the specific heat of water; this will be mentioned again in methods of pro- 

 tection against frost and further in the formation of ice itself, which occurs 

 at zero, or a very little below it, while death sets in only at lower tempera- 

 tures or finally in a change of the cell sap, since a greater quantity of oil, 

 gum and mucilage acts retardingly. 



