Killing of Plant Tissue by Low Temperature 151 



was never able to detect any difference in the amount of killing when 

 thawed rapidly or slowly, except in the case of the fruit of the apple 

 and pear. Molisch\ following the work of Miiller-Thurgau, tried 

 also slow and rapid thawing from the same temperature with a very 

 large number of plants, and found that in nearly all cases the rate of 

 thawing had nothing to do with the killing. In the case of the fruit 

 of the apple and pear, and the leaves of Agave Americana, the slow 

 thawing gave less injury, but even with these, a slightly lower tem- 

 perature than that at which they kill with rapid thawing, would kill 

 them, regardless of the rate of thawing. Miiller-Thurgau observed 

 carefully the freezing of tissue under the microscope and found that 

 ice was very seldom formed within the cell, but usually ice crystals 

 formed outside the cell in the intercellular spaces and continued to 

 increase in length as the temperature went lower, the water passing 

 from the cell into the intercellular spaces increasing the length of 

 the crystals. 



By placing plant tissue frozen to various temperatures in 100 

 cc. of water carefully insulated, and noting the temperatures to which 

 the water was lowered, excluding the losses of heat for warming up 

 the plant tissue, correcting for the heat of the beaker, etc., making 

 use of the fact that eighty gram calories are required to melt one 

 gram of ice, Miiller-Thurgau^ was able to determine, apparently 

 with some accuracy, the percentage of the plant water that is frozen 

 into ice at various temperatures. Only plant tissue with a deter- 

 mined moisture content was used. With the apple he gives the fol- 

 lowing percentages of water frozen out at given temperatures: at 

 -4.5, 63.8 per cent of the water was frozen; at -13, 74.4 per cent of 

 the water was frozen; at -15.2, 79.2 per cent of the water was frozen 

 He also attempted to measure the percentage of the water frozen out 

 of woody tissue by means of frost cracks. His method was to freeze 

 a section of a young tree trunk until a frost crack of a certain width 

 was formed. On thawing of the tissue this crack would close. His 

 next step was to dry the section of tree trunk until a frost crack of 

 the same width was formed. He assumed that the percentage of 

 water loss necessary to form this crack is equal to the percentage 

 taken from the cell during freezing sufficient to form an ice crack of 

 the same width. 



Molisch studied with great care, under the microscope, the 

 freezing of various plant tissues, observing the same i)hen()nicna de- 



'UntorBucluing Ubor daa. etc. 1897. book. (BIbl. No. 76). 

 'Landw. Jahrb. Vol. !, p. 4r,-A. 1880. (IJIbl. No. 78). 



