47 



to -3°, and in Pelargoniwn, after exposure to -6°. In 

 Nicotiaua, one hour at - 12° did not kill the stomatal cells ; 

 they died only when brought to -15° to -17°. In leaves 

 which die by exposure to temperatures of + 2° to + 3°, the 

 stomatal cells were also observed to be alive after the 

 other epidermal cells had been killed. 



3. Tuberous Tissue. The potato tubers, on account of 

 the large masses of homogeneous tissue that they present, 

 were often used in investigations on the freezing point of 

 living material and on the effects of low temperatures. 

 The death point has been investigated, in particular, by 

 the following authors. 



Goppert (1830) reported that when the temperature in 

 the interior of a potato (measured by inserting a ther- 

 mometer in it) had dropped by only one degree below 

 zero and all the sap had been converted into ice, the potato 

 was dead after thawing and turned brown. 



According to Miiller-Thurgau (1886), potatoes exposed 

 to an air temperature of -6° for a time long enough to 

 bring the temperature in their center to - 1.5°, w^ere dead 

 throughout after thawing. A thermometer was inserted 

 in the middle of the potato. Death was diagnosed by the 

 blackening and the soft consistency of the tissue. 



Apelt (1909) who Avas asked by his master, Mez, to test 

 a theory on the existence of a death temperature, specific 

 for each organism and lying below the freezing point, 

 found that each race of potato has its own death point, 

 although the latter can be raised or lowered by the condi- 

 tions of the previous storage. He determined the tem- 

 perature within the material with a thermocouple and 

 diagnosed the vitality of the cells by the plasmolysis 

 method, with potassium nitrate solutions containing a 

 few drops of methylene blue. The low temperatures were 

 produced by ice-salt mixtures. The death points regis- 

 tered extended from -1.71° to -3.63°. They were lower 

 than the freezing points ])y quantities varying from 0.10° 

 to 1.17°. The "hundredth of a degree," alw^ays scrupu- 

 lously given, and the idea of a death point determinable to 



