Killing of Plant Tissue by Low Temperature 187 



In case of the greenhouse grown apple stock there seems to be 

 slight evidence that proteids were precipitated by the low tempera- 

 ture. However, the difference is probably within the range of error. 

 In these cases great care was taken to prevent any change in the 

 soluble proteid content of the sap before analysis began. The 

 unfrozen sap was immediately started for analysis, after being ex- 

 pressed. In all cases the frozen sap v;as thawed on filter paper so 

 that little change could take place after the thawing. 



From the above tables, we are safe in concluding that in the 

 case of the cortex of twigs from fruit trees in autumn or early winter, 

 precipitation of proteids plays no part in the killing since tempera- 

 tures that will kill all the cortex will precipitate no proteids. In the 

 case of some succulent growing plants there seems some evidence 

 that a killing temperature will precipitate a slight percentage of the 

 soluble proteids present, but since increasing the amount of salts 

 present that are supposed to precipitate the proteids increases in- 

 stead of decreasing the resistance to low temperature, we are probably 

 safe in concluding that precipitation of proteids does not explain 

 freezing to death. 



OTHER FEATURES THAT INFLUENCE THE FREEZING TO 



DEATH OF PLANTS. 



Rate of Thawing. The effect of the rate of thawing on the 

 killing from cold has been discussed previously with reference to 

 the works of Miiller-Thurgau,' Molisch,^ and Sachs.^ However, 

 it is not out of place to discuss it briefly as one influence to be dealt 

 with in the killing of certain plants from cold. As mentioned before, 

 Miiller-Thurgau and Molisch found only mature fruits and leaves 

 of Agava Americana to be affected by the rate of thawing. Prac- 

 tically all plants used in freezing experiments to be reported in this 

 paper have been tested with reference to the effect of rapid thawing, 

 particularly because of the effect such thawing would have on the 

 results with other experiments. The following table gives the re- 

 sults with peach fruit buds that were thawed slowly and rapidly: 



'Landw. Jahrb. Vol. I.""), p. 4.5.3, 1886. (Blbl. No. 78). 

 'UnttTsuchutiK liber das Erfrlorcn, otc. Hook, 1H<J7. (Bibl. No. 75). 

 n<r. u. d. Vcr. d. K'on. Sachs. Gesoll. d. Wlss. zu. Leipzig, 1860, Vol. 12, pp. l-.W. (lUbl. 

 No. 94). 



