Killing of Plant Tissue by Low Temperature 171 



Here again the hardiness has been increased by increasing the 

 density of the sap. Of course it should be admitted that even here 

 there is a possibility that some actual change in the protoplasm has 

 taken place by treating it with these solutions. Glycerine has been 

 most effective in increasing the sap density of the tissue and in in- 

 creasing the resistance to cold. It is interesting to observe that in 

 the case of cabbage, the sap density and the hardiness were more 

 greatly increased with salts like potassium chloride than with cane 

 sugar, while in the case of tomatoes, sugar was taken up in larger 

 quantities and caused a greater increase in hardiness. 



According to the theory of Gorke, if killing is due to the salt- 

 ing out of proteids, we should expect the taking up of sugar to increase 

 the hardiness but should not expect that result to follow the taking 

 up of salts. The salts that most readily precipitate certain proteids 

 are ammonium sulphate and zinc sulphate. When roots of tomato 

 plants were kept for twenty-four hours in solutions of a molecular 

 concentration as great as those used in the table above, the hardi- 

 ness of the leaves was not reduced, and zinc sulphate seemed to in- 

 crease the hardiness. These results are not in accord with Gorke 's 

 theory. Potassium nitrate does not increase the hardiness as other 

 substances do. Thus tomato plants with their roots kept in potas- 

 sium nitrate solution of about the same molecular concentration as 

 the solutions used above, seemed to be killed more easily than when 

 the roots were kept in pure water, and corn plants so treated cer- 

 tainly were killed more easily. This lack of protective action is 

 probably due to the high eutectic point of the potassium nitrate 

 since it would precipitate out before the killing temperature of the 

 tissue is reached. 



Apple and peach blossoms were cut from the twigs in such a 

 way that a considerable area of cortex and sap wood adhered to the 

 stem, and these were inserted in solutions of varying strengths of 

 sugar and glycerine and later frozen. The following table gives the 

 results: 



