THE KILLING OF PLANT TISSUE BY LOW TEMPERATURE. 



W. H. Chandler. 



Summary. 



1 . "^he term sap density, as often used in this publication, refers, 

 not to specific gravity, but to molar concentration. These sap den- 

 sities have been determined by the freezing point method, making 

 use of the fact that the molecular weight in grams of any non-elec- 

 trolyte lowers the freezing point 1.86^ C. The sap density is gen- 

 erally given in terms of depression, meaning the number of degrees 

 Centigrade that the freezing point is lower than the freezing point 

 of water. 



2. By the eutectic point is meant the temperature at which 

 the substance in solution crystallizes out. At that temperature 

 there would be at the same time ice, crystals of the solute, and un- 

 frozen solutions. 



3. There are several forms of injury from cold, some of them 

 purely mechanical, such as tearing of tissue due to tension developed 

 at low temperature, or evaporation from the surface when the con- 

 ducting tissue is frozen so as to prevent the movement of water to 

 that tissue, and killing as a result of long continued exposure to low 

 temperature. The term freezing to death, however, is applied here 

 only to a very specific set of phenomena. With all plant tissues, 

 when a certain temperature is reached very shortly after thawing, 

 it will be found that the tissue has taken on a brown, water-soaked 

 appearance, and evaporation from that tissue is much more rapid 

 than from living tissue. These are characteristics of plant tissue 

 frozen to death. 



4. Results of many investigations have shown that during 

 freezing (which may or may not result in freezing to death), ice forms 

 in the tissue, generally not in the cells but in the intercellular spaces, 

 the water moving out of the cells to form crystals in these spaces. 

 The most commonly accepted theory is that killing from cold results 

 from the withdrawal of water from the protoplasm. The amount 

 of water loss necessary to result in death varies with the dilTerent 

 l)lants and different tissues. (Pages 147-155). 



(143) 



