12 MANUAL OF TREE DISEASES 



Protecting the plants by mulching is an efficient means of 

 preventing winter-drying. Where sufficient snow falls early 

 .in the winter, the beds may then be covered with burlap and 

 the snow-mulch retained. Where more open winters are en- 

 countered and little snow can be depended on, a light mulching 

 of straw, buckwheat-hulls and other materials may be used. 



References 



Hartley, Carl. The blights of coniferous nursery stock. U. S. Dept. 



Agr. Bui. 44: 1-21. 1913. 

 'Hedgoock, G. G. Notes on some diseases of trees in our national 



forests. III. Phytopathology 3 : 111-114. 1913. 

 Hartley, Carl. Notes on winterkilling of forest trees. Forest Club 



(Univ. Nebraska) Ann. 4 : 39-50. 1912. 



Feeezing-to-Death 



Caused by low temperatures 



The most common type of low temperature injury to plants 

 is the direct killing of the susceptible parts. A part of the 

 water in the plant is frozen when temperatures below the freez- 

 ing-point are reached, and more and more water is withdrawn 

 from the cells when the temperature becomes lower. The 

 frozen water occurs as ice crystals which are mainly found in 

 the intercellular spaces, but may be within the cells if the freez- 

 ing occurs rapidly. Freezing-to-death has been frequently 

 noticed and discussed for centuries, even before the internal 

 structures of plants were understood. It was also early noted 

 that the formation of ice crystals within the plant always 

 accompanied freezing-to-death. The accepted and seemingly 

 logical explanation of the cause of the death of the plant was 

 that the plant-cells were ruptured, or otherwise mechanically 

 destroyed during the "^formation of the ice crystals. This 

 explanation is totally incorrect, however, although the exact 

 physiological effect on the plant which results in death is as 

 yet undetermined. 



