SEEDLING DISEASES AND INJURIES 13 



Symptoms. 



When plants suffer from low temperatures and ice crystals 

 are formed within the tissues, the first notice.able effect in the 

 foliage is a wilted or flaccid appearance. The affected tissues 

 have a general water-soaked character as soon as the plants 

 are thawed. When the tissues of the buds and stems of plants 

 are killed by ice formation, no evidence of the injury may be 

 seen immediately after thawing. Later, however, affected 

 parts show the water-soaked and browning symptoms. The 

 cells adjacent to the tissues killed remain healthy and there 

 is no enlargement of the lesion. 



Cause. 



As stated above, the death of plant-cells due to low tem- 

 peratures is not a result of mechanical action in the process of 

 ice formation. Nor is the injury due to the rate of thawing, 

 as was long supposed. Carefully planned investigations on 

 this subject have shown that death results when a certain tem- 

 perature is reached. The critical temperature at which death 

 results varies for different kinds of plants and the various parts 

 of the same plant. The injury is due to some physiological 

 effect of the taking of the water from the cell and changing it 

 into ice. The injury has been ascribed, by various investi- 

 gators, to the precipitation of the proteids in the cells, the 

 drying of the plasma-membrane, the plasmolysis of the nuclei 

 and to various other effects. It is known that various tissues 

 of the same plant will withstand different critical temperatures, 

 that plants differ widely in their resistance to low temperature 

 injury, and that the previous treatment of a given plant may 

 make it more resistant or more susceptible to a given degree 

 of temperature. Previous exposure of the plant to temperature 

 just above the killing point makes it more resistant, while 

 lowering the temperature rapidly causes the plant to succumb 

 at a temperature higher than the usual critical temperature for 



