BY P. OLSSON-SEFFER, PH. D. 109 



a separation of the cortical tissues may take place which, how- 

 ever, may occasion but little damage to the plant. 



It is of importance to notice the resulting circumstances at 

 the forming of ice. They are principally the releasing of the melting 

 heat (according to Lemstrom 80 Cal. for every kilogram watery 

 and the increase of volume. The heat released by the freezing is 

 partly utilized by the plant, and the ice formed by the dew is a 

 good coverlet which hinders further loss of heat. Thereby the 

 freezing of the celhvater is for a short time prevented. If the 

 loss of heat still continues the cellwater freezes and causes the 

 death of the plant. 



When a thaw occurs in the frosted parts of a plant the 

 tissues usually regain the conditions which characterized them 

 before the frost appeared. As the water is set free by the 

 melting of the ice it is slowly absorbed by the cell walls and the 

 cell contents, so that when the cells have attained the temper- 

 ature at which chemical processes are possible the normal 

 conditions of imbibition have also been again restored, and the 

 metabolic processes which were temporarily suspended are 

 resumed under the influence of the higher temperature. The 

 case is different, however, when the frosted parts of plants are 

 rapidly thawed, as occurs for instance when they are suddenly 

 warmed by the sun. The rapid accession of heat induces the 

 ice in the intercellular spaces to thaw rapidly, and the ice water 

 being but slowly absorbed by the cellwalls and protoplasm 

 flows into the intercellular spaces and drives out the air, with 

 the result that leaves which are suddenly thawed become trans- 

 lucent. The normal conditions of imbibition have not been 

 restored when the chemical processes start afresh under the 

 influence of the rise in temperature. Instead of these processes 

 assuming the normal features of metabolism, they lead to 

 chemical decomposition in the comparatively dry and withered 

 tissues. In other words the plant is dying. It is therefore 

 emphatically to be recommended that plants affected by night- 

 frost should be protected against a too rapid thaw. 



Views have been divided as to the manner in which death 

 of the plants is caused by frost. It was at one time admitted 

 that destruction took place by the bursting of the walls of the 

 vessels caused by the augmentation of volume which took place 

 at the freezing. Hoffman attributes a part of the mechanical 

 injury from freezing to the separation from the cell-sap of the 

 air previously contained therein. Later researches have shown 



