410 



Plant Physiology 



in the freezing of a plant cell under ordinary conditions, 

 the ice crystals are formed upon the surfaces of the cells. 

 In the case of tissues with intercellular spaces these crystals 

 form in the latter. In this way the protoplasm gives up its 

 water and the mechanical injuries of the ice crystal are not 

 ordinarily exhibited within the protoplast. In the case of 

 very rapid supercooling of large cells it is probable that ice 

 crystals develop within the cell ; thus mechanical harm 

 may result. Similarly, in tissues mechanical injury may 

 sometimes result, and the bark of immature wood may be 

 ruptured when severely frozen. It has been found, how- 

 ever, that the diameter of a frozen twig is usually less than 

 normal. 



In view of all the facts which have been presented by 

 various investigators, it would appear that the ability of a 



plant to withstand cold is in 

 large part determined by the 

 capacity of the cells to give 

 up water without injury 

 during freezing. On the 

 other hand, according to the 

 views of Molisch, death from 

 cold commonly results dur- 

 ing the process of freezing. 



FIG. 110. Frozen leaf-stalk of Lav- . * 



atera, showing ice-masses (black). This refers particularly to 

 [After Muller-Thurgau.] actiye ce ^ Qr herbaceous 



shoots, and is at variance with the popular impression that 

 frozen plants are less injured when thawed out gradually. 

 Many plants are injured at temperatures above the 

 freezing-point. This may be due to a simple disturbance 

 of the water relation, but it is more probable that there 



