INJURIES DUE TO ATMOSPHERIC INFLUENCES 287 



layer of cells that has previously been formed across the petiole 

 of the leaf. Simultaneously with the formation of the ice the 

 connection of the leaf with the tree is severed, the result being 

 that on the following morning there is a general fall of leaves. 



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

 usually regain the condition 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. 

 In many cases, however, it is found that the parts have been 

 killed. Instead of the chemical processes that are revived under 

 the action of a recurrence of heat inducing normal metabolism, 

 they initiate chemical decomposition. The views are divided 

 as to the time when frost proves fatal. While Goppert con- 

 cludes that death occurs during the continuance of the frost, 

 Sachs is of the opinion that the tissues die only after they 

 have thawed, and that a fatal issue depends very much on 

 the manner and rate of thawing. The two views may to 

 a certain extent be reconciled, for it is possible that during 

 winter death occurs during the continuance of the frost, whereas 

 in the case of a late spring frost it appears at the moment 

 of thawing. 



The death of a plant under the action of frost during winter 

 bears a close resemblance to the effects of drought on the 

 tissues. No matter whether the deficiency of water in the 

 tissues is due to the action of frost, or to evaporation being in 

 excess of the absorption of water by the roots, the cells must die 

 if the deficiency exceeds a certain limit. A change is induced 

 in the molecular constitution of the protoplasm, the main 

 feature of which is that the protoplasm is rendered incapable 

 of retaining any considerable quantity of water. This change is 

 probably connected with the dissociation of molecular groups 

 in the protoplasm in consequence of the abstraction of water. 

 In a living condition the micellse * of the protoplasm are 

 surrounded by water, the water and the micellse being held 

 together by that kind of molecular attraction whose action in an 

 organic substance is spoken of as the force of imbibition. It 



*[The hypothetical structural units of an organized body have been 

 termed, among other names, micellae : each micella is supposed to have its 

 own molecular structure also, much as a crystal has. ED.] 



