PROCEEDINGS OF THE ANNUAL MEETING. 363 



as in the case of drought, when this limit is overstepped the cell is unable 

 to recover the water abstracted by the process of freezing, and death 

 •ensues. This may be illustrated by the action of frost upon starch paste. 

 Frost separates the water, but subsequent thawing leaves the water and 

 the starch in a separated condition. Our winters are rarely so cold that 

 our forest trees ever become injured by molecular disorganization of the 

 protoplasm of their cells. These have become inured by long and gradual 

 processes of acclimatization. Not so with exotics, including many 

 varieties of ornamentals and not a few classes of fruit trees, among which 

 we may mention peaches and plums. 



The absorption of water by the roots ceases when the ground is frozen 

 to a depth that is reached by the roots of the young plants. No harm is 

 done if the trees are protected above ground against evaporation, by 

 snow or other covering. The twigs and exposed branches, in cases of 

 extreme frost, then suffer as if affected by severe drought. Warm south 

 winds, causing evaporation during winter, then, have this injurious 

 effect. The limits of forest growth, in my opinion, are as much deter- 

 mined by the action of drought as by low temperature. 



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 normal metabolism 

 (living conditions) they imitate chemical decomposition. Views are 

 divided as to the time when frost proves fatal. Some say during continu- 

 ance of frost. Sachs is of the opinion that the tissues die only after 

 they have thawed, and that the issue depends much upon the manner of 

 thawing. Both theories are probably correct at different periods in the 

 life of the plant. 



First, then, the effect of frost upon green tissue. When the tissues of 

 the leaves or cortex, and, in fact, when any parenchymatous green tissues 

 are frosted, pure water is withdrawn into the adjoining intercellular 

 spaces, but the cells themselves do not generally freeze. The result is 

 that the cells lose their turgidity and at the same time begin to droop. 



In the case of frost affecting growing tissues, the issue depends upon 

 the manner of thawing. Should the plant thaw gradually, the water 

 which has been extracted is reabsorbed by the walls and contents of the 

 cells at the same rate as it is formed from the ice crystals by the gradual 

 accession of heat, so that normal conditions are restored. In the case of a 

 rapid and marked rise of temperature, the ice thaws rapidly and the ice- 

 water flows into and remains in the intercellular spaces, driving out the 

 air and causing, in the case of green leaves, the translucent appearance 

 so well known. Chemical processes start afresh under the influence of 

 the rise in temperature. Instead of these processes going forward in the 

 ordinary manner, decomposition sets in, resulting in dried and withered 

 foliage. 



The manner of destruction of the fruit buds of peaches and plums is 

 undoubtedly analogous to the conditions, causes, and effects outlined 

 above. In many cases the fruit buds, being the tenderer parts in the 

 plant's anatomy, are injured, as the peach-grower learns to his sorrow, 



