472 UNTOWABD INFLUENCES. 



irreparably injured. There are well-known cases in which plants 

 may be thawed quickly without serious injury. 1 



1231. Goppert 2 and others have shown that the flowers of 

 certain orchids, turned blue by the formation of indigo in their 

 cells when they are slightly frozen and suddenly thawed, will 

 preserve their usual colors unchanged if made to thaw very 

 slowly. 8 



1232. As to the length of time during which the vitality of a 

 frozen plant persists, we have no exact observations ; but it is 

 stated that after the recession of a glacier in Chamouni sev- 

 eral plants which had been covered by ice for at least four years 

 resumed their growth. 4 



1233. It is still an open question whether much of the injury 

 to certain plants by freezing is not strictly mechanical, resulting 

 from the expansion during the formation of ice in the cells. 5 



1234. "Winterkilling." The destruction of many plants by 

 exposure to the influences of a variable winter is sometimes 

 attributed to the injurious effects of drying winds rather than 

 to cold alone. It has been shown (748) that the amount of 

 water absorbed by roots is governed largely by the temperature 

 of the soil. Although the exhalation of moisture from the leaves 

 of evergreens in winter is not large, it is, however, sufficient to 

 create a certain demand upon the soil for a supply. This de- 

 mand, slight as it is, is of course greater during very dry 

 weather ; and it is from this that the injuries may be supposed 

 largely to result. 



1235. The behavior of certain plants during exposure to low 

 temperatures affords some of the best illustrations of the adap- 

 tation of vegetation to its surroundings ; and the question as 

 to increasing the tolerance of a given species or variety to the 



1 Sachs has shown that the leaves of cabbage, turnip, and certain beans 

 frozen at a temperature of from 5 C. to 7 C., and placed in water at C., 

 are immediately covered with a crust of ice, upon the slow disappearance of 

 which they resume their former turgescence (Versuchs-Stationen, ii. 1860, p. 

 167). If such frozen leaves are placed in water of 7.5 C. they become flaccid 

 immediately. 



8 Botauische Zeitung, 1871, p. 399. 



8 According to Kunisch (quoted by Pfeffer : Pflanzenphysiologie, ii., p. 

 436), this blue discoloration is observed when the flowers, placed in an atmos- 

 phere of carbonic acid, are subjected to a freezing temperature : in this case, of 

 course, the indigo is produced from chromogen without free oxygen. 



4 Botanische Zeitung, 1843, p. 13. 



6 Hoffmann (Grundziige der Pflanzenklimatologie, 1875, p. 325) attributes 

 i\ part of the mechanical injury from freezing to the separation from the cell- 

 sap of the air previously contained therein. 



