FREEZING AND COLD-RIGOR 233 



forms at iC. is killed at this temperature, whereas an unpeeled one, in 

 which ice-formation takes place only after sub-cooling to 3 or 4 C., can 

 be kept for a few days at a temperature of 2 C. without injury 1 . Many 

 plants behave similarly, and when a formation of ice occurs only at localized 

 points in a leaf sensitive to frost, these areas alone are killed 2 . 



Since an organism could hardly maintain itself indefinitely at a tempera- 

 ture below the minimum for growth, it is to be expected that plants, 

 and especially those from warm regions, should ultimately die at low 

 temperatures not falling below zero. Molisch 3 found in fact that the 

 leaves of Episcia bicolor and of Sciadocalyx Warszewiczii were injured in 

 eighteen to twenty-four hours and killed in five days by a temperature 

 fluctuating between 1-4 and 3-7 C. Under similar conditions the leaves 

 of Trade scantia discolor and of Tournefortia Jiirsutissima were injured in 

 eight days and killed in twenty-seven. Aspidium violascens is still more 

 sensitive, while Epidendrum floribundum, which ceases to assimilate car- 

 bon dioxide at 5 C., and Mimosa pudica, which loses the power of photo- 

 synthesis at 8 C., cannot exist for any length of time at these 

 temperatures 4 . Goppert 5 also mentions that various plants which 

 survive transitory exposure to 2 or 3 C., die when kept for one or 

 two days at 1 C., a temperature at which no ice-formation presumably 

 occurs. Furthermore, the radicles of Cucurbita Pepo and of Phaseolus 

 vulgaris die in parts after remaining for four weeks at from o to i C. 

 It is easy to understand that, owing to the depressant effect of low 

 temperatures upon metabolism, they should take longer to produce a fatal 

 effect than high ones, which steadily accelerate respiration. Indeed all 

 perennial plants whose growth is in nature interrupted by cold seasons 

 must be able to survive these periods of cold-rigor if they are to maintain 

 themselves. 



Death from cold-rigor may take place with or without a formation of 

 ice, and the latter does not always cause death. Frozen plants are by 

 no means unchangeable or non-sensitive, for in the first place they die in 

 time, and a further fall of temperature may bring about this result 

 immediately. For instance, fruit and other trees are killed by unusually 

 cold winters, although they bear the cold of most winters without injury 6 . 



1 Miiller-Thurgau, Landw. Jahrb., 1886, Bd. xv, pp. 488, 505. See also Miiller, Landw. 

 Jahrb., 1880, Bd. IX, p. 133. 



2 Muller-Thurgau, 1. c., p. 505. 



3 Molisch, Unters. ii. d. Erfrieren d. Pflanzen, 1897, p. 61. Goppert, Hardy, and Kunisch 

 (cf. Molisch, 1. c., p. 56) also observed death above zero, but sufficient precautions were not taken 

 to ensure that the temperature did not fall below that indicated by the thermometer. 



4 Ewart, Journ. of Linn. Soc., 1896, Vol. xxi, p. 399. 



* Goppert, \VarmeentwLkelung i. d. Pflanze, 1830, p. 63; Ueber d. Gefrieren u. Erfrieren d. 

 Pflanze, 1883, p. 50. 



6 For observations on animals cf. Welter, Die tiefen Temperaturen, 1895, p. 75; Bachmetjew, 

 Zeitschr. . wiss. Zool., 1899, Bd. LXVI, p. 524; Labbe, La Cytologie, 1898, p. 37. 



