232 THE POWER OF RESISTANCE TO EXTREMES 



SECTION 66. Freezing and Cold-rigor. 



A sufficient rise of temperature always produces death, whereas dried 

 seeds and spores and certain bacteria, even when moist, can withstand 

 temperatures as low as 200 C. Most plants are, however, killed by 

 a sufficient degree of cold, the intensity of which varies considerably in 

 different cases. Thus the aerial parts of Cucumis sativus^ Cucurbita Pepo^ 

 Ricinus communis, Impatiens balsamina, Phascohis namts, and Solatium 

 tuberosum are partly or entirely killed by a single night's exposure to a 

 temperature of from 2 to 4 C. Such plants as Stellaria media, Senecio 

 vulgaris^ L amium ample xicaule^ Urtica urens, and Bellis perennis are only 

 killed after prolonged exposure to temperatures lying between 6 and 

 9 C., while Helleborus foetidus can even withstand 17 C. 1 Similar and 

 even lower temperatures must be withstood by the trees of north Europe 

 during winter. Indeed Larix sibirica, as well as a few other Phanerogams 

 and certain mosses and lichens, grow in the arctic regions where the tem- 

 perature during winter lies between 30 and 50 C., and the plants are 

 often frozen stiff for six months in the year 2 . 



Since plants are poikilothermic organisms in which the small amount 

 of heat produced by metabolism is rapidly lost by radiation and transpi- 

 ration, they assume approximately the same temperature as that of the 

 surrounding medium. Hence trees, and those mosses and lichens which 

 grow on rocks to which the snow does not adhere, follow the changes 

 of temperature of the air more or less rapidly, according to their mass, 

 specific heat, and conductivity, and according to the presence or absence 

 of wind and of moisture. Hence when the external temperature is low, 

 a formation of ice takes place in the plant, and the ice usually suddenly 

 appears after the plant has been cooled a few degrees below the freezing- 

 point of its sap. As a natural physical result, herbaceous plants such as Stel- 

 laria media and Ranunculus glacialis become stiff and brittle when frozen, 

 while the frozen trunk of a tree offers great resistance to the penetration of 

 an axe. In the above and other resistant plants, the formation of ice is 

 not followed by death 3 , as it is in most plants, and also in potatoes, beet- 

 roots, and apples. That death is in many cases the direct result of the 

 formation of ice is shown by the fact that in its absence the same 

 temperature may no longer be fatal. Thus a peeled potato in which ice 



1 Goppert, \Varmeentwickelung i. d. Pflanze, 1830, p. 94; Bot. Ztg., 1875, p. 613; Ueber 

 d. Gefrieren u. Erfrieren d. Pflanze, 1883 ; Frank, Krankheiten d. Pflanzen, 1894, 2. Aufl., Bd. I, 

 p. 197 ; Blisgen, Waldbaume, 1897, p. 43. 



3 See Goppert, 1. c., p. 59; Drude, Handb. der Pflanzengeographie, 1890, p. 24; Schimper, 

 Pflanzengeographie, 1898, p. 45. 



3 This fact has long been known. Cf. Duhamel, Naturgesch. der Eaume, 1765, Bd. II, p. 298; 

 Goppert, 1. c., pp. n, 228. 



