FREEZING AND COLD-RIGOR 237 



capable of withstanding desiccation cannot be killed by any attainable cold l . The 

 exposures were, however, only of short duration, and even Brown and Escombe only 

 used temperatures of -183 to 192 C. lasting for no hours. Longer exposures 

 might produce death, and in fact de Candolle 2 found that after 118 days' exposure 

 to a temperature lying between -37 and 57C., and occasionally rising above 



37 C., the seeds of Lobelia Erinus had lost their vitality, whereas those of Avena, 

 Tritiatm, and others were unaffected. The addition of successive amounts of water 

 produces a gradual decrease of resistance until that of the turgid object is reached. 

 In some cases the plant, or its aerial parts, is killed by cold owing to the fact that 

 it is unable to obtain sufficient water from the frozen soil to cover the loss by 

 transpiration 3 . In arctic climates, where the ground is frozen for long periods, it 

 is of importance that woody plants should possess xerophilous characters to protect 

 them from excessive transpiration 4 . 



Protection against cold. Although plants always ultimately assume the tem- 

 perature of the surrounding medium, nevertheless any means of hindering cooling 

 may afford a temporary protection against low temperatures of short duration 5 . 

 The covering of the plant with earth, snow, or straw is, however, of far greater 

 importance in this respect, especially in preventing the loss of heat by radiation, 

 which may cause the temperature of the plant to fall to 8 C. below that of the 

 surrounding air. Under such circumstances plants may be frozen stiff although 

 the thermometer indicates one or two degrees above zero. The older observations 

 upon the death of plants above zero were probably due to this cause. 



Fog, smoke, and dust all hinder radiation, and the use of smoky fires as a 

 preventive against frost on clear nights was known to Pliny and to the Peruvians 

 before the discovery of America 6 . Obviously the fire should be kindled early in 

 the night, since the insulating value of the smoke lies in the fact that it acts as 

 a check to radiation, and is not due to its hindering the absorption of heat and 

 consequent rapid thawing in the morning 7 . Indeed the slowest thawing is usually 

 of no avail, and the -useful effects of watering the frozen plants are probably due to 

 the more ready restoration of turgidity on thawing. 



1 Pictet (Archives d. sci. phys. et nat. d. Geneve, 1893, 3 e ser., T. xxx, p. 311) attained 



200 C. with liquid air (cf. Brown and Escombe, Proceed, of the Royal Society, 1897, Vol. LXII, 

 p. 160), and Thiselton-Dyer (ibid., 1899, Vol. LXV, p. 362) attained 250 C. by means of liquid 

 hydrogen. In the earlier researches by Pictet, de Candolle, Edwards and Collin, Goppert, the tem- 

 peratures were not so low. 



3 C. de Candolle, Archives d. sci. phys. et nat. de Geneve, 1895, 3" ser., T. xxxm, p. 504. 



3 Cf. the works of Kihlmann (quoted in Vol. I, p. 213) and Stenstrom ; also Goppert, Die 

 Warmeentwickelung in der Pflanze, 1830, p. 58; Molisch, Das Erfrieren der Pflanzen, 1897^.50; 

 Schimper, Pflanzengeographie, 1898. pp. 45, 717. On transpiration at low temperatures cf. Prillieux, 

 Compt. rend., 1872, T. LXXIV, p. 1344. 



* Kihlmann, Stenstrom, 1. c. The bud-scales form no protection against cold, but solely 

 against transpiration and other injurious external agencies. Cf. Griiss, Jahrb. f. wiss. Bot., 1892, 

 Bd. xxm, p. 669. 



5 For details see Miiller-Thurgau, Landw. Jahrb., 1886, Bd. XV, p. 538; Goppert, Ueber das 

 Gefrieren und Erfrieren d. Pflanze, 1883, p. 67 ; Frank, Krankheiten d. Prlanzen, 1895, 2. Aufl., 

 Bd. I, p. 214. 



6 Cf. Goppert, 1. c., 1830, p. 230; Eoussingault, Agronom., china, agricole et physiol., 1862, 

 T. II, p. 384. T Miillei-Thurgau, 1. c., p. 555. 



