FREEZING AND COLD-RIGOR 235 



alpine regions. Neither Miiller-Thurgau l nor Molisch 2 could as a rule 

 detect any injurious effect due to sudden thawing. Molisch found, however, 

 that apples and pears, and also the leaf of Agave americana, after moderate 

 freezing remained living if very gradually thawed, but died when the 

 thawing was rapid. 



Resistant plants withstand rapid and slow cooling equally well, and it 

 is doubtful whether a rapid fall of temperature is more injurious to plants 

 killed by freezing than is gradual cooling. That the injury is not due to 

 the sudden formation of ice after sub-cooling is shown by the fact that 

 a peeled potato is killed by freezing, although no sub-cooling occurs and 

 the ice forms gradually at i C., the freezing-point of the sap 3 . 



No demand is entirely without effect, and hence it is not surprising 

 that repeated freezing and thawing should prove injurious. Thus Goppert 4 

 found that Lamium purpureum^ Stellaria media, and Helleborus niger were 

 killed when frozen at 4 to 5 C. and thawed at room-temperature for 

 more than six times in succession. In the open without these changes the 

 plants withstood temperatures of n to 13 C. 



Goppert 5 concluded that death by cold was the direct effect of freezing, 

 whereas Sachs 6 was of the opinion that death occurred during thawing, and could 

 be avoided by making this process take place slowly. Miiller-Thurgau's investiga- 

 tions have, however, revealed the true state of affairs. 



Usually it can only be told on thawing whether the plant is alive or dead, 

 but in some cases reactions occur which indicate the death of a frozen plant. 

 Thus Goppert 7 showed that the blue colouration due to the separation of indigo 

 in the perianth leaves of Phajus, Calanthe^ and other orchids was a sign of death, 

 whether produced by cold or other agencies. Detmer 8 used the change of colour 

 in Begonia manicata, Molisch 9 that in the red alga Nitophyllum peltatum^ and also 

 the odour due to the liberation of cumarin,, as signs of death. 



Not only does the resistance of different plants vary, but also that of 



1 Miiller-Thurgau, Lan'dw. Jahrb., 1886, Bd. xv, p. 506. 



2 Molisch, Das Erfrieren d. Pflanzen, 1897, p. 34. Any water left in the intercellular spaces on 

 thawing is rapidly reabsorbed. 



3 Miiller-Thurgau, Schweiz. Zeitschr. fur Obst- u. Weinbau, 1894. 



* Goppert, Warmeentwickelung in der Pflanze, 1830, p. 62 ; Ueber das Gefrieren und Erfrieren d. 

 Pflanze, 1883, p. 49. 



5 Goppert,!. c.,p. 232; Bot. Ztg., 1871, p. 73. Cf. Kunisch, Ueber die todtliche Einwirkung 

 niederer Temperaturen, Diss., 1880, p. 42. 



6 Sachs, Sitzungsb. der Sachs. Ges. der Wiss. zu Leipzig, 1860, Bd. xn, p. 27; Versuchsst., 

 1860, Bd. II, p. 175. Duhamel (Naturgeschichte der Baume, 1765, Bd. n, p. 277) made a similar 

 assumption. Cf. the literature quoted by Miiller-Thurgau, 1. c., 1886, p. 506. Miiller-Thurgau also 

 explains how Sachs was misled. 



7 Goppert, Bot. Ztg., 1871, p. 339; Miiller-Tburgau, Landw. Jahrb., 1880, Bd. IX, pp. 162, 166; 

 1886, p. 515; Molisch, 1. c., p. 35. That the blue colouration was due to a formation of indigo 

 was recognized by Marquart in 1830. Kunisch (1. c., pp. 37, 50) states that it also occurs in the 

 absence of oxygen. 



8 Detmer, Bot. Ztg., 1886, p. 521. 9 Molisch, 1. c., pp. 38, 41. 



