242 THE POWER OF RESISTANCE TO EXTREMES 



Plants usually have a lower freezing-point than that of the sap they 

 contain, as can be seen from the following figures given by M tiller l : 



Real freezing- Lowest sub-cooling 



point. point without freezing. 



Potato i-o to i-6C. 2-8 to 5-6 C. 



Apple and Pear . . . . 1-4 to 1-9 C. 2-1 to 5-2 C. 



Grape -3-iC. -6-8 to -7-8 C. 



Phajus grandifolius, labellum ) _ .r8 C -6-oC 



of flower / 



Phaseolus vutgaris, foliage leaf . 0-8 to i-iC. 5-3 to 6-3 C. 



Sempervivum tabulaeforme . . Q<55C. 6-48 C. 



Hence it follows that many plants that are killed by the formation of ice 

 within them can be lowered well below zero without being fatally affected. 

 The possible amount of sub-cooling is slightly or not at all altered by death, 

 and it varies in different cases independently of the concentration of the 

 cell-sap. The latter is of course of primary importance in determining 

 the true freezing-point, but nevertheless other factors enter into play, for the 

 expressed sap and also a dead plant have a slightly higher freezing-point 

 than living objects 2 . Any increase in the concentration of the cell-sap 

 will naturally lower the freezing-point, which is therefore changeable. In 

 the herbaceous plants investigated by Mtiller it lay, however, between 0-15 

 and 8 C., and in most highly succulent plants between i and 2 C. 



When a sub-cooled plant freezes, the formation of ice liberates heat, and 

 if sufficient in quantity this may raise the temperature to the real freezing- 

 point. This actually occurs in plants, and thermometers inserted at 

 different depths in a large potato may show approximately similar readings 

 during freezing, when this takes place rapidly. 



Any localized formation of ice prevents sub-cooling, and it is probably 

 owing to the escape of readily freezing sap that a peeled potato freezes at 

 1 C. without any previous sub-cooling. By rapidly lowering the tempera- 

 ture, the periphery of a thick organ may be frozen before the temperature 

 at the centre has fallen to the freezing-point. Hence in the centre of a 

 beet-root, ice is usually formed without any sub-cooling, but the latter does 

 occur when the temperature is very gradually lowered 3 . 



Methods. Muller-Thurgau * wrapped leaves around the bulb of a sensitive 



1 Muller-Thurgau, Landw. Jahrb., 1886, Bd. xv, p. 490. Molisch (Das Erfrieren d. Pflanzen, 

 1897, ? J 8) states that ice-formation begins in the sporangiophore of Phycomyces nitens at 17, 

 but not at 12 C. In the tracheides of Taxus ice appears, according to Dixon and Joly (Annals 

 of Botany, 1895, Vol. IX, p. 403), at -10 to -11 C. 



2 Miiller, 1. c., p. 478. The freezing-point of a living potato was 0-98 C., after death it 

 became 0-55 C. 



3 Muller-Thurgau, 1. c., 1880, Bd. ix, p. 176 ; 1886, Bd. xv, p. 488. 



4 Muller-Thurgau, 1. c., 1880, pp. 156, 168; 1886, p. 470. Bachmetjew (Zeitschr. f. wiss. 

 Zool., 1899, Bd. LXVI, p. 521) appears to be unaware of the long-known and clearly established 



