244 THE POWER OF RESISTANCE TO EXTREMES 



forces such as capillarity and imbibition. A pressure of i atmosphere only 

 lowers the freezing-point of water 0-0075 C. 1 , so tnat tne hydrostatic pressure 

 due to an osmotic concentration of the sap of 3-5 or of 10 atmospheres 2 lowers 

 the freezing-point by 0-025 or ' O 75C. respectively. Mousson 8 found that in 

 a capillary of 0-4 mm. diameter the freezing-point of water was lowered by o-i 

 to o-2C., while Miiller-Thurgau found that wet filter-paper froze at o-iC. 

 Hence even when the cell-wall is saturated with pure water, the latter will not 

 begin to freeze until the temperature is below zero. In dead plants, owing to 

 the escape of the sap, the freezing-point is usually about the same as that of 

 the expressed sap. 



Sub-cooling is possible in both water and saline solutions when contact with 

 ice is avoided, and it is especially easy to observe in capillary tubes. Thus 

 Mousson found that in a capillary tube of 0-4 mm. diameter the water remained 

 unfrozen at 7 to ioC., while Dufour 4 observed that drops of water a few 

 millimetres in diameter, floating in a mixture of almond oil and chloroform of 

 like specific gravity, remained liquid at 8 to i2C., but immediately turned 

 solid on touching a piece of ice. The sub-cooling in most plants is not greater 

 than is represented by a temperature of 3 to 4C., the temperature then 

 rising to o-iC, owing to the commencing formation of ice, in a particular 

 case in which a living leaf was used. The same sub-cooling is shown by dead 

 plants, so that we are here dealing with a physical phenomenon 5 , which is not 

 appreciably influenced by the life of the cell. The small size of bacterium-cells 

 may appreciably lower their freezing-point, but an experimental answer to this 

 and similar questions is still required. 



SECTION 68. How Cold causes Death. 



The erroneous supposition of Duhamel and Senebier 6 that freezing 

 causes death by bursting the cells was disproved by Goppert, Sachs, and 

 Nageli 7 , who showed that all plants are not killed by freezing, while as 

 a matter of fact the ice is usually formed outside and not inside the cells. 

 Even when the latter is the case the cell-wall is able to stretch sufficiently to 



1 Clausius, Mechanische Warmetheorie, 1876, p. 174; Lehmann, Molecularphysik, 1888, Bd. i, 

 p. 820. 



2 These pressures are produced by solutions isosmotic with LOI or 3.03 per cent, solutions of 

 potassium nitrate. In most plants the turgor is equivalent to from 1.5 to 3 per cent, solutions 

 ofKNO 3 . 



8 Mousson, Die Physik auf Grundlage d. Erfahrung, i. Aufl., 2. Abth., p. 73, and Annal. d. Physik 

 u. Chemie, 1858, Bd. cv, p. 161. 



* Dufour, Annal. d. Physik u. Chemie, 1861, Bd. cxiv, p. 530. 



5 Cf. Ostwald, Lehrbuch, 1891, Bd. I, p. 993 ; Wiillner, Physik, 1885, 4. Aufl., Bd. in, p. 607 ; 

 Tammann, Zeitschr. f. physikal. Chem., 1898, Bd. xxv, p. 441 ; Bachmetjew, Zeitschr. f. wiss. 

 Zoologie, 1900, Bd. LXVII, p. 529. 



See Goppert, Die Warmeentwickelung in der Pflanze, 1830, p. 8; Molisch, Das Erfrieren d. 

 Pflanzen, 1897, p. 66. 



7 Goppert, 1. c., p. 25; Sachs, Versuchsst., 1860, Bd. n, p. 179; Flora, 1862, p. 20; Nageli, 

 Sitzungsb. d. Bayerisch. Akad., 1861, I, p. 267; Nageli and Schwendener, Das Mikroscop, 1877, 

 2. Aufl., p. 455. 



