THE FORMATION OF ICE IN PLANTS 239 



Yeast-cells were observed by Schumacher to be in part killed at H4C. 

 Melsens 1 found that prolonged exposure to 91 C. greatly decreased the fermen- 

 tative activity, and this power was entirely destroyed by an exposure of 108 hours to 

 70 C., followed by one of twenty hours to i3oC. 2 Pictet also states that 

 not only the enzyme of yeast but also the toxins of poisonous bacteria are de- 

 stroyed at extremely low temperatures. 



SECTION 67. The Formation of Ice in Plants. 



Ice is always formed when the temperature falls low enough 3 , and it can 

 not only be seen, but also renders the plant brittle and causes the inter- 

 nal temperature-curve to show a stationary period corresponding to its 

 freezing (or melting) point. This is lower than that of pure water owing to 

 the influence of the salts dissolved in the cell-sap, and usually a certain 

 amount of sub-cooling occurs before freezing begins. 



The ice is usually formed outside the cells, and either fills up intercellular 

 spaces, or forms gaps or fissures in the tissues. In some plants the ice- 

 formation when complete may cause the peripheral tissues to rupture, 

 forming cracks from which masses of ice project 4 . Crystals or masses of 

 ice form on the cut surfaces of succulent parts at low temperatures, as is 

 well shown when slices of a beet-root or cucumber are frozen in a moist 

 chamber. 



The ice-formation has been chiefly studied by Sachs, Prillieux, Miiller-Thurgau, 

 and Molisch 5 . The two last-named authors observed the progress of events under 

 the microscope, which was enclosed in a double-walled metal cylinder containing 

 a freezing mixture 6 . 



Ice usually appears first in the intercellular spaces of parenchymatous tissue, 

 but in the case of wood, within the cavities of the tracheae and tracheides 7 . The 

 ice formed in an intercellular space acts like a wedge, pushing apart the cells and 

 tissues in a manner determined largely by their anatomical arrangement 8 . In 

 plants which can withstand freezing, the structure is probably such as to avoid any 



1 Melsens, Compt. rend., 1870, T. LXX, p. 631. 



2 Pictet and Yung, Compt. rend., 1884, T. xcvm, p. 748 ; Pictet, Archiv. d. sci. phys. et nat. d. 

 Geneve, 1893, 3" ser., T. xxx, p. 312. 



3 This was shown by Schiibler and his pupils in 1823-6 in spite of the contradictions of 

 Hunter and others. Cf. Goppert, Warmeentwickelung in d. Pflanze, 1830, pp. 138, 160. 



4 Prillieux, Ann. d. sci. nat., 1869, 5" ser., T. xn, p. 129. In many cases comb-like masses 

 of ice, i cm. in height, may project. Cf. Caspary, Bot. Ztg., 1854, p. 665; Sachs, Ber. d. Sachs. 

 Ges. d. Wiss. z. Leipzig, 1860, Bd. xn, p. 10; Bay, Botanical Gazette, 1894, Vol. XIX, p. 321. 



5 Miiller-Thurgau, Landw. Jahrb., 1880, Bd. IX, p. 134, and 1886, Bd. xv, p. 453; 

 H. Molisch, Das Erfrieren d. Pflanzen, 1897. 



6 The apparatus used by Molisch is the best. Cf. also Klemm, Jahrb. f. wiss. Bot., 1895, 

 Bd. xxviil, p. 642. 



7 Miiller-Thurgau, 1. c., 1886, p. 481. 



8 Cf. Prillieux, &c. Also Frank, Krankheiten der Pflanzen, 1894, 2. Aufl., Bd. I, p. 178; 

 Dalm^r, Flora, 1895, p. 437 ; Ludwig, Bot. Centralbl., 1899, Bd. LXXX, p. 405. 



