240 



THE POWER OF RESISTANCE TO EXTREME'S 



fatal ruptures being produced by the ice. Prillieux observed that in the stems of 

 Labiatae four masses of ice are commonly formed, each separated from the rest by 

 the harder collenchymatous tissue at the angles of the stem. In the stem of Senetio 

 crassifolius five separate ice-masses usually appear beneath the epidermis, whereas 

 in the stems of many Scrophulariaceae a continuous ring of ice is formed in the 

 outer cortex. Masses of ice also form beneath the epidermis of the petiole of 

 Cynara scolymus and ice-needles radiate from the parenchyma surrounding each 

 vascular bundle into the dark air spaces (Fig. 31). 



Since the cracks and fissures will tend to follow the path of least resistance, 

 they will naturally usually appear between the radial walls of cells arranged in 

 radiating rows. It is also easy to understand why frost should hasten the fall of 

 deciduous leaves. The cracks produced or enlarged by the ice-formation partly 

 or entirely disappear on thawing, but persist when the plant is placed in cold 



alcohol and then thawed. The ice usually appears 

 in the form of needles or columns, which grow out 

 at right angles to the point of origin, and may 

 frequently unite to form large masses. 



The mode of freezing is due to the fact that 

 the thin film of water which covers the outer walls 

 of the cells lining the intercellular spaces freezes 

 first. This causes more water to be withdrawn, 

 and this again freezes. The continuance of this 

 process on moist soil or on a column of plaster 

 of Paris with its base in water results, as in the 

 plant, in the formation and growth of needles 

 or masses of ice 1 . As water is withdrawn from 

 the cell, the cell-sap concentrates, for the salts 



are kept back by the protoplasm so that the ice formed in the intercellular 



spaces is nearly pure 2 . 



The same applies when a cell freezes under water, and hence arises the 



collapse of frozen cells of Spirogyra 3 . The same shrinkage occurs in air, 



and indeed under all circumstances so long as the cell-wall is not too rigid. 



It is owing to their different internal structure that some multicellular organs 



increase in volume, length, or diameter on freezing, whereas others decrease 4 . 



Localized ice-formation may produce bending or curvature, and may also 



FlG. 31. Transverse section of a slow- 

 ly frozen petiole of Cynara scolymus. 

 (After Sachs.) e, epidermis; g; paren- 

 chyma, within which lie vascular bun- 

 dles surrounded by radiating ice- 

 crystals. These are also formed on 

 the parenchyma at K^ and project into 

 the dark cavities. 



1 Cf. Lehmann, Molecularphysik, 1888, Bd. I, p. 347. An analogy used by Le Conte, Mohl, 

 and Sachs, Ber. d. Sachs. Ges. d. Wiss. z. Leipzig, 1860, Bd. xii, p. 6. 



3 Sachs (Lehrb., 4. Aufl., p. 703) states that the ice collected from a cut surface of an 

 artichoke petiole contained about o-i per cent, of solids, and Miiller observed the same in the case 

 of the beet-root. These were merely included matter, since the ice crystallizes from a salt solution as 

 pure water, the salt being deposited separately and at low temperatures often in a hydrated form 

 (NaCl 2H 2 O, &c.). 



3 Molisch, Das Erfrieren d. Pflanzen, 1897, p. 22. 



4 Sachs, Ber. d. Sachs. Ges. d. Wiss. z. Leipzig, 1860, Bd. xii, p. 21 ; Miiller-Thurgau, Lanclw. 

 Jahrb., 1880, Bd. ix, p. 188. 



