DEVELOPMENT AND ALTERATION OF THE TISSUE-STRAINS 67 



produces a shortening of o-i to 2 per cent, in the case of wood-cylinders 1 , 

 although in very swollen walls the decrease of volume on drying is naturally 

 considerably greater. In this way powerful molecular forces may be called 

 into play, leading ultimately to the rupture of the drying wood. 



Temperature. The changes of turgor and of volume produced by alterations 

 of temperature have little influence upon the tissue-strains 2 , although the 

 formation of ice in the plant may cause them to greatly diminish, and may 

 even lead to a pronounced sinking of the branches of trees 3 . 



Very marked curvatures and changes of position may result as physiological 

 reactions to temperature, and these involve alterations in turgor or growth. Presumably 

 a fall of temperature may produce in this way either a decrease or increase of 

 the strains in the tissues, according to the character of the plant concerned. It is 

 indeed possible that the drooping and flaccidity observed in some cases at from 

 i to 2 C., without any formation of ice occurring, result from a physiological 

 depression of turgor. The latter fact, however, has still to be determined, and 

 indeed a physiological depression of turgor does not generally result when the 

 temperature is lowered to the freezing-point, as is shown by the fact that the 

 contractile staminal filaments of Cynareae do not shorten when cooled to o C., 

 nor do the pulvini of Mimosa lose their motility 4 . The pronounced strains 

 leading to cracks and ruptures during severe frosts 5 are not merely the direct 

 result of the contraction caused by the low temperature, but are largely due to 

 the withdrawal of imbibed water from the cell-walls during the formation of 

 ice, causing them to shrink and contract just as when they are dried. 



1 Hildebrand, Ann. d. Physik u. Chemie, iSSS, N. F., Bd. xxxiv, p. 395. According to 

 Villari (ibid., 1868, Bd. cxxxm, pp. 412, 417) the elongation is greater radially than longitudinally, 

 and the same is also the case for the expansion of dry wood by heat. For further literature see 

 Nordlinger, Die technischen Eigenschaften d. Holzes, 1860; Hartig, Holz d. Nadelbaume, 1886, 

 p. 101 ; Kitao, Bull, of College of Agriculture of Tokio, 1898, Vol. Ill, p. 299. 



2 Cf. True, Annals of Botany, 1895, Vol. ix, p. 399. 



3 H. R. Goppert, Warmeentwickelung, 1830, p. 112; Gefrieren u. Erfrieren, 1883, p. 10; 

 Hofmeister, Zelle, 1867, P 2 795 Moll, Influence d. 1. gel^e s. 1. plantes toujours vertes, iSSo, p. 9 

 (repr. from Archiv. Neerlandaises, T. ix). Cf. also Wille, Bot. Centralbl., 1884, Bd. xvni, 

 p. 220 ; Johow, Bot. Jahrb., iSSS, p. 525 ; Vochting, Berichte d. Bot. Ges., 1898, p. 51 ; Geleznow, 

 Rech. s. 1. quant, et 1. repartit. d. 1'eau d. 1. tige d. plant, ligneuses, Melang. biolog. t. d. Bull, 

 d. 1'Acad. d. St.-Petersbourg, 1872, T. IX, p. 667. The older literature is quoted there. See also 

 Bot. Ztg., 1867, p. 383. 



4 In most cases it has not been determined whether the sinking was due to the tissues themselves 

 becoming flaccid, or merely to an insufficient supply of water. According to Kraus (Bot. Ztg., 

 1867, p. 124), a fall of temperature of from 38 to 14 C. produces only a slight, but below 7C. 

 a pronounced, fall of the tissue-strains as measured by the changes of size on isolation, but this 

 requires further explanation. Krabbe's results (Jahrb. f. wiss. Bot., 1896, Bd. xxix, p. 447) do not 

 prove that at from o to 5C. the absorption of water is retarded by the plasmatic membrane, and, 

 in fact, Rysselberghe has shown that plasmolysis is produced by the same strength of solution at 

 oC. as at I5C. 



5 Goppert, Erfrieren d. Pflanzen, 1883, p. 14; Miiller-Thurgau, Landwirt. Jahrb., 1886, Bd. 

 xv, p. 483; Hartig, Forstlich-naturwiss. Zeitschrift, 1894, p. 255; Frank, Krankheiten d. Pflanzen, 

 1895, 2. Aufl., Bd. I, p. 210. Duhamel (De 1'exploitation d. bois, 1764, T. I, p. 324) observed a 

 slight decrease in the circumference of trees when frozen. 



