66 THE TISSUE-STRAINS 



elaters, and in the spores of Equisetum^ when moistened and dried. The layers 

 of starch-grains are also under strain J . 



The growth of isolated tissues. Since the tissues are usually not fully turgid 

 when isolated, they may at first undergo either a further elongation or shortening 

 when placed in water. Tissues which were compressed in the intact organ and 

 their growth prevented, now begin to grow, and in many cases cylinders of pith 

 may become 40 per cent, longer 2 . If growth is inhibited by the absence of 

 oxygen, or by placing the pith in water at o C, only a slight elongation 

 takes place, and this ceases when maximal turgidity is reached 3 . Under these 

 circumstances a split dandelion stalk only curves to a moderate extent, whereas 

 when growth is possible it gradually rolls up into a spiral. 



It is hardly surprising that, as Sachs 4 found, certain parts of the isolated 

 pith-cylinder elongate distinctly in air saturated with water-vapour, for the same 

 thing takes place when the younger parts of a shoot suspended in air grow by 

 withdrawing water from the shrivelling older parts. 



EXTERNAL INFLUENCES may permanently or transitorily affect the 

 tissue-strains, just as in the case of growth and turgor, but the physio- 

 logical reactions involved will be fully discussed in connexion with the 

 processes of growth and movement. The direct physical effects of changes 

 of temperature or of the percentage of water are easily predicted, although 

 it must be remembered that a considerably increased stress may produce no 

 perceptible result when the elasticity of the cell-walls is very high. 



Water-percentage. Since the walls are unequally stretched, and the rigidity 

 of thin-walled cells depends largely upon their turgor, the tissue-strains are 

 usually distinctly affected by a decrease in the percentage of water, and in 

 extreme cases may even be reversed. A decrease of volume also occurs, and 

 G. Kraus 5 showed that tree-stems and fruits swell slightly at night, and decrease 

 again in daytime, when transpiration is active and the percentage of water reaches 

 a minimum. The variations in the diameter of tree-trunks are, however, usually 

 less than i per cent. 



It is only when the loss of water is very great that the water of imbibition 

 is withdrawn from the cell-wall, but even then the decrease of volume only 



1 Nageli, Die Starkekorner, 1858, p. 39; Nageli and Schwendener, Mikroskop, 1877, 2. Aufl., 

 p. 430. 



3 Kraus, Bot. Ztg., 1867, p. 123; N. J. C. Miiller, Bot. Unters., 1872, Bd. I, p. 51. The 

 elongating pith often undergoes a slight decrease of diameter. Cf. Bateson, Annals of Botany, 

 1890-1, Vol. IV, p. 117. 



3 Schwendener and Krabbe, Jahrb. f. wiss. Bot., 1893, Bd. XXV, p. 327; Krabbe, ibid., 1896, 

 Bd. xxix, p. 450. Cf. II, 10. 



* Sachs, Textbook, 2nd Ed., Clar. Press Trans., p. 850; Physiology, 1887, Clar. Press Trans., 



PP- 57 2 . 573- 



6 Kraus, Die tagliche Schwellungsperiode d. Pflanze, 1881 (repr. from Abhandl. d. naturf. 

 Ges. zu Halle, Bd. XV), and Ann. d. Jardin botan. d. Buitenzorg, 1895, T. xn, p. 210. Cf. also 

 Kaiser, Die tagliche Periodicitat d. Baumstamme, Halle, 1879; Reuss, Bot. Centralbl., 1893, Bd. 

 LV > P- 34 8 J Friedrich, Bot. Ztg., 1897, p. 369; Fr. Darwin, Annals of Botany, 1893, Vol. vii, 

 p. 485 (fruits). That stems increase in circumference when supplied with water was first shown by 

 Hales, Statics, 1748, p. 74; Duhamel, De 1'exploitation des bois, 1764, T. I, p. 331. 



