

THE TISSUE-STRAINS 63 



Mimosa ptidica, and shows that the wood-cylinder (A and B, Ji) is compressed 

 by the surrounding tissues. In the latter again the epidermis is stretched by the 

 parenchyma within, as is shown in Fig. 16 B, by the curvatures resulting on slicing 

 away strips of the cortex (c and d] from the wood-cylinder (h) and epidermis (e). 



Pronounced tissue-differentiation is not necessary for the production of 

 these strains, and the peripheral layers of the sporophores of Basidiomycetes 

 are, for example, under tension 1 . In higher plants, as 

 the vascular bundles differentiate they become subjected to 

 longitudinal tension, and this also applies to the bundles 

 of the root, although the tension is much weaker 2 . When, 

 however, a contractile root subsequently shortens owing to 

 the activity of the cortex, the strains are reversed, the 

 cortex being under tension while the epidermis and vascular 

 bundles are subjected to longitudinal compression. 



Conclusions as to the strains in the tissues may be made 

 from the changes of shape on isolation, or on removing 

 certain parts. Thus the surface of a transverse section 

 may become wavy, owing to the protrusion of the com- 

 pressed tissues, and the retraction of those under tension. 

 This can easily be seen in the nodes of grasses, the pulvini 



J FlG. 16. The sections 



of Phaseolus and Mimosa, and also in old roots 3 . The have been previously laid 



in water. (Magnified.) 



transverse strains are more or less modified when the 

 longitudinal strains are removed, for the elongation of the compressed pith 

 which then occurs decreases its diameter, and correspondingly diminishes the 

 radial pressure exerted by it. Similarly, the diameter of the cortex, and hence 

 also the inwardly directed pressure exerted by it, tend to diminish when it 

 is no longer stretched longitudinally. The mode in which these forces, balance 

 in the intact plant is, however, a purely physical problem 4 . 



The magnitude of the shearing stresses becomes, in some cases, so great as to 

 rupture resistant cell-walls and tissues, but even when the limit of elasticity is not 

 exceeded the stresses may amount to 5, or even more than 15 atmospheres. 

 Thus the tangential tension in the bark of trees often reaches 10 atmospheres, 

 for Krabbe found that a weight of 100 grammes was frequently necessary to 

 stretch an isolated millimetre square strip of bark to its original length 5 . 



The stresses in the tissues of stems are often as great as this 8 , and 



1 On algae cf. E. Kiister, Sitzungsb. d. Berl. Akad., 1899, P- 8l 9- 



2 Sachs, Arbeit, d. Bot. Inst. in Wiirzburg, 1873, Bd. I, p. 435. On the strains in the 

 subterranean branches of Yucca and Dracaena cf. Sachs, Lehrbuch, 4th ed., p. 770. 



3 Cf. Pfeffer, I.e., 1893, p. 404; de Vries, Landw. Jahrb., 1880, Bd. ix, p. 41 ; Detlefsen, Arb. 

 d. Bot. Inst. in Wiirzburg, 1878, Bd. II, p. 38. 



* Various problems are clearly set forth by Nageli and Schwendener, Mikroskop, 1877, and ed., 

 pp. 406, 414. 



5 Krabbe, Sitzungsb. d. Berl. Akad., 1882. Cf. Exp. 30 on p. 1,116, in which 200 grammes 

 were necessary per 2 sq. mm., i.e. 10,000 grammes per sq. cm., or a little less than 10 atmospheres 

 (i atmos. = 1,033 grammes per sq. cm.). 



6 The earliest researches are those of Hofmeister (Pflanzenzelle, 1867, p. 276; Flora, 1862, 

 p. 150). Kraus (Bot. Ztg., 1867, Tab., p. 9) did not measure the sectional area of the loaded 



