974 ON FORM AND MECHANICAL EFFICIENCY [ch. 



as the other. And then Schwendener proceeds to shew, in many 

 beautiful diagrams, the various ways in which these strands of strong 

 tensile tissue are arranged in various stems : sometimes, in the simpler 

 cases, forming numerous small bundles arranged in a peripheral 

 ring, not quite at the periphery, for a certain amount of space has 

 to be left for living and active tissue; sometimes in a sparser ring 

 of larger and stronger bundles; sometimes with these bundles 

 further strengthened by radial balks or ridges; sometimes with all 

 the fibres set close together in a continuous hollow cylinder. In 



the case figured in Fig. 461, Schwendener 

 calculated that the resistance to bending 

 was at least twenty-five times as great as 

 it would have been had the six main 

 bundles been brought close together in a 

 solid core. In many cases the centre of 

 the stem is altogether empty ; in all other 

 cases it is filled with soft tissue, suitable 

 for various functions, but never such as 

 to confer mechanical rigidity. In a tall 

 Fig. 461. conical stem, such as that of a palm-tree, 



we can see not only these principles in the construction of the 

 cylindrical trunk, but we can observe, towards the apex, the bundles 

 of fibre curving over and intercrossing orthogonally with one another, 

 exactly after the fashion of our stress-lines in Fig. 460; but of 

 course, in this case, we are still dealing with tensile members, the 

 opposite bundles taking on in turn, as the tree sways, the alternate 

 function of resisting tensile strain*. 



* For further botanical illustrations, see {int. al.) R. Hegler, Einfiuss der Zug- 

 kraften auf die Festigkeit und die Ausbildung meehanischer Gewebe in Pflanzen, 

 SB. sacks. Ges. d. Wiss. 1891, p. 638; Einfluss des mechanischen Zuges auf das 

 Wachstum der Pflanze, Cohn's Beitrdge, vi, pp. 383-432, 1893; 0. M. Ball, Einfluss 

 von Zug auf die Ausbildung der Festigkeitsgewebe, Jakrb. d. wiss. Bot. xxxix, 

 pp. 305-341, 1903; L. Kny, Einfluss von Zug und Druck auf die Richtung der 

 Scheidewande in sich teilenden Pflanzenzellen, Ber. d. hot. Gesellsch. xiv, pp. 378- 

 391, 1896; Sachs, Mechanomorphose und Phylogenie, Flora, Lxxvin, 1894; of. also 

 Pfliiger, Einwirkung der Schwerkraft, etc., uber die Richtung der ZeUtheilung, 

 Archiv, xxxiv, 1884; G. Haberlandt's Physiological Plant Anatomy, tr. by Montagu 

 Drummond, 1914, pp. 1.50-213. On the engineering side of the case, see Angus R. 

 Fulton, Experiments to show how failure under stress occurs in timber, etc., Trans. 

 R.S.E. XLViii, pp. 417-440, 1912; Fulton shews {int. a/.) that "the initial cause of 

 fracture in timbers lies in the medullary rays." 



