680 ON FORM AND MECHANICAL EFFICIENCY [ch. 



stronger bundles ; sometimes with these bundles further strength- 

 ened by radial balks or ridges ; sometimes with all the fibres set 



close together in a continuous hollow 

 cylinder. In the case figured in Fig. 

 333 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 the ascent of sap or other 



functions, but never such as to confer mechanical rigidity. In a 

 tall 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-fines in Fig. 332 ; but of course, in 

 this case, we are still deafing with tensile members, the opposite 

 bundles taking on in turn, as the tree sw^ays, the alternate . 

 function of resisting tensile strain*. 



Let us now" come, at last, to the mechanical structure of bone, 

 of which we find a well-known and classical illustration in the 

 various bones of the human leg. In the case of the tibia, the bone 

 is somewhat widened out above, and its hollow shaft is capped 

 by an almost flattened roof, on which the weight of the body 

 directly rest. It is obvious that, under these circumstances, the 

 engineer would find it necessary to devise means for supporting 

 this flat roof, and for distributing the vertical pressures which 

 impinge upon it to the cyhndrical walls of the shaft. 



* For further botanical illustrations, see (int. al.) Hegler, Einfluss der Zug- 

 kraften auf die Festigkeit und die Ausbildung mechanischer Gewebe in Pfianzen, 

 SB. sacks. Ges. d. Wiss. p. 638, 1891 ; Kny, L , Einfluss von Zug und Druck auf 

 die Richtung der Scheidewande in sich teilenden Pflanzenzellen, Ber. d. hot. 

 Gesellsch. xiv, 1896; Sachs, Mechanomorphose und Phylogenie, Flora, Lxxvm, 

 1894; cf. also Pfliiger, Einwirkung der Schwerkraft, etc., iiber die Richtung der 

 Zelltheilung, Archiv, xxxiv, 1884. 



