180 MECHANICAL SYSTEM 



the contrary (e.g. Phoenix dactylifera, Sdbal Adansoni), the two flanges 

 are so massive that they undoubtedly serve to strengthen the organ as 

 a whole. These internal girders are accompanied by a large number 

 of comparatively stont subepidermal fibrous strands. In the Date 

 Palm the latter are developed approximately to the same extent on 

 both sides of the leaf ; they are, however, placed at irregular intervals, 

 and it only occasionally happens that two strands correspond on 

 opposite sides, and so combine to form a typical girder. The author 

 has observed precisely the same lack of correspondence between the 

 fibrous strands on the opposite sides of the leaf in many other Palms 

 (Livistona chinensis, Sabal Adansoni, Chamaerops excelsa). 



This peculiar arrangement of the mechanical tissues in many 

 Palm leaves seems at first sight irrational, and its true significance is 

 not realised until one has had an opportunity of watching the behaviour 

 of a Palm when exposed to a high wind under natural conditions. 

 The inflexibly constructed stem displays a remarkable degree of rigidity, 

 and oscillates without undergoing any appreciable flexure. The leaves 

 and leaflets behave quite differently : they flutter rapidly to and fro, 

 and in so doing are at one moment violently bent, and at the next, 

 recoil with equal violence, while ultimately they return to their " rest- 

 ing " position without having sustained any permanent deformation. 

 The leaves are, therefore, not inflexible at all, in the sense in which 

 the term applies to the stem ; it would indeed entail far too great an 

 expenditure of material to render an organ like a Palm leaf, which 

 exposes a very large surface to the wind, actually inflexible. Such 

 leaves are accordingly constructed so as to combine strength with 

 pliancy; the withdrawal of the mechanical strands from the periphery of 

 the cross-section to the centre thus fully accords with the behaviour of 

 the leaves in the wind, as Stahl was the first to note. 98 The centrali- 

 sation of the mechanical tissues has the further advantage of increasing 

 the resistance of the pinnae to the longitudinal tensions produced by 

 wind action. 



Another example of the bilateral or dorsiventral type of organ 

 which has to be inflexible in a single plane is provided, according to 

 Worgitzky," by the spirally coiled portions of certain tendrils, which 

 provide a firm and elastic connection between the climber and its 

 support. When such a " watch-spring tendril " is stretched, it is only 

 the straight portion that is subjected to longitudinal tension : in the 

 coiled region the pull is resolved into bending and twisting com- 

 ponents. As the coils are pulled apart, every portion of the spiral 

 res-ion tends to straighten itself, with the result that its concave face 

 is stretched, and its convex face compressed. The curvature always 

 takes place in the same plane and in the same sense. Hence, the 



