152 



MECHANICAL SYSTEM 



do by once more applying the principle of division of labour, or, in 

 other words, by assigning the task of maintaining stability to special 

 tissues. Such mechanical tissues must of course be more or less 

 perfectly adapted to their special function ; this adaptation finds 

 expression not only (quantitatively) in the very considerable thickness of 

 the walls of mechanical elements, but also (qualitatively) in the tenacity 

 of the material employed in the construction of those walls. In this 

 way one can account for the differentiation of specialised mechanical 

 cells or stereides ; collectively, these stereides constitute the mechanical 

 system or stereome, the construction and arrangement of which must 

 next lie considered in detail. 



II. MECHANICAL CELLS. 



A. FORM AND STRUCTURE OF MECHANICAL ELEMENTS. 



1. Bast- fibres (Bast -cells). 



The term bast ss is considerably older than the science of vegetable 

 anatomy. From time immemorial this term has been applied to the 



C D 



% 



Fig. 47 



Bast-fibres. A. Group of bast-fibres from the (fruiting) spadix of Phoenix dactyl'c 

 fera in T.S. A^. Group of bast-fibres from the (fruiting) spadix oi Phoenix dactyli- 

 ftra in L.S. (cell-cavities shaded); the pointed end of a fibre is seen in the middle, a 

 thin septum at/. B. Group of bast -fibres from a twig of Buxus sempervirens ia T.S. 



C. Single bast-fibre of Urena sin uata with irregularly thickened walls ; at Z the cavity 

 is entirely obliterated (after Wiesner). D. Blunt-tipped bast-fibres from the scape of 

 AUivm multibulbosu.m. 



flexible constituents of the rind of certain trees, which can be used as 

 a binding material ; in its original sense at any rate it referred to the 

 conspicuous mechanical properties of the tissues in question. Bast 

 must in fact be regarded as the most widely distributed and important 

 of all mechanical tissues. 



