SCLEREIDES 



L59 



numerous separate strands of bast-fibres which are linked together 

 by tangential arcs of brachysclereides to form a continuous cylinder 

 the composite cylinder (gemischte Bin;/) of Tschirch which is partly 

 responsible for the inflexibility of the organ. It often happens that 

 this composite cylinder or a mechanical cylinder which is primarily 

 homogeneous later becomes interrupted by radial gaps, in consequence 

 of the tangential tensions that result from the growth in thickness of 



Pig 50. 



Development of tlie composite mechanical cylinder in the stem of Aristolochia Sinlio. 

 A. Early stage. On the left a cell of the cortical parenchyma is beginning to penetrate 

 into a narrow radial cleft ; the larger gap on the right is already partly filled by four 

 intrusive cells, which are still thin walled. B. A later stage. Abroad gap almost 

 filled by intrusive selerenchyma. 



the twig : in this event elements of the cortical parenchyma penetrate 

 into these gaps, the adjacent regions of their walls, protruding into the 

 clefts while these are still quite narrow. The growth of the intrusive 

 cells keeps pace with the widening of the gap : they divide, thicken 

 their walls and ultimately become transformed into sclereides, and 

 thus completely repair the broken cylinder (Fig. 50). Another well- 

 known example of isodiametric sclereides is provided by the well- 

 known gritty particles in the flesh of the Pear and other Pomaceous 

 fruits (cf. p. 3). Potonie regards these particles as relics of a stony 

 shell which was fully developed in some ancestral form : in their 

 present condition they may play the same part in the soft flesh of the 



