112 



LYCOPODIALES 



[CH. 



Noticeable features are (i) the small size of the central 

 cylinder (or stele) in proportion to the diameter of the branch, 

 (ii) the production at a comparatively early stage of growth of a 

 zone of secondary wood, x^, which gradually assumes the form 

 of a complete cylinder of unequal breadth, surrounding the 

 primary xylem, x, (iii) the formation of a secondary cortical 

 tissue by a meristematic cylinder (phellogen, pV) situated close 

 to the leaf-cushion region of the outer cortex. On the outer 

 edge the stele consists of narrow tracheae some of which show in 

 longitudinal section the spiral form of thickening characteristic 

 of most protoxylem elements: towards the centre of the stele the 



Fig. 149. Lepidodendron vasculare, a, immature tracheae ; vi, meristem ; 

 mr, medullary ray ; x, xylem. 



A. Longitudinal section through the edge of the secondary wood. 



B. Short tracheae in the centre of the stele. (From a specimen 

 from the Halifax Hard bed in Dr Kidstou's Collection.) 



diameter of the tracheae gradually increases and parenchymatous 

 cells become associated with the elongated scalariform elements. 

 In the central region the stele is composed of parenchymatous 

 tissue arranged in vertical series of short cells, interspersed with 

 short tracheae distinguished by the greater thicknessof theirwalls 

 and by their scalariform and reticulate thickening bands. Some 

 of these short tracheae are shown in vertical section in fig. 149, B : 

 the fine and broken lines connecting adjacent thickening bands 

 probably represent the remains of the original wall. These 

 delicate bands, which have been figured in various species 



