HETERANGIUM 407 



are continued outwards into the cortex, to form the 

 xylem of the foliar bundles. The stele of Heterangium 

 thus differs from that of Lyginodendron in the fact that 

 in the former the xylem extends through the whole 

 central region, forming a continuous mass of primary 

 wood, traversed by irregular bands of conjunctive 

 parenchyma, but without a pith. 



In some stems this primary structure has remained 

 unaltered ; in such cases, the phloem, which, no doubt, 

 formed a zone round the wood, can only be recognised 

 as a ring of imperfectly preserved tissue. Most speci- 

 mens, however, had already produced secondary wood 

 and phloem, though not usually to so great an extent 

 as in Lyginodendron. The secondary wood consists of 

 rather small tracheides, arranged in regular radial series, 

 with medullary rays between them (see Figs. 154, 155, 

 and 1 56, x 2 ). The secondary tracheides bear numerous 

 bordered pits, which are usually limited to the radial 

 walls. Beyond the wood the cambium and secondary 

 phloem are often fairly well preserved (see Fig. 156, 

 cb, f>/i 2 ). In all cases, whether secondary thickening 

 had taken place or not, the stele is surrounded by a 

 well-marked pericycle, consisting of several layers of 

 thin-walled parenchyma. 



'The wide inner cortex has a very characteristic 

 structure. Its ground -mass consists of short -celled 

 parenchyma, but in this tissue numerous horizontal 

 plates of thick-walled mechanical elements are embedded 

 (see Fig. 154, c). The sclerotic plates do not extend 

 all round the stem, but are isolated and ranged in 

 vertical series. Thus, a radial section of the stem may 

 either show a whole row of them, one above the other, 



