LEPIDODENDRON 147 



The former are often no longer than the cells which 

 accompany them. The transverse walls of the short 

 tracheides are reticulately thickened, and form a con- 

 spicuous feature in the transverse sections, by which 

 the species can be easily recognised. 



The phloem, which is fairly preserved in some of 

 the best specimens, forms a zone of thin-walled tissue, 

 including strands of elongated elements, surrounding 

 the wood, and itself surrounded by a somewhat broader 

 band of larger -celled parenchyma, which may be 

 regarded as the pericycle. The leaf-trace bundles, 

 where they cross this zone, are each enclosed in a 

 special sheath, sometimes forming a kind of bridge or 

 trabecula across the more delicate tissues of the phloem 

 and pericycle (Fig. 60). 



The endodermis is simply a layer of tangentially 

 elongated cells. Then we come to the broad zone of 

 inner cortex, which consists of delicate short -celled 

 parenchyma, only preserved in the best specimens, as 

 in that shown in Fig. 59. The more internal layers, 

 however, are of firmer structure, and often persist when 

 the rest has perished. In the middle part of this zone 

 the cells show some trace of radial arrangement. 

 Through this region the leaf-trace bundles pass, still 

 taking a steep upward course (Fig. 59)- Beyond the 

 inner cortex is another broad belt, the outer cortex, the 

 tissue of which is formed of elongated cells with, for 

 the most part, rather thick cell-walls ; this zone, owing 

 to its solid construction, is always well preserved. The 

 leaf-traces, which in this region gradually assume a 

 more horizontal course, often pass through gaps, due 

 to the disappearance of delicate tissue (Fig. 59). 



