PSARONIUS 301 



hardly have been exceeded if they had been dealing 

 with recent instead of with fossil plants. We have here 

 only given the main results for this particular species, 

 which, however, may serve very well as a type of 

 the rest. 



The more minute histology has also been worked' 

 out with a fair amount of detail. The steles have the 

 typical concentric structure, each consisting of a band 

 . of wood surrounded by phloem. The xylem is com- 

 posed of scalariform tracheides with or without xylem- 

 parenchyma, according to the species. 



The ground-tissue is mostly parenchymatous, but 

 internal sclerotic bands occur below the leaf-gaps (see 

 Fig. 113, on the right and left). 



The outer, radiciferous cortex consists of paren- 

 chyma, but each of- the roots by which it is per- 

 meated is surrounded by a dense sclerenchymatous 

 envelope, probably to be regarded as belonging to 

 the root itself. In Psaronius generally, the structure 

 of the roots is polyarch ; in the particular species, 

 P. brasiliensis ; the number of xylem-angles is usually 

 five or six. Phloem and cortex are sometimes very 

 fairly preserved, and in the latter, secretory sacs or 

 canals are conspicuous. The roots strongly recall those 

 of Marattiaceous Ferns now living. 



A species, P Renaulti, occurring in the roof-nodules 

 of the Lower Coal-measures of Lancashire, is interesting 

 from its exceptional antiquity and from the simplicity 

 of its structure. The stem in this species contains a 

 single annular stele, enclosing a large pith, and inter- 

 rupted only at the exit of the leaf-traces, the sole 

 instance, at present known, of a solenostele in a Palseo- 



