XXl] OSMUNDACEAE 329 



increases and the protoxylem strands undergo repeated bifurca- 

 tion ; it assumes in fact the form and general type of structure 

 met with in the leaf-traces of Todea and Osmunda. Numerous 

 diarch roots, given off from the stele at points just below the 

 out-going leaf-traces, pass outwards in a sinuous horizontal 

 course through the cortex of the stem. 



In Zalesskya gracilis the xylem cylinder was probably wider 

 in the living plant than in the petrified stem. In Zalesskya 

 diploxylon^, in all probability from the same Russian locality, 

 there can be little doubt that the xylem was originally solid to 

 the centre (fig. 249). In this species also the phloem forms 

 a continuous band (ph, fig. 249) consisting of four to six layers 

 of sieve-tubes. 



Thamnopteris. 



Thamnopteris Schlechtendalii (Eich.). Figs. 250, 312, A, 

 Frontispiece. 



In 1849 Brongniart^ proposed the name Thamnopteris for 

 a species of fern from the Upper Permian of Russia origin- 

 ally described by Eichwald as Anomopteris Schlechtendalii. A 

 new name was employed by Brongniart on the ground that the 

 fossil was not generically identical with the species previously 

 named by him Anomopteris Mougeotii'. Eichwald's specimen has 

 been thoroughly investigated by Kidston and G wynne-Vaughan*. 

 The stem (Frontispiece) agrees in habit with those of Zalesskya 

 and recent Osmundaceae ; on the exposed leaf-bases the action of 

 the weather has etched out the horse-shoe form of the vascular 

 strands and laid bare numerous branched roots boring their 

 way through the petiole stumps. The centre of the stem is 

 occupied by a protostele 13 mm. in diameter consisting of solid 

 xylem separated by a parenchymatous sheath from a cylinder 

 of phloem. The xylem is composed mainly of an axial column 

 of short and broad reticulately pitted tracheae (fig. 250 b and 

 Frontispiece), distinguished from the sharply contrasted peri- 

 pheral zone of normal scalariform elements, a, by their thinner 



1 Kidston and Gwynne-Vaughan (08) p. 226. 



2 Brongniart (49) A. p. 35. ^ Brongniart (28) A. PI. lxxx. 

 ^ Kidston and Gwynne-Vaughan (09). 



