348 MR. M. D. ZALESSKY ON NEW 
THAMNOPTERIS Brongniart, 1849. 
(Tableaux des genres de Végétaux fossiles, p. 35.) 
THAMNOPTERIS Krpsront Zalessky, n. sp. (Pl. 82; Pl. 84. figs. 6 & 7.) 
The transverse section of the specimen from the Permian rocks of the 
Urals (probably from the mine Kirmensky: the specimen is in the possession 
of my son, George Zalessky) shows that this fern has a central stele about 
11:5 mm. in diameter : the xylem, about 10 mm. in diameter, consists of two 
distinct, regions, an inner and an outer. The inner region is occupied by 
xylem elements wider and shorter than those in the outer region, the elements 
of which present the appearance of normal xylem tracheids. The more 
central elements constitute the greater part of the xylem mass, forming a 
column about 7'5 mm. in diameter; the outer ring of the xylem is about 
2 mm. in breadth (Pl. 32. figs. 2 & 3). 
The outline of the periphery of the whole of the xylem mass is somewhat 
irregular because of the occurrence of prominences at the points of departure 
of leaf-traces which leave the stele at an angle of about 10°. In the inner 
xylem is a cavity due to the destruction of the more internal tissue. All the 
preparations show cells of parenchyma along the edge of this cavity, and 
these are associated in the central part with xylem tracheids. In order to 
make out the distribution of the cells of the parenchymatous tissue in the 
middle of the xylem elements, I prepared one transverse section of the stem 
from the lower part of the specimen where there is no cavity in the centre 
(Pl. 34. figs. 6 & 7). A cavity was found in most of the sections which 
were cut from the upper part of the specimen. The transverse section shows 
that the xylem cylinder of the stele in the lower part of the specimen reaches 
a diameter of 11 mm. The annular zone of the outer xylem is about 2 mm. 
broad ; it surrounds a ring of the same breadth consisting of wider and 
shorter xylem elements, next to which is a narrow zone of parenchymatous 
cells of irregular form distributed in groups among the wider xylem elements. 
This zone appears as a brown border because its cells are darker in colour 
than the xylem elements which extend as far as the centre of the interrupted 
zone of parenchyma. The parenchymatous cells occupy only a limited space 
among the elements of the inner xylem and are distributed in groups, thus 
marking off a central part of the stele which is composed exclusively of wide 
xylem elements. The boundary of the central cavity to which reference has 
been made is situated in this zone of parenchyma, a fact which explains the 
occurrence of parenchymatous cells here and there on the edge of the cavity. 
There is no doubt that the cavity owes its origin to a line of weakness caused 
by the junction of the central part of the inner xylem with the peripheral 
portion where there occur groups of parenchymatous cells. 
The broad zone of the inner parenchymatous cortex, which reaches a 
diameter of about 6 mm., surrounds the stele. Outside the parenchymatous 
cortex is a zone of sclerotic tissue the coloured thick-walled cells of which 
