SPECIES OF PERMIAN OSMUNDACE.%. 349 
mark the boundary between the inner and outer cortex. The stem up to this 
beundary has a radius of about 21 mm. ; the outer cortex occupies a small 
space, so that the stem as a whole does not exceed 46 mm. in diameter. The 
rest of the specimen is composed of petiole-bases investing the stem and 
closely pressed against it : of these, only the most internal are preserved ; the 
more external petiole-bases are shown in another specimen of the fern, which 
consists entirely of a group of these organs. 
The elements of the outer xylem zone are sharply-pointed trache with 
regularly disposed pits which are elongated in the direction of the breadth 
of the tracheæ and are disposed according to their size in two, three, or four 
vertical series ; some portions of the tracheal wall show pits in five or even 
six series. By their transversely elongated pits the xylem elements remind 
one of the scalariform tracheæ which have a single series of transversely 
elongated pits. The narrow protoxylem elements, in which the pits are 
uniseriate, exhibit a specially close resemblance to scalariform tracheæ. 
These protoxylem elements form a part of the outer xylem zone, chiefly in 
the prominences which mark the points of departure of the leaf-traces where 
the elements are grouped together at some distance from the edge of the 
prominences among the metaxylem tracheids. The remaining part of 
the outer xylem zone does not show much decrease in the tracheæ, which 
remain fairly constant from the centre to the periphery. The inner xylem, 
in contrast to the outer xylem, is composed of short and wide tracheæ which 
resemble irregular sacs tapering at the ends, with the result that the boundary 
between two kinds of xylem is obvious in the transverse sections and 
especially so in longitudinal sections. The tracheæ of the inner xylem have 
thinner walls than the elements of the more external xylem. The pits in 
their walls are transversely elongated, but their irregularity of distribution 
gives to the xylem elements the appearance of reticulate rather than scalari- 
form vessels. This reticulate structure of the wall is noticeable not only in 
longitudinal but also in transverse sections, because the pits cover both the 
transverse and longitudinal walls of the tracheæ. All the elements of the 
xylem, both outer and inner, appear to be vessels, but the pits, because of 
the absence of the middle lamella in the walls, appear to be true pores. In 
the reticulate tracheæ each pit represents an independent perforation in the 
wall, but in the elements of the outer xylem zone all the pits of the same 
series communicate with one another by a split in the cell-wall, as in recent 
Osmundaceæ. The cell-wall of the tracheæ of Thamnopteris Kidstoni is 
preserved in such a way that the spaces in it are visible as double black lines 
in the middle part of the membrane, but the wall itself appears to be stained 
a brownish-yellow colour. The central portion of the stele, as I have already 
said, has a cavity along the border of which, and in contact with the short 
tracheæ of the inner xylem, as shown in longitudinal section, one can see 
here and there groups of isodiametrie and rather small cells which have the 
characters of parenchyma, and undoubtedly represent the beginning of 
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