2 58 de Fr aine. — On Medullosa centro fills, a Neiv Species of 
In the lower sections of the series other plant tissues have so intruded 
into the stem that only one of the outer ring of steles is represented in 
entirety ; this is the stele termed a (PL I, Fig. 2), and it undergoes practically 
no change during the whole series ; stele j3 also appears to have been 
materially unaltered. In sections 8 and 9, cut from the middle portion of 
the block, the steles y 1 and y 2 (PI. II, Fig. 2) are represented by two 
fragments ; in the next section F, y 1 has partly fused with y 2 ; this fusion 
takes place very rapidly, so that in section B a single stele y has resulted 
(PI. I, Fig. 1). Such fusion of steles with neighbouring steles appears to be 
of fairly common occurrence in the Permian Medulloseae, and Scott states 
that in Medullosa anglica ‘ there is evidence that the steles in their course 
through the stem underwent both fusion and division, though probably only 
at long intervals’. 1 According to Worsdell, 2 the very sinuous contour of 
many Medullosean stem steles was partly due to the perpetual anastomoses 
between the neighbouring steles. 
IV. The Leaf-trace Bundles. 
The leaf-trace bundles were given off from the peripheral region of the 
outer ring of steles. Although the actual passage of a leaf-trace strand 
through the secondary tissues of the stele could not be traced in the 
sections, yet sufficient evidence was available to show that the leaf-trace 
xylem was chiefly furnished by the peripheral strands of spirally thickened 
tracheides so well marked in the primary xylem of the steles. The xylem 
of the leaf-trace passed out through the zone of secondary wood, and the 
latter rapidly closed up again after its exit. Although several leaf-trace 
strands were present, which had obviously only just been emitted from the 
stele, yet no sign of any secondary tissues could be found in connexion with 
them (PI. XV, Fig. 4, It . 1 ) ; hence it appears extremely probable that the 
outgoing leaf-traces were unaccompanied by secondary xylem. 3 This forms 
an important point of distinction from the leaf-trace in Medullosa anglica , 
in which fossil the outgoing trace is always surrounded by its own zone of 
secondary wood and bast. 4 No phloem is preserved in the stem, hence it is 
impossible to determine whether the outgoing strands are concentric in 
structure, but in any case they immediately begin to divide up into smaller 
collateral bundles, in which the protoxylem lies adjacent to the space repre- 
senting the phloem (cf. It 1 and It. 2 in PL I, Fig. 4). 
In the zone of ill-preserved tissue lying between the ring of steles and 
the periderm, a fairly extensive group of short, reticulately marked tracheides, 
1 M. anglica , loc. cit., p. 91. 
2 Worsdell, W. C. : The Structure and Origin of the Cycadaceae. Ann. of Bot., vol. xx, April, 
1906, p. 129. 
8 In M. pusilla the leaf-trace immediately after its exit showed ‘ no obvious secondary wood ’. 
4 Compare M. anglica , loc. cit., Plate II, Fig. 10. 
