398 Maslen . — The Structure of Mesoxylon Sutcliffii (Scott). 
genus) have spiral or scalariform thickening is a difference between this form 
and Poroxylon , in which not only are the centrifugal elements provided with 
vertical rows of bordered pits, but even in the arcs of primary (centripetal) 
wood the more internal part of each bundle consists of pitted elements 
passing outwards into scalariform ones and these into the spiral tracheides 
of the protoxylem, exactly as is the case in Lyginodendron and Heter- 
angium. 
The sections show that not only does the centripetal xylem gradually 
diminish in quantity and finally die out altogether when the trace bundles 
are followed down the stem, but that the centrifugal wood also gradually 
decreases in amount. Owing to the difference in the nature of the 
tracheides of the centrifugal portion of a bundle and of the centrifugal 
secondary wood outside, it is easy to distinguish them in longitudinal 
sections. 
V. The Xylem, Phloem, and Pericycle ; and the Leaf- 
traces PASSING THROUGH THEM. 
Surrounding the pith and the leaf-trace bundles occurs the zone of 
secondary wood, having a thickness in Mesoxylon Sutcliffii of about 
o*3 cm. (PI. XXXIII, Fig. i, s.x.). The patchy preservation of this tissue 
has already been mentioned (p. 388), and it is illustrated by the sections 
shown on PI. XXXIII, Figs. 1 and 2. As is shown in Fig. 1 the middle 
portion of the wood is often entirely destroyed, resulting in an irregular 
space, i-., while the outer and inner portions may be excellently preserved. 
The same thing is shown in the radial longitudinal section seen in Fig. 2 : 
the secondary wood, s.x., on the left of the section shows a middle empty 
space, while on the right nearly the whole of the xylem has been destroyed. 
Why the middle portion of the wood should be specially liable to 
destruction, while soft delicate tissues such as the phloem and pericycle are 
often well preserved, is not easy to determine. 
The wood consists entirely of radially arranged tracheides of small 
size, and of narrow, generally uniseriate, medullary rays usually from 1-6 
cells in height (see PI. XXXV, Fig. 16). Two or three vertical rows of 
bordered pits are usually seen in radial sections on the radial walls of each 
tracheide (PI. XXXV, Fig. t 5), while in tangential sections no pits are 
visible (PI. XXXV, Fig. 16) since they are confined to the radial walls of 
the elements. The tracheides and narrow medullary rays of the secondary 
xylem are shown in transverse section on PI. XXXIII, Fig. 7, s.x., and 
PI. XXXV, Fig. 17, s.x., &c. 
The similarity in structure of the secondary xylem of Mesoxylon and that 
of other Cordaitales has already been pointed out (see p. 388). Comparison 
of the secondary xylem of Mesoxylon Sutcliffii with that of Poroxylon shows 
that in the latter form the tracheides are much larger, and that, owing to this 
