400 Mas fen.— The Structure of Mesoxylon Sutcliffii (Scott) 
leaf-trace bundles cut across nearly transversely, owing to their nearly 
horizontal course through the wood. Each bundle in traversing the wood 
has its centripetal wood directed upwards and projecting into a space, 
resembling somewhat the well-known rootlet bundles of a Stigmaria as 
seen in tangential sections of the wood. The space into which the centri- 
petal wood projects clearly corresponds with that shown in the transverse 
sections around the bundles surrounding the pith (PL XXXIII, Figs. 3 and 
4, s.), and it was doubtless occupied by delicate conjunctive parenchyma. 
Between this space (as seen in the tangential section) and the secondary 
tracheides of the xylem-ring there is a tissue which is probably continuous 
with the outer pith. 
The transverse sections appear to indicate that in many cases the two 
bundles of a trace may have emerged from the wood at slightly different 
levels, so that when one bundle was well out in the phloem and is cut nearly 
transversely, the other was still embedded in the wood, and so is cut through 
in a more or less longitudinal direction. Thus in the trace at the right of the 
photograph shown on PI. XXXIII, Fig. 1, one of the bundles is seen to be 
fully out while the other has not yet escaped from the secondary xylem. 
Slight obliquity of the section together with the very rapid curving upwards 
of the bundles when they leave the xylem may perhaps be sufficient reason 
to account for this appearance. 
The secondary xylem is followed by the cambium. This tissue, which 
is rarely preserved, is shown on PI. XXXV, Fig. 17, c., and consists of thin- 
walled cells of tabular form, narrower in the radial direction than in the 
tangential one. Traversing the cambium are seen narrow medullary rays 
continuous with those of the secondary xylem. 
Beyond the cambium occurs a continuous zone of secondary phloem 
having a width of about o-6-o*8 mm. when measured at some distance from 
an emerging leaf-trace, "near to which the phloem is usually much disturbed. 
The phloem zone is shown in transverse section on PI. XXXIII, Fig. 7,7)., 
PI. XXXV, Fig. 17, /., and in longitudinal section on PL XXXIII, 
Fig. 2 , p ., and PL XXXIV, Fig. 13, p. It consists mainly or entirely of 
elements which are arranged in radial rows corresponding to those of the 
secondary wood and with similar narrow medullary rays. In addition to 
long thin-walled elements (? sieve tubes) the phloem contains many long 
tubular cells or vessels with dark contents. These latter elements appear 
to be of two kinds. Some present a characteristic square form in transverse 
sections and have very dark contents (Pl. XXXIII, Fig. 7 and PL XXXV, 
Fig. 17, a), while in longitudinal sections they appear as very long straight 
tubes without apparent cross-walls. They may occur in any part of the 
secondary phloem, but are found more especially in the inner portion as 
seen on PL XXXIII, Fig. 7. They frequently occur in tangential rows as 
shown on PL XXXV, Fig. 17. The other tubular elements with contents 
