402 Mas leu . — The Structure of Mesoxylon Sutcliffii (Scott). 
In Poroxylon Boysettii, however (which includes the specimens first dis- 
covered, and described by M. Renault in 1879), the phloem has a different 
structure, 1 showing that the regular structure of the phloem described above 
is only a specific character in Poroxylon. 
Surrounding the phloem there is a fairly thick band of tissue which may 
be interpreted as the pericycle. In transverse sections (see PL XXXIII, 
Fig. 7,/^., and PI. XXXV, Fig. 17 , pe.) this zone is seen to consist mainly 
of rather large cells, most of which are filled with brown contents, and with- 
out any obvious arrangement in radial rows. Longitudinal sections 
(PI. XXXIV, Fig. 13, pe.) show that this tissue consists chiefly of short 
parenchymatous cells without any very definite arrangement excepting that 
in some places the elements occur in vertical rows. There are also a number 
of tubular elements occupied by brown contents and probably formed from 
vertical rows of cells. 
The pericycle is easily distinguished from the phloem in transverse 
sections by the larger size of its cells and the absence of radial arrangement ; 
and also from the cortex outside it (PI. XXXV, Fig. 17, i.c .) by the abun- 
dant contents of the cells. In longitudinal sections it is distinguished from 
the phloem by the shortness of most of its contents-filled cells as well as by 
the larger size of these elements. The pericycle tissue closely resembles 
the greater part of the persistent outer part of the pith (PI. XXXI II, Fig. 6,/".), 
from which it mainly differs in the presence of f vessels ’ with brown contents 
which are absent from the pith, and in the frequent great disturbance of the 
tissues resulting from the emergence of the numerous leaf-traces. 
The leaf -traces in the phloem and pericycle . As we have seen (p. 399), 
the leaf-trace bundles when traversing the zone of secondary xylem ascend 
very slowly, but on reaching the phloem the inclination again increases, so 
that in transverse sections of the stem they are cut nearly transversely. 
Before the exit of the xylem of the trace bundles from the secondary wood, 
and even before they have left the perimedullary position, considerable 
disturbance is visible in the phloem zone opposite to the emerging bundles, 
and the before-mentioned arcs ol dark cells (p. 401) take up their position at 
the outer side of the phloem. 
When once they are free from the xylem cylinder the leaf-trace bundles 
show a distinct collateral structure with external phloem, and this structure 
is preserved as far out as the bundles can be followed into the leaves. On 
reaching the phloem and pericycle, each bundle experiences a marked 
tangential dilatation, and the centrifugal xylem of the bundles becomes of 
a much less compact character than that of the bundles on the margin of the 
pith, or of the secondary xylem of the stele, and consists of a number of 
narrow bands of tracheides separated by broad medullary rays, as shown in 
the bundles seen on PI. XXXIV, Fig. 8, s.b. At the same time the primary 
1 Renault, loc. cit., p. 284. 
