ioi4 
Scott.— The Structure of 
any one transverse section. It must, however, be remembered that the 
great development of periderm in the cortex must give the bundles a more 
horizontal course in this region, so that the conditions, even apart from the 
state of preservation, are not so favourable for observing them in transverse 
section as in specimens where there is less secondary growth. 
On the other hand, the single primary xylem-strands, formed by the 
fused pairs, are present in large numbers around the pith ; in each transverse 
section there are about twenty which show evident centripetal xylem, while 
other prominences, which are without this tissue, no doubt represent the 
lower ends of similar leaf-trace strands or the products of their anastomosis. 
Thus the whole inner margin of the wood is strongly undulated (Fig. i), 
the intermediate secondary xylem only reaching the pith in the bays 
between the prominences. From the great number of primary xylem- 
strands it is evident that the phyllotaxis was complex. 
In the structure of the leaf-traces, the first point to be noticed is the 
centripetal xylem. This is well developed, both in the bundles passing 
through the cortex (PI. LXXXVII, Figs. 6 and 7), and in those which are 
approaching or have reached the edge of the pith (PI. LXXXVII, Fig. 5 ; 
PI. LXXXIX, Fig. 18). It forms, as seen in transverse section, an arc or 
crescent of considerable thickness ; its irregularly grouped elements are 
sharply distinguished from the radial rows of the centrifugal wood (Fig. 18). 
The tracheides of the centripetal xylem are spiral or finely scalariform. It 
appears that the tissue persisted with little diminution for some distance 
downwards after the bundle reached the pith, for many of the circum- 
medullary strands have a well-developed centripetal portion. In others, 
however, it is extremely reduced, while it is altogether absent from many 
of the prominent xylem-strands. There can thus be no doubt that the 
centripetal wood eventually died out in a downward direction, as in other 
species of Mesoxylon and in Poroxylon , Calamopitys Beinertiana, &c. 
The position of the protoxylem appears to have been, as is usually the 
case in this group, in contact with the centripetal wood (PI. LXXXIX, 
Fig. 1 8, px). Here, as in other species, it is accompanied by some thin- 
walled parenchyma, which partially separates the centripetal from the 
centrifugal xylem. 
A characteristic feature of the circum-medullary strands is the presence, 
in many cases, of a definite bundle-sheath separating the xylem from the 
pith. The sheath is limited to those strands which are seen in the upper 
part of their course; it dies out lower down. In a transverse section the 
sheath is found in about two-thirds of the strands which still have definite 
centripetal xylem. The simplest form of sheath occurs low down in the 
course of the strand. Here it consists of an arc, not more than two or three 
cells in thickness, of more or less radially elongated cells, with walls a little 
thicker than those of the adjacent pith-cells (PI. LXXXIX, Fig. 18, sh). 
