224 PROF. F. E, WEISS ON A BISERIATE HALONIAL 
The Stele. 
The centre of the stele is occupied, as is seen from Pl. 24. figs. 6 & 7, by medullary 
tissues, the cells of which are for the most part considerably compressed. Only the 
three or four outermost rows of cells show a definite lumen, and from these the nature 
of the pith-cells can be established. In transverse section they are more or less poly- 
gonal in shape. In longitudinal section it will be seen (Pl. 25. fig. 10) that the primary 
cells have grown considerably in length and fit very closely together with pointed ends. 
These cells have divided up repeatedly by transverse and oblique walls. Sometimes 
two rows of these small polygonal cells are found within the elongated primary cells. 
A pith of this kind has been described and figured by Williamson (1873) (P1. 24. fig. 14) 
for Lepidophloios fuliginosus, and occurs in other lepidodendroid stems. It is not an 
unusual condition for the medulla and cortex of recent plants. The cells seem all 
to be of a parenchymatous character; and I was not able to detect any elements with 
reticulated walls such as occur in Lepidodendron vasculare (Binney) *, and as have been 
-deseribed by Seward (1899) in a stem of Lepidophioios fuliginosus. 
The primary wood which encloses the pith consists of eight or ten layers of tracheids 
decreasing in diameter towards the periphery, where the wood ends in sharp cusps 
composed of the smallest elements (protoxylem). From these the vascular supply of 
the leaf-trace bundles is given off, and in the depression between two consecutive cusps 
there will be seen to the outside of the primary xylem the vascular elements which 
have been given off at a lower level. 
The inner portion of the primary wood is seen to consist of large tracheids with 
scalariform marking. The smaller elements near the periphery seem to be narrow 
tracheids. In transverse section the tracheids are more or less polygonal in outline, and 
often show a splitting apart of the wall between two contiguous cells. Most of the 
cells are devoid of contents, but in some cases the cell-space is filled with a dark brown 
mass. 
The groups of elements which are given off to form the leaf-traces consist of both 
smaller and larger cells, the former generally enclosed by the latter, thus forming a 
mesarch group (Pl. 25. fig. 13). This mesarch arrangement is not always visible 
immediately on leaving the primary xylem. At first the course of the leaf-traces is 
nearly vertical, and it is not until they enter the mid-cortex that they become oblique or 
nearly horizontal. 
The tissue immediately outside the primary wood consists of very small isodiametrical 
cells with dense contents, thus rendering them opaque and preventing their structure 
from being clearly seen. In this tissue lie the leaf-trace bundles just after they are 
given off from the primary wood. Ata little distance from the primary wood the cells 
of the surrounding layer become larger and clearer and are more regularly arranged. 
Their contents are less dense than those of the cells nearer the wood, and they are 
consequently of a lighter colour. These are the layers that are considered from their 
regular arrangement to be of secondary origin. 
* Lepidodendron selaginoides (Will.). 
