818 W. T. GORDON ON RHETINANGIUM ARBERI, A NEW GENUS OF 
elongation of the cell has been so great that it divides into two cells (Pl. III. fig. 23, 
c.w.). Although this elongated parenchyma is very marked in some cases, yet it is not 
always constant over the whole hypodermal region. In PI. II. fig. 15 it is a marked 
feature over the whole area, particularly towards the inner part of the hypoderma, but 
in Pl. II. fig. 14, while present on the right-hand side, it is almost absent on the left- 
hand side of the figure. A similar phenomenon is shown in longitudinal section in PI. III. 
fig. 23 (marked elongation at a) and in Pl. II. fig. 13 (no elongated parenchyma). 
The sclerotic strands, which are not uninteresting in themselves, consist of groups of 
vertically elongated elements in which the lumen has been almost entirely closed by 
deposition of material on the cell walls. The number of these cells in each strand 
varies greatly. The strands unite tangentially at various levels and hence a reticulum 
results. They only unite, however, at considerable distances, so that the meshes of the 
reticulum are long and narrow. Hach strand may also divide radially into two or three 
smaller groups (Pl. Il. fig. 15). Between the sclerotic strands the ordinary cortical 
parenchyma is found, while associated with these fibrous groups, or partly or completely 
sunk in them, are long secretory ducts (PJ. IL, figs. 13, 14, and 15, s.s.). These ducts 
are about 106“ in diameter, and are clearly seen among the smaller-celled cortical 
parenchyma and sclerotic fibres. 
In some cases the hypodermal sclerenchyma abuts directly on the surface of the 
stem, but, as a rule, there are about three layers of small-celled parenchyma external 
to the hypoderma. In certain parts, however, there is an enormous development of 
parenchyma beyond the sclerotic zone (Pl. I. rig. 4, pet. b.), and sometimes the fibrous 
outer cortex disappears entirely at these places. This takes place at the junction of 
the petiole with the stem, and will be considered later. 
HisroLocy oF THE PETIOLE. 
In the centre of the petiole is the vascular strand, and it is quite unlike the 
corresponding strand in other Pteridospermex. In the more complex members of that 
group a great number of small strands pass into the petiole, while in the Lyginodendrex 
the trace consists of a single or double bundle. In Rhetinangiwm, however, the petiole- 
trace appears to consist of several U-shaped xylem groups aggregated into one long 
corrugated band. Usually three such traces are seen round the stem in any transverse 
section, and these are in different stages of development. In PI. I. fig. 2 one trace 
has just been emitted at A, a second is completely differentiated from the stem at B, 
while the third is only forming at C. The first thing to be noted about this last trace 
is that it appears to consist of several peripheral strands of the primary wood. As 
will also be noted, it has passed about half-way through the zone of secondary wood. 
The second trace B has emerged from the secondary xylem, and the various groups 
are more or less U-shaped and connected together either directly or by conjunctive 
parenchyma. The protoxylem groups are on the external edge and are fairly numerous 
