190 Gibson . — Contributions towards a Knowledge 
far as I have been able to determine at present, the central 
cord may be traced also nearly up to the region where 
tracheids first make their appearance in the external cylin- 
drical stele, but there it approaches and seems to be 
separated off from the inner border of the cylindrical stele. 
Here and there also, in the rhizome generally, the central 
cord fuses with the external stele, and the centre is occupied 
by parenchyma (PL XII, Fig. 93). 
4. Structure of the stele and cortex . 
A distinct epidermis covers the stem, the outer walls of 
the epidermal cells being lamellated. There is a well-marked 
cuticle. The hypodermis consists of from eight to twelve 
layers of thick-walled sclerenchymatous cells, not nearly so 
abundant, however, in the rhizome. The cells of the general 
cortex are thick-walled and show a peculiar condition of the 
middle lamella. At the angles of the cells the lamella forms 
a triradiate figure, but the secondary thickening does not 
come quite up to the corners, so that between the cells there 
are minute tubular spaces divided by the lamella into three 
triangular spaces (PI. XII, Fig. 80). 
Next the lacuna the cells are smaller and loosely arranged. 
The lacunae, which are well defined, are occupied by trabe- 
culae, some of which consist simply of an endodermal cell 
with the usual cuticular band, whilst others are connected 
on the cortical side with creeping chlorophyllaceous cells over 
the inner side of the compact cortex (PI. XII, Fig. 81). 
The structure of all the steles of the erect shoot is 
fundamentally the same. There is externally a cuticularized 
pericycle one or two layers deep, the cells of which, as usual, 
contain starch, chlorophyll, &c. Within the pericycle several 
patches of crushed protophloem-elements occur enclosing in 
turn a layer of sieve-tubes, separated from the xylem by one 
or two layers of parenchyma. The sieve-tubes almost surround 
the xylem and actually do so in the accessory steles. Many 
of the accessory steles show two protoxylem-areas, but the 
primary steles one only, which is turned outwards to receive 
the leaf-traces. 
