340 
Wernham .— The Morphology of 
the peduncle is reached, at which point, we shall see, the stele breaks up 
into the vascular strands of the peduncle. 
As we ascend, the original xylem-strand of the young tuber breaks 
away from the main stele ; it ultimately disappears. In Fig. 6, taken 
across the lower part of the leaf region, we have a distinctly U-shaped stele, 
with the gap facing the former site of the young tuber. Five leaves are 
seen in section ; and it will be well, perhaps, at this point, to digress with 
a brief description of the anatomical structure of— 
3. The Leaf. The outline of the transverse section is roughly circular ; 
a distinct peripheral layer is differentiated from the parenchyma which 
occupies the greater portion of the 
section. In the centre is a single 
very small xylem-strand, consisting 
of six or seven wood-elements at 
most. In one case the xylem-strand 
was observed to be surrounded by 
a zone of tissue which was possibly 
phloem, although no sieve-tubes could 
be recognized. 
Stomata occur evenly distributed 
over the epidermal layer, and leading 
into large air-spaces. The parenchyma 
seems to be traversed throughout by 
intercellular spaces ; and the presence, 
in many of the sections, of radially 
elongated parenchymatous cells near 
the periphery suggests the existence 
of a rudimentary palisade tissue. 
Let us now return to a closer 
examination of— 
4. The Stem-stele. The investi¬ 
gation of the structure of the U-shaped 
vascular band yields some interesting results. It is surrounded by 
markedly homogeneous parenchyma, which is continuous into its concavity. 
There appear to be several protoxylem-groups. One is observed 
situated in an exarch position at each free end ; but what is more remarkable, 
several cavities are present in the body of the stele (Figs. 6, 7) ; and the 
presence in some cases of one or two small tracheides clinging to the sides 
of the cavities indicates that the latter represent degraded protoxylem- 
groups, comparable with those seen in Eqiiisetum . These cavities appear 
in Bertrand’s figures, and he expresses the opinion that they represent 
degraded protoxylem-groups. The xylem is thus, of course, mainly mesarch 
in development. 
Fig. 7. The stele in detail. Protoxylem- 
groups appear at the free ends, and degraded 
groups form cavities in the body of the stele. 
A protoxylem-element, px, is seen clinging to 
the edge of one of the cavities. 
