350 MR. M. D. ZALESSKY ON NEW 
the pith in the form of islands among the xylem elements which occupy the 
stele up to the centre. Groups of such cells were also discovered along 
the edge of the cavity in the transverse section of the stem. Outside the 
outer zone of the xylem there are from four to seven layers of small, thin- 
walled cells which represent the xylem-sheath (Pl. 32. fig. 2). In this 
sheath at a distance of one row of cells from the xylem are disposed here and 
there, in one or two layers, cells with brown contents which were no doubt 
specialized cells, because identical cells with brown contents are found in the 
xylem-sheath of the leaf-traces. The sheath is surrounded by a zone of 
phloem consisting of thin-walled, comparatively large cells which appear to 
be sieve-tubes. These sieve-tubes form a band of three or four layers. In 
some places the zone of sieve-tubes reaches a width of seven cells: this 
occurs on the sides of the xylem prominences where the phloem tissue, in con- 
sequence of the junction of the descending leaf-traces with the xylem of the 
stem, is removed from its normal position. The sieve-tubes are connected 
with the small-celled tissue which surrounds the whole stele by a zone of 
three to four layers of cells. 
In some places, it is true, the number of rows is greater, being sometimes 
as many as eight. This zone certainly appears to be pericyelie ; but it is not 
possible, in spite of the wonderful preservation of the tissue, to distinguish a 
peripheral series of cells having the structure of endodermis. This pericyclic 
zone comes out very clearly, being limited towards the centre by larger sieve- 
tubes and towards the periphery by no less large but thicker-walled cells of 
the inner cortex which in this region have black contents. The pericycle 
zone is equally clear in longitudinal section, because its cells have a some- 
what prosenchymatous character and are easily distinguished from the 
isodiametrie, empty cells of the inner cortex and the long and comparatively 
empty sieve-tubes which are clearly contrasted with the narrower and shorter 
cells of the xylem-sheath. The cells of the pericycle, which remind one in 
transverse section of those of the xylem-sheath in longitudinal section, are 
easily recognizable. The pericycle cells have the shape of sacs, while those 
of the xylem-sheath are tubular. The inner cortex is admirably preserved 
throughout its whole breadth up to its junction with the outer cortex. Its 
cells are thin-walled, but not so thin as those of the pericycle and sieve-tubes 
The cells which surround the leaf-traces of the stele have black contents, 
which suggest that they were probably rich in starch. These layers of cells 
with black contents surrounding the leaf-traces and the stele are easily 
recognized, and give one the impression of a sheath. In other places the 
cells of the inner cortex do not contain any black substance and are lighter 
in tint. By this lighter colour they are distinguished from the darker and 
thicker-walled, roundish cells of the outer cortex which in longitudinal 
section appear to be prosenchymatous. The cells in the inner cortex in its 
middle portion are either isodiametric, or somewhat broader than long. 
