410 DR. ERIC DRABBLE ON THE 
parenchyma of pith and cylinder being in all respects similar and quite continuous. 
Immediately internal to such a gap is often a large vessel, sometimes accompanied 
laterally by one or two phloem-groups, sunk in the ground-tissue of the cylinder. At 
other points two phloem-groups may be in close proximity at such a gap, no xylem-group 
intervening. The arrangement of the arcs conveys the impression, borne out by 
examination of the apices, that they have arisen as independent procambial strands in 
the meristem. 
In the large parenchymatous pith are developing strands of fibres, each with a central 
element, of the same nature as those described as occurring in the cortical strands. 
In some cases the aerial roots bear lateral rootlets, which are reduced to spines. The 
structure of these spines is as follows :— 
'The periphery is occupied by a more or less disorganized layer of thin-walled cells, 
beneath which is a zone of parenchyma, followed by three or four layers of lignified 
elements with pitted walls and square ends, and succeeded by about three layers of 
cells of a similar character, but non-lignified. The parenchymatous inner, cortex then 
follows; many of the outer cells show clear evidence of secondary division, as do also 
some of the other cells irregularly scattered throughout this region. "The endodermis is 
in the form of two ares, incurved at either end, where they are not continuous with one 
another, but may project inwards as plates of cells for some distance. The endodermic 
cells are lignified and somewhat thickened. The central cylinder consists of parenchy- 
matous ground-tissue in which are embedded some lignified elements. The phloem-groups 
consist of three to five elements and are arranged peripherally as usual. The xylem is 
very greatly reduced, but the protoxylem may be seen to be exarch and to consist of 
annular elements, those developed more internally being scalariform. The whole ground- 
plan is that of a root, but greatly reduced as to its xylem-constituents. 
(85) HyopHorBE AMARICAULIS, Mart. 
The limiting-layer is thin-walled and lignified, and is followed by a zone of more or 
less elongated and pitted cells, succeeded in turn by the parenchymatous inner cortex, 
through which are scattered individual lignified fibres, not collected into bundles. 
Comparatively small and very irregular air-spaces are present. In the zone immediately 
surrounding the endodermis occurs a complete circle—two or three elements in thickness 
—of very strongly lignified stone-cells with deep simple pits. The endodermic cells are 
comparatively thin-walled and slightly lignified on all except the outer tangential walls. 
The two-layered pericycle is also thin-walled. The central cylinder is laterally depressed 
on one side, and here the endodermis dips in, but is quite continuous. The protoxylem- 
elements are embedded in non-lignified parenchyma, but the ground-mass of the cylinder 
is sclerenchymatous, forming a ring enclosing a central pith into which bays of the 
sclerenchyma project. The pith is parenchymatous, with a few elongated, generally 
Square-ended elements. Through this pith run sclerenchyma-strands, most of which 
surround a central non-lignified vessel. Some of these strands unite externally with 
inwardly ‘projecting bays of sclerenchyma of the ring. In larger roots the central 
cylinder is much lobed, and into each depression the endodermis dips, and at the bottom 
