ANATOMY OF THE ROOTS OF PALMS. 457 
remaining ground-tissue of the central cylinder, excepting a small central pith whose 
limits are not well defined, consists of sclerenchyma, in which are embedded the phloem- 
and xylem-vessels. The vessel-walls are generally well lignified, but in some of the inner- 
most vessels they may still retain their cellulose nature. 
The small irregular central medulla is parenchymatous and surrounds a central 
air-space. 
(4) PnaNix pnAcTYLIFERA, Linn. 
The limiting and next three layers are more or less cuticularized and non-lignified. 
Then follows a lignified zone of elements, some six or seven elements in depth and of the 
usual elongated type. ‘The inner cortex is composed of parenchyma, with large air-spaces 
of no great radial extent, arranged in many irregular zones, and of bundles of fibres with 
their accompanying stegmata. 
The endodermic cells are lignified and very greatly thickened, except on the outer 
tangential walls, the lumina being nearly occluded. 
The endodermic cells are four or five times longer than broad, and the thickening is of 
that nature described by Mohl as “ fibrous.” 
No passage-cells were observed. The pericycle consists of elongated, square-ended, 
lignified cells. A small group of cellulose-walled parenchyma surrounds each protoxylem- 
group. 
A broad zone of sclerenchyma encloses the pith, and in this are embedded the xylem- 
vessels and the phloem. The protoxylem is lignified, but the inner vessels usually retain 
their cellulose walls. Two or more protoxylem-groups often abut on one large vessel, 
and internally are irregularly arranged large vessels which do not appear to belong to any 
xylem-group. "These latter are embedded in masses of lignified ground-tissue, on the 
peripheral side continuous with the sclerenchy matous zone, but internally projecting into 
the medulla. 
The medulla is parenchymatous, with a few air-spaces. Between the internally 
projecting sclerenchymatous masses the cells of the medulla are considerably crushed, 
showing that a differential rate of growth has taken place between the medullary cells 
and the sclerenchymatous masses in these regions. 
(5) ACANTHORHIZA ACULEATA, H. Wendl. 
The limiting-layer is of the usual type. The underlying zone of lignified elongated 
elements shows clearly that lignification commences in the external layer, and shortly 
afterwards appears also in the internal layer. It then proceeds centripetally and 
centrifugally, and in the mature root leaves ouly one or two layers of cells non-lignified in 
the middle of the sclerenchymatous zone. Interior to this zone comes the inner cortex, 
the outer part of which is composed of closely packed and slightly lignified cells. This 
portion of the cortex is relatively much wider than usual. The internal zone of the inner 
cortex with its air-spaces follows. The air-spaces are irregular and comparatively small, 
and usually separated only by a single layer of cells. In this region the only strengthening 
elements are short, square-ended, not greatly thickened, parenchymatous cells. 
