THE ANATOMY OF THE STEM OF THE CALY C A NTHACEiE. 
519 
the phloem of the peripheral bundle.. The sieve-tubes in the latter are very 
numerous, but the phloem parenchyma is less abundant than in the central stele. 
Sieve-plates usually occur on the end walls, but one or two occur on the lateral 
walls also. The xylem of the peripheral bundle consists of spiral, annular, and 
reticulate vessels, and spiral tracheides, but there is no xylem parenchyma. On 
the outer side of the xylem lies a mass of elongated prosenchyma whose cells 
undergo sclerosis as age increases. Externally are several layers of cortical pitted 
parenchyma, while scattered in the parenchyma are large spherical oil-secreting 
cells (fig. 2). Outwardly the cortical parenchyma merges gradually into four or five 
layers of subepidermal collenchyma' containing a few chloroplasts. Silicified bent 
unicellular hairs, their points directed towards the apex of the stem, lie deeply 
embedded among the epidermal cells. Stomata appear to be entirely absent. 
As the shoot elongates, the outer cells of the pericycle become uniformly 
thickened and sclerotic in small isolated groups (fig. 5). These cells must not be 
confused with the sclerotic cells with U-shaped thickenings which one sees clearly 
in the layer continuous with the sclerenchymatous arc of the peripheral bundle. 
The fibres on the outer margin of the peripheral bundles also become strongly 
lignified, and the pits become . extremely narrow canals. The sieve-tubes increase 
greatly in number, and tracheides with bordered pits and spiral thickenings appear. 
In the normal cylinder, the phloem parenchyma increases in volume and constitutes 
the greater part of the bast ; pitted wood parenchyma is also present. The cortex 
and pith remain unaltered. 
In the older stems all the vessels and tracheides of the xylem, as also the xylem 
parenchyma, become greatly lignified. Two kinds of tracheides are met with, viz. 
long, narrow tracheides with pointed ends, whose walls bear uniseriate bordered pits, 
and larger but shorter tracheides with two or three longitudinal rows of bordered 
pits. In some, the pits become so elongated transversely that they appear scalari- 
form. The bordered pits are either in longitudinal rows or scattered irregularly. 
Here and there, among these elements, are elongated, strongly lignified fibres 
without pits. 
The medullary rays, seen in transverse section, are very numerous and uniseriate, 
and from three to five cells deep. The cell walls have numerous simple pits, or 
bordered pits on the side abutting on a tracheide ; in a few cases pits are absent. 
Tyloses are frequent in the large pitted vessels. Most of the tracheides are inter- 
mediate between the elements with ordinary bordered pits and the scalariform 
tracheides so characteristic of Pteridophyta. 
The secondary phloem shows a large development of phloem parenchyma and 
comparatively few sieve-tubes, while the sieve-tubes in the peripheral bundles are 
numerous and are accompanied by relatively little phloem parenchyma. The 
sclerotic cells of the peripheral bundles have greatly thickened walls, as have also 
the pericyclic fibres. The secondary cells are conspicuous, and show slight traces 
