AND HISTOLOGY OF PIPER BETLE. 377 
vertical walls of these cells measure 60 p, and their tangential walls 20 и. 
The radial walls are not so regular as the tangent and vertical ones. Most 
of the cells are, however, radially elongated (30 ш). The rayed arrangement 
is not restricted to the part lying between two masses of xylem, but it 
extends between the phloem positions of the bundles, crosses the outer 
sclerenchyma-ring, and reappears in the cortex that lies in a line with the 
medullary rays. 
The vascular ring is surrounded by an outer sheath of selerenchyma. It 
is, unlike the inner sheath, composite in structure ; the composing constituents 
are bast-fibres, which form semilunar groups about three to six fibres deep 
outside the bast and sclerosed parenchymatous bands, one to two cells deep, 
which link the hard bast across the medullary rays. But their distinctive 
places are not strictly retained by the cells and fibres, as they are often met 
with freely intermixed. The cells are somewhat irregular in shape. Usually 
the vertical and tangential walls are equal to each other and double the 
radial walls. 
This composite ring is reinforced along the middle line of most of the 
medullary rays by buttresses of sclerosed parenchyma projecting inwards into 
the latter. These cells have the same dimensions as those of the ring proper; 
but the relative sizes of the radial and tangential walls are in this case 
reversed, the radial walls being double the tangential walls. Occasionally 
such a buttress projects far inwards, traversing the greater depth of the 
medullary ray. The outline of this outer sheath is erenulate, on account of 
its strong convexity opposite each phloem mass. 
When medullary rays are lignified, the lignification extends from the inner 
to the outer sheath of the sclerenchyma. 
Corter.—The endodermis persists as a continuous ring of cells, with the 
cog-like suberised thickenings on the radial walls. The typical cortical cells 
measure 45 ш across. The rayed arrangement of its cells opposite the 
medullary rays has already been noticed. The secretory cells are present, 
if not abundant, and are somewhat larger than the ordinary parenchyma of 
the cortex. The parenchyma-layer is rather thin, being about 150 ш. It is 
succeeded by bundles of collenchyma, which appear in the form of detached 
ares. These are about 75 p deep. The component cells are only 15 р across. 
The thickening of the walls of some of them takes the form of fibres of 
sclerenchyma (Pl. 18. fig. 31). These are chambered and pitted as the fibres 
elsewhere, but are extremely narrow, being only 6 ш across. They attain a 
length of 800 ш. Occasionally the cortex is traversed by а few small 
concentric cortical bundles. The xylem occupies a peripheral position in 
such a bundle (РІ. 18. fig. 29). 
As regards storage of reserve materials, starch may be seen at certain 
times of the year in almost every cell of the stele and cortex, except the 
large vessels. 
