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only a ring of fibers, separates the vascular cylinder from the cortex. The 
cortex itself is of only limited extent and rarely more than two to four cells 
wide. The outer cortex is differentiated into collenchyma and photo- 
synthetic tissue. The former occupies the ridges of the stem while the 
latter is found in the intervening spaces. The one-celled epidermis is of 
two types: the epidermal cells external to the collenchyma are elongated 
and their tangential walls are thickened; those external to the photosyn- 
thetic region are thin-walled and nearly isodiametric. 
The collenchyma cells are long, pointed, and thickened at the corners 
only and communicate with one another by simple pits. The cortex and 
pith are made up of thin-walled, loosely connecfted parenchyma cells in 
which are often found druses of calcium oxalate (PI. XVII, B). 
Occupying the periphery of the vascular cylinder is a ring of fibers the 
elements of which are of the usual type but vary in size and diameter of 
lumen. They are rarely completely united into a closed ring but rather 
form short bands one to several cells wide. In places the cells of the phloem 
elements of the vascular tissue abut directly on the fibers; most often, how- 
ever, a narrow band of pericyclic tissue intervenes. 
The elements of the vascular cylinder are in general of advanced dicoty- 
ledonary types. The frequent occurrence of transitional forms in xylem 
and intermediary tissue makes the study of this group of plants especially 
interesting. 
The xylem is made up of porous vessels, fibers, and wood parenchyma, 
the last named being vasicentric. The vessels are of two general types 
(PI. XVI, C). The large type, most commonly arranged in uniseriate 
radial rows (PI. XVI, B), is rectangular with end walls nearly transverse. 
The small type of vessel shows less definite arrangement ; it is more elongated 
and its end walls are always more or less oblique. The walls of the vessels 
are heavily pitted. The pits are small, pentagonal, and arranged in alter- 
nate rows (PI. XVI, C, Q). In the small type of vessel, however, the pits 
may not show the symmetrical form and regular arrangement. 
The fibers approach the libriform type. The elements are long and 
pointed but comparatively thin-walled. The walls are but sparingly 
pitted. The longitudinal course of the fibers is not absolutely straight in 
that the ends of the elements diverge obliquely whereby they become partly 
interlocked, which arrangement gives the wood an especially great toughness. 
In the xylem of the leaf- trace bundles we find in addition to the types 
of elements just described the typical elements of the protoxylem with 
transition forms to the pitted vessels. The first formed elements of the 
protoxylem are narrow; the secondary thickenings of their walls are of the 
nature of loose spirals and wide rings (PI. XVI, jE). Much protoxylem, 
however, is made up of larger elements with secondary thickenings in the 
form of close spirals. Gradually the arrangement of the elements becomes 
more definite. The type of element also changes, and instead of close 
