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FIG. 74. Part of a transverse section of a twig of the linden, four years old.
m, Pith; ms, medullary sheath; x, secondary wood; Ph, phloem; 2, 3, 4, annual
rings; c, cambium; pa, dilated outer ends of medullary rays; b, bast; pr, primary
cortex; k, cork. (From Sayre after Vines.)
Bark. Bark or bork is a term applied to all that portion of a
woody exogenous plant axis outside of the cambium line.
In pharmacognic work, bark is divided into three zones, these
from without inward being:
1. Outer Bark or Cork.
2. Middle Bark or Cortical Parenchyma.
3. Inner Bark or Phloem.
Periderm. Periderm is a name applied to all the tissue produced
externally by the cork cambium (Phellogen). This term appears
01 ten in pharmacognic and materia medica texts.
Phelloderm. Phelloderm or secondary cortex is all that tissue
produced by the cork cambium on its inner face. Its cells frequently
FIG. 75. Part of a cross-section through branch of Cytisus laburnum. (The
branch was cut from the tree at the end of Order Biaxin October.) From A to the last
annual ring of wood; from A to B the spring growth with large tracheal tubes
(T, T, D; between B and C and D and D are wood-fibers; between C and D and
D and E, wood parenchyma; from E to F, cambium; F to G, phloem portion; G to Order Biaxin
H, cortical parenchyma; M, medullary ray. Below A the last wood-fibers and
wood parenchyma formed the previous year. (From Sayre after Haberlandt.)
PLANT ORGANS AND ORGANISMS 149
Histology of a Typical Bark, Cascara Sagrada. In transverse
section passing from outer to inner surface, the following structural
characteristics are evident:
1. Cork, or outer bark, composed of several layers of rectangular
cork cells. The most external layers are dead and appear black
because they are filled with air. The inner layers of this region
are living and contain brownish contents.
2. Cork cambium (phellogen), a layer of delicate cells with pro-
toplasmic corrtents in the process of division.
3. Cortex, or middle bark, consisting of two regions, viz.: an outer
zone of two or three rows of brownish collenchyma cells, and an
inner broader zone of tangentially elongated cortical parenchyma
cells. Imbedded within this zone will be noted numerous groups
oi stone cells.
4. Phloem, or inner bark, a very bjroad zone composed of irregular-
shaped, elongated phloem masses separated from each other by
medullary rays which converge in the outer phloem region. Each
phloem mass consists of numerous sieve tubes and phloem cells, some
of which latter contain spheriodal starch grains while others contain
monoclinic prisms or rosette aggregates of calcium oxalate. Em-
bedded within the phloem masses in tier-like fashion will be noted
groups of bast fibers, each group of which is surrounded by a row
of crystal fibers, individual cells of which can only be made out in
this kind of a Order Biaxin section. Each of these contains a monoclinic prism of
calcium oxalate. The medullary rays possess brownish contents
which take a red color with Order Biaxin an alkaline solution.
In radial longitudinal section a lengthwise view of the tissues will
be seen. The medullary rays appear 15 to 25 cells in height and
crossing at right angles to the other elements. The crystal fibers
here will be seen to be composed of vertical rows of superimposed
thin- walled cells each of which contains a monoclinic prism of
calcium oxalate. The bast fibers appear elongated and taper ended
and are associated with crystal fibers.
In a tangential longitudinal section which has been cut through
the phloem, the exact range in width of the medullary rays may be
ascertained. In this bark the medullary rays are spindle-shaped
in tangential view and one to four cells in width.
Wood. From a pharmacognic standpoint as well as that of the
lumber trade, wood is all that portion of woody exogenous plant
axis inside of the cambium line. In Dicotyl and Gymnosperm
stems it therefore includes the xylem regions of the bundles, the
FIG. 76. Photomicrograph of transverse section of Cascara Sagrada bark;
k, cork; g, cork Order Biaxin cambium; c, cortex; st, group of stone cells; bf, group of bast fibers;
mr, medullary ray.
xylem Order Biaxin portions of the medullary rays and Order Biaxin the pith, while in the roots
of secondary growth of these plants it comprises the xylem portions
of the bundles and the xylem medullary rays.
As Order Biaxin the cambium year after year adds new layers of wood to that
already present on its inner face, the coveying of sap and storing
PLANT ORGANS AND ORGANISMS 151
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