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Cognosy 1 – Lecture 5
Bark Introduction
 Definition of bark:
All the tissues of the stem or root of woody plants that are
exterior to the vascular cambium. (i.e. outer & inner bark
[cork & cork cambium, phloem] + cortex & pericycle)
 Formation of bark:
- Bark is formed as a result of a process known as secondary
thickening.
𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑡ℎ𝑖𝑐𝑘𝑒𝑛𝑖𝑛𝑔
- Seedling (structure of herbaceous stem) →
woody plant (structure of bark).
- Secondary thickening: A process that aims to:
1. Increase vascular tissue: Activation of vascular cambium → Formation of 2ry xylem and
2ry phloem.
2. Protect the growing plant: Activation of cork cambium → Formation of cork
Bark formation
Cork cambium
Periderm
Vascular
cambium
Cork
2ry phloem
2ry cortex
2ry xylem
Protection of the plant:
- In herbaceous stems (young):
Epidermis forms the protective outer layer
- In woody stems (old):
Cork forms the protective outer layer
Cork is formed by the activity of cork cambium.
 Cork cambium:
-
A meristematic tissue ( = actively dividing cells) that
divides to produce cork to the outside and secondary
cortex (phelloderm) towards the inside.
Inner bark
N.B:
- Cambium: undifferentiated cells
that are actively & continuously
divided.
- Vascular cambium: divides to give
2ry phloem (to the outside) & 2ry
xylem (to the inside)
- Cork cambium: divides to give
cork & 2ry cortex.
- Cork ‫ الفلين‬is used as insulating
material.
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 Collection of bark:
- In spring or after rainy seasons as bark releases easily (where tissues are soft after
hydration & vascular cambium divides quickly).
- Except wild cherry bark, why? As its active constituents increase in autumn.
 Decortication:
Removal of some outer tissues of the bark such as:
- Cork in canella bark
- Cork and cortical parenchyma in cinnamon.
Why?
- Removed tissues lacks desired constituents (e.g. cork of cinnamon has no volatile oil)
- Removed tissues contain undesired constituents (tannins, resins,..) e.g. in cinnamon.
 Bark drying: Either naturally (air drying) or artificially (ovens).
 Shape of bark:
Shape of
bark
Flat
Curved
Recurved
Channeled
Quill
Compound
quill
Single
Single
Double
Double
 Outer surface: characterized by:
Outer surface
Epiphytes Rhytodoma
Primitive
plants on
stem bark.
Give greyish
or green color
to outer
surface.
Lenticels
openings
for gas
exchange
Exfoliation
separation
of some
outer
tissues
Fissures,
cracks,
furrows
Wrinckles
, ridges
Warts
Raised
corky
patches
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 Inner surface:
-
Smooth: when phloem fibers are absent.
Striated: showing longitudinal lines due to phloem fibers.
Corrugated: when longitudinal shrinkage produces parallel transverse wrinkles.
(when phloem fibers are present at certain areas or schlereids alternating)
Reticulate: random arrangement of fibers forming network
Inner surface may show pieces of attached woods.
 Fracture:
Behavior of the bark when broken transversely and characters of exposed surfaces.
- Short: when fractured surfaces are smooth
- Splintery: easily broken, exposed surface shows fibers.
- Fibrous: when hardly broken, shows fibrous surface e.g. cinchona
- Granular
- Laminated
 Colour:
Usually outer surface is brown but it may be paler due to:
- Epiphytes (give green or silver color) e.g. cinchona
- Ca Ox (in cork cells with no brown pigment) e.g. cascarilla bark
 Tissues of the bark:
From outer to inner:
Tissue
Description
1. Rhytidoma - Collapsed dead tissues of primary origin.
- Result due to division of cork cambium forming cork layer which separate the
outer tissue from the plant ∴ these tissues dies & collapse forming the
rhytidoma tissue. (in quillia bark)
2. Cork
- Protective tissue of secondary origin (cork cambium).
(phellem)
- Regularly arranged flattened cells.
- Suberized walls (sometimes lignified)
- Reddish brown content, may contain CaOX as cascarilla bark.
3. Phellogen
- One row of 2ry lateral meristem.
(cork
- Formed once or several times in plant life.
cambium)
- Developed by activation of parenchyma in the cortex, pericycle or even in
primary phloem.
- Actively divides to give cork cells to the outside and pelloderm to the inside.
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4. Phelloderm 1. Tissue of secondary origin.
(2ry cortex) 2. Several rows, first rows are regularly arranged.
3. Chloroplast, starch granules may present.
4. Parenchyma (as cinchona) or collenchyma (as cascara) or sclerenchyma (as
canella).
5. Cells are not suberized.
5. 1ry cortex
- Mainly consist of parenchyma cells
- May include: fibers, sclereides (as cassia), secretory structures (cells: as cassia
or tubes: as cinchona)
6. Pericycle
- The region outside the vascular bundle.
- Could be formed of:
 Sclerenchyma: Continuous as cinnamon or discontinuous as cassia.
 Parenchyma (undifferentiated) as cinchona.
7. Primary
Phloem (bast):
phloem
- Consists mainly of secondary phloem.
- Phloem elements are:
1. Phloem parenchyma
2. Phloem fibers
8. Secondary
3. Sieve tubes, companion cells
phloem
4. Medullary rays
Sieve tubes:
Functions for conduction of elaborated food to different organs through sieve
plates which is a porous structure.
Callus plate:
- A sieve plate can be temporarily or permanetly blocked by callose mucilage
which is stained red with corallin soda.
- When sieve plate is permanently blocked:
Sieve plate → callus plate
Permanent blockage
Sieve tubes → keratenchyma
of sieve tubes
- Callus plate types:
Permanent
Temporary
Callose is not reCallose re-dissolved
Permanent callus
dissolved and sieve
in the next season
plate
tubes no longer
and sieve tube
function leading to
continues function,
keratenchyma
no keratenchyma is
formation.
formed.
Keratenchyma
9. Vascular
cambium
Keratenchyma:
- Collapsed compact masses of sieve tubes.
- Formed due to permanent blockage of sieve plates by callose mucilage.
- Tested by corallin soda.
- Present in cinnamon cascara.
__
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N.B:
-
Outer bark: rhytidoma, cork, phellogen, phelloderm.
Inner bark: Primary & secondary phloem.
Periderm: means phellogen and its products of division i.e. cork & phelloderm.
-
Incomplete bark: Lacking some of the previous layers except phloem. E.g.:
1. Decotricated bark (cinnamon, vanilla)
2. Inner bark (quillia)
-
Sclerenchyma: highly lignified cells, present in two forms:
 Sclereids (stone cells):
In phelloderm (canella), in pericycle (cassia, cinnamon), in phloem (hamamelis)
 Fibers (elongated cells with tapering ends):
In cortex, pericyle or phloem (known as bast fibers)
-
Tissues from outer to inner:
Rhytodoma
Cork (phellem)
Outer bark
Phellogen (cork cambium)
Phelloderm (secondary cortex)
1ry cortex
Pericycle
1ry phloem
Inner bark
2ry phloem
Vascular cambium
Tissues of
secondary origin
Tissue of
secondary origin
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