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Bark
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Article Contents
Simcha Lev-Yadun, University of Haifa-Oranim, Tivon, Israel
. Introduction
. Structure and Development
. Regulation of Cork Development
. Dilatation
Online posting date: 16th May 2011
Bark comprises all the tissues outside the vascular cambium of a vascular plant. The majority of the bark of
woody plants develops from three meristems: the vascular
cambium that gives rise to the secondary phloem, the
phellogen that gives rise to the cork and the dilatation
meristem that produces parenchyma cells to prevent
cracking when the axis increases in diameter. Bark tissues
have a critical role in defending plants from pathogens
and herbivores through their physical and chemical
properties. They also defend from environmental hazards
such as sun irradiation, desiccation, wind, flooding, hail,
snow and even fire by forming a thick cork layer. The bark
has a critical role in storage and transport of organic
molecules and in many plants the bark also contributes to
photosynthesis. Many of the various defensive and toxic
substances found in barks are used by humans as medicines, spices and for various industries. Gene exploring in
barks is expected to result in many beneficial molecules
for agriculture, medicine, food and industry.
Introduction
Bark, which includes all tissues formed outside the vascular
cambium, is structurally, physiologically and functionally a
very complex part of the plant. The major functions of the
bark are translocation and storage of organic materials,
water storage and wound healing, protection from herbivores and pathogens, protection from environmental hazards, and in the shoot, photosynthesis. In many leafless or
almost leafless plants, all or most of a plant’s photosynthesis
is performed by the bark. An ecologically important function of bark is to protect trees from fires. Thick barks, being
poor conductors of heat, isolate the sensitive live tissues of
many tree species from fires. See also: Epidermis: Outer Cell
Layer of the Plant
ELS subject area: Plant Science
How to cite:
Lev-Yadun, Simcha (May 2011) Bark. In: Encyclopedia of Life Sciences
(ELS). John Wiley & Sons, Ltd: Chichester.
DOI: 10.1002/9780470015902.a0002078.pub2
Structure and Development
The bark includes primary and secondary phloem, cortex,
first periderm, sequent periderms (rhytidome) and tissues
formed by dilatation growth (Esau, 1965, 1969; Roth, 1981;
Fahn, 1990; Junikka, 1994). The structure of the bark in
roots of a given species is usually somewhat less complicated than in the shoot, and the description here refers to the
shoot rather than to the root. The wealth of known structures and functions of the bark is never found in a single
species, and there are also many variations at different ages
and with changing growth conditions in the same individual
(Borger, 1973; Roth, 1981; Lev-Yadun and Aloni, 1990).
Additional variation is expressed following wounding or
pathogen attacks (Borger, 1973). See also: Phloem Structure and Function
In woody plants, two regions are distinguished within
the bark: the inner bark, which is alive and where certain
cells may redifferentiate and become meristematic or
change their fate (e.g., parenchyma cells that turn into
sclereids); and the outer, dead bark cut off from live
tissues by dead isolating cork layers – the rhytidome. We
distinguish between primary bark (originating from
primary meristems – protoderm, ground meristem and
procambium) and secondary bark (originating from secondary meristems – the vascular cambium, the phellogen
and dilatation meristem) (Esau, 1969; Fahn, 1990).
The contribution of the primary meristems to the primary bark is as follows. The protoderm gives rise to the
epidermis, which may exist for many years or may be
replaced by cork (periderm). The ground meristem gives
rise to the cortex, made of parenchyma, collenchyma,
fibres, sclereids, idioblasts of various types, resin ducts,
gum ducts or laticifers. The procambium gives rise to the
primary phloem, including the primary phloem fibres. In a
number of plants, the border between the cortex and the
primary phloem is marked by the starch sheath, a layer rich
in starch grains, considered to be homologous with the
endodermis of the root. An endodermis, with its typical
casparian strip, is known from the shoot of only a small
number of plants. In addition to these components, in
shoots, leaf traces composed of xylem and phloem cross the
cortex before they fuse with the central vascular cylinder
(Esau, 1965; Fahn, 1990). See also: Cork; Starch and Starch
Granules
The vascular cambium (a secondary lateral meristem) is
the origin of the secondary phloem. The distinction
ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net
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Bark
between the primary and secondary phloem is easy in most
gymnosperms and dicotyledons since the secondary
phloem has a radial component (the vascular rays) in
addition to the axial component, whereas the primary
phloem does not include rays. However, hundreds of species produce secondary phloem with no rays as a special
adaptation (Larson, 1994; Lev-Yadun and Aloni, 1995).
The secondary phloem of many species has many bands of
fibres (bast fibres) (Esau, 1965, 1969). When cambial
activity produces large amounts of xylem, the phloem is
pushed outward, and the old, nonconducting and soft sieve
cells collapse and flatten, but the bands of fibres, axial
parenchyma, sclereides and rays remain intact (Esau, 1965,
1969; Fahn, 1990). See also: Lateral Meristems; Meristems
In many conifers, resin ducts develop in both the primary
and secondary bark. They produce resin with species or
even genotype-specific chemical composition. In many
angiosperms, laticifers or gum ducts develop and produce
special resins and latexes. Wounding or pathogen attacks
result in the differentiation of additional (traumatic) resin
ducts in conifers and gum ducts in many dicotyledons
(Fahn, 1979; Fink, 1999). See also: Conifers; Gymnosperms; Latex and Laticifers; Plant Gums
At a certain stage of shoot development, a periderm
(cork tissue) may appear. The periderm is a secondary
tissue formed from a secondary meristem – the phellogen,
also known as cork cambium. The function of the periderm
is to isolate and thus protect the live tissues from both biotic
and abiotic damaging factors (Fahn, 1990; Sandved et al.,
1993). In certain trees, the periderm may reach a thickness
of dozens of centimetres, but usually it is only several
millimetres to several centimetres thick. A well-known case
is the giant sequoia of the western USA, in which bark
thickness of mature trees ranges from 25 to 80 cm. The
periderm is composed mostly of cork (phellem) cells, which
die after differentiation, and their secondary cell walls
are rich with suberin. The periderm also includes a certain
amount of parenchyma (phelloderm) cells, and sometimes, for instance in pines, layers of hard, heavily lignified
sclereids within the phellem zone (Fahn, 1990). As with
resin and gum ducts, wounding and pathogenesis induces
the formation of additional cork tissues (wound periderm)
(Borger, 1973). See also: Cork
In many plant species, there are many lenticels, which
serve as gas exchange shafts through the almost impermeable cork. Lenticels are made of many loosely arranged
cells that form a continuity of intercellular spaces with the
inner tissues and appear as dots and stripes on the bark
surface (Fahn, 1990; Figure 1).
Initials of the phellogen mostly divide outward to produce phellem (cork) cells, but in many species a small
fraction of its cell divisions are inward, to form the parenchymatic phelloderm. Derivatives of the phellogen are
usually arranged in radial files. Initiation of periderm starts
at different distances from the shoot apex and from different
tissues. The site of phellogen initiation is in the pericycle in
the roots, but in the shoot it can initiate in the epidermis,
subepidermis or in much deeper layers, such as the cortex or
2
Figure 1 Cross-section of the stem of a small tree of Calotropis procera
showing a microscopic view of the secondary xylem with the pores of the
water conducting vessels (red bottom part); live part of the bark with the
band of latex-forming ducts in the middle and the outer layers of cork with
a typical lenticel in the centre (the green stained).
old phloem (Fahn, 1990). Although for each organ and
species there is a typical cell layer in which phellogen is
initiated, there are many exceptions. Genetic, physiological
and environmental factors largely influence the timing and
location of phellogen initiation. In some plants, phellogen
initiation occurs within a short time (only a few days) after
an organ is formed, but in others it can be delayed for
decades. In some plants, only a first periderm is formed, and
this periderm may continue its activity for the rest of the
plant’s life. The best-known case of such periderm is the
cork oak of the Iberian peninsula, from which most of
the global commercial cork originates (Fahn, 1990).
In many woody plants, there is a second stage of cork
formation – the rhytidome or subsequent periderms. In
plants that form subsequent periderms, live parenchyma
cells of the secondary phloem redifferentiate and form an
internal zone of phellogen. The activity of this phellogen
produces a layer of cork cells that isolate all tissues outward
to them from nutrients and so cause them to die. All tissues
(cork and other tissues) found outward from the innermost
subsequent periderm compose the rhytidome (Fahn, 1990).
See also: Parenchyma
In several tropical tree species, especially trees of the
savannah, spines develop on the surface of the trunks (e.g.,
Hura crepitans). Usually these spines are made of a special
type of cork that develops from islands of phellogen (Roth,
1981).
In trees, bark tissues usually comprise a much smaller
fraction of the trunk volume than wood. Similarly, the
effort to study bark development, anatomy and physiology
is only a small fraction of that given to wood production.
Thus, we do not know much concerning bark biology. The
best model plant, Arabidopsis thaliana, has both primary
and secondary bark, although it is a rather small annual
and most of its secondary bark tissues are formed in the
main root. Certain aspects of bark development, however,
can be studied in this plant. The secondary phloem, which
composes most of the inner bark in trees, is formed by
ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net
Bark
cambial activity that usually produces 5–15 times more
secondary xylem than phloem. Some species, that is, the
cork oak, form growth-rings in the cork. However, dating
various events using cork rings is not a common practise.
The structure of the periderm influences the morphology of
the outer surface of woody plants, especially trees. The
bark can be smooth or covered by scales of various shapes
and sizes. It can peel in small, medium or very large sheets
or blocks and can differ in colour (Borger, 1973; Sandved
et al., 1993). The functionality of the specific types of bark
structure, morphology, shape and size of scales, colour
and chemistry are almost not studied and therefore
unknown. See also: Arabidopsis thaliana as an Experimental Organism
Regulation of Cork Development
Several environmental and endogenous conditions induce
cork formation: submergence in water, direct strong sun
irradiation, mechanical and biotic wounding and ageing.
All these factors are known to increase the production of
gaseous phytohormone ethylene. It was therefore proposed
that ethylene is the major activator for phellogen initiation
and activity (Lev-Yadun and Aloni, 1990). Since the cork
layer formed is almost impermeable to gases, as a cork layer
is formed, the inner cell layers are exposed to increasing
levels of ethylene, and more cork formation is induced.
In many plant species, there is a gap in phellogen initiation around buds in the nodal region, or a cork-free region
in stems and branches beneath buds and major veins of
leaves. Therefore, it was proposed that the basipetal polar
auxin transport inhibits cork formation in these regions
(Lev-Yadun and Aloni, 1990). These cork-free regions later
enable suppressed buds to develop quickly and form new
branches after damage to the canopy, and not to be disturbed by a heavy, hard layer of cork (Lev-Yadun
and Aloni, 1993). See also: Plant Growth Factors and
Receptors
Dilatation
When trees and shrubs or even thick annuals grow in
diameter, the outer tissues expand to a certain but limited
degree. With additional diameter growth, there is a danger
that cracks will be formed, and the sensitive inner tissues
will be exposed to biotic and abiotic agents that can
endanger the plant. To avoid the formation of such cracks,
a special tissue is formed in the outer bark – a dilatation
tissue (Esau, 1965, 1969; Roth, 1981; Fahn, 1990). Dilatation is the outcome of one or two processes that co-occur
in many species. The first process is dilatation growth as the
result of cell expansion. Certain parenchyma cells of the
cortex, of the axial phloem parenchyma or the phloem ray
parenchyma expand to keep the bark intact (Figure 2). In
other cases, groups of these cells enter a phase of cell divisions and form a dilatation meristem. When dilatation
occurs in the phloem rays, a distinct funnel-shaped phloem
Figure 2 Longitudinal tangential section of the bark of a large tree of Ficus
sycomorus showing a microscopic view of the bark in a region of old
phloem where dilatation started. The axial fibres and parenchyma form
a net of strands while the radial component, the rays (which are
spindle-shaped), start to dilate.
ray dilatation occurs (Esau, 1965, 1969; Roth, 1981; Fahn,
1990). Dilatation may be very irregular, resulting in the
formation of whirled tissues and various shapes of rays.
Similarly, meristematic dilatation zones in various other
orientations and from other origins occur within the bark
in the process of dilatation (Roth, 1981; Lev-Yadun and
Aloni, 1992). Dilatation activity results in the production
of parenchyma, and later, many of these parenchyma cells
may redifferentiate to sclereids (Roth, 1981). Dilatation
activity is suppressed in the lower side of leaning trunks,
probably by the higher-than-usual auxin levels. Application of ethylene, wounding, or environmental conditions
that induce ethylene production results in higher dilatation
activity. See also: Phloem Structure and Function
The regulation of traumatic resin and gum
ducts, and wound cork formation
At least two plant hormones (ethylene and jasmonic acid),
which are expressed at higher levels than normal following
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Bark
wounding and pathogen attacks, induce the formation of
traumatic resin and gum ducts and wound cork. Ethylene,
one of the first known plant hormones, was recognised first
as a major inducer of traumatic resin and gum ducts (Fahn,
1988). The role of ethylene in inducing cork formation
(Lev-Yadun and Aloni, 1990) and dilatation activity (LevYadun and Aloni, 1992) followed. Later, methyl jasmonate
was found to be involved in traumatic resin and gum duct
formation (Franceschi et al., 2002; Hudgins et al., 2004).
The stress hormone ABA seems to have a role in inducing
suberin synthesis (Ginzberg, 2008). The role of other phytohormones and their regulatory networks in these processes, if any, is not clear. The general picture emerging is
that the three stress/wounding/defence phytohormones
ethylene and ABA are deeply involved in nontraumatic and
traumatic bark formation and methyl jasmonate in traumatic bark tissue formation. The role of methyl jasmonate
in nontraumatic bark formation is not yet known. The
precise hormonal regulation of bark formation at the
molecular level is still far from being understood.
Gene expression in the bark
The study of gene expression and genes involved in regulation of bark formation has practically only begun. We
have only a preliminary understanding of the genetic and
molecular processes involved in bark biology (e.g., Roach
and Deyholos, 2007; Soler et al., 2007, 2008; Barel and
Ginzberg, 2008; Wildhagen et al., 2010; Duan et al., 2010).
The chances of finding commercially important molecules
in the bark and the renewed interest in bast fibres have
resulted in a recent modest increase in such studies of the
bark but much more should be done. Genomic studies of
differentiating cork in the most important source for
commercial cork, the cork oak resulted in the classification
of about 50 genes belonging to the main pathways for cork
biosynthesis (Soler et al., 2007). The seasonal expression of
some of these genes was also studied and found to show
highest expression in June, a crucial month for cork
development in cork oak (Soler et al., 2008). In potato
tuber skin, another important model for periderm formation, many of the expressed proteins are not only known
plant defence components, but also various enzymes
involved in cork formation (Barel and Ginzberg, 2008).
In Hevea brasiliensis, the most important source of
natural rubber, induced by bark wounding, the treatments
of wounding, application of methyl jasmonate or ethylene
specifically induced the expression of various genes with
two genes up-regulated by all three treatments (Duan et al.,
2010). Autumnal storage of proteins in poplar bark
after the re-translocation of amino acids from senescing
leaves is correlated with day length and temperature.
Some of these proteins are known to be storage proteins,
some have various physiological functions and the function of others is not known yet (Wildhagen et al., 2010).
A much broader and deeper understanding of the genes
involved in all aspects of bark biology is expected in the
coming years.
4
Fibres in the bark
Two types of fibres are found in the bark (bast fibres).
Some plants, for example, flax, produce only primary
phloem fibres of procambial origin. Others, for example,
hemp, kenaf and many trees produce in addition secondary
phloem fibres of cambial origin (Esau, 1965, 1969; Fahn,
1990).
Uses of bark
Products made of bark have been important since ancient
times. Several plants are famous for their bast fibres
(primary and secondary phloem fibres): flax, hemp, jute,
ramie and kenaf. Their fibres are used for textiles, cordage,
matting, fishing nets, sails, sacks, paper production, composite materials, etc. Before the invention of plastics, these
and other plant fibres were of utmost importance. Cork
from the cork oak (Quercus suber) and a long list of secondary cork-based products are used all over the world.
Sealing wine bottles is only a small fraction of the uses of
cork. Tanning hides with bark tannins to produce leather
has been practiced for millennia. Before the age of rubber
and plastics, leather was extremely important, and its
production commonly needed bark tannins (Hill, 1952).
Rubber is mainly produced from the latex exuded from the
artificially wounded bark of H. brasiliensis. Many other
species produce latex in the bark, but only a small number
of species are used for rubber production (Loadman, 2005).
Several barks provide chemicals used for medicine. Quinine, the cure for malaria, still a major killer in the tropics,
is produced from the bark of several species of the genus
Cinchona. Taxol, a drug used against several types of
cancer, is extracted from the bark of the Pacific yew
(Taxus brevifolia). Curare, probably the best-known muscle-relaxing poison and a source for medicines, is produced
by mixing barks of several tropical species. Cinnamon,
camphor and several other spices are also produced from
tree barks (Sandved et al., 1993). See also: Plant Secondary
Metabolism; Taxol; Vegetable Tannins
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Further Reading
Dusotoit-Coucaud A, Kongsawadworakul P, Maurousset L et al.
(2010) Ethylene stimulation of latex yield depends on the
expression of a sucrose transporter (HbSUT1B) in rubber tree
(Hevea brasiliensis). Tree Physiology 30: 1586–1598.
da Ponte-e-Sousa JCA, de A and Neto-Vaz AM (2011) Cork and
metals: a review. Wood Science and Technology 45: 183–202.
Saveyn A, Steppe K, Ubierna N and Dawson TE (2010) Woody
tissue photosynthesis and its contribution to trunk growth and
bud development in young plants. Plant, Cell and Environment
33: 1949–1958.
Schreiber L (2010) Transport barriers made of cutin, suberin and
associated waxes. Trends in Plant Science 15: 546–553.
Serra O, Figueras M, Franke R, Prat S and Molinas M (2010)
Unraveling ferulate role in suberin and periderm biology by
reverse genetics. Plant Signaling & Behavior 5: 953–958.
ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net
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