Parenchyma
Parenchyma tissue (ground tissue) is made-up of parenchyma cells. Parenchyma cells are the least
differentiated plant cells and can give rise to all other plant cell types during development or after
wounding of the plant (they are totipotent - maintaining all possible cell development fates within
themselves). Other parenchyma cells become specialised for storage of material (storage
parenchyma), secretion (secretory parenchyma) or for photosynthesis, as in the mesophyll
parenchyma cells of the leaf. Parenchyma also provides support for young, fleshy and green plant
parts, like herbaceous stems. When the cells are turgid (swollen with water), they support these plant
parts, and when they are plasmolysed (contain insufficient water to generate positive pressure) these
plant parts will wilt. Photosynthetic parenchyma contain green chloroplasts. Parenchymatous tissues
contain air spaces for the diffusion of gases. Metabolically active parenchyma contains large air
spaces for gas exchange, especially in the spongy mesophyll of the leaf, which requires oxygen and
carbon dioxide exchange for photosynthesis.
Parenchyma tissue (ground tissue) is made-up of parenchyma cells. Parenchyma cells are the least
differentiated plant cells and can give rise to all other plant cell types during development or after
wounding of the plant (they are totipotent - maintaining all possible cell development fates within
themselves). Other parenchyma cells become specialised for storage of material (storage
parenchyma), secretion (secretory parenchyma) or for photosynthesis, as in the mesophyll
parenchyma cells of the leaf. Parenchyma also provides support for young, fleshy and green plant
parts, like herbaceous stems. When the cells are turgid (swollen with water), they support these plant
parts, and when they are plasmolysed (contain insufficient water to generate positive pressure) these
plant parts will wilt. Photosynthetic parenchyma contain green chloroplasts. Parenchymatous tissues
contain air spaces for the diffusion of gases. Metabolically active parenchyma contains large air
spaces for gas exchange, especially in the spongy mesophyll of the leaf, which requires oxygen and
carbon dioxide exchange for photosynthesis.
Parenchyma cells are approximately isotropic and storage parenchyma especially are isodiametric
and orthotetrakaidecahedron in shape. An orthotetrakaidecahedron has 14 sides, 8 hexagonal and
6 quadrilateral, with 30 edges and angles of 120 degrees between adjacent faces and 109 degrees 28’
16’’ between adjacent edges. Tetrakaidecahedrons can be stacked endlessly without interstices and
also maximise wall-wall contact area. In published studies (see bibliography), parenchyma of elder
(Sambucus canadensis) was measured to have a mean of 13.97 faces (n = 100 cells).
Parenchyma is compressible and has a degree of elasticity. The elastic modulus (E) is a measure of
material stiffness and the elastic modulus of parenchyma is proportional to the tissue turgor pressure
(measured at 8 MNm-2 for a pressure of 0.31 MPa and 19 MNm-2 for 0.67 MPa). Measurements in
meristem parenchyma give E = 19-40 MNm-2. E also increases for larger samples. Parenchyma is also
less compressible at higher strain rates.
Parenchyma is a non-linear elastic material and shows short-term elastic recovery, long-term plasticity,
stress relaxation and creep and so is viscoelastic. Its plasticity is due to loss of water from the cells.
The degree of elasticity (the ratio of recovered elastic deformation to total deformation under loading
by a given stress) for potato tuber parenchyma is 0.46-0.60. (A perfectly elastic material has a degree
of elasticity of 1 and a perfectly plastic material 0). Elastic hysteresis of parenchyma is high; for
example potato tuber parenchyma dissipates 72-90% of the total energy gained during a loading cycle.
The Poisson ratio (v) for parenchyma lies in the range 0.23-0.5, for example for apple flesh it has the
range 0.21-0.34. For v = 0.5, shear occurs at 45 degrees to the axis of tension. Mechanical failure of
parenchyma occurs by cell rupture or by failure of cell-cell bonding in softer tissues. Fruit softening
results from a reduction in the fraction of insoluble pectin in the middle lamella. The soluble pectins
hydrate and reduce the binding strength. The middle lamella exhibits plastic behaviour.
Plant tissues are cellular solids. Gas-filled interstitial spaces lower density. Cell fluids can move into
these spaces when the tissue is under stress. The relative density (total density over the density of
the solid component) is < 1 for plant tissues. Spongy mesophyll is an open-walled cellular solid, whilst
xylem, phloem, cork and wood are closed-walled cellular solids. Cork has a relative density of about
0.09, heartwood ranges from 0.09 - 0.94, and balsa wood has a relative density of about 0.13.
Parenchyma may behave as a pressurised cellular solid or as a hydrostat. A hydrostat is a thin-walled
inflatable structure and is a suitable means of providing support in aquatic plants. For example the
alga Caulerpa is a single-celled alga up to 20 m long and 1 m tall and behaves as a hydrostat. To
behave like a hydrostat the wall thickness to cell radius ratio needs to be < 20%. If the ratio exceeds
20% then the pressure difference across the cell wall plays little or no role in wall stresses. If the ratio
equals 20% then the material behaves as a pressurised cellular solid. Hydrophytes tend to have
hydrostatic tissues, whilst xerophytes have more thick-walled ‘dead’ tissues.
Turgid cells convert compression forces into tensile forces in the cell walls, which is useful since
cellulose is strong in tension. Plant cell walls act as beams/struts. In an open-walled cellular solid E
decreases as the interstitial volume increases. The solid phase exhibits plastic behaviour. Willow
(Salix) has relatively thin cell walls and so is susceptible to fracture. This aids its propagation and
dispersal, since broken twigs and branches that fall into water can root further downstream. This is
especially characteristic of crack willow.
Spongy mesophyll provides aeration and support for leaves. There is less spongy mesophyll in sun
leaves than in shade leaves. Sun leaves face greater water and wind stresses and by having less
spongy mesophyll their stiffness is less dependent on hydration.
Aerenchyma is an open-walled cellular solid with a low relative density of about 10-3. The elastic
modulus of aerenchyma has been measured at 2.26 MNm-2 for Juncus effusus. Compare this to 19-
40 MNm-2 for meristem parenchyma. Aerenchyma therefore lacks stiffness. Its bulk modulus (K)
approaches zero and although aerenchyma is mechanically weak it provides aeration and buoyancy
and has low self-loading.
Collenchyma
Collenchyma cells are modified parenchyma cells that have thickened walls and are often elongated
cells. Often the corner walls are especially thickened. Collenchyma cells retain living protoplasts and,
like parenchyma, can produce other cell types during healing of wounds. Collenchyma forms the
vascular sheath around vascular tissues, in ribs accompanying the larger veins in leaves, and often
occurs under the epidermis of young non-woody stems either as a continuous layer or in ribs/ridges.
Function: collenchyma has strengthened, but flexible cell walls and provides support for growing
leaves and growing stems. It is plastic and stretchable and develops more in growing tissues subjected
to greater mechanical stresses. Collenchyma in celery stems sticks between your teeth!
Collenchyma supports rapidly growing organs. It is highly plastic and has a low elastic range. It has an
elastic modulus E = 22 MNm-2 and a breaking stress, measured in celery, of 23.3 MNm-2 (cf. 4.1 MNm-
2 for primary xylem). Collenchyma strengthens older leaves. Older collenchyma is less plastic. Young
collenchyma is a non-linear viscoelastic material that stretches and flows as the leaf grows.
and orthotetrakaidecahedron in shape. An orthotetrakaidecahedron has 14 sides, 8 hexagonal and
6 quadrilateral, with 30 edges and angles of 120 degrees between adjacent faces and 109 degrees 28’
16’’ between adjacent edges. Tetrakaidecahedrons can be stacked endlessly without interstices and
also maximise wall-wall contact area. In published studies (see bibliography), parenchyma of elder
(Sambucus canadensis) was measured to have a mean of 13.97 faces (n = 100 cells).
Parenchyma is compressible and has a degree of elasticity. The elastic modulus (E) is a measure of
material stiffness and the elastic modulus of parenchyma is proportional to the tissue turgor pressure
(measured at 8 MNm-2 for a pressure of 0.31 MPa and 19 MNm-2 for 0.67 MPa). Measurements in
meristem parenchyma give E = 19-40 MNm-2. E also increases for larger samples. Parenchyma is also
less compressible at higher strain rates.
Parenchyma is a non-linear elastic material and shows short-term elastic recovery, long-term plasticity,
stress relaxation and creep and so is viscoelastic. Its plasticity is due to loss of water from the cells.
The degree of elasticity (the ratio of recovered elastic deformation to total deformation under loading
by a given stress) for potato tuber parenchyma is 0.46-0.60. (A perfectly elastic material has a degree
of elasticity of 1 and a perfectly plastic material 0). Elastic hysteresis of parenchyma is high; for
example potato tuber parenchyma dissipates 72-90% of the total energy gained during a loading cycle.
The Poisson ratio (v) for parenchyma lies in the range 0.23-0.5, for example for apple flesh it has the
range 0.21-0.34. For v = 0.5, shear occurs at 45 degrees to the axis of tension. Mechanical failure of
parenchyma occurs by cell rupture or by failure of cell-cell bonding in softer tissues. Fruit softening
results from a reduction in the fraction of insoluble pectin in the middle lamella. The soluble pectins
hydrate and reduce the binding strength. The middle lamella exhibits plastic behaviour.
Plant tissues are cellular solids. Gas-filled interstitial spaces lower density. Cell fluids can move into
these spaces when the tissue is under stress. The relative density (total density over the density of
the solid component) is < 1 for plant tissues. Spongy mesophyll is an open-walled cellular solid, whilst
xylem, phloem, cork and wood are closed-walled cellular solids. Cork has a relative density of about
0.09, heartwood ranges from 0.09 - 0.94, and balsa wood has a relative density of about 0.13.
Parenchyma may behave as a pressurised cellular solid or as a hydrostat. A hydrostat is a thin-walled
inflatable structure and is a suitable means of providing support in aquatic plants. For example the
alga Caulerpa is a single-celled alga up to 20 m long and 1 m tall and behaves as a hydrostat. To
behave like a hydrostat the wall thickness to cell radius ratio needs to be < 20%. If the ratio exceeds
20% then the pressure difference across the cell wall plays little or no role in wall stresses. If the ratio
equals 20% then the material behaves as a pressurised cellular solid. Hydrophytes tend to have
hydrostatic tissues, whilst xerophytes have more thick-walled ‘dead’ tissues.
Turgid cells convert compression forces into tensile forces in the cell walls, which is useful since
cellulose is strong in tension. Plant cell walls act as beams/struts. In an open-walled cellular solid E
decreases as the interstitial volume increases. The solid phase exhibits plastic behaviour. Willow
(Salix) has relatively thin cell walls and so is susceptible to fracture. This aids its propagation and
dispersal, since broken twigs and branches that fall into water can root further downstream. This is
especially characteristic of crack willow.
Spongy mesophyll provides aeration and support for leaves. There is less spongy mesophyll in sun
leaves than in shade leaves. Sun leaves face greater water and wind stresses and by having less
spongy mesophyll their stiffness is less dependent on hydration.
Aerenchyma is an open-walled cellular solid with a low relative density of about 10-3. The elastic
modulus of aerenchyma has been measured at 2.26 MNm-2 for Juncus effusus. Compare this to 19-
40 MNm-2 for meristem parenchyma. Aerenchyma therefore lacks stiffness. Its bulk modulus (K)
approaches zero and although aerenchyma is mechanically weak it provides aeration and buoyancy
and has low self-loading.
Collenchyma
Collenchyma cells are modified parenchyma cells that have thickened walls and are often elongated
cells. Often the corner walls are especially thickened. Collenchyma cells retain living protoplasts and,
like parenchyma, can produce other cell types during healing of wounds. Collenchyma forms the
vascular sheath around vascular tissues, in ribs accompanying the larger veins in leaves, and often
occurs under the epidermis of young non-woody stems either as a continuous layer or in ribs/ridges.
Function: collenchyma has strengthened, but flexible cell walls and provides support for growing
leaves and growing stems. It is plastic and stretchable and develops more in growing tissues subjected
to greater mechanical stresses. Collenchyma in celery stems sticks between your teeth!
Collenchyma supports rapidly growing organs. It is highly plastic and has a low elastic range. It has an
elastic modulus E = 22 MNm-2 and a breaking stress, measured in celery, of 23.3 MNm-2 (cf. 4.1 MNm-
2 for primary xylem). Collenchyma strengthens older leaves. Older collenchyma is less plastic. Young
collenchyma is a non-linear viscoelastic material that stretches and flows as the leaf grows.
| Plant Cell and Tissue Types |
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Sclerenchyma
Sclerenchyma develop from parenchyma or collenchyma cells. In addition to the cellulose cell wall (so-
called primary cell wall – which forms first) sclerenchyma have lignified walls (secondary cell walls
added to the inside of the primary walls). Sclerenchyma may form tough fibres (many of which are
used to make ropes and cords) or exist as stone cells or sclereids (sclereids make pears gritty and
the clove scales of garlic hard). Sclerenchyma cells may still have living protoplasts, or they may be
hollow (‘dead’) – though their walls are so thick that little lumen may remain. Sclerenchyma have hard,
rigid walls.
Function: sclerenchyma occurs in protective structures, such as the hard shells of nuts and the stony
endocarp of stone fruits (like peach stones), bark fibres and also provide support for older non-
elongating plant parts. Flax stem fibres are used in cloth, money paper and cigarette paper.
Sclerenchyma possesses shear-resistant lignified walls. Wet phloem fibres have an elastic modulus, E
about 19 GNm-2 (in tension) whilst dry phloem fibres have an elastic modulus in the range 51-60
GNm-2 (depending on how well they are dried).
Sclerenchyma develop from parenchyma or collenchyma cells. In addition to the cellulose cell wall (so-
called primary cell wall – which forms first) sclerenchyma have lignified walls (secondary cell walls
added to the inside of the primary walls). Sclerenchyma may form tough fibres (many of which are
used to make ropes and cords) or exist as stone cells or sclereids (sclereids make pears gritty and
the clove scales of garlic hard). Sclerenchyma cells may still have living protoplasts, or they may be
hollow (‘dead’) – though their walls are so thick that little lumen may remain. Sclerenchyma have hard,
rigid walls.
Function: sclerenchyma occurs in protective structures, such as the hard shells of nuts and the stony
endocarp of stone fruits (like peach stones), bark fibres and also provide support for older non-
elongating plant parts. Flax stem fibres are used in cloth, money paper and cigarette paper.
Sclerenchyma possesses shear-resistant lignified walls. Wet phloem fibres have an elastic modulus, E
about 19 GNm-2 (in tension) whilst dry phloem fibres have an elastic modulus in the range 51-60
GNm-2 (depending on how well they are dried).
Xylem
Xylem is tissue specialised in the transport of water and minerals from the roots to other parts of the
plant. Xylem consists of several cell types, including parenchyma and sclerenchyma fibres, and
water-conducting cells. The water-conducting cells, or tracheary elements, are of two types: vessel
members that join end-to-end to form vessels, and elongated narrower fibrous tracheids. The
tracheary elements have no living protoplasts, but instead have empty lumens through which xylem
sap flows. In vessel elements the end-walls have all but disappeared (and form so-called perforation
plates) to allow the xylem sap to flow freely from vessel member to vessel member, along the vessel.
In tracheids, the end-walls persist, but are porous. Conifers have only tracheids and no vessel
elements. Many angiosperms (flowering plants) have both tracheids and vessel elements. Tracheary
elements have lignified secondary cell walls.
In non-woody stems the xylem (along with the phloem) forms vascular bundles. In woody stems, the
xylem is the wood.
Function: xylem both conducts water and minerals from the roots to other plant parts, and provides
support. Large diameter vessels are best suited to conducting water, but narrower vessels and
tracheids are better at providing support. The largest vessels may be up to 0.5 mm in diameter.
Xylem is tissue specialised in the transport of water and minerals from the roots to other parts of the
plant. Xylem consists of several cell types, including parenchyma and sclerenchyma fibres, and
water-conducting cells. The water-conducting cells, or tracheary elements, are of two types: vessel
members that join end-to-end to form vessels, and elongated narrower fibrous tracheids. The
tracheary elements have no living protoplasts, but instead have empty lumens through which xylem
sap flows. In vessel elements the end-walls have all but disappeared (and form so-called perforation
plates) to allow the xylem sap to flow freely from vessel member to vessel member, along the vessel.
In tracheids, the end-walls persist, but are porous. Conifers have only tracheids and no vessel
elements. Many angiosperms (flowering plants) have both tracheids and vessel elements. Tracheary
elements have lignified secondary cell walls.
In non-woody stems the xylem (along with the phloem) forms vascular bundles. In woody stems, the
xylem is the wood.
Function: xylem both conducts water and minerals from the roots to other plant parts, and provides
support. Large diameter vessels are best suited to conducting water, but narrower vessels and
tracheids are better at providing support. The largest vessels may be up to 0.5 mm in diameter.
Wood
Wood is a non-pressurised (gas-filled) cellular solid. Gas is compressible and so the apoplast
provides most of the support. The secondary lignified walls strengthen the apoplast. Lignin increases
compressive strength and reduces water infiltration. The reduction in water infiltration lowers the
elastic modulus. Compression expels air, densifying the tissue and increasing E. Lignin in the middle
lamella increases bonding strength. The middle lamella has a typical shearing modulus of G of about
77GNm-2.
The sequential growth-rings of trees give them a polylaminate construction. Neighbouring rings have
slightly different grain orientations and the interfaces between the layers acts as a barrier to fracture
propagation. The denser secondary xylem formed at the end of each growth season adds further to
this heterogeneity.
Wood is anisotropic, specifically orthotropic, having three Poisson ratios and three elastic moduli. This
arises from the three mutually perpendicular planes of symmetry: longitudinal (along the grain) radial
and tangential to the grain, each with different material properties. The growth rings result in wood
having six Poisson ratios: vLR, vRL, vLT, vTL, vRT and vTR. (The first subscript designates the axis
parallel to the direction of the applied force, and the second subscript is the axis of transverse strain).
For balsa wood: vLR = 0.229, vRL = 0.488, vLT = 0.665, vTL = 0.217, vRT = 0.011and vTR = 0.007.
For yellow birch: vLR = 0.426, vRL = 0.451, vLT = 0.697, vTL = 0.447, vRT = 0.033 and vTR = 0.023.
In woods vLT is typically the largest.
Balsa wood has a shear modulus G = (GLR, GLT, GRT) = (0.169, 0.115, 0.0156) GNm-2 and an
elastic modulus E = (EL, ER, ET) = (3.12, 0.144, 0.0468) GN m-2. Typical values for the secondary
cell wall matrix are: G = 0.77 GNm-2, v = 0.30, and E = 2 GN m-2.
Pine wood has an elastic modulus of 8.51 GNm-2 and an elastic limit of 0.045 GNm-2.
Typical wood has elastic moduli of EL = 11.3 GNm-2, ET = 0.487 GNm-2 and ER = 0.926 GNm-2 and
shear moduli of GLT of about 0.98 GLR and GRT of about 0.24 GLR. Thus, the ratio of shear to
elastic moduli < 1 and shearing failure is likely to occur in bending/torsion. As tissue water content
increases, E decreases and thus dry branches are weaker and more brittle.
Cork
Cork, like wood, is a non-pressurised (gas-filled) cellular solid. Cork is also anisotropic, but is
axisymmetric, with two Poisson ratios and two elastic moduli. This arises from the two mutually
perpendicular planes of symmetry with different material properties. The longitudinal and tangential
planes have equivalent properties, but cork is very compressible in the radial direction (v of about 0 in
the radial direction).
Wood is a non-pressurised (gas-filled) cellular solid. Gas is compressible and so the apoplast
provides most of the support. The secondary lignified walls strengthen the apoplast. Lignin increases
compressive strength and reduces water infiltration. The reduction in water infiltration lowers the
elastic modulus. Compression expels air, densifying the tissue and increasing E. Lignin in the middle
lamella increases bonding strength. The middle lamella has a typical shearing modulus of G of about
77GNm-2.
The sequential growth-rings of trees give them a polylaminate construction. Neighbouring rings have
slightly different grain orientations and the interfaces between the layers acts as a barrier to fracture
propagation. The denser secondary xylem formed at the end of each growth season adds further to
this heterogeneity.
Wood is anisotropic, specifically orthotropic, having three Poisson ratios and three elastic moduli. This
arises from the three mutually perpendicular planes of symmetry: longitudinal (along the grain) radial
and tangential to the grain, each with different material properties. The growth rings result in wood
having six Poisson ratios: vLR, vRL, vLT, vTL, vRT and vTR. (The first subscript designates the axis
parallel to the direction of the applied force, and the second subscript is the axis of transverse strain).
For balsa wood: vLR = 0.229, vRL = 0.488, vLT = 0.665, vTL = 0.217, vRT = 0.011and vTR = 0.007.
For yellow birch: vLR = 0.426, vRL = 0.451, vLT = 0.697, vTL = 0.447, vRT = 0.033 and vTR = 0.023.
In woods vLT is typically the largest.
Balsa wood has a shear modulus G = (GLR, GLT, GRT) = (0.169, 0.115, 0.0156) GNm-2 and an
elastic modulus E = (EL, ER, ET) = (3.12, 0.144, 0.0468) GN m-2. Typical values for the secondary
cell wall matrix are: G = 0.77 GNm-2, v = 0.30, and E = 2 GN m-2.
Pine wood has an elastic modulus of 8.51 GNm-2 and an elastic limit of 0.045 GNm-2.
Typical wood has elastic moduli of EL = 11.3 GNm-2, ET = 0.487 GNm-2 and ER = 0.926 GNm-2 and
shear moduli of GLT of about 0.98 GLR and GRT of about 0.24 GLR. Thus, the ratio of shear to
elastic moduli < 1 and shearing failure is likely to occur in bending/torsion. As tissue water content
increases, E decreases and thus dry branches are weaker and more brittle.
Cork
Cork, like wood, is a non-pressurised (gas-filled) cellular solid. Cork is also anisotropic, but is
axisymmetric, with two Poisson ratios and two elastic moduli. This arises from the two mutually
perpendicular planes of symmetry with different material properties. The longitudinal and tangential
planes have equivalent properties, but cork is very compressible in the radial direction (v of about 0 in
the radial direction).
Phloem
Phloem conducts sugary sap from photosynthesizing leaves (sources) to non-photosynthesizing plant
parts (sinks) that can not make their own sugar. Thus, phloem mostly flows down the stem of a plant,
but may flow upwards if the plant is utilising stored carbohydrates in storage organs like root tubers
(e.g. potato), for example. Phloem also delivers the sugars to growing fruit. Phloem, like xylem, is a
complex tissue made-up of several different cell types: parenchyma, sclerenchyma, transfer cells,
small companion cells, and sieve tube members (STM) that make-up sieve tubes. Sieve tube
members lose their nuclei at maturity, and depend on their companion cells to regulate their
metabolism. Cytoplasm low in organelle content and consisting mostly of cytosol and strands of
protein (P-protein) remains. The STM end-walls are highly porous and are called sieve plates.
Transfer cells are parenchyma cells modified to transfer sugars to and from the phloem.
In non-woody stems and leaves the phloem occurs in the vascular bundles, together with the xylem. In
woody stems, there is a thin layer of phloem constituting the innermost layer of the bark (sometimes
this phloem is considered to be separate from the bark). Removing a ring of bark from the whole
circumference of a tree trunk will kill the tree as no phloem remains to send sugars to the roots.
(Though sometimes the roots will live long enough to put out new shoots through the soil).
Phloem conducts sugary sap from photosynthesizing leaves (sources) to non-photosynthesizing plant
parts (sinks) that can not make their own sugar. Thus, phloem mostly flows down the stem of a plant,
but may flow upwards if the plant is utilising stored carbohydrates in storage organs like root tubers
(e.g. potato), for example. Phloem also delivers the sugars to growing fruit. Phloem, like xylem, is a
complex tissue made-up of several different cell types: parenchyma, sclerenchyma, transfer cells,
small companion cells, and sieve tube members (STM) that make-up sieve tubes. Sieve tube
members lose their nuclei at maturity, and depend on their companion cells to regulate their
metabolism. Cytoplasm low in organelle content and consisting mostly of cytosol and strands of
protein (P-protein) remains. The STM end-walls are highly porous and are called sieve plates.
Transfer cells are parenchyma cells modified to transfer sugars to and from the phloem.
In non-woody stems and leaves the phloem occurs in the vascular bundles, together with the xylem. In
woody stems, there is a thin layer of phloem constituting the innermost layer of the bark (sometimes
this phloem is considered to be separate from the bark). Removing a ring of bark from the whole
circumference of a tree trunk will kill the tree as no phloem remains to send sugars to the roots.
(Though sometimes the roots will live long enough to put out new shoots through the soil).
| See also: Plant Biomechanics Plant Cells |