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 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). In other words,
when the parenchyma cells are full of water and turgid, the stiffness of the tissue increases. (Note that
the name 'elastic modulus' is unfortunate as it actually measures stiffness). The elastic modulus is
measured in N per metre-squared. Measurements in meristem parenchyma give E = 19-40 MNm^-2. E
also increases when larger samples of tissue are measured, however. Parenchyma is also less
compressible at higher strain rates. Strain rate is the rate at which the tissue deforms under stress, so
if it stretched or compressed quickly, then it appears stiffer.

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) is a measure of the tendency for material to spread out sideways when
compressed (or conversely to narrow when stretched like an elastic band). For a perfectly
incompressible material, v = 0.5 and failure occurs by shearing at 45 degrees to the axis of tension.
For parenchyma lies in the range 0.23-0.5, for example for apple flesh it has the range 0.21-0.34.
Thus, parenchyma is slightly compressible. In comparison, rubber has a poisson ratio close to 0.5 and
for cork it is close to zero, meaning that cork expands sideways very little when compressed.
Mechanical failure of parenchyma occurs by cell rupture or by failure of cell-cell bonding in softer
tissues. Plant cells are cemented or glued together by the
middle lamella - a layer of pectin-rich
biological glue between adjacent cell walls.
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, found in the middle layer of leaf
blades, 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 only 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 (water-loving
plants) 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) and especially crack willow (Salix fragilis) 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 0.001. It is tissue with
many large extracellular air spaces to allow ample gas-exchange in plants that grow in or near water
and may thus be submerged. Without aerenchyma these plants would suffocate and drown. The
elastic modulus of aerenchyma has been measured at 2.26 MN m^-2 for
Juncus effusus. Compare this
to   19^-40 MN m^-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. As an example, the
willow root contains aerenchyma.

In addition to its mechanical functions, parenchyma cells adopt a variety of other functions. Any
parenchyma cells may take on the role of food reserve storage, often in the form of
starch grains
typically stored in modified chlorophyll-lacking 'chloroplasts' called
amyloplasts.
Plant Cell and Tissue Types
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).
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 / elements
that join end-to-end to form vessels, and elongated narrower fibrous
tracheids. The tracheary elements have no living protoplasts when mature, 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.


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. In a living tree, the outer layers of sap-wood are of course filled with
xylem sap being transported from the roots to the rest of the plant. Heart wood may also become
occluded by various toxic and waste materials dumped into it by the parenchyma rays. How much
wood in the living plant is air-filled is difficult to say. Xylem vessels eventually cease conducting as
they cavitate and fill with bubbles of air. In many plants these defunct vessels are filled by resins,
gums or tyloses. Conifers have
valves (bordered pits) which close to seal off the vessel and may fill
the vessel, at least partially, with
resins. Hardwood trees may fill defunct vessels with gums, however,
trees with wider vessels tend to plug them with
tyloses. Tyloses are balloon-like outgrowths of
neighbouring parenchyma cells which penetrate and block the vessel lumen. These mechanisms
prevent air spreading throughout the vascular system and also help reduce the spread of infection.
The Red Oak,
Quercus Rubra, does not block its vessels and it is possible to blow air through a piece
of red oakwood.

Xylem vessels, whether primary or secondary, have
primary cellulose walls, as do all plant cells, but
secondary walls containing lignin are deposited within these whilst the protoplasm is still present.
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 the elastic modulus. Lignin in the middle lamella
increases bonding strength. The middle lamella has a typical shearing modulus of G of about 77 GN
m^-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. Companion and transfer cells are often seen as modified parenchyma cells, but their
functions are so specialised that they are probably best considered as distinct cell types. 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).
secondary xylem in Tilia stem
secondary xylem in Tilia stem
helianthus stem primary xylem
tracheids in pine
tracheids in pine
Helianthus (sunflower) stem pith
sunflower stem pith
iris root parenchyma
Iris root pith parenchyma
Helianthus stem pith parenchyma
Ray parenchyma in woody stem of Tilia
(L.S.)
pinus root xylem
secondary xylem in Tilia stem
Above: Primary xylem vessels in a (non-woody) stem of Helianthus (sunflower) in longitudinal section (L.S.).  Note the
spiral thickenings which prevent the xylem vessels from collapsing under negative pressure, but allow the xylem in young
and growing organs, such as these, to stretch as the organ grows and also to bend and flex. In more mature organs,
these annular or spiral thickenings may disappear as the whole xylem wall becomes thickened.

In non-woody stems the xylem (along with the phloem) forms vascular bundles. In woody stems, the xylem is the wood.
Xylem in the stem is continuous with that in the cores of roots and xylem traces extend into leaves and floral appendages.
Above: secondary xylem in a woody stem of a sapling of the lime-tree (Tilia). The vessels (stained red) are large and
thick-walled and make up the wood of the sapling, along with the plates of parenchyma cells (stained green) which form
radial rays extending from the inner bark toward the centre of the tree (like the spokes of a wheel).
primary xylem in helianthus stem
primary xylem in helianthus stem
Above: left and centre - secondary xylem in the Scots pine (Pinus sylvestris), consisting of tracheids (stained red) with
bordered pits (see ) and parenchyma rays (stained green) as seen in L.S. Right - secondary xylem in an old woody root
of
Pinus.
Click photomicrographs to view full sized images.
sclerenchyma in Tilia
Tilia phloem
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 MN m^-2 and a breaking stress, measured in celery, of 23.3 MN m^-2 (cf. 4.1
MN m^-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.
starch in parenchyma of Tilia stem pith
Above: Storage parenchyma - left: starch storing parenchyma in the pith of a Tilia sapling stem; the
starch is stained black. Adjacent to the outer cylinder of phloem on the right of the image is a single
layer of starch-laden parenchyma cells. Centre - starch grains in parenchyma cells of iris rhizome.
Throughout the tissue were scattered extracellular crystals with the appearance of silica (though they
may not be silica as other materials can resemble this). Silica deposits in plant tissues are not
uncommon and such crystals are called phytoliths. In
Arum maculatum (wild arum) needle-like crystals
of saponin oxalates are abundant and these account for much of the toxicity of these plants as they
are extremely irritating. Right - starch grains in parenchyma of
Ranunculus (buttercup) root.

Parenchyma is also important in
transport - transporting water and other materials across the plant,
either for local transport across short distances within an organ, or for slow transport over larger
distances. Examples include water transport across the root cortex (see transport in plants) and the
rays of wood.
Phloem also functions as the nervous system of the plant by conducting electrochemical action
potentials, which travel at much slower velocities than are typical for animal nervous systems,
however. There seems to be some uncertainty as to whether the companion cells or the sieve tube
elements are the site of action potential propagation.