Tissue (ENGLISH POST)
Tissue Triphosa dubitataTissue
In biology, tissue is
a cellular organizational level intermediate between cells and a
complete organ. A tissue is an ensemble
of similar cells from the same origin that together carry out a specific
function. Organs are then formed by the functional grouping together of
multiple tissues.
The study of tissue is
known as histology or, in connection
with disease, histopathology. The classical tools for
studying tissues are the paraffin block in which tissue is
embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of
decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the
detail that can be observed in tissues. With these tools, the classical
appearances of tissues can be examined in health and disease, enabling
considerable refinement of medical diagnosis and prognosis.
Contents
[hide]
Animal tissues
Animal tissues are
grouped into four basic types: connective, muscle, nervous, and epithelial. Collections of tissues
joined in structural units to serve a common function compose organs. While all
animals can generally be considered to contain the four tissue types, the
manifestation of these tissues can differ depending on the type of organism.
For example, the origin of the cells comprising a particular tissue type may
differ developmentally for different classifications of animals.
The epithelium in all birds and
animals is derived from the ectoderm and endoderm with a small
contribution from the mesoderm, forming the endothelium, a specialized type of
epithelium that composes the vasculature. By contrast, a
true epithelial tissue is present only in
a single layer of cells held together via occluding junctions called tight junctions, to create a selectively
permeable barrier. This tissue covers all organismal surfaces that come in
contact with the external environment such as the skin, the airways, and the
digestive tract. It serves functions of protection, secretion, and absorption,
and is separated from other tissues below by a basal lamina.
Connective
tissue[]
Connective tissues are
fibrous tissues. They are made up of cells separated by non-living material,
which is called an extracellular matrix. This matrix can be
liquid or rigid. For example, blood contains plasma as its matrix and bone's
matrix is rigid. Connective tissue gives shape to organs and holds them in
place. Blood, bone, tendon, ligament, adipose and areolar tissues are examples
of connective tissues. One method of classifying connective tissues is to
divide them into three types: fibrous connective tissue, skeletal connective
tissue, and fluid connective tissue.
Muscle
tissue
Muscle cells form the active
contractile tissue of the body known as muscle tissue or muscular tissue.
Muscle tissue functions to produce force and cause motion, either locomotion or movement within
internal organs. Muscle tissue is separated into three distinct categories:
visceral or smooth
muscle,
found in the inner linings of organs; skeletal muscle, typically attached to
bones and which generates gross movement; and cardiac muscle, found in the heart where it contracts
to pump blood throughout an organism.
Nervous
tissue
Cells comprising
the central nervous system and peripheral nervous system are classified as
nervous (or neural) tissue. In the central nervous system, neural tissues form
the brain and spinal cord. In the peripheral
nervous system, neural tissues forms the cranial nerves and spinal nerves, inclusive of the motor neurons.
Epithelial
tissue
The epithelial tissues
are formed by cells that cover the organ surfaces such as the surface of
the skin, the airways, the reproductive tract, and the inner lining of
the digestive
tract.
The cells comprising an epithelial layer are linked via semi-permeable, tight junctions; hence, this tissue
provides a barrier between the external environment and the organ it covers. In
addition to this protective function, epithelial tissue may also be specialized
to function in secretion, excretion and absorption. Epithelial tissue helps
to protect organs from microorganisms, injury, and fluid loss.
Functions of epithelial
tissue:
·
the cells of the body's surface form the
outer layer of skin.
·
inside the body, epithelial cells form the
lining of the mouth and alimentary canal and protect these organs.
·
epithelial tissues help in absorption of
water and nutrients.
·
epithelial tissues help in elimination of
waste.
There are many kinds of
epithelium, and nomenclature is somewhat variable. Most classification schemes
combine a description of the cell-shape in the upper layer of the epithelium
with a word denoting the number of layers: either simple (one layer of cells)
or stratified (multiple layers of cells). However, other cellular features,
such as cilia may also be described in the classification system. Some common
kinds of epithelium are listed below:
·
Simple squamous epithelium
·
Stratified squamous epithelium
·
Simple cuboidal epithelium
·
Transitional epithelium
·
Pseudostratified columnar epithelium (also
known as Ciliated columnar epithelium)
·
Columnar epithelium
·
Glandular epithelium
·
Ciliated columnar epithelium
Mineralized
tissue
Plant tissues
Cross-section of a flax plant stem with
several layers of different tissue types:
1. Pith,
2. Protoxylem,
3. Xylem I,
4. Phloem I,
5. Sclerenchyma (bast fibre),
6. Cortex,
7. Epidermis
1. Pith,
2. Protoxylem,
3. Xylem I,
4. Phloem I,
5. Sclerenchyma (bast fibre),
6. Cortex,
7. Epidermis
Plant tissues are
categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue.
·
Vascular tissue - The primary
components of vascular tissue are the xylem and phloem. These transport fluid
and nutrients internally.
·
Ground tissue - Ground tissue is
less differentiated than other tissues.
Ground tissue manufactures nutrients by photosynthesis and stores reserve
nutrients.
Plant tissues can also be
divided differently into two types:
1. Meristematic tissues
2. Permanent tissues.
Meristematic
tissues
Meristematic tissue consists of
actively dividing cells, and leads to increase in length and thickness of the
plant. The primary growth of a plant occurs only in certain, specific regions, such
as in the tips of stems or roots. It is in these regions that meristematic
tissue is present. Cells in these tissues are roughly spherical or polyhedral,
to rectangular in shape, and have thin cell walls. New cells produced by
meristem are initially those of meristem itself, but as the new cells grow and
mature, their characteristics slowly change and they become differentiated as
components of the region of occurrence of meristimatic tissues, they are
classified as:
·
Apical Meristem - It is present at the
growing tips of stems and roots and increases the length of the stem and root.
They form growing parts at the apices of roots and stems and are responsible
for increase in length, also called primary growth. This meristem is
responsible for the linear growth of an organ.
·
Lateral Meristem - This meristem
consist of cells which mainly divide in one plane and cause the organ to
increase in diameter and growth. Lateral meristem usually occurs beneath the
bark of the tree in the form of Cork Cambium and in vascular bundles of dicots
in the form of vascular cambium. The activity of this
cambium results in the formation of secondary growth.
·
Intercalary Meristem - This meristem is
located in between permanent tissues. It is usually present at the base of
node, inter node and on leaf base. They are responsible for growth in length of
the plant and increasing the size of the internode, They result in branch
formation and growth.
The cells of meristematic
tissues are similar in structure and have thin and elastic primary cell wall
made up of cellulose. They are compactly
arranged without inter-cellular spaces between them. Each cell contains a
dense cytoplasm and a
prominent nucleus. Dense protoplasm of meristematic
cells contains very few vacuoles. Normally the meristematic cells are
oval, polygonal or rectangular in
shape.
Meristemetic tissue cells
have a large nucleus with small or no vacuoles, they have no inter cellular
spaces.
Permanent
tissues
The materialistically
that take up a specific role lose the ability to divide. This process of taking
up a permanent shape, size and a function is called cellular differentiation. Cells of meristematic
tissue differentiate to form different types of permanent tissue. There are 3
types of permanent tissues:
1. simple permanent tissues
2. complex permanent tissues
3. special or secretory
tissues (glandular).
Simple
tissues
A group of cells which
are similar in origin; similar in structure and similar in function are called
simple permanent tissue. They are of four types:
Parenchyma
Parenchyma (para -
'beside'; chyma - 'in filling, loose, unpacked') is the bulk
of a substance. In plants, it consists of relatively unspecialised living cells
with thin cell walls that are usually loosely packed so that intercellular
spaces found between cells of this tissue. This tissue provides support to
plants and also stores food. In some situations, a parenchyma contains
chlorophyll and performs photosynthesis, in which case it is called a
chlorenchyma. In aquatic plants, large air cavities are present in parenchyma
to give support to them to float on water. Such a parenchyma type is called
aerenchyma. shape:Each individual parenchymatous cell may be
isodiametric,spherical,oval,cylindrical,rectangular,stellate or long spindle
like structure.
Collenchyma
Cross section of
collenchyma cells
Collenchyma is Greek word where
"Collen" means gum and "chyma" means infusion. It is a
living tissue of primary body like Parenchyma. Cells are thin-walled
but possess thickening of cellulose, water and pectin substances (pectocellulose) at the corners where
number of cells join together. This tissue gives a tensile strength to the
plant and the cells are compactly arranged and have very little inter-cellular
spaces. It occurs chiefly in hypodermis of stems and
leaves. It is absent in monocots and in roots.
Collenchymatous tissue
acts as a supporting tissue in stems of young plants. It provides mechanical
support, elasticity, and tensile strength to the plant body. It helps in
manufacturing sugar and storing it as starch. It is present in the margin of
leaves and resist tearing effect of the wind.
Sclerenchyma
Sclerenchyma is Greek word where
"Sclrenes" means hard and chyma" means infusion. This tissue
consists of thick-walled, dead cells. These cells have hard and extremely thick
secondary walls due to uniform distribution of lignin. Lignin deposition is so
thick that the cell walls become strong, rigid and impermeable to water.
Epidermis
The epidermis is the
outer (epi in Greek meaning "over" or "upon") of the two
layers that make up the skin (or cutis), the inner layer being the dermis.[1]
It provides a barrier to infection from environmental pathogens[2] and
regulates the amount of water released from the body into the atmosphere
through transepidermal water loss (TEWL).[3] The outermost part of the
epidermis is composed of a stratified layer of flattened cells,[4] that overly
a basal layer (stratum basale) composed of columnar cells arranged perpendicularly.
In humans, it is thinnest
on the eyelids at 0.05 mm (0.0020 in) and thickest on the palms and soles at
1.5 mm (0.059 in).[5] It is ectodermal in origin.
Complex
permanent tissue
The complex tissue
consists of more than one type of cells which work together as a unit. Complex
tissues help in the transportation of organic material, water and minerals up and
down the plants. That is why it is also known as conducting and vascular
tissue. The common types of complex permanent tissue are:
·
Xylem or wood
·
Phloem or bast.
Xylem and phloem together
form vascular bundles.
Xylem
Xylem consists of:
·
Tracheid
·
Vessel members
·
Xylem fibers
·
Xylem parenchyma
Xylem serves as a chief
conducting tissue of vascular plants. It is responsible for
the conduction of water and mineral ions/salt.
Cross section of 2 year
old Tilia Americana, highlighting xylem ray shape and orientation
Xylem tissue is organized
in a tube-like fashion along the main axes of stems and roots. It consists of a
combination of parenchyma cells, fibers, vessels, tracheids, and ray cells.
Longer tubes made up of individual cells are vessels (tracheae), while vessel members
are open at each end. Internally, there may be bars of wall material extending
across the open space. These cells are joined end to end to form long tubes.
Vessel members and tracheids are dead at maturity. Tracheids have thick
secondary cell walls and are tapered at the ends. They do not have end openings
such as the vessels. The tracheids ends overlap with each other, with pairs of
pits present. The pit pairs allow water to pass from cell to cell.
Though most conduction in
xylem tissue is vertical, lateral conduction along the diameter of a stem is
facilitated via rays. [1] Rays are horizontal
rows of long-living parenchyma cells that arise out of the vascular cambium. In
trees and other woody plants, rays radiate out from the center of stems and
roots, and appear like spokes on a wheel in cross section. Rays, unlike vessel
members and tracheids, are alive at functional maturity. [2]
Phloem
Phloem consists of:
·
Phloem fiber
·
Phloem parenchyma.
Phloem is an equally
important plant tissue as it also is part of the 'plumbing system' of a plant.
Primarily, phloem carries dissolved food substances throughout the plant. This
conduction system is composed of sieve-tube member and companion cells, that
are without secondary walls. The parent cells of the vascular cambium produce
both xylem and phloem. This usually also includes fibers, parenchyma and ray
cells. Sieve tubes are formed from sieve-tube members laid end to end. The end
walls, unlike vessel members in xylem, do not have openings. The end walls,
however, are full of small pores where cytoplasm extends from cell to cell.
These porous connections are called sieve plates. In spite of the fact that
their cytoplasm is actively involved in the conduction of food materials,
sieve-tube members do not have nuclei at maturity. It is the companion cells
that are nestled between sieve-tube members that function in some manner
bringing about the conduction of food. Sieve-tube members that are alive
contain a polymer called callose, a carbohydrate polymer, forming the callus
pad/callus, the colourless substance that covers the sieve plate. Callose stays
in solution as long as the cell contents are under pressure. Phloem transports
food and materials in plants upwards and downwards as required.
This
article is about biological tissue. For the moth, see Triphosa dubitata. For other uses,
see Tissue.
In biology, tissue is
a cellular organizational level intermediate between cells and a
complete organ. A tissue is an ensemble
of similar cells from the same origin that together carry out a specific
function. Organs are then formed by the functional grouping together of
multiple tissues.
The study of tissue is
known as histology or, in connection
with disease, histopathology. The classical tools for
studying tissues are the paraffin block in which tissue is
embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of
decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the
detail that can be observed in tissues. With these tools, the classical appearances
of tissues can be examined in health and disease, enabling considerable
refinement of medical diagnosis and prognosis.
Contents
[hide]
Animal tissues
Animal tissues are
grouped into four basic types: connective, muscle, nervous, and epithelial. Collections of tissues
joined in structural units to serve a common function compose organs. While all
animals can generally be considered to contain the four tissue types, the
manifestation of these tissues can differ depending on the type of organism.
For example, the origin of the cells comprising a particular tissue type may
differ developmentally for different classifications of animals.
The epithelium in all birds and
animals is derived from the ectoderm and endoderm with a small
contribution from the mesoderm, forming the endothelium, a specialized type of
epithelium that composes the vasculature. By contrast, a
true epithelial tissue is present only in
a single layer of cells held together via occluding junctions called tight junctions, to create a selectively
permeable barrier. This tissue covers all organismal surfaces that come in
contact with the external environment such as the skin, the airways, and the
digestive tract. It serves functions of protection, secretion, and absorption,
and is separated from other tissues below by a basal lamina.
Connective
tissue
Connective tissues are
fibrous tissues. They are made up of cells separated by non-living material,
which is called an extracellular matrix. This matrix can be
liquid or rigid. For example, blood contains plasma as its matrix and bone's
matrix is rigid. Connective tissue gives shape to organs and holds them in
place. Blood, bone, tendon, ligament, adipose and areolar tissues are examples
of connective tissues. One method of classifying connective tissues is to
divide them into three types: fibrous connective tissue, skeletal connective
tissue, and fluid connective tissue.
Muscle
tissue
Muscle cells form the active
contractile tissue of the body known as muscle tissue or muscular tissue.
Muscle tissue functions to produce force and cause motion, either locomotion or movement within
internal organs. Muscle tissue is separated into three distinct categories:
visceral or smooth
muscle,
found in the inner linings of organs; skeletal muscle, typically attached to
bones and which generates gross movement; and cardiac muscle, found in the heart where it contracts
to pump blood throughout an organism.
Nervous
tissue
Cells comprising
the central nervous system and peripheral nervous system are classified as
nervous (or neural) tissue. In the central nervous system, neural tissues form
the brain and spinal cord. In the peripheral
nervous system, neural tissues forms the cranial nerves and spinal nerves, inclusive of the motor neurons.
Epithelial
tissue
The epithelial tissues
are formed by cells that cover the organ surfaces such as the surface of
the skin, the airways, the reproductive tract, and the inner lining of
the digestive
tract.
The cells comprising an epithelial layer are linked via semi-permeable, tight junctions; hence, this tissue
provides a barrier between the external environment and the organ it covers. In
addition to this protective function, epithelial tissue may also be specialized
to function in secretion, excretion and absorption. Epithelial tissue helps
to protect organs from microorganisms, injury, and fluid loss.
Functions of epithelial
tissue:
·
the cells of the body's surface form the
outer layer of skin.
·
inside the body, epithelial cells form the
lining of the mouth and alimentary canal and protect these organs.
·
epithelial tissues help in absorption of
water and nutrients.
·
epithelial tissues help in elimination of
waste.
There are many kinds of
epithelium, and nomenclature is somewhat variable. Most classification schemes
combine a description of the cell-shape in the upper layer of the epithelium
with a word denoting the number of layers: either simple (one layer of cells)
or stratified (multiple layers of cells). However, other cellular features,
such as cilia may also be described in the classification system. Some common
kinds of epithelium are listed below:
·
Simple squamous epithelium
·
Stratified squamous epithelium
·
Simple cuboidal epithelium
·
Transitional epithelium
·
Pseudostratified columnar epithelium (also
known as Ciliated columnar epithelium)
·
Columnar epithelium
·
Glandular epithelium
·
Ciliated columnar epithelium
Mineralized
tissue
Plant tissues
Cross-section of a flax plant stem with
several layers of different tissue types:
1. Pith,
2. Protoxylem,
3. Xylem I,
4. Phloem I,
5. Sclerenchyma (bast fibre),
6. Cortex,
7. Epidermis
1. Pith,
2. Protoxylem,
3. Xylem I,
4. Phloem I,
5. Sclerenchyma (bast fibre),
6. Cortex,
7. Epidermis
Plant tissues are
categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue.
·
Vascular tissue - The primary
components of vascular tissue are the xylem and phloem. These transport fluid
and nutrients internally.
·
Ground tissue - Ground tissue is
less differentiated than other tissues.
Ground tissue manufactures nutrients by photosynthesis and stores reserve
nutrients.
Plant tissues can also be
divided differently into two types:
1. Meristematic tissues
2. Permanent tissues.
Meristematic
tissues
Meristematic tissue consists of
actively dividing cells, and leads to increase in length and thickness of the
plant. The primary growth of a plant occurs only in certain, specific regions, such
as in the tips of stems or roots. It is in these regions that meristematic
tissue is present. Cells in these tissues are roughly spherical or polyhedral,
to rectangular in shape, and have thin cell walls. New cells produced by
meristem are initially those of meristem itself, but as the new cells grow and
mature, their characteristics slowly change and they become differentiated as
components of the region of occurrence of meristimatic tissues, they are
classified as:
·
Apical Meristem - It is present at the
growing tips of stems and roots and increases the length of the stem and root.
They form growing parts at the apices of roots and stems and are responsible
for increase in length, also called primary growth. This meristem is
responsible for the linear growth of an organ.
·
Lateral Meristem - This meristem
consist of cells which mainly divide in one plane and cause the organ to
increase in diameter and growth. Lateral meristem usually occurs beneath the
bark of the tree in the form of Cork Cambium and in vascular bundles of dicots
in the form of vascular cambium. The activity of this
cambium results in the formation of secondary growth.
·
Intercalary Meristem - This meristem is
located in between permanent tissues. It is usually present at the base of
node, inter node and on leaf base. They are responsible for growth in length of
the plant and increasing the size of the internode, They result in branch
formation and growth.
The cells of meristematic
tissues are similar in structure and have thin and elastic primary cell wall
made up of cellulose. They are compactly
arranged without inter-cellular spaces between them. Each cell contains a
dense cytoplasm and a
prominent nucleus. Dense protoplasm of meristematic
cells contains very few vacuoles. Normally the meristematic cells are
oval, polygonal or rectangular in
shape.
Meristemetic tissue cells
have a large nucleus with small or no vacuoles, they have no inter cellular
spaces.
Permanent
tissues
The meristematic tissues
that take up a specific role lose the ability to divide. This process of taking
up a permanent shape, size and a function is called cellular differentiation. Cells of meristematic
tissue differentiate to form different types of permanent tissue. There are 3
types of permanent tissues:
1. simple permanent tissues
2. complex permanent tissues
3. special or secretory
tissues (glandular).
Simple
tissues
A group of cells which
are similar in origin; similar in structure and similar in function are called
simple permanent tissue. They are of four types:
Parenchyma
Parenchyma (para -
'beside'; chyma - 'in filling, loose, unpacked') is the bulk
of a substance. In plants, it consists of relatively unspecialised living cells
with thin cell walls that are usually loosely packed so that intercellular
spaces found between cells of this tissue. This tissue provides support to
plants and also stores food. In some situations, a parenchyma contains
chlorophyll and performs photosynthesis, in which case it is called a
chlorenchyma. In aquatic plants, large air cavities are present in parenchyma
to give support to them to float on water. Such a parenchyma type is called
aerenchyma. shape:Each individual parenchymatous cell may be
isodiametric,spherical,oval,cylindrical,rectangular,stellate or long spindle
like structure.
Collenchyma
Cross section of
collenchyma cells
Collenchyma is Greek word where
"Collen" means gum and "chyma" means infusion. It is a
living tissue of primary body like Parenchyma. Cells are thin-walled
but possess thickening of cellulose, water and pectin substances (pectocellulose) at the corners where
number of cells join together. This tissue gives a tensile strength to the
plant and the cells are compactly arranged and have very little inter-cellular
spaces. It occurs chiefly in hypodermis of stems and
leaves. It is absent in monocots and in roots.
Collenchymatous tissue
acts as a supporting tissue in stems of young plants. It provides mechanical
support, elasticity, and tensile strength to the plant body. It helps in
manufacturing sugar and storing it as starch. It is present in the margin of
leaves and resist tearing effect of the wind.
Sclerenchyma
Sclerenchyma is Greek word where
"Sclrenes" means hard and chyma" means infusion. This tissue
consists of thick-walled, dead cells. These cells have hard and extremely thick
secondary walls due to uniform distribution of lignin. Lignin deposition is so
thick that the cell walls become strong, rigid and impermeable to water.
Epidermis
The epidermis is the
outer (epi in Greek meaning "over" or "upon") of the two
layers that make up the skin (or cutis), the inner layer being the dermis.[1]
It provides a barrier to infection from environmental pathogens[2] and
regulates the amount of water released from the body into the atmosphere
through transepidermal water loss (TEWL).[3] The outermost part of the
epidermis is composed of a stratified layer of flattened cells,[4] that overly
a basal layer (stratum basale) composed of columnar cells arranged perpendicularly.
In humans, it is thinnest
on the eyelids at 0.05 mm (0.0020 in) and thickest on the palms and soles at
1.5 mm (0.059 in).[5] It is ectodermal in origin.
Complex
permanent tissue
The complex tissue
consists of more than one type of cells which work together as a unit. Complex
tissues help in the transportation of organic material, water and minerals up and
down the plants. That is why it is also known as conducting and vascular
tissue. The common types of complex permanent tissue are:
·
Xylem or wood
·
Phloem or bast.
Xylem and phloem together
form vascular bundles.
Xylem consists of:
·
Tracheid
·
Vessel members
·
Xylem fibers
·
Xylem parenchyma
Xylem serves as a chief
conducting tissue of vascular plants. It is responsible for
the conduction of water and mineral ions/salt.
Cross section of 2 year
old Tilia Americana, highlighting xylem ray shape and orientation
Xylem tissue is organized
in a tube-like fashion along the main axes of stems and roots. It consists of a
combination of parenchyma cells, fibers, vessels, tracheids, and ray cells.
Longer tubes made up of individual cells are vessels (tracheae), while vessel members
are open at each end. Internally, there may be bars of wall material extending
across the open space. These cells are joined end to end to form long tubes.
Vessel members and tracheids are dead at maturity. Tracheids have thick
secondary cell walls and are tapered at the ends. They do not have end openings
such as the vessels. The tracheids ends overlap with each other, with pairs of
pits present. The pit pairs allow water to pass from cell to cell.
Though most conduction in
xylem tissue is vertical, lateral conduction along the diameter of a stem is
facilitated via rays. [1] Rays are horizontal
rows of long-living parenchyma cells that arise out of the vascular cambium. In
trees and other woody plants, rays radiate out from the center of stems and
roots, and appear like spokes on a wheel in cross section. Rays, unlike vessel
members and tracheids, are alive at functional maturity. [2]
Phloem
Phloem consists of:
·
Phloem fiber
·
Phloem parenchyma.
Phloem is an equally
important plant tissue as it also is part of the 'plumbing system' of a plant.
Primarily, phloem carries dissolved food substances throughout the plant. This
conduction system is composed of sieve-tube member and companion cells, that
are without secondary walls. The parent cells of the vascular cambium produce
both xylem and phloem. This usually also includes fibers, parenchyma and ray
cells. Sieve tubes are formed from sieve-tube members laid end to end. The end
walls, unlike vessel members in xylem, do not have openings. The end walls,
however, are full of small pores where cytoplasm extends from cell to cell.
These porous connections are called sieve plates. In spite of the fact that
their cytoplasm is actively involved in the conduction of food materials,
sieve-tube members do not have nuclei at maturity. It is the companion cells
that are nestled between sieve-tube members that function in some manner
bringing about the conduction of food. Sieve-tube members that are alive
contain a polymer called callose, a carbohydrate polymer, forming the callus
pad/callus, the colourless substance that covers the sieve plate. Callose stays
in solution as long as the cell contents are under pressure. Phloem transports
food and materials in plants upwards and downwards as required.
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