Transcript TOPIC_6_TISSUES_OF_THE_PLANT_BODY

Review of the typical Plant Cell
Major cells of the plant body
Parenchyma cells
Collenchyma cells
Sclerenchyma cells
◦ Fibers
◦ Sclereids
Water-conducting cells of the Xylem
◦ Tracheids
◦ Vessel elements
Food-conducting cells of the Phloem
◦ Sieve-tube members
◦ Companion cells
Cells and tissues
All organs are composed of similar structural units (cells and
tissues)
Development of tissues of the primary plant
body
Tissues  organizations of cells of one or more types that
have a common origin and common collective function
Differentiation  process of development of cell types and
tissues
Tissues of the plant body
The primary plant body is made up of four tissue types.
Tissue type
Functional roles
Meristem tissue division of new cells for new growth or
repair
Ground tissue
storage, processing, physical
support
Dermal tissue
protection, nutrient absorption
Vascular tissue
movement of fluids/food and physical
support
Meristem cells
Meristem cells are those that retain the ability to divide
for the entire life of a plant..
Division of meristem cells results in plant growth.
Cells of the meristem remain embryonic.
New cells are formed by cell division.
One of the daughter cells (initial) retains meristematic
activity, the other (derivative) differentiates
Meristem tissues
Meristem tissues are responsible for cell division
(multiplication). They are zones of actively dividing cells.
Growth in plants includes two stages:
• production of new cells
• expansion of cells via uptake of water by vacuoles
(differentiation)
Meristem regions
Apical meristems: These are located at root and shoot tips,
and are directly involved in elongation. They create
derivatives which form primary growth.
Apical meristems are of three types:
Protoderm: forms outer dermal layer of tissues
Ground meristem: forms cortical cells
Procambium: forms vascular tissue
Derivatives of primary meristems
Lateral meristems
a. Vascular meristem- internal
growth in girth which involves
secondary tissues (xylem and
phloem)
In the fasicular region,
cambial cells divide toward
the center to form xylem
tissue, and towards the
outside to form phloem
tissue.
Interfasicular indicates the
cambium between the
'fasciles of xylem & phloem.
b. Cork cambium- Cells form
the characteristic corky layer
as well as an internal layer.
Fasicular and interfasicular cambium
Intercalary meristem
Intercalary meristem occurs between mature tissues
(sections in the vicinity of nodes or leaf attachment):
These are common in grasses (occur at bases of nodes).
They are involved in regeneration of parts removed by
herbivores, lawnmowers, etc.)
Ground Tissue
Ground tissue is simple,
non-meristematic tissue,
made up of 3 cell types:
parenchyma,
collenchyma and
sclerenchyma.
This tissue generally
forms either the pith or
cortex (bulk of leaf) –
the mesophyll.
Ground tissues
Ground meristem
• Comprise the greater portion of the meristematic tissue
• Consists of large, thin-walled, isodiametric cells
Found in the following regions
Pith: in the very centre of the stem
Cortex: in a cylinder just beneath the epidermis and surrounding the
vascular tissue
Cortex
• Complex region
• Forms beneath the epidermis as a cylindrical zone that extends inwards
to the primary phloem
Parenchyma cells
These are the most abundant cells in the plant body.
They are spherical cells which flatten at point of contact.
They are alive at maturity.
They have large vacuoles for storage of starch, fats, and
tannins.
They form primary sites of metabolic functions
(photosynthesis, respiration, protein synthesis).
They constitute "ready reserves" from which a plant makes
specialized cells to meet its changing needs
Parenchyma cells
 Typical plant cell – most abundant in plants
 Thin walled (walls contain cellulose, not lignin)
 Unspecialized – can either photosynthesize, or store starch
◦ Can be found in leaves – contain chloroplasts and carry out photosynthesis
(mesophyll cells are an example)
◦ Can also be found in roots and other non-photosynthesizing parts, and store starch in
amyloplasts (related to chloroplasts) – in stems they are called the pith
amyloplasts
chloroplasts
Specialized parenchyma
Chlorenchyma- photosynthetic cells, have high density of
chloroplasts
Aerenchyma- prominent intercellular spaces that improve
gas exchange capacity; provide maximum support with
minimal metabolic requirement
Transfer cells- specialized for short distance transfer of
solutes between cells; have secondary cell walls. These
form inner extensions of walls that increase surface
area; occur in areas of high solute transport, such as
secretory tissues (e.g. nectar and mucilage cells)
Chlorenchyma- photosynthetic cells with high
density of chloroplasts
Collenchyma cells
Unevenly thickened primary cell
walls; elongate cells
Live protoplasm.
Cells are longer than wide. They are
located just beneath epidermis.
Function to support growing organs
(floral parts, border veins); their
non-lignified cell walls can stretch
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Collenchyma cells
Usually lie just under the
epidermis of leaves, stems and
roots
They are collectively called the
cortex
They Cells are columnar in
shape, lack lignin in their walls,
but have thicker walls than
parenchyma cells
They give younger plants or plant
parts support
Parenchyma
Collenchyma
Because they have thick walls
but lack lignin, they are able to
provide support without
restricting growth – hence found
Epidermis
in young, growing parts
Collenchyma: a summary
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Main supporting tissue of actively growing organs
Similar to parenchyma
Can resume meristematic activity
Occurs immediately beneath or near surface of young
stem/petioles
Lying beneath the epidermis
Elongated cells
May contain chloroplasts
Living at maturity
Thick, non-lignified primary walls
Cell walls thickened at the corners
Thickenings flexible and can stretch
Serves as a strengthening tissue in young expanding stems
and leaf petioles
Parenchyma vs. Collenchyma cells
Sclerenchyma cells
Sclerenchyma - ( Greek for skleos = hard)
- most are dead at maturity.
- rigid, thick, lignified, non-stretchable secondary cell walls.
There are 2 types of sclerenchyma cells:
• sclereids or stone cells: short; variable shape; form hard layers
such as the shells of nuts and seed coats; produce the gritty
texture of pears and guavas
• Fibers: long, slender; occur in strands or bundles; tiny cavity or
lumen; the different hardness fibers are used to make coarse
rope, linen, cloth
Sclerenchyma
Sclereids
Fibers
Three Tissue System
• The cell types we have so covered (parenchyma, collenchyma,
sclerenchyma, sieve tube cells, etc.) can be placed into 3 main
tissue categories or systems:
– Dermal tissue system
– Vascular tissue system
– Ground tissue system
Three Tissue System, cont’d.
Tissue System
and Its Functions
Dermal Tissue System
• protection
• curtailment of water loss
Ground Tissue System
• photosynthesis
• food storage
• regeneration
• support
• protection
Vascular Tissue System
• transport of water and
minerals
• transport of food
Component Tissues
Epidermis
Periderm (in older
stems and roots)
Parenchyma tissue
Collenchyma tissue
Sclerenchyma tissue
Xylem tissue
(Tracheids and vessel
elements)
Phloem tissue (Sievetube members and
Location of Tissue
Systems
Cambial tissue
Procambium develops into vascular tissue
Appear first as isolated groups of cells arranged in a circle among the
ground meristem cells
Differentiates into two primary vascular tissues phloem and xylem
Vascular tissues are primarily specialized for conducting materials
Also provide mechanical strength to organs
Phloem conducts foods that are synthesized by photosynthesis
Xylem conducts water and mineral salts
Primary vascular tissues
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Well-defined
2 complex tissues
Xylem and phloem which occur side by side
Differentiated from a procambium strands
The primary xylem
• The conducting cells that occur in primary xylem of vascular
plants are tracheids and vessel elements which conduct
water and mineral salts
• Associated with them may be fibres (xylem fibres) and
parenchyma (xylem parenchyma)
Sclereids & Fibres
Sclereids
• Variable in shape
• Isodiametric to irregular, branched
• Very thick, lignified walls
• Stone cells are a common type
• Branched, resembling very irregular stars
Fibres
• Long narrow cells/tapering or pointed ends
• Massed together in long strands
• Walls lignified or not lignified
• Thickened walls
Two types:: xylary fibres  occur in the xylem tissue
extraxylary  occur in any other tissue
Tracheids – long, slender spindle-shaped cells with
“pits” or breaks in the secondary cell wall. Water
flows vertically and horizontally through the pits.
Vessel elements – short, fat cells with perforations.
Form pipes through which water flows between roots
and shoots.
There are two types of pits in xylem cells: simple pits
and bordered pits
Simple pits occur in
fibres, sclereids and
in parenchyma cells
when they have
secondary walls.
Pits form opposite
each other in the
secondary walls of
adjacent cells
Tracheids are
spindle shaped
and have pits.
Longitudinal section
Vessel elements are
short and wide
and have perforations
as well as pits.
Longitudinal section
Tracheids and
vessel elements
together in
vascular tissue
Sieve-tube elements and companion cells of
the phloem
Sieve-tube elements and companion
cells of the Phloem
Phloem
Unlike xylem cells, phloem cells can contain protoplasts (either
complete or incomplete)
Two types:
– Sieve-tube members – chains of cells that conduct food (partial
protoplasts -lack nuclei and ribosomes)
– Companion cells – connected to sieve-tube cells, contain nuclei
and ribosomes, so help maintain sieve-tube cells; have a normal
protoplast with a full complement of organelles, these cells
possibly regulate the metabolic activity of the sieve-tube
members that have no nucleus.
– Plasmodesmata connect the protoplast of companion cells and
sieve-tube member
Vascular Bundles with xylem & phloem
Maize– vein in cross
section
Summary on primary tissues
• Primary meristematic tissues are protoderm,
ground meristem and procambium.
• The primary plant body is composed of
• Epidermis differentiated into epidermal, guard
cells and epidermal hairs;
• Cortex composed of collenchyma, sclerenchyma
(fibres and sclereids) and parenchyma;
• Vascular tissue  composed of xylem (fibres,
tracheids, vessel members and parenchyma) and
phloem (fibers, sieve-tube members, companion
cells, and parenchyma
Pith is composed largely of parenchyma and sometimes
accompanied by sclereids
pith rays, composed of parenchyma cells.
Parenchyma stores water and food, and conducts materials for
short distances.
Collenchyma, sclereids, and fibers are strengthening or
mechanical tissue elements.
Tracheids and vessels (series of vessel members) conduct
water and mineral salts.
Sieve tubes (series of sieve-tubes members) conduct
foods.
Collenchyma and sclerenchyma may occur in patches or
completely surround the stem just underneath the
epidermis.
Sclerenchyma is frequently associated with vascular
bundles.
Dermal tissue
Protoderm
Outermost layer of cells
• Develops into epidermis
Epidermis
• Single superficial layer of cells that covers and protects all
underlying primary tissues
• Prevents excessive water loss/drying out/mechanical injury
• Allows for exchange of gases in photosynthesis and respiration
• Surface layer usually coated with cutin
• Specialized epidermal cells include guard cells
• Two guard cells plus pore comprise one stoma (plural stomata)
• Extra epidermal structures include trichomes and glandular cells