Transcript REngl_-_Lab_2

Plant tissues
The vascular plant sporophyte is usually composed of three
kinds of vegetative organs:
1.
roots
2.
stems
3.
leaves
These organs are constructed by some types of plant tissues.
•
Plant tissue - is a group of cells that are structurally
and/or functionally distinct, and perform a common function.
Tissue systems
Meristematic tissues
I. According to their origin:
•primary
•secondary
•Primary meristems - set extension of the plants organs - apical
& intercalary meristems - determine the primary growth of plants
•Secondary meristems - determine the thickening – lateral
meristems - define the secondary growth
II. According to their position in the plant:
•Apical - situated at the growing tip of the stems & roots and form
shoot apex & root apex . In the leaves it forms the leaves apices.
•lateral - define the increasing of root and stem diameter :
vascular cambium (cambium ) & cork cambium (phellogen)
•Intercalary – define elongation of nodal regions
• Primary meristems - (germinal) - all the cells of the embryo are
meristems. The cells of primary meristems retain the ability to
divide throughout life and in the cases when plant organs are
damaged by predators, they resume the ability to divide and
differentiate into permanent tissues. Cell division and cell
elongation in the primary meristem are defined as primary growth
and its result is elongation of plant organs and their differentiation.
Importance - to reproduce asexually
- to regenerate damaged parts of the plant body
- a basic role in normal plant growth - give rise to
other types of permanent tissues
- formation of new organs and their correct
placement
Cell characteristics:
- spherical,oval,polygonal or rectangular in shape,
- compactly arranged & without any intercellular space
- with dense or abundant cytoplasm
Examples for primary meristems – protoderma,
procambium and ground meristem
1. Protoderm: lies around the outside of the stem and
develops into the epidermis
2. Procambium:
• lies inside of the protoderm and develops into
primary xylem and primary phloem.
• produces the vascular cambium and cork cambium secondary meristems.
3. Ground meristem: develops into the cortex and
the pith. Composed of parencyma, collenchyma and
sclerenchyma cells.
Relationships between apical meristems, primary meristems and
primary grouth (stem apex (tip)
Apical meristems in root apex
•Secondary meristems - determine the secondary growth
(thickening ) of the plant parts. The secondary growth is
characterized for Dicots and it is not found in Monocots.
Examples: lateral meristems vascular cambium and cork
cambium
Cork cambium (= bark cambium, pericambium and phellogen ):
•a tissue found in many vascular plants
•As a part of the periderm
•lateral meristem, responsible for secondary growth
•replaces epidermis in roots and stems
• found in woody and many herbaceous dicots , gymnosperms and some monocots
•acts in two directions: outside forms phellem (cork) – dead at maturity; inward
phelloderm - composed of living parenchyma cells
Result – in this way the stem and root gradually thicken towards the bark.
•outwards secondary phloem
•inwards secondary xylem
The vascular cambium (plural cambia) – acts in the central cylinder
Result – the stem and root gradually thicken towards the central cylinder.
Intercallary meristem - a type of primary meristem, exactly the
remnant of the apical meristem. It is common of the nodal
regions – especially at the base of the leaves. They occur only in
Monocots (particularly grass).
The main function :
• elongation of nodal
regions
• rapid growth and regrowth
of leaf and stems
(after damage due to
herbivores, heavy rains,
storms, and lawn mowers).
Permanent plant tissues – they consist of cells that lost
their ability to divide and perform certain functions – roof,
conductive, protective.
Permanent tissue types:
•Simple
•Complex
•Special (secretory)
Simple permanent tissues - A group of cells which are similar in
origin, in structure and in function
Simple permanent tissues – types:
• Parenchyma
• collenchyma
• Sclerenchyma
1. Parenchyma:
Parenchyma - the most common tissue which is unspecialized.
Living tissue, that fills the internal volume of the plant bodies and
placed between other types of tissues.
Parenchyma cells are spread everywhere in the plant body - in
epidermis, cortex, pericycle, pith etc. They are responsible for
photosynthesis, storage of food, secretion etc.
Due to the diverse functions it performs, parenchyma is divided into 4
types:
1. Assimilatory parenchyma - contains chlorophyll and performs
photosynthesis;
2. Storage parenchyma – for food storage
3. Aerenchyma – characterized by the aquatic plants, with large air cavities ,
for support to floating
4. Water storage parenchyma – cacti, stone crop, sedum etc. – with succulent
stems and leaves
Observation of parenchyma cells in apple (Malus domestica), carrot
(Daucus carota) , potato (Solanum tuberosum).
Collenchyma - cells are thin-walled, but the thickening of the cell
wall begins - mostly this is done at the corners where the cells join
together. In that case the pectine substances accumulate
between the celulose fibers. It is present in the leaf margin to
resist tearing effect of the wind.
Collenchyma functions:
• provides mechanical support, elasticity, and tensile strength to
the plant body – especially in the young plants with growth
opportunities
• arranges the cells in a compact mass, with a very little intercellular spaces
• helps in a starch storage
Observation of collenchyma cells in cucumber (Cucumis sativa)
Sclerenchyma - gives support to the plant and provides hardness
and protective covering to seed and nuts. Its consists of thickwalled, dead cells with cells walls, rich of lignin. Lignin
deposition is so thick that the cell walls become strong, rigid
and impermeable to water.
Main features:
- closely packed without inter-cellular spaces
Distribution: mainly occur in hypodermis, perycycle,
secondary xylem and phloem.
Types of sclerenchyma:
•
Sclereids - short cells with thick, lamellated, lignified walls,
gathered in groups. Form plum and cherry pits, the shells of
almond and walnut, placed in groups in the flesh of the pear &
quince, named stone cells.
•
Fibres - elongated cells with pointed ends. They are
placed in the stem of flax, hemp - textiles fibers.
Observation of sclereids in quince ( Cydonia oblonga)
Ground tissue system
Ground tissue system - includes the above mentioned three
types of simple tissues : parenchyma, collenchyma and
sclerenchyma.
Main function: fills all interior spaces in plant organs and store
organic products.
Complex permanent tissues
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.
Examples - xylem and phloem
•Xylem or wood
•Phloem or bast
Xylem and phloem are
elements of vascular
(conductive) tissue system
Xylem and phloem together
form vascular bundles.
•Xylem is responsible for transporting water and certain nutrients from the root
to the rest of the plant - in upward direction. The second function – to provide
mechanical support to the plant.
• Phloem carries soluble organic material, i.e. food for the plant, which is
produced in the leaves by photosynthesis to the other parts of the plant – in
downward direction.
Xylem elements
Xylem consists of non-living tissue at
maturity
• Tracheary elements: Tracheids &
vessels
• wood parenchyma – for storage and
secretion
• xylem fibres - for mechanical support
• libriform fibers – for mechanical
support
1.Tracheids - phylogenetically oldest
conductive vessels
- elongated cells with tapering ends
and lignified walls with pits. found in
Gymnosperms & Angiosperms
2. Tracheas - long, non-living tubes, parallel to the main axis of the stem, and built by
vessel members.
The oblique cell walls between the adjacent vessel members becomes partially or
completely perforated - this is the perforation plate. Found mainly in Angiosperms.
The vessel originates from the vascular cambium.
Phloem - made of living sieve - tube elements that lack a
nucleus, ribosomes, or vacuoles; their metabolic functions are
provided by companion cells.
Phloem elements:
•Sieve cell - similar to tracheids –
they are living, elongated cells
•Sieve tube - parallel to the
tracheas of the xylem (occurring
only in Angiosperms) and
consists of sieve-tube
elements. Their transverse walls
are oblique and perforated and
the sieve plate is formed. Sieve
plate is penetrated by pores .
•Companion cell
•Phloem fibers
Vascular bundels - In majority of plants vascular elements cluster into
bundles.
Bundels types:
•Simple bundles: xylem and phloem elements are located at separate bundles
- young roots.
• Compound bundles: contain both xylem and phloem
- Concentric: xylem and phloem form concentric rings in cross section
- ferns
- Collateral: bundles containing xylem and phloem adjoining each
other side by side, xylem facing to the centre of the organ, phloem facing
outwards.
- collateral closed bundle - procambium completely
differentiated into xylem or phloem, so no dividing cell remains in the bundle.
This bundle is incapable of secondary thickening (monocots).
- collateral open bundles – procambium differentiation is not
complete, thus a thin layer of dividing procambium remains between the xylem
and the phloem. These bundles may take part in secondary thickening (dicots).
- bicollateral bundles - phloem occurs on either side of the
xylem (Cucurbitaceae)
Cross section of the stem:
• Dicot - vascular bundles arranged in a ring. In plants with stems that live for more
than one year, the individual bundles grow together and produce the characteristic
growth rings.
• Monocots - the vascular bundles are randomly scattered throughout the ground tissue
Helianthus stem
Observation of different kinds of vascular bundlespermanent slides
Dermal tissue system - forms the outmost layer of a plant. It
includes both epidermal and peridermal tissues.
Function – prevents water loss and acts as a barrier to fungi
and other invaders.
Dermal tissue – types:
I. Primary dermal tussue – epidermis
II. Secondary dermal tissue – periderm
III.Terthiary dermal tissue – the bark
I. Primary dermal tissue - epidermis - thin layer of cells that
cover the surfaces of leaves, stems, and roots. In Monocots
cover all above ground parts of the plants, and in Dicots cover
only young branches and the leaves. Other function is
controlling water and gases exchange.
Origin – from protoderm,in woody plants - replaced by
periderm.
Epidermis – elements:
1. Pavement cells
2. Stomata
3. Trichomes
1.
Pavement cells - tightly linked to each other and provide
mechanical strength and protection
• Cuticle - a thin hydrophobic layer which covers the outermost surface of
epidermis cells of land plants – its functions are to reduce water loss, to protect
from UV radiation, as well as from bacterial and fungal pathogens.
The cuticle is synthesized by epidermal cells and its framework is
provided by cutin - insoluble compound of C atoms (16 -18), epoxy fatty acids
and glycerol.
2. Stomata - most differentiated cells of the epidermis,
regulates the exchange of gases and water vapors.
Stoma complex - a pore, guard cells (with chloroplast),
accessory cells (lack chloroplasts)
Situation - Typically - in dry land plants the stomata are
located on the lower epidermis
- floating leaves - most or all stomata are on the
upper surface
- Vertical leaves (grasses) - there are equal
numbers of stomata on both surfaces.
Guard cells - bounded the
stomata pore
Mechanism of stomata opening:
- in sunlight the sugars form - the
concentration of potassium ions (K+) increases
- this causes water potential lowering
in the guard cells
- water from other cells enters the
guard cells by osmosis - they swell and
become turgid
- the swollen guard cells become
curved and pull the stomata open.
- At night, the sugar is used up
- water leaves the guard cells - they
become flaccid and the stomatal pore closes.
Observation of epidermal cells and stomata in Tradescantia virginica
3. Trichomes (Plant hairs) - unicellular or multicellular
•Trichomes for absorbtion - cover the plants root and are called root hairs.
•Trichomes for protection – covered plant stems and leaves:
Papillae - on the petals of flowers. They give the petals a soft, velvety
appearance and prevent them from becoming wet.
Stinging hairs - The cell walls consist of calcite and silica, the cell itself is full
of complex mixture of formic acid, histamine and serotonine. In case of breakage of the
hairs tip the mixture is poured and causes swellings and flushings of the skin surface nettle
Emergences - the thorns on roses
•Trichomes for secretion - form some of the most important
glands found in the plant kingdom. The most common
glandular hairs are multicellular – a number of cells forming
a stalk and a head. The secreted substances are very
different – resins, gums, volatile oils, mucilage. Sometimes
the oils are scented and give plants their characteristic
fragrance, e.g. lavender.
•Trichomes for seed dispersal - cotton seed hairs, willow and
willow herb
water and gases
- replaces the epidermis during growth in thickness of
stems and roots of gymnosperms and dicots ( secondary growth).
- originates from the lateral meristem cork cambium
(phellogen)
- periderm - includes 3 layers - the phellem (cork),
phellogen (cork cambium) and phelloderm
III.Tertiary dermal tissue (rhytidome) – forms from periderm,
that arise at greater depths causing the accumulation of dead
tissue on the surface. Contains dead tissue layers and no longer
growing periderms. It is the outermost layer of the trees – the
bark.
* Lenticells - interruptions in the phellem through which gas
exchange occurs. They form at the places of stomata, and
produce a filling tissue, consisting of loosely arranged cells.
Observation of lenticells and periderm in one year old
stem of
Plant secretory tissue
Function – includes the secretion of gums, resins, volatile oils, nectar,
latex, and other substances in plants.
Divided into two groups:
1. Laticiferous tissues
2. Glandular tissues
The secreted substances – they are no further utilized by the plant (resins,
rubber, tannins, and various crystals)
- take part in the functions of the plant (enzymes and
hormones)
- may remain in the cell
- may be excreted to the surface of the plant or into
intercellular cavities or canals.
1. Lactiferous tissues - thin walled, greatly elongated and many-branched
ducts containing a milky or yellowish colored juice known as latex. Function act as food storage organs or as reservoir of waste products.
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Latex cells - also called "non-articulate latex ducts“. These ducts are
independent units which extend as branched structures for long distances in
the plant body, but do not fuse together. Thus a network is not formed as in
latex vessels.
•
Latex vessel - Also called "articulate latex ducts", these ducts or
vessels are the result of anastamosing of many cells together. They form a
complex network with anastamose branching. Latex vessels are commonly
found in many angiosperm families Papaveraceae,Compositae, Euphorbiaceae, Moraceae.
2. Glandular tissues - consist of special structures - the glands. These glands contain
some secretory or excretory products. A gland may consist of isolated cells or small
group cells with or without a central cavity.
They are of various kinds and may be internal or external.
- Internal glands are:
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Oil-gland secreting essential oils - in the fruits and leaves of orange, lemon.
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Mucilage secreting glands - in the betel leaf
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Glands secreting gum, resin, tannin, etc.
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Digestive glands secreting enzymes or digestive agents
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Special water secreting glands at the tip of veins
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- External glands are commonly short hairs tipped by glands. They are
water-secreting hairs or glands,
Glandular hairs secreting gum like substances as in tobacco, plumbago, etc.
Glandular hairs secreting irritating, poisonous substances,as in nettles
Honey glands, as in carnivorous plants.
Observation of Oil-gland secreting essential oils in orange
(Citrus X sinensis)
Glands and trichomes in tomato leaf
Glands in Drosera rotundifolia
Mint oil glands and trichomes