chapter 5 tissue
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Brief lecture notes of Chapter 5TRANSCRIPT
CHAPTER 5
TISSUE ORGANIZATION
5.1 ORGANIZATION OF THE ANIMAL
BODY
• Animals have same general body plan – “tube within a tube”:
– Digestive tract – tube running from mouth to anus.
– Tube suspended in coelom – internal body cavity.
– Coelom divided into two cavities by diaphragm:
• Thoracic cavity – heart and lungs.
• Abdominal cavity – stomach, intestine, and liver.
5.1.1 General Body Architecture
• Body supported by internal skeleton, endoskeleton - jointed bones that grow as body grows:
– Skull – surrounds brain
– Vertebrae, column consisting of separate bones – surrounds spinal cord.
Four levels of organization in vertebrate body:
Cells →→→→ tissues →→→→ organs →→→→ organ systems
• Most animals are composed of specialized cells organized into
tissues that have different functions
• Tissues make up organs, which together make up organ systems
• Tissues = a group of cells similar in structure and function.
• Embryo differentiates into three fundamental embryonic tissues, the
germ layers:
– Endoderm – innermost layer
– Mesoderm – middle layer
– Ectoderm – outermost layer
5.1.2 Tissues Structure And Function
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• Germ layers differentiate into different types of cells.
• Cells organized into primary tissues
• Different tissues have different structures that are suited to their
functions
• Tissues are classified into four main categories:
– Epithelial tissues
– Connective tissues
– Muscle tissues
– Nerve tissues
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5.1.3 Organ And Organ System
• In all but the simplest animals, tissues are organized into organs
• In some organs, the tissues are arranged in layers
• Organs = body structures composed of several different tissues that form a structural and functional unit.
• Example: Heart
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• Organ systems = a group of organs that function together to carry out
the major activities of the bodies
• Example: Digestive system
Lumen of
stomach
Mucosa: an epithelial
layer that lines the
lumen
Submucosa: a matrix of
connective tissue that
contains blood vessels
and nerves
Muscularis: consists
mainly of smooth muscle
tissue
Serosa: a thin layer of
connective and epithelial
tissue external to the muscularis0.2 mm
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5.2 EPITHELIAL TISSUES
• Occurring in sheets of tightly packed cells, epithelial tissue covers
the outside of the body and lines organs and cavities within the
body.
– The cells of an epithelium are closely joined and in many
epithelia, the cells are riveted together by tight junctions.
– The epithelium functions as a barrier protecting against
mechanical injury, invasive microorganisms, and fluid loss.
• The cells at the base of an epithelial layer are attached to a
basement membrane, a dense mat of extracellular matrix.
– The free surface of the epithelium is exposed to air or fluid.
5.2.1 Covering Epithelium
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• Epithelia are classified by the number of cell layers and the shape of the cells on the free surface.
– A simple epithelium has a single layer of cells, and a stratified epithelium has multiple tiers of cells.
– A “pseudostratified” epithelium is single-layered but appears stratified because the cells vary in length.
• The shapes of cells on the exposed surface may be cuboidal (like dice), columnar (like bricks on end), or squamous (flat like floor tiles).
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Characteristics:
1) Epithelium (epithelial membrane) covers internal and external surfaces of vertebrate body.
• Derived from all three germ layers:
− Ectoderm → epidermis – outer portion of skin
− Endoderm → inner surface of digestive tract
− Mesoderm → inner lining of blood vessels = endothelium.
• True epithelium arises from ectoderm or endoderm
• Epitheliums arising from mesoderm are not true epithelium.
2) Provides a barrier that prevents passage of some substances while facilitating passage of others.
Example:
• Epidermis
– Protects from dehydration → relatively impermeable to water.
– Protects from airborne pathogens.
• Epithelium of digestive tract
– Allows selective entry of digestive products
– Barrier to toxins
• Lungs
– Allows rapid gaseous diffusion
3) May be modified into glands→ secretion.
4) Cells are tightly bound – very little intercellular spaces.
– Lower layers of cell rest on basement membrane – compose of network of collagenous fibers.
– Have free surface on the other side.
– Blood vessels cannot pass through adjacent cells.
– Depends on diffusion of nutrient and O2 from blood vessels in nearby tissues.
– This limits thickness of epithelium – one or a few layers thick.
5) Cells can regenerate.
• Cells constantly replaced.
• Example:
– Liver – gland formed from epithelium
– Can regenerate after portions of it are surgically removed.
– Epidermis – renewed every two weeks
– Epithelium inside stomach – replaced every two to three days.
1. Simple epithelium
a) Squamous epithelium
• Cells thin and flattened.
• Little cytoplasm – encloses centrally placed disc-shaped nucleus.
• Margins tessellated (irregular).
• Adjacent cells often bound firmly together by protoplasmic
connections.
• Location and function:
– Bowman’s capsules, alveolar lining of lungs, and blood capillarywalls – enables diffusion of materials through it.
– Blood vessels and heart chambers – provides smooth lining that allows relatively friction-free passage of fluid through them.
b) Cuboidal epithelium
• Least specialized epithelia.
• Cube shaped.
• Pentagonal/hexagonal from surface view.
• Central spherical nucleus.
• Location: Pancreatic duct, collecting duct of kidney, and salivary
duct, salivary, mucus, sweat, and thyroid glands
• Function: secretion and absorption
c) Columnar Epithelium
• Cells tall and quite narrow.
• Have more cytoplasm.
• Nucleus at basal end.
• Mucus-secreting goblet cells are interspersed amongst the cells.
• (A layer of moist epithelium containing goblet cells, together with the underlying connective tissue, is called a mucous membrane or mucosa).
• Epithelium may be secretory and/or absorptive in function.
• Free surface end of cell usually has microvilli – increases surface area for absorption and secretion.
• Location and function:
– Stomach lining – mucus secreted by goblet cells protects stomach lining from acidic contents of stomach and from digestion by enzymes.
– Intestine lining – protects from digestion by enzymes, lubricates passage of food, and absorption of digested food (small intestine).
– Kidney ducts – protection.
d) Ciliated epithelium
• Columnar, but with cilia at free surface.
• Mucus-secreting goblet cells produce fluids.
• Movement of cilia produces current in fluid.
• Location and function: Oviducts, ventricles of brains, spinal canal, and respiratory passages – moves materials from one location to another by ciliary action.
e) Pseudostratified epithelium
• Cells appear to be at different layers because not all cells reach the free surface.
• All cells attached to basement membrane.
• Location:
– Linings of urinary tract, trachea, epididymis – pseudostratifiedcolumnar.
– Linings of respiratory passages – pseudostratified columnar ciliated.
– Olfactory mucosa
• Function: Secretion and movement by ciliary action
2. Stratified Epithelium
• Several layer of cells.
• Forms tough impervious layer.
• Cells formed by mitotic division of germinal layer on basement
membrane.
• First-formed cells cuboid in shape.
• As cells are pushed towards free surface, they become flattened and
are called squames.
• Squames eventually flake off and replaced by new ones from
beneath.
• Types (depending on shape of uppermost layer cells):
– Stratified squamous – outer layer of skin, parts of esophagus, lining of mouth - protection
– Stratified cuboidal – sweat gland ducts - protection
– Stratified columnar – mammary gland ducts – protection and secretion
– Stratified transitional – lining of urinary bladder – permits distention
Transitional epithelium
• Modified stratified epithelium.
• 3 – 4 layers of cells of similar size except free surface cells, which are more flattened.
• Free surface cells do not flake off.
• All cells able to change shape under differing condition.
• Example: urinary bladder and ureter.
– Cells change shape when wall of bladder is stretched as it fills up with urine.
– Thickness of tissues prevents urine escaping in surrounding tissues.
5.2.2 Glandular Epithelium
• Some epithelia, called glandular epithelia, absorb or secrete
chemical solutions.
– The glandular epithelia that line the lumen of the digestive and
respiratory tracts form a mucous membrane that secretes a
slimy solution called mucus that lubricates the surface and keeps
it moist.
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Exocrine gland
• Epithelial connection between gland and surface epithelium remains as a tube (duct).
• Deepest cells become secretory cells → discharge secretions into duct.
• Secretory cells may form spherical sack or tube, which may show various degree of branching (to increase area of secretory surface).
• Example: sweat, oil, wax, mammary and digestive glands, pancreas– produces mucus, oil, wax, milk, or digestive enzymes.
Endocrine gland
• Epithelial connection disappears.
• Secretory cells forms close association with blood capillaries.
• Secretion (hormones) passes into bloodstream.
• Example: pituitary and thyroid glands, pancreas, ovary, and testes –produces hormones.
5.3 CONNECTIVE TISSUES
• Connective tissue functions mainly to bind and support other tissues.
– Connective tissues have a sparse population of cells scattered through an extracellular matrix.
– The matrix generally consists of a web of fibers embedding in a uniform foundation that may be liquid, jellylike, or solid.
– In most cases, the connective tissue cells secrete the matrix.
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Functions:
1. Provides supportive framework for body, for example, skeletal, bone, and cartilage tissue.
2. Binds other tissues together, example binds skin with underlyingtissues. (Thus, tissue is strong.)
3. Forms sheath around organs of body, separating them so that theydo not interfere with each other’s activities.
4. Embedding and protecting blood vessels and nerves where they enter or leave organs.
5. Protection against wounding or bacterial infection – areolar tissue.
6. Insulation of body against heat loss – adipose tissue.
7. Producing blood
• There are three kinds of connective tissue fibers, which are allproteins: collagenous fibers, elastic fibers, and reticular fibers.
• Collagenous fibers are made of collagen, the most abundant protein in the animal kingdom.
– Collagenous fibers are nonelastic and do not tear easily when pulled lengthwise.
• Elastic fibers are long threads of elastin.
– Elastin fiber provides a rubbery quality that complements the nonelastic strength of collagenous fibers.
• Reticular fibers are very thin and branched.
– Composed of collagen and continuous with collagenous fibers, they form a tightly woven fabric that joins connective tissue toadjacent tissues.
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• The major types of connective tissues in vertebrates are loose
connective tissue, adipose tissue, fibrous connective tissue,
cartilage, bone, and blood.
– Each has a structure correlated with its specialized function.
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5.3.1 Connective Tissue Proper
• Loose connective tissue binds epithelia to underlying tissues and
functions as packing material, holding organs in place.
– Loose connective tissue has all three fiber types.
• Two cell types predominate in the fibrous mesh of loose connective
tissue.
– Fibroblasts secrete the protein ingredients of the extracellular
fibers.
– Macrophages are amoeboid cells that roam the maze of fibers,
engulfing bacteria and the debris of dead cells by phagocytosis.
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Areolar Tissue
• Matrix – transparent semi-fluid containing:
– Mucin
– Hyaluronic acid
– Chondroitin sulphate
• Fibres:
– Collagen
– In wavy bundles.
– Scattered throughout matrix.
– Flexible but inelastic.
• Elastin
– Thin straight fibres.
– Forms a loose anastomosing network.
– Flexible but elastic
• Both provide considerable tensile strength and resilience to tissue.
Cells – interspersed in the matrix:
• Fibroblast
– Flattened, spindle-shaped, with oval nucleus.
– Produce fibres.
– Located close to fibres but can migrate towards wounded tissues
– secretes fibres to seal injured area.
• Macrophage (histiocyte)
– Polymorphic cell – capable of amoeboid movement → engulf
bacteria or foreign particles.
– Generally immobile – but at times can wander to areas of
bacterial invasion → provides body defense.
• Mast cell
– Small, oval shaped, with granular cytoplasm.
– Secretes matrix, heparin and histamine.
– Found close to blood vessels.
– Heparin – anticoagulant → prevents conversion of prothrombin to thrombin.
– Histamine – released from injured/disrupted tissues → causes vasodilation, contraction of smooth muscle and stimulates gastric secretion.
• Plasma cell
– Rare.
– Products of mitotic cell division by migratory lymphocytes.
– Components of body immune system - produce antibodies.
• Chromatophore
– In specialized areas – skin and eye.
– Branched and densely packed with melanin granules.
• Fat cell
– Contains large lipid droplet.
– Cytoplasm and nucleus confined to margins.
• Mesenchyme cell
– Reserve of undifferentiated cells.
– Can be stimulated to transform into one of the above cell types when needed.
• Location: upper dermis, blood vessels, nerves, around body organs.
• Function: Gives strength, elasticity, and support.
• Adipose tissue is a specialized form of loose connective tissue
that stores fat in adipose cells distributed throughout the matrix.
– Adipose tissue pads and insulates the body and stores fuel as
fat molecules.
– Each adipose cell contains a large fat droplet that swells when
fat is stored and shrinks when the body uses fat as fuel.
Adipose Tissue
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Fibrous Connective Tissue
• Fibrous connective tissue is dense, due to its large number of
collagenous fibers.
– The fibers are organized into parallel bundles, an arrangement
that maximizes nonelastic strength.
– This type of connective tissue forms tendons, attaching muscles
to bones, and ligaments, joining bones to bones at joints.
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5.3.2 Special Connective Tissues
• Cartilage has an abundance of collagenous fibers embedded in a rubbery matrix made of a substance called chondroitin sulfate, a protein-carbohydrate complex.
– Chondrocytes secrete collagen and chondroitin sulfate.
– The composite of collagenous fibers and chondroitin sulfate makes cartilage a strong yet somewhat flexible support material.
– The skeleton of a shark and the embryonic skeletons of many vertebrates are cartilaginous.
– We retain cartilage as flexible supports in certain locations, such as the nose, ears, and intervertebral disks.
• Three types of cartilage: hyaline, yellow elastic, and white fibrous cartilage.
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Hyaline Cartilage
• Matrix semi-transparent – consist of chondroitin sulphate and fine collagen fibrils.
• Peripheral chondrocytes are flattened and arranged in parallel rows.
• Those situated internally are bigger and scattered.
• Chondrocyte is contained in lacunae – each encloses one, two, four, or eight chondrocytes.
• No blood vessels – exchange of materials between chondrocytesand matrix is by diffusion.
• Elastic and compressible tissue.
• Location: ends of bones (sternum of ribs), nose, air passages ofrespiratory system (larynx and trachea) and in parts of ear.
• Function: Provides movement at joints, flexibility, and support.
Yellow Elastic Cartilage
• Semi-opaque matrix containing network of elastic fibres.
• Chondrocytes close to one another.
• Very elastic and flexible – allows tissue to recover its shape after distortion.
• Location: External ear, eustachian tube, epiglottis, and cartilages of pharynx.
• Function: Gives support and maintains shape.
White Fibrous Cartilage
• Matrix contains large numbers of densely packed collagen fibres.
• Very little chondrocytes.
• Have great tensile strength and a small degree of flexibility.
• Location: intervertebral discs (provides cushioning effect), symphysispubis (the region between two pubic bones of the pelvis), and ligamentous capsules of joins.
• Function: Support and fusion.
• The skeleton supporting most vertebrates is made of bone, a mineralized connective tissue.
– Bone-forming cells called osteoblasts deposit a matrix of collagen.
– Calcium, magnesium, and phosphate ions combine and harden within the matrix into the mineral hydroxyapatite.
– The combination of hard mineral and flexible collagen makes bone harder than cartilage without being brittle.
– The microscopic structure of hard mammalian bones consists of repeating units called osteons.
• Each osteon has concentric layers of mineralized matrix deposited around a central canal containing blood vessels and nerves that service the bone.
• Two types of bone: compact and spongy bone.
Bone
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i) Compact bone
• Consists of units called Harvesian system or osteon.
• Characteristic of the Harvesian system:
– Contains a central Harvesian canal containing nerves, arteries,
veins and lymph vessels.
– Canal surrounded by numerous concentric cylinders called
Harvesian lamellae.
– Interspersed between the lamellae are numerous lacunae
containing osteoblasts. When not active, they are called
osteocytes – can be activated and differentiate into osteoblasts.
– Fine channels called canaliculi radiate from each lacuna. Canaliculi contain cytoplasm and may link up with Haversiancanal, with other lacunae or pass from one lamella to another.
– Capillaries branch from the arteries and veins in the Haversiancanal and pass via canaliculi to the osteoblasts in the lacunae –facilitate passage of nutrients, metabolic waste and respiratorygases towards and away from the cells.
• In longitudinal section, the Haversian canals are linked to one another by transverse canals called Volkmann canals.
• The bone is covered by a dense connective tissue called periosteum.
ii) Spongy bone
• Found in larger bones and is always surrounded by compact bone.
• Consists of sheets of bones called trabeculae, interspersed with
large spaces occupied by bone marrow.
• Trabeculae contain osteoblasts.
Blood
• Blood functions differently from other connective tissues, but it does have an extensive extracellular matrix.
– The matrix is a liquid called plasma, consisting of water, salts, and a variety of dissolved proteins.
– The liquid matrix enables rapid transport of blood cells, nutrients, and wastes.
– Suspended in the plasma are erythrocytes (red blood cells), leukocytes (white blood cells), and cell fragments called platelets.
• Red cells carry oxygen.
• White cells function in defense against viruses, bacteria, and other invaders.
• Platelets aid in blood clotting.
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Plasma
• Pale yellow liquid.
• 90% water and 10% solutes.
• Solutes:
– Metabolites – glucose, amino acids, vitamins.
– Wastes – nitrogen compounds, CO2.
– Hormones (– regulate cellular activities).
– Ions – especially sodium, chloride, and bicarbonate.
– Proteins – albumin, globulins (carriers of lipids and steroid hormones), and fibrinogen (blood clotting).
• If fibrinogen is removed, blood plasma is called serum.
• Function: Provides medium for exchange of substances.
Erythrocytes (Red blood cells)
• 5 million per mm3 blood.
• Diameter: 7 - 8 µm.
• No nucleus (and organelles).
• Shape: biconcave disc.
• Plasma membrane thin and flexible.
• Cell filled with haemoglobin – lack of nucleus permits more haemoglobin to be packed into cell → 250 million molecules of haemoglobin per cell.
• Life span: 120 days.
• New cells manufactured in red bone marrow – 1½ million per second.
• Function: O2 and CO2 transport – biconcave disc provides a large surface-volume ratio for absorption.
Leukocytes (White blood cells)
• Larger the erythrocytes.
• 7000 per mm3 blood.
• Have nucleus.
• Short life span – a few days.
• Function: Body defenses.
• Two main groups: granulocytes and agranulocytes.
ii) Granulocytes
• Contains lobed nucleus and granular cytoplasm.
• Capable of amoeboid movement.
• Three types:
Neutrophils
– 7 – 9 µm.
– 70% of leukocytes.
– Nucleus with 3 – 5 lobes.
– (Can squeeze between cells of capillary walls (diapedesis) and move to infected area.)
– Function: Phagocytosis - engulf and digest pathogen
Eosinophils
– (Granules stained red with eosin dye.)
– 1.5% of leukocytes.
– 9 – 12 µm.
– Nucleus sometimes “Z” shaped.
– Function: Anti-histamine properties.
Basophils
– (Granules stained blue with methylene blue.)
– 0.5% of leukocytes.
– ≈ 10 µm.
– Nucleus sometimes “S” shaped.
– Function: Produce heparin and histamine (causes inflammatory response).
ii) Agranulocytes
• Non-granular cytoplasm.
• Oval or bean-shaped nucleus.
• Two types:
Monocytes
– 4% of leukocytes.
– 9 – 12 µm.
– Bean-shaped nucleus.
– (Can migrate from bloodstream to inflamed areas, acting in same manner as neutrophils.)
– Function: Phagocytosis - engulf bacteria.
Lymphocytes
– 24% of leukocytes.
– 6 – 8 µm.
– Big, rounded nucleus with little cytoplasm.
– Function: Antibody production and cellular immune response.
Platelets
• Irregularly shaped membrane-bound cell fragments.
• Formed from megakaryocytes, large bone marrow cells.
• 0.25 million per mm3 blood.
• Usually without nucleus.
• Life span: 7 – 8 days.
• Function: Blood clotting.
5.4 MUSCLE TISSUES
• Muscle tissue is composed of long cells called muscle fibers that
are capable of contracting when stimulated by nerve impulses.
– Arranged in parallel within the cytoplasm of muscle fibers are
large numbers of myofibrils made of the contractile proteins actin
and myosin.
– Muscle is the most abundant tissue in most animals, and muscle
contraction accounts for most of the energy-consuming cellular
work in active animals.
• There are three types of muscle tissue in the vertebrate body:
skeletal muscle, cardiac muscle, and smooth muscle.
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5.4.1 Skeletal Muscle
• Attached to bones by tendons, skeletal muscle is responsible for
voluntary movements.
– Skeletal muscle consists of bundles of long cells called fibers.
• Each fiber is a bundle of strands called myofibrils.
– Skeletal muscle is also called striated muscle because the
arrangement of contractile units, or sarcomeres, gives the cells a
striped (striated) appearance under the microscope.
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5.4.2 Smooth Muscle
• Smooth muscle, which lacks striations, is found in the walls of the digestive tract, urinary bladder, arteries, and other internal organs.
– The cells are spindle-shaped.
– They contract more slowly than skeletal muscles but can remain contracted longer.
– Controlled by different kinds of nerves than those controlling skeletal muscles, smooth muscles are responsible for involuntarybody activities.
• These include churning of the stomach and constriction of arteries.
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5.4.3 Cardiac Muscle
• Cardiac muscle forms the contractile wall of the heart.
– It is striated like skeletal muscle, and its contractile properties are
similar to those of skeletal muscle.
– Unlike skeletal muscle, cardiac muscle carries out the
unconscious task of contraction of the heart.
– Cardiac muscle fibers branch and interconnect via intercalated
disks, which relay signals from cell to cell during a heartbeat.
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5.5 NERVOUS TISSUES
• Nervous tissue senses stimuli and transmits signals from one part of the animal to another.
– The functional unit of nervous tissue is the neuron, or nerve cell, which is uniquely specialized to transmit nerve impulses.
– A neuron consists of a cell body and two or more processes called dendrites and axons.
• Dendrites transmit impulses from their tips toward the rest of the neuron.
• Axons transmit impulses toward another neuron or toward an effector, such as a muscle cell that carries out a body response.
– In many animals, nervous tissue is concentrated in the brain.
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5.5.1 Neurons
• The cell body contains nucleus and cytoplasm.
• Cell body is enclosed within a plasma membrane.
• Also contains Nissl’s granules (involved in protein synthesis), ribosomes, and other organelles.
• The dendrites are cytoplasmic processes extending from cell body –they make synaptic connections with other neurons.
• A nerve fibre is a long cytoplasmic extension from cell body that transmits impulse.
• Nerve fibres that transmit impulses away from the cell body are called axons while those that transmit impulses towards cell body are called dendrons.
• The cytoplasm of the cell body contains large amount of ribosomes –synthesize proteins which is supplied to the nerve fibres.
• The cytoplasm of the nerve fibre is continuous with the cytoplasm of the cell body and lacks ribosomes.
• The plasma membrane of the nerve fibre is continuous with that of the cell body.
• Nerve fibres may or may not be surrounded by a fatty myelin sheath, formed from Schwann cells.
• Sheath is constricted at intervals along the nerve fibre by nodes of Ranvier.
• The myelin sheath insulates the nerve fibre and speed up the transmission of impulses along it.
• A tough inelastic membrane called neurilemma surrounds the sheath.
Classification of neuron
Based on function (direction in which impulse is transmitted)
• Sensory or afferent neuron
– Transmits impulse from the receptor to the central nervous system.
• Motor or efferent neuron
– Transmits impulse from the central nervous system to the effector.
• Intermediary neuron
– Transmit impulse from sensory to the motor neuron.
Based on structure (number of fibres)
• Multipolar neuron
– Neuron has one axon and several dendrites.
• Bipolar neuron
– Neuron has one axon and one dendrite at opposite ends of the
cell body.
• Pseudo-unipolar neuron
– Neuron has only one extension that divides into two, dendrite and
axon.
5.5.2 Neuroglia
• Ten times more numerous than neurons.
• Found throughout the central nervous system.
• Function:
– Provides mechanical support to neurons.
– Supply nourishment to neuron fibres.
– Some are involved in the memory process – stores information in the form of RNA.
– Some form the myelin sheath.
5.6.1 PLANT CELL TYPES
The plant body has a hierarchy of organs,
tissues, and cells
• Plants, like multicellular animals, have organs composed of different
tissues, which are in turn are composed of cells
5.6 THE STRUCTURE AND FUNCTION
OF PLANT TISSUES
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The Three Basic Plant Organs: Roots,
Stems, and Leaves
• The basic morphology of vascular plants reflects their evolutionary
history as terrestrial organisms that inhabit and draw resources
from two very different environments.
– Plants obtain water and minerals from the soil.
– They obtain CO2 and light above ground.
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• To obtain the resources they need, vascular plants have evolved two systems: a subterranean root system and an aerial shoot systemof stems and leaves.
• Each system depends on the other.
– Lacking chloroplasts and living in the dark, roots would starve without the sugar and other organic nutrients imported from the photosynthetic tissues of the shoot system.
– Conversely, the shoot system (and its reproductive tissues, flowers) depends on water and minerals absorbed from the soil by the roots.
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(1) Roots
• Functions of roots:
– Anchoring the plant
– Absorbing minerals and water
– Often storing organic nutrients
• In most plants, absorption of water and minerals occurs near the root
tips, where vast numbers of tiny root hairs increase the surface area
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• Most eudicots and gymnosperms have a taproot system, consisting of
one large vertical root (the taproot) that produces many small lateral, or
branch, roots.
• In angiosperms, taproots often store food that supports flowering and
fruit production later.
• Seedless vascular plants and most monocots, including grasses, have
fibrous root systems consisting of a mat of thin roots that spread out
below the soil surface.
• A fibrous root system is usually shallower than a taproot system.
• Grass roots are concentrated in the upper few centimeters of soil. As
a result, grasses make excellent ground cover for preventing erosion.
• Sturdy, horizontal, underground stems called rhizomes anchor large
monocots such as palms and bamboo.
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• The root system helps anchor a plant.
• In both taproot and fibrous root systems, absorption of water and
minerals occurs near the root tips, where vast numbers of tiny root
hairs enormously increase the surface area.
• Root hairs are extensions of individual epidermal cells on the root
surface.
� Absorption of water and minerals is also increased by
mutualistic relationships between plant roots and bacteria
and fungi.
• Some plants have modified roots. Some arise from roots while
adventitious roots arise aboveground from stems or even from
leaves.
� Some modified roots provide additional support and
anchorage. Others store water and nutrients or absorb
oxygen or water from the air.
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Prop roots.
Storage roots.
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“Strangling” aerial roots.
Buttress roots.
Pneumatophores.
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(2) Stems
• A stem is an organ consisting of
– An alternating system of nodes, the points at which leaves are
attached
– Internodes, the stem segments between nodes
• An axillary bud is a structure that has the potential to form a lateral
shoot, or branch
• A terminal bud is located near the shoot tip and causes elongation of
a young shoot.
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• The presence of a terminal bud is partly responsible for inhibiting the
growth of axillary buds, a phenomenon called apical dominance.
– By concentrating resources on growing taller, apical dominance is
an evolutionary adaptation that increases the plant’s exposure to
light.
– In the absence of a terminal bud, the axillary buds break
dominance and give rise to a vegetative branch complete with its
own terminal bud, leaves, and axillary buds.
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• Modified shoots with diverse functions have evolved in many plants.
• These shoots, which include stolons, rhizomes, tubers, and bulbs,
are often mistaken for roots.
• Stolons, such as the “runners” of strawberry plants, are
horizontal stems that grow on the surface and enable a plant
to colonize large areas asexually as plantlets form at nodes
along each runner.
• Rhizomes, like those of ginger, are horizontal stems that
grow underground.
• Tubers, including potatoes, are the swollen ends of rhizomes
specialized for food storage.
• Bulbs, such as onions, are vertical, underground shoots
consisting mostly of the swollen bases of leaves that store
food.
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Stolons.
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(3) Leaves
• The leaf is the main photosynthetic organ of most vascular plants
• Leaves generally consist of
– A flattened blade and a stalk
– The petiole, which joins the leaf to a node of the stem
• Grasses and other monocots lack petioles. In these plants, the base
of the leaf forms a sheath that envelops the stem.
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• Most monocots have parallel major veins that run the length of the blade, while eudicot leaves have a multibranched network of major veins.
• Plant taxonomists use floral morphology, leaf shape, spatial arrangement of leaves, and the pattern of veins to help identify and classify plants.
– For example, simple leaves have a single, undivided blade, whilecompound leaves have several leaflets attached to the petiole.
• The leaflet of a compound leaf has no axillary bud at its base.
– In a doubly compound leaf, each leaflet is divided into smaller leaflets.
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• Most leaves are specialized for photosynthesis.
– Some plants have leaves that have become adapted for other
functions.
– These include tendrils that cling to supports, spines of cacti for
defense, leaves modified for water storage, and brightly colored
leaves that attract pollinators.
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Simple leaf
Axillary bud
Petiole
Compound leaf
Axillary bud
Petiole
Leaflet
Doubly compound leaf
Axillary bud
Petiole
Leaflet
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• Some plant species have evolved modified leaves that
serve various functions
Tendrils.
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The Three Tissue Systems: Dermal,
Vascular, and Ground
• Each organ of a plant has three tissue systems: dermal, vascular,
and ground.
– Each system is continuous throughout the plant body.
• The dermal tissue is the outer covering.
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• In nonwoody plants, it is a single layer of tightly packed cells, or epidermis, that covers and protects all young parts of the plant.
• The epidermis has other specialized characteristics consistent with the function of the organ it covers.
– For example, the root hairs are extensions of epidermal cells near the tips of the roots.
– The epidermis of leaves and most stems secretes a waxy coating, the cuticle, which helps the aerial parts of the plant retain water.
• In woody plants, protective tissues called periderm replace the epidermis in older regions of stems and roots.
Dermal Tissue
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• Vascular tissue, continuous throughout the plant, is involved in the
transport of materials between roots and shoots.
• The two vascular tissues are xylem and phloem
– Xylem conveys water and dissolved minerals upward from roots into
the shoots.
– Phloem transports food (organic nutrients) made in mature leaves
to the roots; to nonphotosynthetic parts of the shoot system; and to
sites of growth, such as developing leaves and fruits (from where
they are made to where they are needed).
Vascular Tissue
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• The vascular tissue of a root or stem is called the stele.
• In angiosperms, the stele of the root forms a solid central
vascular cylinder, while
• The stele of stems and leaves is divided into vascular
bundles, strands consisting of xylem and phloem.
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• Ground tissue is tissue that is neither dermal tissue nor vascular
tissue.
– In eudicot stems, ground tissue is divided into pith, internal to
vascular tissue, and cortex, external to the vascular tissue.
– The functions of ground tissue include photosynthesis, storage,
and support.
– For example, the cortex of a eudicot stem typically consists of
both fleshy storage cells and thick-walled support cells.
Ground Tissue
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Common Types of Plant Cells
Like any multicellular organism, a plant is characterized by cellular differentiation, the specialization of cells in structure and function
• Some major types of plant cells:
– Parenchyma
– Collenchyma
– Sclerenchyma
– Water-conducting cells of the xylem
– Sugar-conducting cells of the phloem
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(i) Parenchyma
• Mature parenchyma cells have primary walls that are relatively thin
and flexible, and most lack secondary walls.
– The protoplast of a parenchyma cell usually has a large central
vacuole.
– Parenchyma cells are often depicted as “typical” plant cells
because they generally are the least specialized, but there are
exceptions.
– For example, the highly specialized sieve-tube members of the
phloem are parenchyma cells.
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• Parenchyma cells perform most of the metabolic functions of the
plant, synthesizing and storing various organic products.
– For example, photosynthesis occurs within the chloroplasts of
parenchyma cells in the leaf.
– Some parenchyma cells in the stems and roots have colorless
plastids that store starch.
– The fleshy tissue of most fruit is composed of parenchyma cells.
– Most parenchyma cells retain the ability to divide and differentiate
into other cell types under special conditions, such as the repair
and replacement of organs after injury to the plant.
– In the laboratory, it is possible to regenerate an entire plant from
a single parenchyma cell.
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(ii) Collenchyma
• Collenchyma cells have thicker primary walls than parenchyma
cells, though the walls are unevenly thickened.
– Grouped into strands or cylinders, collenchyma cells help support
young parts of the plant shoot.
– Young stems and petioles often have strands of collenchyma just
below the epidermis, providing support without restraining growth.
– Mature collenchyma cells are living and flexible and elongate with
the stems and leaves they support.
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(iii) Sclerenchyma
• Sclerenchyma cells have thick secondary walls usually strengthened
by lignin and function as supporting elements of the plant.
– They are much more rigid than collenchyma cells.
– Unlike parenchyma cells, they cannot elongate.
– Sclerenchyma cells occur in plant regions that have stopped
lengthening.
• Many sclerenchyma cells are dead at functional maturity, but they
produce rigid secondary cells walls before the protoplast dies.
– In parts of the plant that are still elongating, secondary walls are
deposited in a spiral or ring pattern, enabling the cell wall to stretch
like a spring as the cell grows.
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• Two types of sclerenchyma cells, fibers and sclereids, are
specialized entirely for support.
– Fibers are long, slender, and tapered, and usually occur in
groups.
• Those from hemp fibers are used for making rope, and those
from flax are woven into linen.
– Sclereids are irregular in shape and are shorter than fibers.
• They have very thick, lignified secondary walls.
• Sclereids impart hardness to nutshells and seed coats and
the gritty texture to pear fruits.
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• The water-conducting elements of xylem, the tracheids and vessel
elements, are elongated cells that are dead at functional maturity.
– The thickened cell walls remain as a nonliving conduit through
which water can flow.
• Both tracheids and vessels have secondary walls interrupted by pits,
thinner regions where only primary walls are present.
(iv) Xylem
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• Tracheids are long, thin cells with tapered ends.
– Water moves from cell to cell mainly through pits.
– Because their secondary walls are hardened with lignin, tracheids
function in support as well as transport.
• Vessel elements are generally wider, shorter, thinner walled, and
less tapered than tracheids.
– Vessel elements are aligned end to end, forming long micropipes
or xylem vessels.
– The ends are perforated, enabling water to flow freely.
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• In the phloem, sucrose, other organic compounds, and some mineral ions move through tubes formed by chains of cells called sieve-tube members.
– These are alive at functional maturity, although a sieve-tube member lacks a nucleus, ribosomes, and a distinct vacuole.
– The end walls, the sieve plates, have pores that facilitate the flow of fluid between cells.
(v) Phloem
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– Each sieve-tube member has a nonconducting nucleated
companion cell, which is connected to the sieve-tube member
by numerous plasmodesmata.
– The nucleus and ribosomes of the companion cell serve both that
cell and the adjacent sieve-tube member.
– In some plants, companion cells in leaves help load sugar into
the sieve-tube members, which transport the sugars to other
parts of the plant.
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PARENCHYMA CELLS
Parenchyma cells in Elodea
leaf, with chloroplasts (LM) 60 µm
80 µmCortical parenchyma cells
Collenchyma cells (in cortex of Sambucus,
elderberry; cell walls stained red) (LM)
COLLENCHYMA CELLS
SCLERENCHYMA CELLS
SUGAR-CONDUCTING CELLS OF THE PHLOEM
WATER-CONDUCTING CELLS OF THE XYLEM
5 µm
Fiber cells (transverse section from ash tree) (LM)
25 µm
Sclereid cells in pear (LM)
Cell wall
Sieve-tube members:longitudinal view
30 µm
15 µm
Companion
cell
Companion
cell
Sieve-tubemember
Plasmodesma
Sieveplate
Sieve plate with pores (LM)
Nucleus
Cytoplasm
Sieve-tube members:longitudinal view(LM)
Vessel elements withperforated end walls
Vesselelement
Tracheids
Pits
Tracheids and vessels(colorized SEM)
TracheidsVessel 100 µm
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5.6.2 TYPES OF MERISTEM
Meristems generate cells for new organs
• A major difference between plants and most animals is that plantgrowth is not limited to an embryonic period.
• Most plants demonstrate indeterminate growth, growing as long as the plant lives.
• In contrast, most animals and certain plant organs, such as flowers and leaves, undergo determinate growth, ceasing to grow after they reach a certain size.
• Indeterminate growth does not mean immortality.
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• Annual plants complete their life cycle—from germination through flowering and seed production to death—in a single year or less.
– Many wildflowers and important food crops, such as cereals and legumes, are annuals.
• The life of a biennial plant spans two years.
– Often, there is an intervening cold period between the vegetative growth season and the flowering season.
• Plants such as trees, shrubs, and some grasses that live many years are perennials.
– Perennials do not usually die from old age, but from an infection or some environmental trauma.
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• A plant is capable of indeterminate growth because it has perpetually
embryonic tissues called meristems in its regions of growth.
– These cells divide to generate additional cells, some of which
remain in the meristematic region, while others become
specialized and are incorporated into the tissues and organs of
the growing plant.
– Cells that remain as wellsprings of new cells in the meristem are
called initials.
– Those that are displaced from the meristem, derivatives,
continue to divide for some time until the cells they produce
differentiate within developing tissues.
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• The pattern of plant growth depends on the location of meristems.
• Two major types of meristems
– Apical meristems
– Lateral meristems (also known as cambiums)
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(i) Apical Meristem
• Apical meristems, located at the tips of roots and in the buds of shoots, supply cells for the plant to grow in length.
• This elongation, primary growth, enables roots to extend through the soil and shoots to increase their exposure to light and carbon dioxide.
• In herbaceous plants, primary growth produces almost all of the plant body.
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• Lateral meristems add thickness to woody plants, a process called secondary growth (progressive thickening of roots and shoots where primary growth has ceased.)
• There are two lateral meristems: the vascular cambium and the cork cambium
• The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem
• The cork cambium replaces the epidermis with periderm, which is thicker and tougher
(ii) Lateral Meristems
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Shoot apicalmeristems(in buds)
Vascularcambium
Corkcambium
Lateralmeristems
Primaryphloem
Periderm
Corkcambium
Secondaryxylem
Primaryxylem
Pith
Pith
Cortex
Secondary growth in stems
Secondaryphloem
Vascular cambium
Primary phloem
Primary xylem
Cortex
Primary growth in stems
Epidermis
Root apicalmeristems
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5.6.3 STRUCTURE OF DICOTYLEDONOUS
& MONOCOTYLEDONOUS
(Angiosperm Diversity)
• Angiosperms have diversified into more than 250,000 species thatdominate most terrestrial ecosystems.
• Until the late 1990s, flowering plants were divided into monocots and dicots on the basis of number of cotyledons or seed leaves.
• Current research supports the view that monocots form a clade but reveals that dicots are not monophyletic.
• The majority of plants traditionally called “dicots” form a clade now known as “eudicots.”
• The remaining plants are divided into several small lineages.
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• Three of these lineages are called basal angiosperms, because
they include the oldest known lineages of flowering plants.
– Amborella is a basal angiosperm that lacks vessels that are found
in more derived angiosperms.
• Another lineage is the magnoliids.
– Magnoliids include 8,000 species, including magnolias.
– These angiosperms share primitive traits such as spiral
arrangement of floral parts with the basal angiosperms.
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• One quarter of angiosperms are monocots.
– Monocot traits include single cotyledons, parallel venation, scattered vascular bundles, fibrous root systems, pollen grains with a single opening, and floral parts in multiples of three.
• More than two-thirds of angiosperms—170,000 species—are eudicots.
– Eudicot traits include two cotyledons, netlike venation, vascular bundles arranged as a ring, a taproot, pollen grains with three openings, and floral parts in multiples of four or five.
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Southern magnolia(Magnoliagrandiflora)
HYPOTHETICAL TREE OF FLOWERING PLANTS
MAGNOLIIDS
Amborella
Star anise
and relatives
Water lilies
Magnoliids
Monocots
Eudicots
Orchid(Lemboglossumrossii)
Monocot
Characteristics
One cotyledon
Embryos
Two cotyledons
Eudicot
Characteristics
Californiapoppy(Eschscholziacalifornia)
EUDICOTSMONOCOTS
Pygmy date palm (Phoenix roebelenii)
Veins usually
parallel
Vascular tissuescattered
Stems
Pyrenean oak(Quercuspyrenaica)
EUDICOTSMONOCOTS
Leaf
venation
Vascular tissueusually arranged
in ring
Veins usually
netlike
Dog rose (Rosa canina), a wild rose
Root systemusually fibrous(no main root)
Roots
Lily (Lilium“Enchantment”)
EUDICOTSMONOCOTS
Taproot (main root)usually present
Barley (Hordeum vulgare), a grass
Floral organsusually in
multiples of three
Pollen
Zucchini (CucurbitaPepo), female(left), andmale flowers
EUDICOTSMONOCOTS
Pollen grain withthree openings
Stigma
Ovary
Anther
Filament
Floral organs usuallyin multiples offour or five
Pollen grain withone opening
Flowers
Pea (Lathyrusnervosus, LordAnson’sblue pea), a legume
Tissue Organization of Stems
• In gymnosperms and most eudicots, the vascular tissue consists of vascular bundles that are arranged in a ring
• In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring
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Structure of dicotyledonous stem:
• Epidermis (dermal tissue): – Outer surface covered with cuticle.
– May be perforated with stomata.
• Cortex (ground tissue)– Several layers of collenchyma cells immediately below the epidermis
– Parenchyma cells below collenchyma cells.
• Vascular bundles– Arranged in a ring.
– Xylem towards the inner side, and phloem towards the outside.
– Xylem and phloem separated by cambium.
• Pith (ground tissue)– Made up of living parenchyma cells.
• Secondary growth– Woody dicot plants undergo secondary growth, an increase in the girth of stems.
– Cambium divides, forming secondary xylem (wood) on the inside and secondary phloem (inner bark) on the outside.
Structure of monocotyledonous stem:
• Epidermis (dermal tissue): – Outer surface covered with cuticle.
– May be perforated with stomata.
• Does not have distinct area of cortex
• Vascular bundles– Are not arranged in a circle/ring but instead are scattered throughout ground tissues
– Does not possess lateral meristems (cambium) that give rise to secondary growth.
– Monocot does not produce wood.
Tissue Organization of Leaves
• The epidermis in leaves is interrupted by stomata, which allow CO2exchange between the air and the photosynthetic cells in a leaf
• The ground tissue in a leaf is sandwiched between the upper and lower epidermis
• The vascular tissue of each leaf is continuous with the vascular tissue of the stem
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Internal structure of dicotyledonous leaf:
• Epidermis– On upper and lower surface.
– Outer surface covered with cuticle.
– Lower epidermis perforated by stomata.
• Palisade mesophyll.– Very little air spaces between palisade cells.
– Cells contain chloroplast –performs most of the photosynthesis.
• Spongy mesophyll– Loosely arranged – allows CO2 to diffuse easily.
– Cells contain fewer chloroplasts.
• Vascular bundles– Xylem and phloem surrounded by bundle sheath.
– Xylem and phloem in leaves form in strands called veins.
– Most dicots have netted venation.
Internal structure of monocotyledonous leaf:
• Epidermis
– On upper and lower surface.
– Outer surface covered with cuticle.
– Lower epidermis perforated by stomata.
• Although most monocots have both palisade and spongy mesophylls, some monocots lack distinct regions of palisade and spongy mesophylls.
• Vascular bundles
– The leaves of most monocots have parallel venation.
Tissue Organization of Roots
• In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes.
• The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem.
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Internal structure of dicotyledonous root:
• Epidermis
– No cuticle.
– Have root hairs.
• Cortex
– Contains thin-walled parenchyma cells.
• Endodermis
– One layer of cells. Each cell has a special bandlike region, called a Casparian strip.
• Pericycle
– One layer of cells.
– Surrounds the vascular bundle.
• Vascular cylinder
– Xylem arranged like a star in transverse section with several “spokes”.
– Phloem located in between “spokes” of xylem
Internal structure of monocotyledonous root:
• Epidermis– No cuticle.
– Have root hairs.
• Cortex– Contains thin-walled parenchyma cells.
• Endodermis– One layer of cells.
• Pericycle– One layer of cells.
– Surrounds the vascular bundle.
• Vascular cylinder– Xylem does not arrange like a star in transverse section.
– Phloem and xylem are in separate alternating bundles arranged around the central pith.
– Pith consists of parenchyma cells.