Instructions for printer-friendly copy.

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(click on pic for source)

    Plant Tissues

    A tissue is defined as an aggregation of cells
    coordinated to perform a particular function or set of functions.

    Plant tissues are categorized as...

    • meristem
        • apical
        • lateral
        • intercalary

    • simple tissues
      • parenchyma
      • collenchyma
      • sclerenchyma

    • complex tissues
      • dermal tissues
        • epidermis
        • periderm

      • vascular tissues
        • xylem
        • phloem

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(click on pic for source)

    Annual Herbaceous Plants

    All plants begin their development as herbaceous (non-woody) sprouts.

    Herbaceous plants are pliable, having little or no lignin.
    Woody plants develop lignified stems and roots,
    becoming rigid with maturity.

    An annual plant

    • lives, reproduces, and dies in about a year
    • grows only from primary meristems
    • produces only primary growth.
    • remains herbaceous

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(click on pic for source)

    Perennial Woody Plants

    A perennial plant
    • lives for more than one year
    • grows from primary meristems
    • may also have secondary (lateral) meristems
    • may produce both primary and secondary growth
    • may or may not become woody

    Many types of plants are "woody", but only plants
    with a vascular cambium produce true, botanical wood.

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    Three Tissue Systems

    Meristem cells
    • are totipotent, and can differentiate into any other type of cell
    • increase plant length (apical, intercalary meristems)
    • increase plant girth (lateral meristems)

    Simple tissue
    • is composed of a single type of cell

    Complex tissue
    • is composed of more than one type of cell.

    These are arranged into three systems.

    • The ground system forms the soft tissue "bulk" of the plant.

    • The dermal system forms the external "skin" of the plant.

    • The vascular system provides water and nutrient transport.

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    Meristem

    Plants exhibit indeterminate growth: they produce and differentiate
    new tissues throughout their lifespan.

    The source of new tissues is meristem:

    • embryonic and totipotent
    • can differentiate into any other type of cell

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    Primary Meristem

    Totipotent apical meristem is found at the tip of every root and shoot.
    As it produces new cells, the root or stem increases in length.
    This is known as primary growth.

    Apical meristem daughter cells mature into three primary meristems:

    • protoderm will give rise to epidermis (multipotent)
    • procambium will give rise to vascular tissues (multipotent)
    • ground meristem will give rise to ground tissues (multipotent)

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    Growth Zones

    An elongating stem or root has three growth regions:
      Zone of Cell Division
      • location of the apical meristem
      • meristem cells undergoing rapid mitosis to produce
        • one daughter cell remains meristematic
        • one daughter cell begins a path of differentiation

      The Zone of Elongation

      • location of the three primary meristems
      • (recall: protoderm, procambium, ground meristem)
      • cells in this region are elongating and beginning to differentiate

      The Zone of Maturation

      • location of mature cells that have assumed their final identity

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CLICK ON PIC for a larger view

    Secondary Meristems

    In woody plants, the procambium also gives rise to
    two secondary (=lateral) meristems, or cambia.

    • vascular cambium (multipotent)
      • located between xylem and phloem
      • gives rise to xylem and phloem

    • cork cambium (multipotent)
      • located between phloem and dermal layer
      • gives rise to bark

    Lateral meristem growth increases the plant's diameter.

    This is known as secondary growth.

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    Ground Tissue

      There are three types of simple ground tissue.

      Each is named for the cell type comprising it.

      • Parenchyma cells make up parenchyma tissue.
      • Collenchyma cells make up collenchyma tissue.
      • Sclerenchyma cells make up sclerenchyma tissue.

      Each of these cell types also comprise components
      of complex tissues and plant organs.

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(click on pic for source)


(By Robert Kohlman, Wikimedia Commons)


(click on pic for source)

    Our Friend Parenchyma

    Parenchyma cells are the most abundant plant cells.

    Parenchyma cells

    • are found in most tissues of land plants.
    • are spherical alone, but multi-sided when squashed together.
    • (most have 14 sides!)
    • have a large central vacuole
    • have thin, flexible walls
    • store starch grains, oils, and other plant products
    • form most of the soft/crispy plant parts
    • can divide and multiply long after they are mature.
    • have long lifespans. Some can reach 100 years of age!

    Specialized parenchyma:

    • Chlorenchyma
      • is rich in chloroplasts
      • is found near the surface of leaves and herbaceous stems
    • Aerenchyma
      • has large, air-filled intercellular spaces
      • This can provide flotation (as in water lily leaves.
    • Transfer cells
      • move substances into other cells via plasmodesmata.
      • Companion cells load substances into vascular tissue for transport.
      • provide support and nourishment to vascular tube cells.
    • Spongy mesophyll
      • facilitates water movement through vascular tissue.

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(click on pic for source)


(click on pic for source)

    Collenchyma

    Collenchyma cells provide pliable strength to plant tissues.

    Collenchyma cells

    • are usually found just under the epidermis
    • tend to be elongated, rather than round.
    • have a thicker primary wall than parenchyma cells.
    • have particularly thickened corners
    • are long-lived, like parenchyma.

    Examples:

    • flexible outer layer of broccoli stem
    • celery strings
    • outer walls of growing roots and shoots

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(click on pic for source)


(click on pic for source)

    Sclerenchyma

    Sclerenchyma cells provide compressional strength and support to plant tissues.

    Sclerenchyma cells
    • lose their living protoplast at maturity.
    • (Parenchyma and collenchyma cells retain their living protoplasts.)
    • have very thick, hard secondary walls at maturity.
    • highly lignified
    • come in two flavors:

      Sclerids
      • five different types (some unique to a particular taxon)
      • may be scattered in other plant tissues.
      • may be concentrated in specific areas.
        • the "grit" in pear parenchyma/flesh ("stone cells")
        • peach, apricot, cherry, etc. pits
        • nut shells

      Fibers
      • are the main structural support elements in plants.
      • are thin and elongate.
      • narrow lumen.
      • found in many tissues and organs.
        • xylary - found in xylem
        • extraxylary - found in tissue other than xylem
      • used by humans to make textiles, paper

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(click on pic for source)

    Complex Tissue: The Dermal System

    The epidermis and periderm serve as a plant's outer covering.
    • Epidermis covers young stems, young leaves, and mature leaves.
    • Periderm covers roots and stems after secondary growth.

    Herbaceous plants retain epidermis all their lives.

    Woody plants start their lives as herbaceous plants.
    When young, they have epidermis.

    As a woody plant matures, however, the epidermis sloughs off
    and is replaced by periderm.

    Periderm is found only in woody plants.

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(click on pic for source)

    Epidermis

    Epidermis is usually only once cell layer thick.
    Leaves have both upper and lower layers of epidermis.

    A few plants have thicker epidermis in specific locations

    • Orchidaceae velamen
      • acts as aa sponge to absorb water and minerals

    • Piperaceae (Pepper Family) and some Ficus (fig) species
      • thick leaf epidermis protects against desiccation
      • These are ususally subtropical dry forest species.

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(click on pic for source)

    Epidermal Cells

    Epidermis is one cell layer thick.
    It contains specialized cell types and structures.

    • pavement cell - the main epidermal cells are thin and flattened
    • guard cells - border the stomates
    • trichomes - bear hairlike extensions
      • can deliver toxins
      • can confer pubescence

    Epidermal cells lack chloroplasts.

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(click on pic for source)

    The Cuticle

    Epidermis secrete an aceulluar, waxy cuticle
    composed of cutin that helps prevent desiccation.

    Vascular plants have much thicker cuticle than non-vascular plants.

    It is generally much thicker on the upper leaf epidermis
    than on the lower leaf epidermis.

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(photo by Dennis Kunkel)

    Guard Cells and Stomates

    A stomate is a small opening in the epidermis
    through which gas exchange takes place.

    It is bordered by two guard cells.

    In vascular plants, the guard cells can change shape with turgor pressure.

    • turgid guard cells open the stomate
    • flaccid guard cells close the stomate

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(click on pic for source)

    Trichome

    The trichome gets its name from the Greek trich, meaning "hair".

    A trichome is an epidermal cell bearing a hairlike extension of cytoplasm.

    Trichomes serve a wide variety of functions, including

    • increasing surface area (especially in roots)
    • confering pubescence
    • secreting toxins and other substances

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(click on pic for source)

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(click on pic for source)

    Periderm

    In woody plants, the epidermis is eventually replaced by periderm.

    This is the woody/corky layer commonly referred to as "bark".
    But the bark is only one of several periderm layers.

    • Phellem (cork) is the outer layer.
      Its cells are dead and hollow at maturity.

    • Phellogen (cork cambium) is the lateral meristem
      that generates periderm.

    • Phelloderm, located just internal to the cork cambium,
      is composed of living parenchyma.

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(click on pic for source)


(click on pic for source)

    Lenticels: Breathing Through Bark

    Before the phellem cells die, they secrete waxy suberin into their walls.

    The suberin makes the cork both
      • waterproof
      • nearly impermeable to air

    The latter problem is solved by lenticels.

    Lenticels are pockets of living, unsuberinized parenchyma cells.
    Gas exchange takes place through these openings in the periderm.

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(By Nefronus - Own work, Wikimedia Commons)

    Xylem and Phloem

    Plants lack a re-circulating fluid transport system.

    Instead, plants have tissues composed of longitudinal bundles of tubes.

    • Xylem - moves xylem sap (water, minerals) upwards from roots to leaves
    • Phloem - moves phloem sap (water, photosynthates) in multiple directions

    Water moves through a plant

      --> from the root
      xxxx--> through the stem
      xxxxxxxx--> through the leaf
      xxxxxxxxxxxx--> out into the atmosphere

    Xylem sap movement is unidirectional.
    Phloem sap movement is multidirectional.

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(click on pic for source)


(click on pic for source)

    Xylem Cells

    Xylem conducts water and minerals through
    • tracheids (more primitive)
      • dead & hollow at maturity
      • tapered at each end
      • perforated with many pores
      • relatively narrow diameter lumen
      • lie end-to-end with tapered ends meeting
      • form long conducting tubes

    • vessel elements (more derived)
      • dead & hollow at maturity
      • open at each end
      • perforated with many pits
      • relatively wide diameter lumen
      • lie end-to-end to form xylem tubes
      • perforation plate separates pairs of vessel elements

    Xylem metabolism and structural support is provided by
    • parenchyma (form live rays that conduct water laterally)
    • collenchyma
    • sclerenchyma

    Xylem is highly lignified.

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(By Kelvinsong, Wikimedia Commons)

    Phloem

    Phloem conducts water and organic substances through
    • sieve tube elements
      • contain a living protoplast
      • lack organelles or nucleus
      • relatively wide lumen
      • lie end-to-end to form sieve tubes

    • companion cell
      • is a type of transfer cell
      • is paired with a single sieve tube element (its sister cell)
      • living cell with all organelles and nucleus
      • provides metabolic support to its partner sieve tube
      • loads photosynthetically produced molecules into phloem

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(Photo by Rosanne Jordan, via Fine Art America)

    Plant Organs

    The simple and complex tissues of plants are
    organized into three different types of organs.

    In order of evolutionary appearance, they are:

      • stem

      • root
      • leaf

    Each of these has evolved specializations within and among species.
    Sometimes they've become unrecognizable as stem, root, or leaf.

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(click on pic for source)

    Stem: The First Plant Organ

    The stem is the main body or stalk of a plant.
    The primitive stem condition is to rise vertically from the ground.

    External Anatomy

    • apical meristem - meristem located at the center of a bud
    • terminal bud - growing tip on the end of each shoot
    • axillary bud - growing tip located in each leaf axil
    • leaf axil - the angled space where leaf meets stem
    • node - point on a stem where a leaf is attached
    • internode - stem between the nodes

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    Stem Layers in Cross section (cs)

    A stem is basically a cylinder made up of concentric layers of tissues.

    From external to internal layer, a generalized stem consists of

    • epidermis - complex tissue is a protective barrier
    • cortex - composed mostly of parenchyma
    • phloem - transports water and organic molecules made by the plant
    • vascular cambium - lateral meristem
    • xylem - transports water and minerals from the soil
    • pith - composed of parenchyma

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    Mature Herbaceous Stem (cs)

    A cross section of an herbaceous stem reveals
    a somewhat more complex structure.
    • epidermis - the "skin"
    • cortex - ground tissue infrastructure
      • mostly parenchyma
      • may be fortified with collenchyma and some sclerenchyma
    • vascular bundles/fascicles
      • phloem - transports organic solutes in water
      • xylem - transports inorganic solutes and minerals in water
    • pith - composed of parenchyma

    There is no vascular cambium if this plant remains
    herbaceous throughout its lifespan.

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    Young Woody Stem (cs)

    The architecture of a young woody stem is similar to that of an herbaceous stem.
    • epidermis - will eventually be replaced by bark
    • cortex - will eventually be replaced by bark
    • phloem - transports organic solutes in water
    • vascular cambium - gives rise to secondary xylem & phloem
    • xylem - transports inorganic solutes and minerals in water
    • pith - will eventually be replaced by xylem

    But in a wood species, the vascular cambium will give rise to

    • secondary xylem (wood rings)
    • secondary phloem (the innermost layer of bark)

    These are absent in an herbaceous plant.

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    Spring Wood and Summer Wood

  • spring wood
    • large vessel lumens
    • develops during spring/early summer
  • summer wood
    • smaller vessel lumens
    • develops during late summer

    The line formed between new spring wood and the previous year's summer wood creates a visible marker of each annual ring.

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(click on pick for source)

    Mature Wood: Heartwood and Sapwood

    After several years of growth, the vascular cambium has replaced
    all primary tissue with secondary growth.

    Secondary xylem:

    • sapwood - the outer cortex of a stem where xylem tubes are open and conducting water and minerals coming from the roots
    • heartwood - the central core of a stem where xylem tubes are clogged with resins and no longer conducts water

    Secondary xylem cell walls contain toxic
    secondary metabolites that deter herbivory.

    Secondary phloem:

    • grows outwards,
    • forms a band around the vascular cambium.
    • lies just beneath the bark.

    Careless lawn crews working too close with a weed whacker
    can girdle a tree, severing phloem all the way around.

    A girdled tree is doomed.

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Many species of plants have specialized stems that perform specific functions.
(required homework link!)

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    The Root

    The root is the first organ to emerge from a seed.
    Its functions are
    • anchor the plant to its substrate
    • absorb water and inorganic substances from the substrate
    • conduct the above upwards to the rest of the plant
    • production (in meristems) of certain hormones
    • production of secondary metabolites
    • storage of nutrients as carbohydrates and/or lipids

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(click on pick for source)


(click on pick for source)

    The Root Cap

    The root cap (= calyptra) is a sheath of parenchyma cells
    surrounding the growing root tip.

    The root cap

    • is produced by the apical meristem
    • protects the delicate tip and apical meristem from abrasion
    • tells the root tip which way is down (roots are gravitotropic)
      • Specialized root cap cells (statocytes) act as gravity sensors
      • Statocytes contain amyloplasts (w/ starch granules).
      • Inside the cell, the granules fall towards gravity.
      • Root tip grows in the direction of the amyloplasts.
      • Remove the root cap, and the root tip will grow in random directions.

    Outer root cap cells produce slippery mucilage that

    • lubricates the root as it grows through the soil
    • provides a growth medium for beneficial nitrogen-fixing bacteria

    Still, the root cap cells are constantly abraded off and replaced from within.

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    Root Cross Section

    Like the stem, the root has three main, cylindrical layers.
    • epidermis (water uptake)
    • cortex (nutrient storage)
    • stele (vascular tissue)

    But roots also have two special layers not present in stems

    • endodermis
    • pericycle

      Another difference between roots and stems:
      Stems have nodes, whereas roots do not.

      Only stems can sprout side branches from their nodes.

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(click for larger image)

    Endodermis: A Selective Filter

    The endodermis serves as a "filter" that allows
    only water and specific molecules into the stele.

    This is done by filling intercellular spaces with waxy suberin.

    Each endodermal cell bears a "belt" of suberin called a Casparian strip.

    Water and other molecules cannot pass through suberin.
    Their only path is through the plasma membrane.

    The plasma membrane is highly selective about what it allows through.

    • Polar substances must pass through very picky membrane channels.
    • Many non-polar substances can pass through the membrane itself.

    Thus, the endodermis filters the solvents and solutes
    entering the root vascular tissue.

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    Lateral Roots Arise from Pericycle

    The pericycle is a cylinder of meristematic cells
    lying just internal to the endodermis.

    It is a lateral meristem responsible for generating

    • lateral (branch) roots
    • part of the root vascular cambium

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    Water Uptake: Epidermis

    Root epidermis is the surface that meets the environment.
    It is the first barrier encountered by fluids entering the root from the soil.

    Surface area is increased by trichomes that form root hairs.

    Root hairs are found primarily in the Region of Maturation.
    They die off as the cells age.

    Thus, most water enters the root very close to the root tip.

    Although epidermal cell walls contain waxy suberin,
    water and minerals can pass easily between the cells of the epidermis.

    Most filtration is performed by the endodermis.

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(Source: Pearson Education)

    Root Symbiosis: The Nitrogen Cycle

    Earth's atmosphere is abou 78% nitrogen (N2 gas).

    Nitrogen is a critical component of

    • nucleic acids (DNA and RNA)
    • proteins

    But most organisms cannot sequester gaseous nitrogen.

    Only a few genera of bacteria are able to perform nitrogen fixation,
    the conversion of gaseous nitrogen into forms that plants can use.

    Plants incorporate these inorganic compounds into
    biological macromolecules that feed all other life forms on the planet.

    Without nitrogen fixation, life on earth as we know it would not exist.

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(click on pic for source)

    Root Nodules: Home of Nitrogen Fixers

    Here's how they set up shop...


    (Source: Pearson Education)

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Many species of plants have specialized roots that perform specific functions.
(required homework link!)

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(click on pic for source)

    The Leaf

    The leaf is the plant kingdom's most evolutionarily recent organ.
    It is the primary site of
    • photosynthesis
    • gas exchange

    The leaf has undergone more evolutionary diversification
    and specialization than either stem or root.

    There's a modified leaf for just about everything.

    • protection from herbivores
    • reducing water loss
    • storing water
    • storing food
    • holding toxins
    • climbing other plants
    • attracting pollinators for sex
    • trapping and digesting live animals
    • add your favorite here

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    The Leaf: External Anatomy

    A leaf consists of
    • blade
    • petiole

    The wedge-shaped space between the stem and petiole is the leaf axil.
    An axillary bud is located on the stem at each petiole's base.
    It contains an apical meristem, the source of side branches.

    Some leaves lack a petiole, and are attached directly to the stem.
    These are said to be sessile.

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    Anatomy of a Leaf

    From top surface downward
    • cuticle - a noncellular, waxy sheet of cutin secreted by the epidermis
    • epidermis - one-cell thick, lacks chloroplasts.
    • pallisade mesophyll - columnar, photosynthetic parenchyma
    • spongy mesophyll - cuboid, photosynthetic parenchyma.
    • vascular tissue - xylem on top, phloem on bottom
    • lower epidermis - contains most of the stomates
    • lower cuticle - usually thinner than the upper cuticle

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    Leaf Arrangement

    Phyllotaxy is the arrangement of leaves
    along the length of the stem at the nodes.

    • opposite
      • two leaves per node
      • offset in opposite directions

    • alternate
      • one leaf per node
      • offset in alternating directions

    • whorled
      • multiple leaves per node
      • attached in a radial pattern.

    • helical
      • one leaf per node
      • each offset to form a helical pattern

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Many species of plants have specialized leaves that perform specific functions.
(required homework link!)

And since you just spent half the semester learning about
leaves and transpiration in BIL 161 labs, we'll leaf it at that.

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