Weber State University          Department of Botany

                                                    BOTANY  LS1203 - PLANT  BIOLOGY

Plant Cells and Tissues

cell = smallest living unit; engages in metabolism, reproduction; interacts with and responds to environment
eukaryotic cell ==>  nucleus (vs. prokaryotic bacterial cells)

tissue = collection of cells that cooperate in performing one of more functions
The tissues of the angiosperms provide functions that make them highly successful land plants:
water acquisition
water conservation
support
gas exchange
optimize light collection
minimize heat load and damage from solar radiation

Three vegetative organs:  root, stem, leaf

Four Types of Tissues:
Meristematic:  apical meristem, primary meristems (procambium, protoderm, ground meristem), lateral meristems (vascular cambium, cork cambium)
Ground Tissue:  pith, cortex, mesophyll
Dermal Tissue:  epidermis, periderm (cork, cork cambium, phelloderm)
Vascular Tissue:  xylem, phloem

meristem = a collection of dividing cells found in specific places in a plant

Primary Growth (herbaceous growth)
performed by all plants at some point in their lives
occurs at tips of roots and stems
results in increase in plant length
divisions of cells in the apical meristem produce three moderately differentiated primary meristems that make the three primary tissues:
    apical meristem —>  primary meristems:  —>  primary tissues:
                                     ground meristem                mesophyll, pith, cortex, ground tissue; parenchyma, collenchyma, sclerenchyma
                                     protoderm                         epidermis
                                     procambium                      1° xylem and 1° phloem

Secondary Growth (woody growth)
not performed by all plants
lateral meristems produce increase in plant girth
provide new vascular and protective tissues
new cells are laid down in rings
vascular cambium —>  2° xylem and 2° phloem
cork cambium   —>  cork and phelloderm

Ground Tissue
    food or water storage, photosynthesis, respiration, support, mechanical protection
    might be divided into regions of cortex and pith by the vascular tissue; called mesophyll in leaves

Parenchyma Cells
    alive at maturity
    found in all tissues, esp. ground tissue
    thin primary cell wall, polyhedral shape
    large vacuole
    photosynthesis, food storage, water storage, respiration
    least specialized cell, therefore shows totipotency best:  wound healing, vegetative propagation

Collenchyma Cells
    alive at maturity
    generally found just inside the epidermis, in strands or cylinders
    unevenly thickened 1° cell walls:  provide flexible support for young plant parts

Sclerenchyma Cells
    usually dead at maturity
    thick 2° walls, often lignified
    two types:  fibers (long, thin) and sclereids (boxy, branching)
    provide mechanical protection, support, sometime storage

Dermal Tissues
    outermost cell layers
    mechanical protection, chemical protection, gas exchange, water conservation

Epidermis

    epidermal cells
    guard cells (monocot vs. dicot), stoma
    trichomes:  one cell, multicellular, glandular
    bulliform cells
    cuticle:  cutin, wax

Periderm
    cork cambium: produces cork to the outside and phelloderm to the inside
    cork:  cork cells, dead at maturity, suberized, no air spaces, gas exchange via lenticels
    phelloderm:  parenchyma cells, alive at maturity, photosynthesis


Vascular Tissues

Xylem
    transport of water and dissolved minerals obtained from soil
    parenchyma cells
    fibers
    tracheids (found in all vascular plants)
    vessel elements/vessel members (found mostly in just the angiosperms)

Phloem
    transport of sucrose and other soluble organic molecules
    parenchyma cells
    fibers
    sieve tube elements/sieve tube members with sieve plate; no nucleus at maturity
    companion cells
    (sieve cells and albuminous cells in non-flowering plants)

Roots

Functions
anchorage:  deep roots or spreading roots
absorption of water and minerals
storage (food or water)
conduction


Radicle:  the embryonic  root

Dicots:  taproot system
    the radicle grows into a single primary root; additional roots branch off of the primary root
    can grow very deep into soils
    can enlarge radially for storage

Monocots:  fibrous root system
    after the radicle begins to grow, additional roots grow from the stem (adventitious roots) that are uniform in diameter with the radicle; still have extensive branch roots
    tend to be shallower than taproots
    very good at anchorage and preventing soil erosion

Adventitious Roots
    grow on organs other than roots: stems, leaves
    on cuttings, growth is enhance by auxins (a class of plant hormones)
    can perform all root functions
    vegetative propagation

Root Tip
    region of cell maturation:  where cells become specialized; has root hairs
    region of cell elongation:  where cells enlarge
    region of cell division:  where the apical and primary meristems are located
    root cap:  
        protects the meristems as expendable cells face the ripping action of the soil
        senses gravity
        secretes mucigel
            prevents dessication
            better contact with soil for water and mineral absorption
            lubricates the soil for easier root passage
            provides a specific microhabitat (rhizosphere) for beneficial soil microorganisms

Epidermis
    covers the entire root surface except the root cap
    usually lacks stomata
    no or very thin cuticle:  thin cuticle lets water and minerals in, provides protection from harmful bacteria and fungi; thicker cuticle on older roots near the soil surface where drying is a problem  
    root hairs:  extensions of epidermal cells; greatly increase the root surface area for absorption

Cortex
    ground tissue between the epidermis and the vascular cylinder
    three layers:  
        hypodermis (sometimes); might have suberin to prevent water loss
        storage parenchyma cells
        endodermis
            tightly packed cells
            Casperian strip of suberin and lignin to force filtering of dissolved minerals

Vascular Cylinder (stele)
    everything inside the endodermis
    pericycle
        a meristematic tissue
        source of lateral (branch) roots (arise internally from mature root)
        contributes to lateral meristems in woody root systems
    primary xylem and phloem
    in monocots:  pith of  parenchyma cells
    in dicots:  procambium persists

Modified Roots
    Storage:  beet, turnip, radish, carrot, sweet potato, dandelion
    Propagation:  adventitious buds of aspen, apple
    Parasitism:  haustoria of mistletoe

Beneficial Root/Microbe Interactions
    Mycorrhizae (fungi) and Rhizobial Bacteria:  to be covered later
        
Uses of roots
    veggies:  carrots, turnips
    staple food crops:  sweet potato, cassava
    flavors:  licorice, sarsparilla
    drugs:  ipecac, ginseng
    erosion control
    dyes:  madder

Stems

with leaves ==>  shoot system

Stem Functions
    support and display leaves (for photosynthesis) and flowers (for pollination)
    conduction between the leaves and roots via xylem and phloem
    photosynthesis
    storage (food, water)

External features of stems
    nodes vs. internodes
    axillary (lateral) buds
        spare apical meristems
        branches, with next set of leaves
    phyllotaxy:  alternate, opposite, whorled
    lenticels
    leaf scar
    vascular bundle scar
    bud scale scar

Internal Stem Anatomy

Epidermis
    epidermal cells
    stomata with guard cells
    trichomes
    cuticle

Ground Tissue
    parenchyma
    fibers
    collenchyma
    pith, cortex

Vascular Tissue
    vascular bundles, xylem towards in the inside and phloem towards the outside
    procambium persists in dicots
    In most dicots, the vascular bundles are arranged in a ring and divide the ground tissue into pith and cortex regions.
    In most monocots, the vascular bundles are found throughout the ground tissue.

Secondary Growth in Stems
    new cells are produced internally, in rings
    external most cells are continually destroyed and must be replaced; internal most cells accumulate as wood

vascular cambium
    2° xylem and 2° phloem
cork cambium
    cork:  dead at maturity, suberized, gas exchange via lenticels
    phelloderm:  parenchyma cells, alive at maturity, photosynthesis

wood = 2° xylem
hardwood (dicots) vs. softwood (conifers)
sapwood vs. heartwood
growth rings:  spring (early) wood + summer (late) wood
knots

bark = all tissues to the outside of the vascular cambium; includes the periderm and the phloem
    splits and gets crushed as secondary growth continues; does not accumulate like xylem does
    periderm =  cork cambium + cork + phelloderm

Modified Stems
    tendrils:  to anchor climbing plants (some tendrils are modified leaves)
    succulence:  for water storage, e.g. cacti
    runner:  horizontal, above ground stem (strawberry)
    rhizome:  horizontal, under ground stem (iris, begonia)
    bulb:  rosette stem with fleshy leaves (onion)
    tubers:  underground, swollen stem tips for storage (potato)
    thorns:  protection

Uses of stems
    sugar cane
    flax (for linen fibers)
    potato tubers
    from secondary growth:
        lumber
        paper
        fuel
        spices:  cinnamon (from bark)
        medicines:  quinine (from bark)
        as intact trees:  ceremonial, commemorative, shade, etc.

Leaves
    
Primary function = PHOTOSYNTHESIS; problem = transpiration

Leaf = blade + petiole
    can be sessile; sheath
    simple leaf = undivided blade (blade margin can be smooth or with various indentations)
    compound leaf = blade divided into leaflets  (leaflets on same plane, no buds)
        pinnately compound ==> rachis
        palmately compound
    veins:  netted venation (dicots = “broad leafed plants”), parallel venation (monocots = “narrow leafed plants”)

Epidermis
    usually transparent
    no chloroplasts (exceptions:  guard cells)
    lots of stomata
        horizontal leaves ==> preferential distribution to lower epidermis, especially in leaves of woody dicots
        vertical leaves ==>  even distribution to lower and upper epidermis
        in rows between parallel veins of monocots; scattered in dicots
        transpiration/photosynthesis compromise
    bulliform cells
    trichomes
    cuticle

Vascular Tissue
    xylem and phloem in bundles (veins);  xylem toward upper epidermis

Ground Tissue = Mesophyll
    sclerenchyma
    parenchyma with lots of chloroplasts (chlorenchyma)
    dicot:  palisade mesophyll, spongy mesophyll
    monocot:  uniform mesophyll


Moisture affects leaf development and features:   xerophytes, hydrophytes, mesophytes

Mesophytes:  grow best in moist, but not wet, environments
    find structures for moderate conservation of water:  adequate cuticle, stomata on lower epidermis

Xerophytes:  grow in seasonal or persistent arid conditions
    water conservation vs. water storage (succulence)
    small, thick leaves
    multiple palisade and spongy mesophyll layers
    multiple epidermis
    very thick cuticle
    lots of stomata (for maximum CO₂ collection = photosynthesis = when water is available)
    sunken stomata, with trichome overlays
    lots of sclerenchyma cells for support when water (and therefore turgor pressure) are low
    special type of photosynthesis found in most succulents
    deciduous in dry season

Hydrophytes:  grow partially or completely submerged in water
        problems getting enough light and CO₂ for photosynthesis
    Elodea:  submerged ==>  no cuticle or stomata, only a few cells thick, no extensive vascular tissue
    large, thin leaves
    poorly developed mesophyll
    lots of aerenchyma for gas exchange and buoyancy
    thin (if any) cuticle
    stomata on air-exposed surfaces, e.g. the upper epidermis of floating leaves (water lily)
    low in xylem, support cells
    thin cell walls
    reduced root systems
    leaf dimorphism in emerged vs. submerged leaves of the same plant

Leaf Abscission
    deciduous vs. evergreen
    
Abscission zone
    fewer sclerenchyma cells
    separation layer forms in the zone
    cells get suberin that seals off the vascular tissue and other cells —> leaf scar

Signs:  loss of chlorophyll; leaves change color to yellow, red, etc.
    also losing proteins, nucleic acids, etc.
Where do the leaf materials go?

Modified leaves
    tendrils (pea):  compound leaves of peas:  the end leaflets are tendrils
    stipules:  at the base of petioles; photosynthesis, tendrils
    spines (cactus):  for protection
    bud scales:  overlapping, tough, waterproof.  Protect buds from frost, drying, pathogens
    storage (succulence, for water storage; food storage of onion)
    insect trapping by carnivorous plants:  pitchers, sticky surfaces, fast movements
    asexual reproduction (e.g. Kalenchoë), propagation (Begonia, Sedum)
    cotyledons:  storage, absorption
    flower parts

Uses of leaves
    veggies:  lettuce, cabbage, spinach
    herbs:  parsley, bay, thyme
    drinks:  tea, herbal teas
    dyes:  henna
    fibers:  sisal (Agave)
    drugs:  digitalis (foxglove), belladonna (atropine), tobacco, marijuana

Review

Cells and tissues
    primary tissues:  epidermis, ground tissue, vascular tissue
    functions of the tissues; know the cell types found in each tissue; know the tissue specific functions of the cells
    primary meristems:  protoderm, ground meristem, procambium
    be able to identify, describe, and give the function(s) of various plant cell types:  parenchyma, collenchyma, sclerenchyma (fiber, sclereid), vessel element, sieve tube element, trichome, guard cells
    
Roots:    functions; types of root systems
    modifications and their functions
    root regions
    development (apical meristem —> 1° meristems  —>   1° tissues)
    functions of tissues
    cell types found in tissues
    endodermis, Casparian strip, pericycle
    root hairs
    origin of branch (lateral) roots

Stems:  functions
    modifications and their functions
   development (apical meristem —> 1° meristems  —>   1° tissues)
    compare tissue arrangements in monocots and dicots
    functions of tissues
    cell types found in tissues
    How does the origin of branch stems differ from the origin of branch roots?

Secondary growth
    secondary tissues (2° xylem, 2° phloem, cork, and phelloderm), their functions, and their cell types
    wood:    early wood vs. late wood; sapwood vs. heartwood; hardwood vs. softwood
    How do knots form?
    bark vs. wood; bark vs. periderm
    external features of stems
    Where are the youngest tissues located?  The oldest?        

Leaves:  functions
    modifications and their functions
    development (apical meristem —> 1° meristems  —>   1° tissues)
    compare tissue arrangements in monocots and dicots
    functions of tissues
    cell types found in tissues
    abscission
    venation patterns:  parallel, netted
    petiole, blade, stipules, sessile, compound leaves (palmate and pinnate), simple leaves, sheath

mesophytes vs. xerophytes vs. hydrophytes:  be familiar with the anatomical differences between the leaves of these three groups of plants.  How do the anatomical features provide a survival advantage to a xerophyte or hydrophyte in its particular environment?

Be able to trace the lineage of  tissues from the apical meristem to 1° tissues, both forwards and backwards.

What products do we obtain from various plant organs?  What features of the anatomy of the plants make them good for these uses?

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30 August 2004.