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    Plant Architecture

    Most of the plants we see every day are seed plants.

    However, not all plants produce seeds,
    and not all plants have the familiar appearance
    of a typical seed plant.

    But seed plant anatomy is a good starting point
    for exploring plant construction.

Simple Tissues

    Ground tissues make up the bulk of the plant.
    • parenchyma
    • collenchyma
    • sclerenchyma

Complex Tissues

    Complex tissues make up the dermal and vascular structures.

      Epidermis contains several specialized
      cell types and structures.

    • epithelial cell - flattened, no chloroplasts
    • stomates - bordered by guard cells
    • trichomes - multicellular, hairlike extensions
      • confer pubescence
      • deliver toxins

      (Glandular trichomes are epidermal outgrowths
      that can secrete or store large amounts of
      specific metabolites, from sticky mucilage to stinging toxins.)

    • Vascular tissue comprises xylem and phloem.
      Each of these is composed of multiple cell types.

      xylem is composed of

      • tracheids (dead & hollow; water conducting)
      • vessel elements (dead & hollow; water conducting)
      • parenchyma, collenchyma and sclerenchyma (structural support)

      phloem is composed of

      • sieve elements (alive, but with few organelles; sap conducting)
      • companion cells (fully equipped cell that feeds a sieve element)

Plant Organs

Simple and complex tissues combine to form plant organs.
There are only three. In order of evolutionary appearance, they are:

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


    Stem: Herbaceous vs. Woody

    The stem provides structural support against gravity,
    and provides transport via xylem and phloem.

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

    <--- Herbaceous plants are pliable, having little or no lignin.

    An annual plant

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

    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 become woody

    Many types of plants are "woody", however...
    <--- Only plants with a vascular cambium produce true, botanical wood.


CLICK ON PIC for a larger view

    Stem: Primary vs. Secondary Meristem

    As the apical meristem produces new cells, the stem grows upwards.

    The meristem cells mature into three primary meristems:

    • protoderm gives rise to epidermis
    • procambium gives rise to vascular tissues
    • ground meristem gives rise to ground tissues

    In woody plants, the procambium also gives rise to two secondary meristems:

    • vascular cambium
      • located between xylem and phloem
      • gives rise to xylem and phloem
    • cork cambium
      • located between phloem and dermal layer
      • gives rise to bark

    A secondary meristem

    • generates secondary/lateral growth, increasing a plant's diameter/girth.
    • may also be called cambium or lateral meristem.



    Herbaceous Stem, cross section

    A cross section of an herbaceous stem reveals tissue rings
    • 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

    Young Woody Stem, cross section

    The stem of a young woody plant is very similar to that of an herbaceous plant.
    • 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 wood

    The vascular cambium is a secondary meristem.
    It will give rise to secondary growth, increasing the stem's diameter with

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

    Stem: External View

  • apical bud - new growth from the apical meristem
  • axil - the wedge-shaped space between stem and leaf
  • node - points on stem where leaves sprout
  • internode - stem region between nodes
  • axillary bud - new growth from meristem at the node

    Woody stem anatomy is essentially the same.
    However, scars mark regions of annual growth. --->

    In temperate species, specialized leaves form
    bud scales that protect apical and axillary buds
    over winter.

    Secondary Growth in Woody Plants

    As a plant with a vascular cambium ages, its pith is replaced by secondary xylem.

    One ring of xylem forms over every growing season.
    Different growing conditions over the course of a year result in
    physically distinguishable regions in each xylem ring:

  • spring wood
    • large vessel lumens
    • develop during spring/early summer
  • summer wood
    • smaller vessel lumens
    • develop 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.

    Mature Wood

    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 is loaded with toxic secondary metabolites that deter herbivory.

    Secondary phloem:

      Secondary phloem grows outwards,
      forming a band around the vascular cambium.
      It 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.

    Stems Breathe

    How does a very large tree acquire enough oxygen for its needs?
    Some species have lenticels, spongy areas
    that replace groups of stomates present on the young herbaceous stem.

    A few familiar fruits and vegetables have visible lenticels.

    A potato is an underground stem called a tuber.

    The fleshy part of an apple is derived from
    the flower's receptacle, an evolutionarily
    modified stem.


Specialized Stems

Although the stem is the most evolutionarily ancient plant organ,
many plant species have highly derived stems with specialized form and function.


    A rhizome is an underground stem.
    It can be distinguished from a root
    by the presence of nodes,
    from which new aerial stems may emerge.

    Ferns and many other plants propagate
    asexually via rhizomes.


    A tuber is a rhizome modified to store
    nutrients and starch.

    The most familiar example is a potato,
    but wild tubers have been an important
    food source for hunter-gatherer humans
    for thousands of years.


    A tendril is a stem modified for clinging and climbing.

    (Tendrils also can be modified leaves.)

    Tendrils are positively thigmotropic:
    they grow towards areas of physical contact.


    A stolon is an aboveground stem that grows horizontally.

    It takes root at points along its length and sprouts new plants.

    Stolons are another means by which plants can propagate asexually, producing a clone.


    A bulb is an underground stem consisting of
    • a dense basal plate (short stem axis)
    • shoot primordium
      ...enclosed by thick, fleshy leaves.

    In some cases, the leaves function as
    nutrient storage reservoirs during dormancy.

    Old leaves dry and form a protective tunic
    around the fleshy, nutrient storage leaves.

    One particular bulb plant will be so
    familiar to you, it will make you cry.







    Like a bulb, a corm contains
    • a basal plate
    • a thin tunic
    • a central shoot primordium

    It lacks visible storage leaves,
    which distinguishes it from a bulb.

    Plants that develop from corms include
    gladiolus, freesias, crocus, and hyacinth.


    A pseudobulb is a modified stem unique to orchids.

    They are thickened, bulblike stems that store both water and nutrients.


    A cladophyll is a flattened stem that serves the photosynthetic function of a leaf.

    The term is derived from the Greek

    • clad, "branch"
    • phyll, "leaf"



    A thorn is a sharp, protective modified stem.

    Not all plant poky-bits are stems.
    They also can be derived from

    • sclerified, pointed epidermis ("prickle")
    • sclerified, pointed leaf ("spine")
    • sclerified, pointed trichome ("bristle")

    Thorns and spines have vascular tissue traces.
    Prickles do not.


Click on the Bonus Icon.

    The Root

    This organ is the first 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 that are transported other parts of the plant
    • production of secondary metabolites (e.g., nicotine in tobacco, which is transported to the leaves for deposition as an herbivore deterrent)
    • storage of nutrients as carbohydrates and/or lipids

    The most familiar plants develop a taproot,
    a large, central root from which lateral roots emerge.


    Root Meristems

    When any meristem cell divides
    • one daughter cell remains meristematic
    • its sister cell develops into a tissue lineage

  • Region of Cell Division - apical meristem
  • Region of Elongation - primary meristems (cells elongating)
  • Region of Maturation - mature, differentiated tissues

    Water Uptake: Epidermis

    Root epidermis is the surface that meets the environment.
    It is the first selectively permeable membrane the plant uses to filter solutes.

    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.
    Most water uptake occurs 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.

    Further filtration is performed by deeper layers of the root.

(Click on pic for larger view and explanations.)

    Water Uptake: Selective Filtration

    Like the stem, the root has three main layers:
    • epidermis - water uptake
    • cortex - nutrient storage
    • vascular tissue - water & nutrient transport

    But roots also have two special layers not found in stems

    • endodermis - selectively permeable layer
    • pericycle - lateral meristem generates side branch roots

    Fluids travel via

    • symplast - the connection formed by plasmodesmata
    • apoplast - the continuous surface formed by adjoining cell walls
    • tonoplast - pathway formed by vacuole plasma membranes

    The endodermis prevents apoplastic passage of solutes and water.
    Fluids following the symplast are selectively filtered.

    Only inorganic solutes and water can enter the root.
    Organic solutes ordinarily cannot cross the endodermis.

Specialized Roots

Like stems, roots have evolved diverse form and function among different plants.

    Food Storage Roots

    Foor Storage Roots sequester starch,
    The plant draws on this store for metabolic activities
    later in the season, such as flowering.

    Carrots, beets, turnips, and sweet potatoes
    are familiar examples.

    Water Storage Roots

    Many plants living in very arid habitats have roots
    that collect large amounts of water during rainy season.

    The plant draws on its stores throughout the dry season.

    Adventitious Roots

    Adventitious Roots arise from a non-root organ (stem or leaf).

    Adventitious roots may be produced either

    • during normal development
    • in response to stress conditions
      • flooding
      • nutrient deprivation
      • wounding

    Propagative Roots

    Propagative roots have meristematic regions from which
    new, genetically identical plantlets sprout.

    These regions are not the same as axillary buds at stem nodes. They lack a true apical meristem.

    <--- Agave propagates from roots.


    Pneumatophores (Greek pneum, "breathe" and phor, "to bear")
    are aerial roots whose tips are equipped with numerous lenticels.

    The root tips protrude from water-logged soil
    to provide gas exchange surface.

    You can visit pneumatophores on our local shorelines in the mangrove forests.

    Prop Roots

    Prop roots grow from the lower stem or trunk and provide extra support.

    They are most commonly seen in

    • plants with relatively flexible, tall stems
    • plants that live in softer soils

    Aerial Roots

    Aerial roots are typically found in epiphytes such as orchids and bromeliads.

    They sometimes can grow adventitiously from branches of certain species of soil-anchored plants.

    Buttress Roots

    Buttress roots are wall-like extensions from the base of the trunk.

    They provide support against physical assault from high winds
    exerting force on the canopy.

    Our native Strangler Fig (Ficus aurea) and some popular ornamentals
    such as the Royal Poinciana (Delonix regia) tend to develop these
    when grown in shallow soil over a solid coral rock base.

    Contractile Roots

    Contractile roots are most often found at the base
    of a corm, bulb, or other underground structure.

    During drought, they expand and contract vertically,
    pulling the bulb deeper into the soil to a level appropriate
    for its species and the environmental conditions.


    Haustoria are specialized, peg-like modified roots that tap into a host plant's vascular bundles to extract water, carbohydrates, and nutrients.

    Parasitic dodders (Cuscuta spp., Convolvulaceae) have little or no chlorophyll, and must feed on a host plant via haustoria.


    The Leaf

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

    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.

    Leaf Shapes

    Leaf shape is often a useful character for plant identification.

    There are many different leaf shapes, only some of which are shown here.

    <--- Leaves may be entire (no teeth or indentations).


    XXXXXXXXXXXXXXXXXXXXXXLeaves may be lobed. --->

    Leaves may be simple or compound
    in any given shape.

    A compound leaf is composed of multiple leaflets attached to a central rachis.

    How can you tell the difference
    between a leaf and a leaflet?

    Look for the axillary bud at the base of the structure.
    Axillary buds are located

    • ONLY at the base of a petiole/rachis
    • NEVER at the base of a leaflet

    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 in in a helical pattern

    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.
      x Copious air spaces facilitate transpiration .
    • vascular tissue - xylem on top, phloem on bottom
    • lower epidermis - contains most of the stomates
    • lower cuticle - usually thinner than the upper cuticle

    Leaf Modifications and Specializations

    Leaves are highly plastic in form.
    The same plant may have leaves of different size, shape, and color,
    depending on the leaves' growing conditions.

    Leaves have undergone more evolutionary diversification and specialization
    than other plant organs in response to natural selection.

    • cactus spines
    • climbing tendrils
    • prey traps
    • nutrient storage
    • water storage
    • pollinator attraction
    • drip tips to shed excess rainwater

    Plant leaf morphology reflects the plant's natural history.

    • A xerophyte is adapted for life in a very arid environment.
    • A hydrophyte is adapted for life in a very wet environment.
    • A mesophyte is adapted for life in a moderate environments.

    But the most important evolutionary innovation was
    the sporophyll.

    A flower is a whorl of leaves called sporophylls
    that are specialized for reproduction.