Plant Structure

Plant Body Organization

A vascular plant consists of

$Root System$

• Anchors the plant.
• Used to absorb water and ions.

$Shoot System$

• Consists of supporting stems, photosynthetic leaves, and reproductive flowers.
• Repetitive units consist of internode, node, leaf, and axillary bud.

Plant Cell Types

Distinguishing plant cell types based on

• Size of vacuoles.
• Living or not at maturity.
• Thickness of secretions found in their $cellulose cell walls.$
  • Some cells have only a primary cell wall of cellulose, synthesized at the cell membrane of the $protoplast$ (the term for a plant cell w/o the cell wall).
  • Some cells have more heavily reinforced cell walls with multiple layers of cellulose.

New Growth Occurs at Meristems

$Meristems$

• Clumps of small cells with dense cytoplasm and large nuclei.
• Act as stem cells do in animals.
• One cell divides producing a $differentiating cell$ and another that remains $meristematic$.
• Extension of shoot and root produced by $apical meristems.$
• $Lateral meristems$ produce an increase in shoot and root diameter.

Apical Meristems

• Located at tips of stems and roots
• Give rise to primary tissues which are collectively called the primary plant body
• Apical meristems composed of delicate cells that need protection
  • $Root cap$ protects root apical meristem.
  • $Leaf primordia$ shelter shoot apical meristem.

Lateral Meristems

• Found in plants that exhibit $secondary growth$
• Give rise to secondary tissues which are collectively called the $secondary plant body$
• $Woody plants$ have two types
  • $Cork cambium$ produces $outer bark$.
  • $Vascular cambium$ produces $secondary vascular tissue.$
  • Secondary xylem is the main component of $wood$.

Plant Tissues

Three main types of tissue

• $Dermal.$

  • On external surfaces that serves a protective function.

• $Ground$.

  • Forms several different internal tissue types and can participate in photosynthesis, serve a storage function, or provide structural support.

• $Vascular.$

  • Conducts water and nutrients

Dermal Tissue

• Forms the $epidermis$
• One cell layer thick in most plants
• Forms the outer protective covering of the plant
• Covered with a waxy $cutin$ layer constituting the cuticle
• Contains special cells, including $guard cells$, $trichomes$, and $root hairs$.

Guard Cells in Dermal Tissue

• Paired sausage-shaped cells
• Flank a $stoma$ (pl.stomata) - epidermal opening
  • Passageway for oxygen, carbon dioxide, and water vapor.

Trichomes in Dermal Tissue

• Cellular or multicellular hairlike outgrowths of the epidermis
• Occur frequently on stems, leaves, and reproductive organs
• Reduce evaporation by covering stomatal openings
• Protect leaves from high-intensity light and ultraviolet radiation
• Can buffer against temperature fluctuations
• May be a single cell or multicellular
• Some are glandular, secreting substances that deter herbivory

Root Hairs in Dermal Tissue

• Tubular extensions of individual epidermal cells
• Greatly increase the root’s surface are and efficiency of absorption
• Should not be confused with lateral roots

Ground Tissue

3 cell types

$Parenchyma$

• Function in storage, photosynthesis, and secretion

$Collenchyma$

• Provide support and protection

$Sclerenchyma$

• Provide support and protection

Vascular Tissue

$Xylem$

• Conducts water and dissolve minerals throughout plant

$Phloem$

• Conducts a solution of carbohydrates - mainly sucrose - used by plants for food
• Also transports hormones, amino acids, and other substances necessary for plant growth

Xylem Cells

$Vessels$

• Continuous tubes of dead cylindrical cells arranged end-to-end.

$Tracheids$

• Dead cells that taper at the end and overlap one another
• Vessel members tend to be shorter and wider than tracheids

In addition to conducting cells, xylem typically includes fibers and parenchyma cells (ground tissue cells)

Phloem Cells

Contains two types of elongated food-conducting cells

• $Sieve cells$ (seedless vascular plants and gymnosperms) and $sieve tube members$ (angiosperms).
• Living cells that contain clusters of pores called sieve areas or sieve plates.
• Sieve-tube members are more specialized (more efficient).
  • Associated with companion cells, which carry out some of the metabolic functions needed to maintain the sieve-tube member.

Roots

-Simpler pattern of organization and development than stems

-Four regions are commonly recognized:

• $Root cap$.
• Boundaries not clearly defined.
  • $Zone of cell division$
  • $Zone of elongation$
  • $Zone of maturation$

Modified Roots

Most plants produce either/or:

• $Taproot system$ - single large root with small branch roots (Most Eudicots).
• $Fibrous root system$ - many small roots of similar diameter (Most Monocots)

Some plants, however, produce modified roots with specific functions

• Some are adventitious roots (roots that arise from any place other than the plant’s root).

Stems

-Like roots, stems contain the 3 types of plant tissue

-Also undergo growth from cell division in apical and lateral meristems

-Shoot apical meristem initiates stem tissue and intermittently produces primordia

• Develop into $leaves$, other $shoots$, and even $flowers$.

Type of Leaf Arrangement

Alternate: Ivy

Opposite: Periwinkle

Whorled: Sweet woodruff

External Stem Structure

• $Node$ - point of attachment of leaf to stem
• $Internode$ - area of stem between two nodes
• $Blade$ - flattened part of leaf
• $Petiole$ - stalk of leaf
• $Axillary bud$ - develops into branches with leaves or may form flowers
• $Terminal bud$ - extends the shoot system buring the growing seasom

Steam Vascular Tissue

Major distinguishing feature between monocot and eudicot stems is the organization of the vascular tissue system

• Monocot vascular bundles are usually scattered throughout ground tissue system.
• Eudicot vascular tissue is arranged in a ring with internal ground tissue ($pith$) and external ground tissue ($cortex$)

Vascular Tissue Arrangement

Vascular tissue arrangement is directly related to the stem’s ability for $secondary growth$

• In eudicots, a vascular cambium develops between the primary xylem and phloem.

  • Connects the ring of primary vascular bundles.

• In monocots, there is no vascular cambium - no secondary growth.

Tree Stump Rings

Rings in the stump of a tree reveal annual patterns of vascular cambium growth

• Cell size depends on growth conditions.
• In woody eudicots and gymnosperms, the cork cambium arises in the outer cortex.
  • Produces boxlike cork cells on outside and parenchyma-like phelloderm cells on inside.
  • Collectively called the $periderm$.

Lenticels

$Lenticels$ - Cork cambium produces unsuberized cells that permit gas exchange to continue

Modified Stems

$Bulbs$ - swollen underground stems, consisting of fleshy leaves

$Corms$ - superficially resemble bulbs, but have no fleshy leaves

$Rhizomes$ - horizontal underground stems, with adventitious roots

$Runners and stolons$ - horizontal stems with long internodes

$Tubers$ - swollen tips of rhizomes that contain carbohydrates

$Tendrils$ - twine around supports and aid in climbing

$Cladophylls$ - flattened photosynthetic stems resembling leaves

Leaves

• Initiated as $primordia$ by the $apical meristems$
• Principal site of photosynthesis
• Expand by cell enlargement and cell divison
• $Determinate$ in structure - growth stops at maturity
• Different patterns adaptive in different enviornments

Eudicot Leaves

• Most eudicot leaves are flattened
• Slender stalk called $petiole$
• Leaf flattening increases photosynthetic surface
• Flattening of the leaf blade also reflects a shift from radial symmetry to dorsal - ventral (top-bottom) symmetry

Veins

• Vascular bundles in leaves.
• Main veins are a parallel in most monocot leaves.
• Veins of eudicots form an often intricate network.

Leaf Epidermis

The leaf’s surface is covered by transparent epidermal cells

• Most have no chloroplasts

Epidermis has a waxy $cuticle$

• Different types of glands and trichomes may be present

Lower epidermis contains numerous stomata flanked by $guard cells$

Mesophyll in Eudicots and Monocots

Most eudicot leaves have 2 types of $mesophyll$ (ground tissue)

• $Palisade mesophyll$ - usually two rows of tightly packed chlorenchyma cells.
• $Spongy mesophyll$ - loosely arranged cells with many air spaces in between.
  • Function in gas exchange and water vapor exit.

Monocot leaves - mesophyll is usually not differentiated into palisade/spongy layers.

Modified Leaves

Floral leaves ($bracts$) - surround true flowers and behave as showy petals.

$Spines$ - reduce water loss and may deter predators

$Reproductive leaves$ - plantlets capable of growing independently into full-sized plant

$Insectivorous leaves$ - trap insects

• Pitcher plants have cone-shaped leaves that accumulate rainwater.
• Sundews have glands that secrete sticky mucilage.
• Venus flytrap have hinged leaves that snap shut.