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Plant Organs and Modular Growth

Plant Organs and Modular Growth

Overview of Plant Growth

  • Plants, unlike many animals (e.g., lion, Panthera leo L.), exhibit modular growth.

  • Modular growth involves adding repeated units derived from cell division in meristems.

    • Example: Giant sequoia, Sequoiadendron giganteum (Lindl.) J. Buchh. (Cupressaceae), demonstrates this growth pattern.

Classification of Plant Organ Types

Plants possess distinct organ types classified into two main categories:

  1. Vegetative Organs:

    • Stems

    • Leaves

    • Roots

  2. Reproductive Organs:

    • Flowers/cones

    • Seeds

    • Fruits

    • Note: Reproductive organs will be covered in separate plant reproduction modules.

Vegetative Organs: Stems

  • Definition: One of the two main structural axes of a plant, typically growing aboveground.

  • Functions:

    • Provides support for leaves and reproductive organs.

    • Transports fluids (water, nutrients, sugars).

    • Stores materials (e.g., starches).

    • Produces new tissue through meristematic activity.

  • Key Features:

    • Node: The point on a stem where leaves or flowers originate and proliferate.

    • Internode: The segment of a stem located between two successive nodes.

    • Petiole: The stalk that attaches a leaf blade to the stem.

Specialized Stem Examples

  • Rhizome:

    • A horizontally growing, belowground stem.

    • Capable of producing roots and leaves at its nodes.

  • Stolon:

    • A horizontally growing, aboveground stem.

    • Often called "runners."

    • May produce roots and leaves at its nodes.

    • Example: Buffalograss (Bouteloua dactyloides (Nutt.) Columbus (Poaceae)) in Wind Cave National Park, SD, shows prominent stolons.

  • Thorn:

    • A reduced, stiffened stem with a sharp point and a rounded shape.

    • Occurs on the upper side of an axil, which is a branch position.

    • Crucifixion thorn (Castela emoryi (A. Gray) Moran & Felger (Simaroubaceae)) from Maricopa County, AZ, is an example.

    • Key Distinction: A thorn is botanically a modified stem.

  • Cladophyll (or Cladode):

    • Photosynthetic stems that function as and may resemble leaves.

    • Example: Found on Vancouver Island, B.C.

Vegetative Organs: Leaves

  • Definition: One of the two main structural axes of a plant, primarily an aboveground organ usually specialized for photosynthesis.

  • Parts:

    • Blade/Lamina: The flattened, expanded portion of the leaf, where most photosynthesis occurs.

    • Petiole: The stalk connecting the blade to the stem.

    • Sheath: In some plants (like grasses), the basal part of the leaf that wraps around the stem.

Specialized Leaf Examples

  • Grasses (Poaceae):

    • Exhibits a distinct blade (lamina) and a sheath, as seen in Northern Sonora, MX.

  • Conifers:

    • "Scale leaves" are typical of Cupressaceae families.

    • "Needles" are characteristic of Pinaceae families.

  • Spine:

    • A stiff, slender, sharp-pointed modified leaf.

    • Arises from below the epidermis on the lower side of an axil, which is a leaf position.

    • Key Distinction: A spine is botanically a modified leaf.

Distinguishing Thorns vs. Spines vs. Prickles

  • Thorn: Modified stem.

  • Spine: Modified leaf.

  • Prickle: A small, sharp outgrowth of the epidermis or bark, not a modified organ in itself.

    • Example: The sharp protections on a rose (Rosa sp. (Rosaceae)) are prickles.

Leaf Morphology Terms

Botany utilizes specific terms to describe leaf characteristics:

  • Shape:

    • Acicular (needle-shaped)

    • Falcate (hooked/sickle-shaped)

    • Orbicular (circular)

    • Rhomboid (diamond-shaped)

    • Acuminate (tapering to a long point)

    • Flabelate (fan-shaped)

    • Ovate (egg-shaped, wide at base)

    • Rosette (leaflets in tight circular rings)

    • Alternate (leaflets arranged alternately)

    • Hastate (triangular with basal lobes)

    • Palmate (like a hand with fingers)

    • Spatulate (spoon-shaped)

    • Aristate (with a spine-like tip)

    • Lanceolate (pointed at both ends)

    • Pedate (palmate, with divided lateral lobes)

    • Spear-shaped (pointed, barbed base)

    • Bipinnate (leaflets also pinnate)

    • Linear (parallel margins, elongate)

    • Peltate (stem attached centrally)

    • Subulate (tapering point, awl-shaped)

    • Cordate (heart-shaped, stem in cleft)

    • Lobed (deeply indented margins)

    • Perfoliate (stem appearing to pierce leaf)

    • Trifoliate/Ternate (leaflets in threes)

    • Cuneate (wedge-shaped, acute base)

    • Obcordate (heart-shaped, stem at point)

    • Odd Pinnate (leaflets in rows, one at tip)

    • Tripinnate (leaflets also bipinnate)

    • Deltoid (triangular)

    • Obovate (egg-shaped, narrow at base)

    • Even Pinnate (leaflets in rows, two at tip)

    • Truncate (squared-off apex)

    • Digitate (with finger-like lobes)

    • Obtuse (bluntly tipped)

    • Pinnatisect (deep, opposite lobing)

    • Unifoliate (having a single leaf)

    • Elliptic (oval-shaped, small or no point)

    • Opposite (leaflets in adjacent pairs)

    • Reniform (kidney-shaped)

    • Whorled (rings of three or more leaflets)

  • Margin:

    • Ciliate (with fine hairs)

    • Crenate (with rounded teeth)

    • Dentate (with symmetrical teeth)

    • Denticulate (with fine dentition)

    • Doubly Serrate (serrate with sub-teeth)

    • Entire (even, smooth throughout)

    • Lobate (indented, but not to midline)

    • Serrate (teeth forward-pointing)

    • Serrulate (with fine serration)

    • Sinuate (with wave-like indentations)

    • Spiny (with sharp stiff points)

    • Undulate (widely wavy)

  • Venation:

    • Arcuate (secondary veins bending toward apex)

    • Cross-Venulate (small veins connecting secondary veins)

    • Dichotomous (veins branching symmetrically in pairs)

    • Longitudinal (veins aligned mostly along the long axis of the leaf)

    • Palmate (several primary veins diverging from a point)

    • Parallel (veins arranged axially, not intersecting)

    • Pinnate (secondary veins paired oppositely)

    • Reticulate (smaller veins forming a network)

    • Rotate (in peltate leaves, veins radiating)

Vegetative Organs: Roots

  • Definition: An organ typically growing underground; it lacks leaves and nodes.

  • Functions:

    • Primary function is to absorb water and nutrients from the soil.

    • Anchors the plant securely in the substrate.

    • Stores nutrients and other plant products.

Types of Root Systems

  • Fibrous Root System:

    • Composed of numerous thin, moderately branching roots that originate directly from the stem.

    • Characterized by no single dominant (large) root.

  • Taproot System:

    • Features an enlarged, somewhat straight to tapering root that grows vertically downward.

    • This central taproot forms a main axis from which other, smaller lateral roots grow.

Activity

  • As a practical exercise, observe and identify examples of the described plant organs (stem, node, internode, rhizome, stolon, thorn, cladophyll, leaf, petiole, blade, sheath, scale, needle, spine, prickle, root, fibrous root, taproot) in your daily environment.

  • Images of these observations can be shared and discussed for feedback.

Transition to Photosynthesis

  • The next module will cover the process of photosynthesis, which is critical for plant life.