LectureTopic21-Plant-Structure

Plant Structure Overview

Are Plants Computers?

• Plants like romanesco grow according to genetically determined, repetitive programs.

• Plant development is highly dependent on environmental conditions, unlike animals, which emphasizes resource acquisition.

I. Developmental Plasticity

• Definition: The ability of an organism to change form in response to environmental conditions.

• Example: Water lily leaves demonstrate adaptations based on their surroundings.

Importance of Developmental Plasticity

• More pronounced in plants than in animals.

• Natural selection has led to adaptations in plant morphology:

  • Water retention features such as spines, water storage organs, and specific shapes.

II. Basic Plant Organs

• Three basic organs: roots, stems, and leaves.

  • Morphology reflects evolution for nutrient acquisition from both above and below ground.

  • Organized into two systems:

    • Root system

    • Shoot system

• Roots depend on sugars from photosynthesis in the shoot system, while shoots rely on water and minerals from the root system.

Structure of Plant Organs

• Reproductive shoot components:

  • Flower (reproductive shoot)

  • Apical bud (tip of the shoot)

  • Node (point of leaf attachment)

  • Internode (segment between nodes)

• Vegetative shoot components:

  • Leaf

  • Blade (the broad part of the leaf)

  • Petiole (stalk joining leaf to stem)

  • Axillary bud (potential for lateral shoot)

• Root components:

  • Taproot and lateral branch roots.

III. Roots

Functions of Roots

• Multicellular organs with three primary functions:

  1. Anchoring the plant in place.

  2. Absorbing minerals and water from the soil.

  3. Storing organic nutrients (e.g., sugars).

Types of Root Systems

• Taproot system: One main vertical root with lateral roots.

• Adventitious roots: Arising from stems or leaves.

• Fibrous root system: Typically in seedless vascular plants and monocots, with thin lateral roots and no main root.

Absorption

• Water and minerals absorption occurs primarily near root hairs.

• Root hairs significantly increase the surface area for effective uptake.

Root Modifications

• Prop roots: Support tall plants (e.g., corn).

• Storage roots: Example: carrots and beets.

• Pneumatophores: Air roots for plants like mangroves.

• Buttress roots: Provide stability for many rainforest trees.

• Strangling roots: Grow from branches and can harm host trees.

IV. Stems

Structure of Stems

• Composed of nodes (points where leaves attach) and internodes (stem segments between nodes).

• Bud types:

  • Axillary bud: Potential to form lateral shoot/branch.

  • Apical bud: Located at the shoot tip, promotes elongation of the young shoot.

• Apical dominance: Helps maintain dormancy in non-apical buds to prioritize growth of the main shoot.

Stem Modifications

• Rhizomes: Horizontal stems below the surface.

• Bulbs: Underground shoots with large storage leaves.

• Stolons: Horizontal shoots on the soil surface for asexual reproduction (runners).

• Tubers: Enlarged rhizomes or stolons for food storage.

V. Leaves

Structure of Leaves

• Leaves are the primary photosynthetic organs of vascular plants.

• Generally consist of:

  • Flattened blade

  • Petiole: stalk connecting leaf to stem node.

Types of Leaves

• Simple leaves: Single undivided blade.

• Compound leaves: Multiple leaflets from a single petiole; no axillary bud at the base.

• Doubly compound leaves: Leaflets are further divided.

Leaf Modifications

• Tendrils: Provide support.

• Spines: Offer protection, reduce surface area, and shade.

• Storage leaves: Store water and food.

• Reproductive leaves: Can develop into plantlets that grow roots.

• Bracts: Surround flowers and attract pollinators.

VI. Tissues in Plants

Types of Plant Tissue

• Three main categories of tissue systems:

  • Dermal tissue: Protective outer 'skin' of the plant.

  • Vascular tissue: Responsible for fluid transport.

  • Ground tissue: Involves all other functions including photosynthesis, storage, and support.

VII. Vascular Tissue System

Major Types of Vascular Tissue

1. Xylem: Conducts water and dissolved minerals upwards from roots to shoots.

2. Phloem: Transports organic nutrients made during photosynthesis to roots and growth sites.

Structure of Xylem

• Composed of tracheids and vessel elements, both dead at maturity:

  • Tracheids: Long, thin cells that conduct water.

  • Vessel elements: Shorter, wider cells aligned to form vessels for efficient water transport.

Structure of Phloem

• Conducts sugars primarily:

  • Sieve plates: Porous end walls facilitating fluid flow.

  • Companion cells: Serve both themselves and sieve-tube elements, providing necessary support and function.

VIII. Common Types of Plant Cells

Major Plant Cell Types

• Parenchyma: Thin-walled, most metabolic functions, undifferentiated initially.

• Collenchyma: Support young plant parts, thicker uneven cell walls providing flexibility.

• Sclerenchyma: Thickened walls for support, often dead at maturity (includes xylem and support fibers).

Study Questions

• Define developmental plasticity and its functions.

• Identify the three basic plant organs and their functions.

• Explain the functions of roots and the different types of roots.

• Describe the structure and types of stems and their bud types.

• Explain the composition and function of leaves, including leaf modifications.

• Discuss the categories of plant tissues and their respective roles.

• Describe the two types of vascular tissues and their functions.

• List the major types of plant cells, their characteristics, and functions.