Plant Reproduction, Fruit Development, and Tissue Systems

Flower Reproduction and Fruit Development

  • Pollination and Reproduction: When flowers are pollinated, it triggers reproduction, leading to seed formation.

  • Ovule and Ovary Development:

    • Pollen lands on the stigma, a pollen tube forms, and ovules within the ovary begin to develop.

    • Simultaneously, the ovary starts to swell and develop.

  • Fruit vs. Seed Origins:

    • Seed: Develops from the ovules (mature ovules).

    • Fruit: Develops from the ovary (the ovary develops into the fruit).

  • Examples of Fruit and Seed:

    • Cantaloupe, Peas, Kiwi: The visible seeds are the mature ovules, and the surrounding juicy part is the developed ovary (the fruit).

    • Apple:

      • Picture A shows remnants of an apple flower, dissected to reveal developing ovules.

      • The hypanthium (which is considered part of the ovary in apples) expands into the fleshy, delicious part of the apple.

      • Picture G shows mature, developed seeds encased within the fruit.

  • Botanical vs. Horticultural Fruit: Botanically, many structures (like acorns) are fruits. Horticulturally, the term "fruit" is often reserved for sweet, fleshy, edible structures, while other botanical fruits might be classified as vegetables (e.g., tomatoes, cucumbers). This distinction will be discussed later.

Seed Anatomy and Types

  • Seeds: Part of the mature ovary, developed from ovules, and contain a baby plant.

  • Monocot Seed (e.g., Corn):

    • Appearance: Dry, hard, difficult to peel apart.

    • Internal Structure (if dissected):

      • Plumule: Embryonic shoot.

      • Radicle: Embryonic root (not just a "root").

      • Cotyledon: Surrounds the baby plant, provides nutrition.

      • Endosperm: Largest part, stores nutrients (not photosynthetic).

      • Seed Coat: Outermost protective layer.

      • Pericarp: Fruit wall, fused with the seed coat in corn (very thin and inseparable into distinct layers).

  • Dicot Seed (e.g., Peanut):

    • Appearance: Easily split into two pieces after removing the papery covering.

    • Internal Structure:

      • Cotyledons: Two parts (as it's a dicot), provide nutrition.

      • Embryonic Leaf: Small, visible structure.

      • Radicle: Embryonic root.

      • Hypocotyl: Embryonic stem, clearly visible.

      • Testa: The seed coat around the outside of the seed (same as seed coat).

Fruit Morphology (Pericarp)

  • Pericarp: The fruit wall, developed from the ovary wall.

  • Differentiation in Fleshy Fruits (e.g., Peach): The pericarp has three distinct layers that are very exaggerated.

    • Exocarp: The outermost layer, the skin.

    • Mesocarp: The middle, juicy, delicious part.

    • Endocarp: The innermost layer, which forms the hard pit surrounding the seed. The seed itself is inside the endocarp, requiring force (e.g., a hammer) to extract.

  • Prefixes: Exo-\text{Exo-} (outside), Meso-\text{Meso-} (middle), Endo-\text{Endo-} (inside).

Fruit Classification Overview

  • Simple Fruits: Develop from a single ovary or several fused ovaries.

    • Can be dry or fleshy.

  • Multiple Fruits: Develop from the fused ovaries of multiple flowers in an inflorescence.

  • Aggregate Fruits: Develop from a single flower with multiple separate ovaries.

  • Note: Detailed identification and classification will be covered in laboratory sessions.

Plant Tissues: Organs to Cells

  • Hierarchy: Organs are made of tissues, and tissues are made of cells.

  • Types of Tissues: Differentiated into meristematic and non-meristematic tissues.

Meristematic Tissues (Actively Dividing Cells)

  • Meristem: Areas in a plant where cells are continually reproducing, leading to growth.

  • Cell Differentiation: Initially, meristematic cells are undifferentiated, but they eventually develop specific functions (e.g., dermal, vascular).

  • Types of Meristematic Tissue:

    • Apical Meristem:

      • Function: Responsible for primary growth (increase in length).

      • Location: Found at the tips of shoots (shoot apical meristem) and roots (root apical meristem).

      • Horticultural Significance: The shoot apical meristem suppresses the growth of axillary buds (lateral suppression, causing plants to grow tall and leggy).

      • Pinching: Removing the shoot apical meristem promotes the growth of axillary buds, leading to a bushier, branchier plant (a common horticultural practice for basil, poinsettias, etc.).

      • Reproductive Growth: The dome shape of the apical meristem changes when the plant transitions to reproductive growth.

      • Root Apical Meristem: Found at root tips; responsible for root elongation and the formation of new root hairs.

    • Intercalary Meristem:

      • Function: Responsible for primary growth (length) in specific plants.

      • Location: Found in grasses, located at the base of leaves and nodes, above the ground.

      • Evolutionary Adaptation: This low position protects the meristem from grazing animals (or mowing), allowing grasses to continue growing after being eaten from the top.

    • Lateral Meristem:

      • Function: Responsible for secondary growth (increase in width/girth).

      • Types:

        • Vascular Cambium: A thin sheet that produces xylem to the inside and phloem to the outside, causing the stem to widen.

        • Cork Cambium (Phellogen): Fills in the gaps created by expanding growth, producing cork (bark) to the outside and phelloderm to the inside.

Non-Meristematic Tissues (Non-Dividing Cells, Specialized for Support/Functions)

  • Dermal Tissue System:

    • Function: Outer protective covering of the plant.*

    • Location: Outer layer of stems, roots, leaves, and all other plant parts.

    • Primary Growth (Epidermis):

      • Cuticle: Waxy layer on the surface to prevent water loss.

      • Trichomes: Little hairs on the dermal tissue (e.g., root hairs from roots, specialized sticky hairs on cannabis or sundews).

      • Gas Exchange: Regulates gas exchange with the environment.

    • Secondary Growth (Periderm/Bark):

      • Periderm Layers:

        • Cork: Outermost layer, the actual bark.

        • Cork Cambium: Produces cork.

        • Phelloderm: A messy layer of cells just inside the cork cambium.

  • Vascular Tissue System (Plumbing/Transport):

    • Function: Transports water, minerals, hormones, and organic compounds throughout the plant.

    • Components: Always found adjacent to each other.

      • Xylem:

        • Function: Transports water and minerals from the roots up the plant.

        • Cell Type: Composed of dead, hollow cells (primarily cell walls).

        • Helpers: Requires companion cells called tracheary elements (e.g., tracheids) to aid function.

      • Phloem:

        • Function: Transports organic compounds (e.g., sugars produced during photosynthesis) from the source (where they're made, like leaves) to the sink (where they're needed, like roots for growth, or flowers).

        • Cell Type: Composed of sieve cells.

        • Helpers: Requires companion cells to aid function.

  • Ground Tissue System (Fundamental Tissue):

    • Function: The bulk of the plant, primarily for support, filler, and storage.

    • Location: Includes mesophyll in leaves, central stele in some roots, and pith (center of the stem).

    • General Classification: Tissues that don't fit into dermal or vascular categories.

Plant Cell Types (Introduction)

  • Plants have three main types of cells that comprise all tissues:

    • Parenchyma cells

    • Collenchyma cells

    • Sclerenchyma cells

  • Detailed examination of these cell types will occur in an upcoming lecture.