Plant Growth and Development

Plant Growth and Development

I. Meristems: Where the Growth Occurs

  • Definition of Meristems:

    • Meristems are defined as perpetually embryonic tissue that allows for indeterminate growth.

    • Rare cases exist where plants exhibit determinate growth.

  • Types of Meristems:

    • Apical Meristems:

    • Location: Found at the tips of roots and shoots, as well as at the axillary buds of shoots.

    • Function: Responsible for elongating shoots and roots, a process termed primary growth.

    • Lateral Meristems:

    • Function: Add thickness to woody plants, referred to as secondary growth.

    • Types of Lateral Meristems:

      • Vascular Cambium:

      • Adds layers of vascular tissues called secondary xylem (wood) and secondary phloem.

      • Cork Cambium:

      • Replaces the epidermis with periderm, which is thicker and tougher than the original epidermis.

II. Primary Growth: Roots

  • Vascular System:

    • Known as the stele in roots or stems.

    • In angiosperms, the stele of the root is organized into a vascular cylinder.

  • Outcomes of Primary Growth in Roots:

    • Produces epidermis, ground tissue, and vascular tissue.

  • Cortex:

    • Fills the region between the vascular cylinder and epidermis.

    • Endodermis:

    • The innermost layer of the cortex.

  • Root Structure Overview:

    • Overview of root parts includes:

    • Epidermis

    • Ground tissue

    • Vascular tissue

    • Zones:

      • Zone of differentiation

      • Zone of elongation

      • Zone of cell division, featuring mitotic cells (including apical meristem).

      • Root cap: Protects the meristem during root growth.

III. Primary Growth: Shoots

  • Shoot Apical Meristem:

    • Characterized as a dome-shaped mass of dividing cells located at the shoot tip.

  • Leaf Development:

    • Leaves arise from leaf primordia positioned along the sides of the apical meristem.

  • Axillary Buds Development:

    • Develop from the meristematic cells that are left at the bases of leaf primordia.

IV. Tissue Organization of Stems

  • Development of Lateral Shoots:

    • Occur from axillary buds located on the stem's surface.

  • Vascular Tissue Arrangements:

    • In most eudicots:

    • Vascular tissue consists of vascular bundles arranged in a ring formation.

    • In most monocots:

    • Vascular bundles are scattered throughout the ground tissue, not forming a ring.

  • Key Components:

    • Phloem: Transports sugars and nutrients.

    • Xylem: Conducts water and minerals.

    • Pith: Central area consisting of parenchyma cells providing storage and support.

V. Tissue Organization of Leaves

  • Epidermis:

    • Functions to reduce water loss.

  • Stomata:

    • Facilitate CO₂ exchange between air and photosynthetic cells.

    • Stomatal pores are flanked by two guard cells that regulate opening and closing.

  • Mesophyll:

    • The ground tissue in leaves, divided into:

    • Palisade Mesophyll: Located in the upper part for photosynthesis.

    • Spongy Mesophyll: Loosely arranged for gas exchange.

  • Vascular Tissue:

    • Leaf’s vascular tissue connects with stem vascular tissue to transport sugars and receive water and nutrients.

  • Veins:

    • Function as the skeleton of the leaf, running as vascular bundles.

VI. Secondary Growth

  • Definition:

    • Secondary growth enhances and contributes to the girth of stems and roots in woody plants.

  • Secondary Plant Body:

    • Composed of tissues produced by the vascular cambium and cork cambium.

  • Characteristics:

    • Secondary growth is a common feature of gymnosperms and many eudicots but only occurs in a few monocots.

  • Visual Representation:

    • Cross-sections of stems demonstrating primary and secondary growth indicate vascular cambium, cork cambium, secondary xylem, and phloem.

Review Points

  • Key questions addressed for understanding:

    • What are meristems? Types and specific lateral meristems explained.

    • Difference between primary and secondary growth.

    • Mechanism of root growth, outcomes of primary growth, and formation of lateral roots.

    • Definition and process of shoot apical meristem development, and relationships to axillary buds.

    • Stem tissue organization in eudicots versus monocots.

    • Leaf tissue organization and vascular function within each leaf.

    • Mechanisms behind secondary growth and the role of associated meristems.

    • Identification of crucial stem parts in cross-section: primary/secondary xylem, phloem, vascular cambium, cork cambium, bark, and cortex.