LectureTopic22-Plant+growth

Plant Growth and Development

I. Meristems: Where the Growth Occurs

• Definition: Meristems are perpetually embryonic tissue which allows for indeterminate growth, although a few plants exhibit determinate growth, which is rare.

  • Types of Meristems:

    • Apical Meristems: Located at tips of roots and shoots, and at axillary buds of shoots. They are responsible for elongating shoots and roots, termed primary growth.

    • Lateral Meristems: Responsible for adding thickness to woody plants, referred to as secondary growth.

      • Types of Lateral Meristems:

        • Vascular Cambium: Adds layers of vascular tissue, called secondary xylem (wood) and secondary phloem.

        • Cork Cambium: Located in stems/trunks, replaces the epidermis with periderm, which is thicker and tougher.

II. Primary Growth: Roots

• Stele: The vascular system of a root or stem, organized into a vascular cylinder in angiosperms.

• Primary Growth Outcomes:

  • Produces epidermis, ground tissue, and vascular tissue.

  • Ground Tissue: Fills the cortex, the region between vascular cylinder and epidermis. The innermost layer of the cortex is called the endodermis.

• Zones of Growth:

  • Zone of Cell Division: Contains mitotic cells including the apical meristem.

  • Zone of Elongation: Where cells begin to elongate.

  • Zone of Differentiation: Cells mature and take on specific functions.

III. Primary Growth: Shoots

• Shoot Apical Meristem: Dome-shaped mass of dividing cells at the shoot tip.

• Development:

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

  • Axillary buds develop from meristematic cells left at the bases of leaf primordia.

IV. Tissue Organization of Stems

• Lateral Shoots: Develop from axillary buds on the stem's surface.

• Vascular Tissue Arrangement:

  • In most eudicots, vascular tissue is organized into vascular bundles arranged in a ring.

  • In most monocot stems, vascular bundles are scattered throughout ground tissue:

V. Tissue Organization of Leaves

• Epidermis: Reduces water loss.

• Stomata: Allow CO2 exchange between the air and photosynthetic cells; regulated by two guard cells.

• Mesophyll: The ground tissue in leaves, consists of:

  • Palisade Mesophyll: Upper part of the leaf, tightly packed for photosynthesis.

  • Spongy Mesophyll: Loosely arranged for gas exchange.

• Vascular Bundles (Veins): Continuously connected to stem vascular tissue, providing water and nutrients, as well as transporting sugars.

VI. Secondary Growth

• Definition: Adds girth to stems and roots in woody plants and consists of tissues produced by the vascular cambium and cork cambium.

• Characteristics:

  • Typical in: Gymnosperms and many eudicots; few monocots exhibit secondary growth.

Questions to Consider:

• What are meristems and their types?

• Differences between primary and secondary growth?

• Mechanism of root and lateral root growth?

• Structure and development of shoot apical meristems, leaves, and branches?

• Tissue organization in eudicots vs monocots?

• Organization of leaf tissues and secondary growth processes?