12.3 Plant Stems

Plant Stems - Chapter 12.3

1. Overview of Plant Stems

• Stems play a critical role in connecting the vascular tissue of leaves to roots.

• They support leaves and reproductive organs.

• In some plants, stems are modified for storage (e.g., water or carbohydrates).

  • Example: Cactus stems store water; they perform photosynthesis due to green stems.

• Stems have adaptations for protection against injury and herbivores.

2. Types of Seeds: Monocots vs. Dicots

2.1 Cotyledons

• Cotyledons are structures that provide nutrients to the plant embryo.

• Monocots: One cotyledon.

• Dicots: Two cotyledons.

• Recent classification of dicots into four groups:

  • Amborellales: One surviving species, Amborella tricopoda.

  • Nymphaeales: Water lilies.

  • Other early angiosperms.

  • Eudicots: True dicots, a larger group with advanced features.

3. Stem Structure

3.1 Herbs vs. Woody Plants

• Herbaceous Plants:

  • Non-woody stems that are pliable.

  • Photosynthesize and have a thin epidermis.

• Woody Plants:

  • Stems that contain wood; hard and have bark.

  • Do not usually perform photosynthesis.

  • Examples:

    • Gymnosperms always have woody stems.

    • Eudicots are mostly woody angiosperms.

    • Monocots like palms and bamboo have stem tissues resembling wood.

3.2 Vascular Tissue in Stems

• Herbaceous Stems:

  • Vascular tissue arranged in distinct bundles within ground tissue.

  • In each vascular bundle:

    • Xylem is closer to the center.

    • Phloem is closer to the outside.

  • Monocots have scattered vascular bundles; eudicots have a ring formation.

4. Woody Stem Anatomy

4.1 Layers of a Woody Stem

• Sapwood: Younger xylem for transporting water and minerals.

• Heartwood: Older, rigid, non-transporting xylem filled with resins.

• Bark: Protective outer layer containing cork, which prevents water loss.

  • Cork cambium produces cork, which cracks and flakes off.

4.2 Growth and Structure

• Woody stems grow thicker due to the vascular cambium.

  • Composed of meristematic cells dividing to form new xylem and phloem each year.

• Xylem develops inside the vascular cambium; phloem develops outside.

5. Growth Rings Formation

• Growth is seasonal, primarily in spring and summer.

  • Spring wood: Rapid growth produces large, thin-walled xylem.

  • Summer wood: Slower growth, creating thicker-walled cells.

• These layers create distinct growth rings, indicating annual growth cycles.

6. Cells in Vascular Tissue

6.1 Xylem Cells

• Xylem cells are thick-walled and dead at maturity.

• Rich in lignin, providing structural strength.

• Two types of xylem cells:

  • Tracheids: Long, tapered cells allowing water and solute movement via pits.

  • Vessel Elements: Shorter, wider, with perforation plates for efficient water transport.

6.2 Phloem Cells

• Three types of phloem cells:

  • Sieve Cells: Have narrow pores and retain organelles.

  • Sieve Tube Elements: Contain cytoplasm, but lack a nucleus.

  • Companion Cells: Always associated with sieve tube elements, providing necessary organelles.

• Together, they form long conducting tubes for nutrient transport.