Botany Chapter 6: Stems

Introduction to Stems

• Stems are a primary source of wood in household items.
• Grafting is a technique used to unite different plant varieties.
• Some plants exhibit indeterminate growth, allowing them to continue growing indefinitely.

External Form of a Woody Twig

• Woody twigs consist of an axis with attached leaves, defined by nodes and internodes.
• Node: Location where leaves attach; can be alternate, opposite, or whorled.
• Internode: Region between nodes on the twig.
• Leaves consist of a flattened blade attached to a twig via a petiole.

Anatomy of a Woody Twig

• Axil: Angle formed between the petiole and stem, containing an axillary bud, which can develop into branches or flowers.
• Terminal Bud: Located at the twig tip, promotes elongation of the twig.
• Bud scales protect developing buds, while stipules are leaf-like appendages at the base of leaves.

Deciduous Trees and Shrubs

• Deciduous varieties shed all leaves annually, leaving dormant axillary buds with leaf scars.
• Bundle scars within leaf scars indicate presence of food and water-conducting tissues.

Origin and Development of Stems

• The stem tip contains apical meristem, which contributes to lengthening and is dormant until the growing season.
• Leaf primordia are tiny embryonic leaves that develop into mature leaves, protected by bud scales.

Primary Meristems

• Apical meristem produces three primary meristems:
  • Protoderm: Forms the epidermis.
  • Procambium: Gives rise to primary xylem and phloem.
  • Ground Meristem: Produces pith and cortex (comprised mainly of parenchyma cells).

Leaf and Bud Primordia

• Leaf and bud primordia derive from the primary meristem and contain traces of xylem and phloem that connect to leaves and buds.

Vascular Cambium

• A layer between primary xylem and phloem can differentiate into vascular cambium.
• Cells from the vascular cambium form secondary xylem (wood) inward and secondary phloem outward.

Cork Cambium

• Cork cambium produces cork cells containing suberin (for waterproofing) and phelloderm cells.
• Functions to reduce water loss and protect the stem; lenticels facilitate gas exchange.

Tissue Patterns in Stems (Steles)

• Stele: The central cylinder comprising primary xylem, phloem, and possibly pith.
• Types of steles:
  • Protostele: Solid core with phloem surrounding xylem; found in primitive seed plants.
  • Siphonosteles: Tubular with pith, common in ferns.
  • Eusteles: Composed of discrete vascular bundles; typical in flowering plants and conifers.