Chapter 6 - Stems

Chapter 6 - Stems: Connecting Everything Together

External Anatomy of Stems

  • Terminal Bud

    • Located at the end of the stem, also known as the apical meristem.

  • Node

    • The area on the stem where leaves and branches (lateral buds) attach.

  • Internode

    • The space between nodes along the stem.

  • Lenticels

    • Small openings that allow for gas exchange; play a crucial role in the stem's ability to expand.

  • Terminal Bud Scale Scars

    • Rings left behind on the stem following growth of the terminal bud.

Plant Types

  • Annual Plants

    • Complete their lifecycle from seed to seed within a single year.

  • Perennial Plants

    • Live for multiple years and regrow from their root systems each year.

  • Biennial Plants

    • Take two years to complete their lifecycle; store food in the first year and bloom in the second.

  • Spring Ephemerals

    • Complete their lifecycle from seed to seed in just a few weeks.

Types of Stems

  • Dicot Stem

    • Typically circular in cross-section. See reference picture on phone for visual representation.

    • Main components:

    • Epidermis/Cuticle: Protective outer layer.

    • Wood: Composed of secondary xylem.

    • Vascular Cambium: Produces vascular tissue (xylem and phloem).

    • Vascular Bundle: Arrangement of xylem and phloem tissues.

    • Pith: Central region used for storage; consists of undifferentiated cells.

Dendrochronology

  • The study of tree rings to determine the age and growth conditions of trees over time.

  • Key components involved in dendrochronology include:

    • Heartwood: The non-living, central part of the tree that provides structural support.

    • Sapwood: The outer part of the wood that actively participates in water conduction.

    • Vascular Cambium: This layer produces secondary xylem (inside) and phloem (outside).

  • Conducting Tissues:

    • Xylem: Conducts water.

    • Phloem: Conducts food (nutrients).

Monocot Stems

  • Vascular Arrangement: Vascular bundles are scattered throughout the stem, contrasting with the organized arrangement in dicots.

  • Growth Patterns in Tropical Rainforests

    • Trees in tropical rainforests exhibit continuous growth year-round due to favorable conditions.

Specialized Stem Structures

  • Rhizomes:

    • Underground stems that grow horizontally, contributing to plant spread and vegetative reproduction.

  • Runners (Stolons):

    • Aboveground stems that can root at nodes, facilitating plant propagation across the ground.

  • Tubers:

    • Modified stems used for food storage; example: potato.

  • Bulbs:

    • Stem is a disk and is accompanied by thickened leaves, as seen in onions.

  • Corms:

    • Bulbous structures where the stem is part of the plant's structure, with leaves forming on top. A new corm can develop on top of the first one.

  • Cladophylls:

    • Structures resembling leaves but are actually modified stems; common in cacti and other succulents.

Wood Structure in Trees

  • Springwood:

    • Formed when water is plentiful; characterized by lighter-colored and less dense growth.

  • Latewood (Summerwood):

    • Darker wood produced later in the growth season, indicating less favorable growth conditions.

Summary of Monocot Stems

  • Monocot stems have a more complex vascular system where vascular tissues (xylem and phloem) are scattered randomly throughout the stem structure, diverging from the distinct circular arrangement found in dicots.

Note: The information provided elaborates on the external anatomy, plant types, specialized structures, and fundamental growth patterns associated with stems. These details are crucial for understanding the morphological diversity in plant biology and the functional significance of various stem types.