Chapter 7: Stems

Support and Structure: Stems are responsible for keeping the plant in an erect posture and increasing the overall length of the plant.
Metabolic Activities: Stems can store food reserves and are capable of performing photosynthesis in certain instances.
Transport Mechanisms:
- Xylem: Facilitates the upward transport of water and essential minerals from the roots to the leaves.
- Phloem: Facilitates the bidirectional transport (up and down) of food and organic nutrients between the roots and the leaves.
- Sugar Transport: Specifically responsible for moving sugars produced in the leaves to the roots for storage or metabolic use.
Reproductive and Vegetative Roles:
- The stem bears the essential organs of the plant, including buds, leaves, and fruits.
- Stems assist in the vegetative spread and propagation of plants through specialized structures such as stolons and rhizomes.

Definition and Function: A lenticel functions as a pore, providing a medium for the direct exchange of gases between the internal tissues and the atmosphere.
Purpose: It allows the plant to bypass the periderm, which is a layer that would otherwise prevent the exchange of gases between the internal environment and the outside air.

External Morphology:
- Terminal Bud: Located at the apex of the stem, responsible for primary vertical growth.
- Bud Scales: Protective coverings for the buds.
- Lateral Bud (Axillary Bud): Found in the axil of a leaf, capable of forming new branches or flowers.
- Node: The point on the stem where leaves, buds, and branching occur.
- Internode: The segment of the stem between two consecutive nodes.
- Leaf Scar: A mark left on the stem after a leaf falls off.
- Stipule: An outgrowth, sometimes fused to the petiole, found at the base of the leaf.
- Lenticels: Small pores visible on the surface of the stem.
Internal Tissue Organization:
- Pith: The soft, spongy central tissue in the stems of vascular plants.
- Cortex: The tissue layer between the epidermis and the vascular tissue.
- Outer Skin: The protective exterior layer (epidermis or bark).
- Cambium: A layer of tissue that provides cells for secondary growth.

Shoot Apical Meristem: The region of actively dividing cells at the tip of the shoot.
- Protoderm: The primary meristem that gives rise to the epidermis.
- Ground Meristem: The primary meristem that develops into the pith and cortex.
- Procambium: The primary meristem that differentiates into the primary xylem and primary phloem.
Meristematic Regions:
- Apical Meristem: Responsible for terminal growth.
- Axillary Bud Primordium: Young buds developing in the axils.
- Young Leaf Primordium: Immature leaves forming near the apex.
- Central Mother Cells: A specific group of cells within the meristematic region.
- Expansion Zone: The area where cells increase in size, contributing to the growth in length.
Intercalary Meristem: Meristematic tissue located at the base of nodes or leaf blades, allowing for rapid regrowth and elongation of the internode.

Definition: The stele is the central cylinder of vascular tissue—comprising xylem and phloem—in a plant's stem and root, typically surrounded by an endodermis.
Components:
- Primary Vascular System: Includes tissues originating from the procambium.
- Xylem: Inner vascular tissue.
- Phloem: Outer vascular tissue.
- Endodermis: A boundary layer surrounding the stele; some plants possess an inner and outer endodermis.
- Traces and Gaps:
  - Leaf Trace: A strand of vascular tissue that extends from the stem into a leaf.
  - Root Trace: Vascular tissue extending into the root.
  - Leaf Gap: A break in the vascular cylinder of the stem where the leaf trace departs.

Woody Stem Composition:
- Heartwood: The older, non-living central wood of a tree, often darker in color.
- Sapwood: The outer, living layers of the secondary xylem that transport water and minerals.
- Rays: Radial files of cells that transport materials horizontally across the wood.
Bark and Periderm:
- Bark: All tissues external to the vascular cambium, including the living phloem and periderm.
- Living Phloem: Responsible for sugar transport.
- Periderm: The protective outer layer replacing the epidermis in woody stems, consisting of the Cork Cambium and Cork.
Cambium Types:
- Vascular Cambium: Produces secondary xylem and phloem.
- Fascicular Cambium: Cambium within the vascular bundles.
- Interfascicular Cambium: Cambium between the vascular bundles.
Growth Rings: Patterns in the wood resulting from seasonal changes.
- Rainy Season: Faster growth, resulting in wider rings.
- Dry Season: Slower growth, resulting in narrower rings.
- Anomalies: Scars from occurrences such as forest fires can be recorded within the growth ring history.
Sectional Orientations:
- Cross section / Transverse section: Cut perpendicular to the long axis.
- Tangential section: Cut longitudinal and tangent to the growth rings.
- Radial section: Cut longitudinal through the center (radius) of the stem.

Rhizomes: Horizontal underground stems with scale leaves, nodes, internodes, and adventitious roots.
Runners (Stolons): Horizontal stems that grow along the surface of the ground (e.g., Strawberry), producing roots and new plants at nodes.
Stem Tubers: Enlarged underground storage stems (e.g., Potato). Features include "eyes" which are germinating buds, and terminal buds.
Bulbs: Short stems surrounded by fleshy leaf bases (scale leaves) that store food.
- Tunicated Bulb: Example: Onion ( $Allium$ ). Features a tunic, fleshy scale leaves, a disc (stem), and adventitious roots.
- L.S. of Bulb: Shows the apex, axillary buds, and fleshy scales.
Corms: Thick, vertical, underground stems (e.g., $Colocasia$ ). Features include daughter corms, adventitious roots, and scars of scale leaves.

Cladophylls / Phylloclades: Flattened or cylindrical stems that perform photosynthesis and take on the appearance and function of leaves (e.g., $Opuntia$ ). They often feature leaf spines at the nodes.
Thorns: Hard, woody, pointed stems for protection (e.g., $Citrus$ known as Khatta, and $Duranta$ ). These arise from axillary or accessory buds.
Stem Tendrils: Slender, coiling structures used by weak-stemmed plants for climbing and support (e.g., $Muchlenbeckia$ ). These are often axillary in origin.
Succulent Stems: Thickened, fleshy stems adapted for water storage in arid environments.
Prickles: Small, sharp outgrowths from the epidermis or cortex of the stem for defense.