Stems

Origin of Stems
- Stems originate from the epicotyl area of the embryo.
  - Epicotyl: Stem portion above the cotyledon attachment.
  - Plumule: Tip of the embryo that develops into the shoot.
  - Hypocotyl: Stem below the cotyledon attachment (emerges first).
  - Radical: Tip of the embryo that develops into the root.
Development of Stems
- Involves the apical meristem and three primary meristems.
- Primary Meristems:
  - Protoderm: Produces the epidermis (primary tissue).
  - Pro-cambium: Produces primary xylem and phloem.
  - Ground Meristem: Produces cortex, pith, or ground tissue.

External Morphology
- Stem/Branches: Main body extending from the roots and supporting leaves.
- Node: Location where leaves arise; can occur anywhere on the stem.
- Internode: Region situated between nodes.
- Leaf Blade: Whole leaf structure.
  - Veins: Vascular tissues that transport water, nutrients, and sugars throughout leaves.
  - Petiole: Stalk attaching leaf to the stem.
- Sessile Leaves: Leaves directly attached to the stem without a petiole.
- Apical Buds: Youngest growing part at the top of the stem.
- Axillary Buds: Located at the angle between a leaf and stem, may develop into new shoots.

Example: Dicotyledons (annuals).
Main Tissues:
- Primary tissues dominate in herbaceous stems including epidermis, cortex, pith, and vascular bundles.

Siphonostele vs. Solenostele
Eustele:
- Herbaceous dicots have discrete vascular bundles arranged in a cylinder.
- Presence of vascular cambium (V.C.) and Cork cambium (C.C.).
Atactostele:
- Primary xylem and phloem in scattered vascular bundles.
- Monocots lack vascular cambium and cork cambium; produce no wood or bark.

Similar initial arrangement of primary tissues in young herbaceous dicots, woody dicots, and gymnosperms.
Secondary Meristems:
- As vascular cambium and cork cambium develop, the plant increases girth.
- Creates structures like bark, secondary phloem, and wood (secondary xylem).
Xylon: Refers to wood.
Secondary Tissues:
- Annual rings comprised of early wood and late wood.
- Wood includes heartwood and sapwood (functional water transport).
- Bark includes periderm and lenticel.

Lenticels:
- Raised areas of loosely packed cells, facilitating gas exchange.
Laticifers:
- Latex-secreting cells originating from meristems; sourced for rubber, chewing gum, and some drugs.

Girdling:
- Removal of vascular cambium can kill trees.
Dendrochronology:
- Study of growth rings to interpret tree history.
- Use of increment borer for analyzing growth.

Characteristics:
- Many plants exhibit atypical secondary growth; monocots typically do not have it.
- Example: Palm trees grow primarily through thickening rather than secondary growth.

Functionality:
- Higher plants often have erect shoots but many have specialized modified stems based on environmental adaptation.
Examples of Below-Ground Modifications:
- Rhizomes: Fleshy, creeping underground stems (e.g., Ginger).
- Corms: Thickened stems under papery leaves (e.g., Gladiolus).
- Stolons: Horizontal slender stems.
- Tubers: Enlarged tips of stolons storing starch.
- Bulbs: Buds with multiple fleshy leaves that store food (e.g., Onions, lilies).

Cladophylls:
- Flattened, stem-like structures (e.g., Greenbriars, some orchids).
Thorns:
- Modified stems/branches (e.g., honey locust, hawthorn).
Spines:
- Modified leaves (e.g., barberry); cacti have both thorns and spines.
Prickles:
- Epidermal outgrowths from bark (e.g., rose, blackberry).
Tendrils:
- Slender, sensitive structures in climbing plants that provide support; tendrils can be modified stems, leaves, or roots.

As Food:
- Examples include potato and Brussels sprouts.
For Sugar Production:
- Sourced from sugar beet and sugarcane, as well as from maple sap.
Medicinal Uses:
- Quinine extracted from cinchona bark.
Spices and Other Products:
- Cinnamon from bark; ginger and turmeric also utilized.
Commercial Uses:
- Stems used for lumber, particle board, fibers (linen, jute, and hemp), dyes, and paper.