General Structure of Stems

General Structure of Stems

• Stems develop from the plumule and are key to the plant’s aerial structure.
• Stems consist of an ascending axis, nodes, internodes, and buds.
• Types of stems include aerial, sub-aerial, and underground variations.
• Mechanical classifications include weak, herbaceous, and woody stems.

Weak Stems

• Creepers grow flat on the ground, e.g., grasses.
• Climbers require support and use tendrils or hooks, e.g., Piper betel.
• Twinners coil around supports, e.g., Ipomoea.

Herbaceous and Woody Stems

• Herbaceous stems are soft, e.g., sunflowers; woody stems are hard, e.g., mango trees.
• Functions include photosynthesis through leaf production and nutrient transport.

External Structure of Stems

• Woody twigs feature an axis with attached leaves.
• Leaf arrangements: alternate (spiral), opposite (pairs), and whorled (groups).
• Nodes are where leaves attach; internodes are the spaces between nodes.
• Leaves consist of a blade and are attached via a petiole.
• Axillary buds develop in the axil, potentially forming flowers or branches.
• Terminal buds, larger than axillary ones, promote twig lengthening.

Leaf and Bud Scars

• Scars from terminal buds indicate twig age and history.
• Stipule scars may encircle the twig or be inconspicuous.
• Deciduous trees show dormant axillary buds with visible leaf scars post-fall.
• Tiny bundle scars indicate vascular tissue locations within leaf scars.

Internal Structures of Stems

• Apical meristem is located at the stem tip, responsible for growth; protected by scales and leaf primordia.
• Primary meristems include:
  • Protoderm: forms epidermis and cuticle.
  • Procambium: produces primary xylem (for water) and phloem (for nutrients).
  • Ground meristem: develops pith and cortex (for storage).

Lateral Meristems

• Vascular cambium adds girth, producing secondary xylem and phloem.
• Cork cambium (Phellogen) produces protective cork.

Types of Steles

• Protostele: solid core of conducting tissues.
• Siphonostele: tubular with vascular tissue at the center.
• Eustele: arranged vascular bundles.

Woody vs. Herbaceous Stems

• Herbaceous dicotyledons have primarily primary tissues; woody dicotyledons develop secondary tissues, visible as annual rings.

Growth Rings

• Spring wood has larger vessels and a lighter color, while summer wood is darker with smaller vessels.

Heartwood vs. Sapwood

• Heartwood is non-conductive, older wood at the center; sapwood is younger and functional near the cambium.

Monocotyledonous Stems

• Lack vascular or cork cambium, producing no secondary tissue with vascular bundles scattered throughout.

Functions of Stems

• Conducting tissues include xylem (water transport) and phloem (nutrient transport).
• Cork and bark provide protection; wood offers structural support.
• Pith and cortex serve as storage locations for nutrients.

Stem Modifications

Underground Modifications

• Rhizome: Horizontal growth with thick structures and nodes, e.g., ginger.
• Tuber: Swollen structures with vegetative buds (eyes), e.g., potatoes.
• Bulb: Food stored in fleshy scales; new plants develop in spring, e.g., garlic.
• Corm: Stout vertical structure with buds in axils and adventitious roots, e.g., saffron.

Sub-aerial Modifications

• Runner: Creeping growth that roots at nodes, e.g., strawberries.
• Stolon: Lateral branches grow horizontally with nodes and internodes, e.g., jasmine.
• Offsets: Short, thick horizontal stems producing new plants.

This quiz explores the essential characteristics and types of stems in plants. Learn about the roles of nodes, internodes, buds, and the distinctions between weak, herbaceous, and woody stems. Test your knowledge on how these structures contribute to plant growth and development.