Overview
This lecture covers the structure and function of vascular tissues in plants, emphasizing xylem and phloem roles in water and nutrient transport, along with tissue arrangement in stems and roots.
Vascular Tissues in Plants
- Not all plants have vascular tissue; those that do possess specialized tubes called xylem and phloem.
- Xylem transports water from roots to shoots in one upward direction.
- Phloem transports sugars and organic molecules produced by the plant.
Xylem Structure and Function
- Xylem employs cohesion (water-water attraction) and adhesion (water-xylem wall attraction) for water movement.
- Water evaporates from leaves (transpiration), creating a pulling force called transpirational pull.
- Xylem consists of dead, hollow cells for maintaining a continuous water column.
- Lignin, a polysaccharide, reinforces xylem walls and prevents collapse under pressure.
- Pits in the xylem allow water movement between xylem and phloem.
Arrangement of Plant Tissues in Stems
- Transverse sections of stems reveal vascular bundles arranged in a ring.
- Each vascular bundle contains xylem (closer to the center) and phloem (closer to the periphery).
- The pith is found in the very center, cortex surrounds the bundles, and the epidermis forms the outer layer.
Arrangement of Plant Tissues in Roots
- Root transverse sections show xylem in the center (often x-shaped), surrounded by phloem and cortex.
- The epidermis with root hairs increases water absorption surface area.
- Cortex between epidermis and vascular tissue supports and stores nutrients.
Key Terms & Definitions
- Xylem — Vascular tissue that transports water upward from roots to shoots.
- Phloem — Vascular tissue responsible for transporting sugars and organic molecules.
- Cohesion — Attraction between water molecules.
- Adhesion — Attraction between water molecules and xylem walls.
- Transpiration — Evaporation of water from the leaves.
- Transpirational Pull — The pulling force for water movement generated by transpiration.
- Lignin — A strengthening polysaccharide in xylem walls.
- Pith — Central region in stems, mainly for bulk, not transport.
- Cortex — Tissue between vascular bundles and epidermis, supports and may perform photosynthesis.
- Epidermis — Outermost protective layer of plant tissue.
Action Items / Next Steps
- Practice identifying xylem, phloem, cortex, pith, and epidermis in stem and root micrographs.
- Review structural differences in tissue arrangement between stems and roots.