Overview
This lecture covers the structure and function of xylem and phloem tissues in plants, their differences, similarities, and how to identify them in different plant parts.
Xylem Structure and Function
- Xylem is a plant tissue made up of dead cells arranged end-to-end, forming a long, hollow tube.
- Xylem cells lack a nucleus and organelles, so no metabolic activities such as protein synthesis occur.
- Xylem vessels are strengthened by thickened cell walls containing lignin (a polysaccharide) to prevent collapse.
- Perforated end plates allow water and mineral ions to move up through the xylem.
- Xylem parenchyma are living cells surrounding vessels, storing food and tannin to deter herbivores.
- Xylem vessels have bordered pits to allow lateral water movement to adjacent cells.
- Practical identification: Use colored dye in water and cut thin sections to observe dye movement through xylem.
Phloem Structure and Function
- Phloem transports food molecules such as sucrose and amino acids throughout the plant.
- Phloem tissue consists of living sieve tube elements, connected by sieve plates (perforated cell walls).
- Sieve tube elements lack a nucleus and most organelles, but are kept alive by companion cells which provide metabolic support.
- Companion cells contain nuclei and mitochondria, connected to sieve tubes by plasmodesmata.
- Phloem is classified as living tissue due to companion cells.
Comparing Xylem and Phloem
- Xylem is made of dead cells; phloem is made of living cells (with help from companion cells).
- Xylem contains bordered pits and perforated end walls; phloem has sieve plates.
- Xylem transports water and minerals upwards; phloem transports food in both directions.
- In vascular bundles, xylem is typically located inside (closer to the stem center) and phloem outside (closer to stem edge).
Identification and Plant Anatomy
- In roots, xylem is X-shaped in the center; look for the larger hollow tubes.
- In stems, xylem is inside the vascular bundle, phloem is outside, and cambium (meristem cells) is between them.
- In leaves, xylem is at the top of the vascular bundle, closer to the palisade mesophyll for efficient water supply to photosynthetic cells.
Key Terms & Definitions
- Xylem — Plant tissue of dead, hollow cells transporting water and minerals up from the roots.
- Lignin — A strengthening polysaccharide in xylem cell walls.
- Xylem Parenchyma — Living cells in xylem that store food and tannins.
- Bordered pits — Small openings in xylem allowing lateral water movement.
- Phloem — Plant tissue transporting sugars and amino acids throughout the plant.
- Sieve tube elements — Main phloem cells, joined end-to-end by sieve plates, lacking nucleus.
- Sieve plates — Perforated walls between sieve tube elements.
- Companion cells — Living cells next to sieve tubes providing metabolic support.
- Cambium — Layer of meristematic cells between xylem and phloem that can differentiate into either tissue.
Action Items / Next Steps
- Practice drawing and labeling xylem and phloem in roots, stems, and leaves.
- Create a comparison table of xylem and phloem features.
- Review practical skills for observing xylem flow with dyed water in plant stems.