Sieve Tube Elements: These living cells are joined end-to-end to form long tubes. Unlike xylem, they have perforated end walls called sieve plates, which allow the flow of cytoplasmic sap containing sucrose and amino acids.
Companion Cells: Because sieve tube elements lack a nucleus and most organelles to maximize flow space, they are paired with companion cells. These cells perform the metabolic functions (like protein synthesis and ATP production) required to maintain the sieve tube.
Plasmodesmata: Numerous microscopic channels called plasmodesmata connect companion cells to sieve tube elements, facilitating the active loading and unloading of solutes.
Roots: The vascular tissue is located in a central cylinder (stele). The xylem typically forms an 'X' or star shape in the center to withstand the pulling forces of the plant as it grows, with phloem found in the gaps between the xylem arms.
Stems: Vascular bundles are arranged in a ring near the outer edge. The xylem is positioned toward the center of the stem to provide internal support, while the phloem is positioned toward the outside.
Leaves: The vascular bundles form the midrib and veins. In a leaf cross-section, the xylem is located on the upper side (closer to the upper epidermis) and the phloem is on the lower side.
| Feature | Xylem | Phloem |
|---|---|---|
| Substances Transported | Water and Mineral Ions | Sucrose and Amino Acids |
| Direction of Flow | Unidirectional (Roots to Leaves) | Bidirectional (Source to Sink) |
| Cell Vitality | Dead at maturity | Living cells |
| Structural Support | High (Lignified walls) | Low (Thin walls) |
| Driving Force | Transpiration Pull (Passive) | Active Loading / Pressure Flow |
Identification: In diagrams, always look for the cell wall thickness. Xylem vessels have much thicker, lignified walls and larger diameters compared to phloem cells.
Orientation Check: Remember the 'Inside-Out' rule for stems: Xylem is Inside, Phloem is Outside. In leaves, Xylem is Upper, Phloem is Lower.
Functional Logic: If a question asks why xylem cells are dead, explain that living contents (cytoplasm/nucleus) would create resistance to the mass flow of water.
Common Error: Do not confuse 'source' and 'sink'. A source is where sugar is produced (leaves) or released from storage; a sink is where it is used (growing buds) or stored (roots).
