Plant Transport Systems

Xylem

• Vascular tissue that moves water and dissolved minerals upward from roots only (unidirectional)
• Forms the woody (supportive) part of stems
• No end walls between adjacent conducting cells

Phloem

• Vascular tissue that moves sugars and other metabolic products; flow is bidirectional (source ➜ sink)
• Conducting cells possess end walls with perforations (sieve plates)

Sieve Tube Elements & Companion Cells

• Sieve tube: elongated, living cells forming the main food-conducting channel in phloem
• Companion cell: metabolically supports its adjacent sieve tube element

Vascular Bundle Structure

• Bundled strand containing xylem (inner side) and phloem (outer side), often wrapped by supporting sclerenchyma and embedded in ground tissue

Guard Cells & Stomata

• Guard cells flank each stoma; regulate gas exchange and water loss by changing shape
• Stomata are microscopic pores on leaves and stems that open/close based on guard-cell turgor

Capillary Action

• Spontaneous upward movement of water in narrow xylem vessels against gravity, driven by cohesion, adhesion, and surface tension

Transpiration

• Continuous evaporation of water vapor from leaf stomata pulls a water column upward through xylem (transpiration pull)

Translocation

• Active transport of soluble organic compounds (mainly sucrose) through phloem from sources (e.g., leaves) to sinks (e.g., roots, buds, fruits)

Root Pressure

• Osmotically generated positive pressure in root xylem that can push sap upward, supplementing xylem ascent when transpiration is low