Xylem: The Water Transport System

Xylem: The Water Transport System

Structure of Xylem Tissue

Xylem tissue is made up of specialized cells called xylem vessels or tracheids. These have several key features:

• Hollow tubes: Xylem cells die and lose their cytoplasm, creating continuous hollow tubes for water flow
• Lignin reinforcement: Cell walls contain lignin, a strong polymer that prevents collapse under negative pressure
• No end walls: Adjacent xylem cells have their end walls broken down to form continuous vessels
• Pits and perforations: Allow water to move between vessels if one becomes blocked

How Water Moves Through Xylem

Water transport in xylem relies on the transpiration-cohesion-tension theory:

1. Transpiration: Water evaporates from leaf surfaces through stomata
2. Tension creation: This evaporation creates negative pressure (suction) in the xylem
3. Cohesion: Water molecules stick together due to hydrogen bonding
4. Adhesion: Water molecules also stick to xylem walls
5. Mass flow: The entire water column is pulled up through the plant

This system can pull water up trees over 100 meters tall - imagine drinking through a straw that long!