Plant Vascular Systems: Xylem and Phloem

• Plants possess specialized vascular systems to transport water, minerals, and photosynthetic products throughout their bodies

Xylem Function

• Xylem facilitates water and mineral transport from roots to aerial plant parts
• Its composition includes dead cells called tracheids and vessel elements
• Tracheids are elongated with tapered ends, featuring pits for water movement
• Vessel elements are wider and shorter than tracheids, and form continuous tubes
• Lignin in xylem cell walls provides structural support, preventing collapse

Phloem Function

• Phloem transports sugars (photosynthesis products) from sources to sinks
• Sources are areas of sugar production (e.g., leaves)
• Sinks are areas of sugar use or storage (e.g., roots, fruits, developing leaves)
• Phloem is made of living cells: sieve tube elements and companion cells
• Sieve tube elements connect end-to-end, to form sieve tubes
• Sieve plates are porous structures found between sieve tube elements
• Companion cells support sieve tube elements, providing metabolic aid

Water Transport Mechanisms

• Water transport in plants uses cohesion, adhesion, and transpiration
• Transpiration is the evaporation of water from leaves, creating tension
• Cohesion is the attraction between water molecules due to hydrogen bonds
• Adhesion is the attraction between water molecules and xylem walls
• Transpiration pull draws water up the xylem, from roots to leaves
• Root pressure can aid water movement, especially at night
• Guttation is the exudation of water droplets from leaves due to root pressure

Nutrient Transport Processes

• Mineral nutrients are absorbed by roots from soil
• Active transport is needed for ion uptake against concentration gradients
• Mycorrhizae (fungal associations with roots) enhance nutrient absorption
• Nutrients are transported through the xylem with water
• Nutrients are distributed to various plant parts, depending on demand

Translocation In Plants

• Translocation includes movement of sugars through the phloem
• The pressure flow hypothesis explains translocation
• Sugars load into sieve tubes at source tissues, increasing solute concentration
• Water enters sieve tubes from the xylem by osmosis, increasing pressure
• Pressure drives the flow of sugars and water toward sink tissues
• Sugars unload at sink tissues, with decreasing solute concentration
• Water exits sieve tubes and returns to the xylem, decreasing pressure
• Phloem sap contains sugars (mainly sucrose), amino acids, and other solutes

Plant Vascular Systems

• The arrangement of xylem and phloem varies in different plant parts
• The stele is the central vascular cylinder in roots
• Xylem is in the stele center; phloem surrounds it
• Vascular bundles in stems and leaves contain both xylem and phloem
• In dicot stems, vascular bundles are in a ring
• In monocot stems, vascular bundles scatter throughout the stem
• Vascular cambium is a lateral meristem, producing secondary xylem and phloem, increasing stem thickness in woody plants