Phloem

Phloem is plant vascular tissue that transports sugars and other organic nutrients from sources such as leaves to growing or storage tissues.

Pressure-flow hypothesis: A model explaining phloem transport as bulk flow driven by pressure differences between sources and sinks. It explains how sugar loading at leaves drives sap toward tissues that consume or store it.
Xylem: Plant vascular tissue that conducts water and dissolved minerals, usually from roots toward shoots. Xylem transports water and minerals, whereas phloem distributes organic nutrients between sources and sinks.
Source–sink relationship: The relationship between tissues that export assimilates and tissues that import or use them. Phloem links sugar-producing leaves with growing, respiring, or storage tissues.
Aphid: A small sap-feeding insect that uses piercing mouthparts to extract plant fluids, often from phloem. Aphids exploit nutrient-rich phloem sap and can transmit plant viruses while feeding.
Phloem loading: The movement of sugars from photosynthetic cells into the sieve elements of phloem. Loading establishes the high solute concentration that helps generate transport pressure.
Vascular bundle: A strand of plant tissue containing xylem and phloem, often with associated support tissues. Phloem occurs within these vascular strands in stems, leaves, and roots.
Girdling: The removal of a ring of bark that interrupts phloem transport around a stem or trunk. It demonstrates that sugars accumulate above a girdle when their downward transport is blocked.
Phytoplasma: A wall-less bacterium that lives in plant phloem and is often transmitted by sap-feeding insects. Its restricted location illustrates how phloem can serve as a habitat and transport route for pathogens.
Phloem unloading: The movement of transported sugars out of phloem into sink tissues. Unloading supplies growing tissues and storage organs while reducing pressure at destinations.
Vascular cambium: A lateral meristem that produces secondary xylem inward and secondary phloem outward. Its activity adds new phloem during the secondary growth of woody plants.

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