Phloem is the living transport tissue in vascular plants that moves dissolved sugars and other organic molecules from sites of production to sites of use or storage. In most vascular plants this flow carries sucrose and related sugars as well as amino acids and other organic compounds and nutrients. The sugars originate in leaves and other photosynthetic tissues by photosynthesis, and are allocated throughout the plant to growing shoots, roots, developing fruits, and storage organs.
Structure and components
Phloem is a complex tissue composed of several cell types. The transport conduits in angiosperms are sieve-tube elements — elongated, living cells joined end-to-end with porous sieve plates — that conduct sap. Each sieve element is closely associated with one or more companion cells, which maintain metabolism and help with loading and unloading of solutes. Supporting cells such as phloem parenchyma and fibres provide storage and mechanical strength. In trees the phloem occupies the inner layers of bark, a fact reflected in the word phloem, derived from the Greek φλοιος (phloios); in many woody species this region is vital for transporting carbohydrates from leaves to roots and woody tissues (trees, bark).
How transport works
Long-distance movement in the phloem generally follows a source-to-sink pattern: cells that make or release sugars (sources) export them into the phloem, creating osmotic gradients that draw water in and generate hydrostatic pressure that drives bulk flow toward sinks where sugars are consumed or stored. This pressure-flow concept is widely used to explain phloem transport, though loading and unloading can occur by different cellular pathways and are regulated. Transport can be dynamically redirected as growth and storage demands change, so flow is not strictly one-way and may be bidirectional along different strands.
Ecological interactions and notable facts
Phloem is a food source for specialized animals and microbes. Sap-feeding insects such as aphids and other insects insert slender mouthparts into sieve elements to tap the nutrient-rich sap; the high pressure in phloem often causes sap to flow into the insect. Excess sugar excreted as honeydew can coat leaves and surfaces and encourage growth of sooty fungus, affecting plant health and appearance.
- Relationship with xylem: phloem works alongside the xylem, which transports water upward and supplies the water that helps drive phloem pressure, supporting photosynthesis and whole-plant function.
- Analogy and distinction: phloem moves organic solutes and signals but is not directly analogous to animal blood vessels; it is a plant-specific living tissue with unique cellular arrangements.
Phloem's roles extend beyond carbon allocation: it transmits hormones, small RNAs and protein signals that coordinate development and responses to stress, and it is a pathway for some pathogens. Practical implications include the effects of girdling (removal of phloem) on crop and tree health, challenges in managing phloem-feeding pests, and research interest in improving phloem loading for better yields. Experimental techniques such as tracer studies and the use of insect stylets help scientists study phloem function in intact plants. For further reading see introductory resources on plant vasculature and transport (vascular plants, photosynthesis).