# Phloem

> The plant's living transport tissue that carries sugars, amino acids, hormones and mobile nutrients from leaves to roots, buds and other growing parts.

- Canonical: https://genetixbay.com/knowledge/glossary/phloem.html
- Also written: phloem tissue, sieve tubes, sugar transport tissue, phloem sap
- Glossary category: Plant biology · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Phloem is the vascular tissue that moves the products of [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md). Living sieve-tube cells, supported by companion cells, carry a sugar-rich sap (mostly sucrose) from "sources" such as mature leaves to "sinks" such as roots, new shoots and developing flowers. Flow runs whichever way supply and demand dictate, so phloem moves material both up and down.

## In practice

Phloem is the reason nutrients are called mobile or immobile. Nitrogen, phosphorus, potassium and magnesium move readily in phloem, so a plant short of them strips old leaves to feed new growth, and [deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) shows low on the plant first. Calcium, boron and largely iron are poorly phloem-mobile, so their shortages show on new growth.

In late flower, buds are the main sink. [Leaf senescence](https://genetixbay.com/knowledge/glossary/senescence.md) and nutrient remobilisation into flowers run through phloem, which is the biology behind both the natural fade and the GenetixBay [nitrogen taper](https://genetixbay.com/knowledge/glossary/nitrogen-taper.md).

Sap-sucking pests such as [aphids](https://genetixbay.com/knowledge/glossary/aphid.md) and [whiteflies](https://genetixbay.com/knowledge/glossary/whitefly.md) feed directly on phloem sap; their [honeydew](https://genetixbay.com/knowledge/glossary/honeydew.md) is excess sugar.

## Don't confuse with

- **Xylem**: moves water and ions up from roots.
- **Bark**: outer tissue that, on woody plants, contains phloem.
- **Resin**: made in [trichomes](https://genetixbay.com/knowledge/glossary/trichome.md), not carried in phloem.

## Related terms

[Xylem](https://genetixbay.com/knowledge/glossary/xylem.md), [Senescence](https://genetixbay.com/knowledge/glossary/senescence.md), [Photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md), [Nitrogen taper](https://genetixbay.com/knowledge/glossary/nitrogen-taper.md)

## Read more

- [Mobile vs immobile nutrients: reading old vs new leaves](https://genetixbay.com/knowledge/nutrients/mobile-vs-immobile-nutrients.md)

## Sources

1. Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th edition. Sinauer Associates.