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How does phloem transport food through a tree?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

In a tree, the phloem—often referred to as the inner bark—serves as the primary pathway for transporting products of photosynthesis, primarily sugars, from the leaves to other parts of the plant, such as growing tissues, fruits, flowers, and storage organs like roots and tubers.123

The most widely accepted explanation for this movement is the pressure-flow hypothesis. According to this model, a pressure gradient is established between "source" regions (where sugars are produced or mobilized) and "sink" regions (where sugars are consumed or stored):

Source to Sink: The accumulation of sugars in the sieve tubes at source regions increases the osmotic pressure. Conversely, the removal of sugars at sink regions lowers the pressure.1
Flow: This difference in pressure drives the solution through the sieve tubes from the source to the sink.1

This transport is highly variable and under metabolic control, allowing the tree to distribute nutrients dynamically based on its current growth and storage needs.1