Phloem Transport: Flow from Source to Sink

Phloem Transport — From Source to Sink

Food, mostly in the form of the sugar sucrose, travels through the plant via the vascular tissue phloem, moving from a source to a sink. The source is generally whichever part of the plant is actively making food — typically a leaf, through photosynthesis — while the sink is whichever part currently needs the food, or is storing it for later.

Crucially, which organ acts as source and which as sink is not fixed — the roles can reverse depending on season or the plant's physiological needs. Sugar stored in the roots, for example, can itself become a source of food in early spring, when swelling buds — urgently needing energy to build new growth — act as the sink. …