bio

Phloem

The system of pipes that carries sugar from the leaves to everything that is not a leaf: roots, trunk, growing tips, fruit. If the xylem is the straw that brings water up, the phloem is the freight line that takes food down — and it too has no pump. The plant controls only the two ends: loading sugar in at a leaf and taking it out at a root, while the travelling looks after itself thanks to the pressure difference those two operations create. Earth phloem sap moves at something like 0.2 to 2 metres an hour — a walking pace measured in days, not seconds — so for a tree hundreds of metres tall this is a real constraint.