Means of Transport

Two Traffic Systems, One Plant

A plant has no heart and no blood vessels, yet it must move water, mineral ions, the sugars it makes in photosynthesis, and chemical messengers (hormones) to every living cell in its body. Because there is no pump-driven circulatory system, plants rely on two very different strategies depending on how far a substance has to travel.

• Short distances — within a single cell, from one cell to its neighbour, or across a membrane — are handled by diffusion, cytoplasmic streaming, and active transport working together.
• Long distances — root to shoot tip, or leaf to root — need the two specialised conducting tissues, xylem and phloem. Movement through these tissues is called translocation.

The direction of travel is not the same for every substance. Water and minerals absorbed by the roots travel through the xylem essentially in one direction only — upward, from root to shoot. Sugars made in the leaves, by contrast, are exported in many directions at once, reaching every growing or storage region, and can later be re-exported from storage when needed. Minerals absorbed by the roots mostly move up, but can also be withdrawn from an ageing organ and redirected to a young, actively growing one. Hormones move in tiny amounts and are sometimes strictly polarised, travelling only from where they are produced to where they act. So although there is no central pump, the whole plant runs an orderly, coordinated traffic system in which every organ is simultaneously a sender and a receiver.