Phloem consists of living cells arranged end to end. Phloem vessels contain cytoplasm, and this goes through holes from one cell to the next.
Phloem transports sucrose up and down the plant. This is called translocation.
Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation.
The sucrose from the source are usually translocated to the nearest sink (root) through the phloem sieve tube elements.
https://organismalbio.biosci.gatech.edu
The phloem tissue is the principal sugar conductive (傳導的) tissue in plants. Over 90 years ago, Ernest Münch (1930) proposed the now widely accepted mechanism for phloem transport. According to his theory, the mass flow in the phloem is driven by an osmotically generated pressure gradient. As the sieve pores interconnect the protoplasts (cells which have had their cell wall removed) of the sieve tubes, the transport in the sieve tube itself is a mass flow driven by a pressure gradient.
The solute concentration are controlled by release/retrieval mechanisms in the sieve element–companion cell complexes (SECCCs). Differential release/retrieval balances control the influx/efflux of sugars and water in the various phloem zones. In the collection phloem, the uptake or retrieval dominates; in the release phloem, the release dominates. In the transport phloem, having a dual task (nourishment of lateral and terminal sinks), the balance between release and retrieval varies with the requirements of the plant.
The gradual loss of solutes and corresponding amount of water towards the sink (root), where massive delivery of water and solutes takes place, has been attributed to the relative size reduction of CCs along the source to sink path, which may explain a decreasing retrieval capacity of the SECCCs in the direction of the sink.
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