transports solutes (dissolved substances), mainly sugars like sucrose from source to sink
phloem is purely a transport tissue and isn't used for support
sieve tube elements
living cells that form the tube for transporting sugars through the plant.
joined end to end to form sieve tubes.
The 'sieve' parts are the end walls, which have lots of holes in them to allow solutes to pass through.
have no nucleus, a very thin layer of cytoplasm and few organelles.
The cytoplasm of adjacent cells is connected through the holes in the sieve plates.
companion cells
The lack of a nucleus and other organelles in sieve tube elements means that they can't survive on their own.
So there's a companion cell for every sieve tube element
Companion cells carry out the living functions for both themselves and their sieve cells. For example, they provide the energy for the active transport of solutes.
