Sieve tubes and companion cells
The phloem tube is built the same way as the xylem tube, with one difference — and that difference has a name.
The sieve tube
Sieve tube cells join end to end, and the cross walls between them dissolve incompletely, forming a sieve tube. The cross walls that remain, pierced with pores, are called sieve plates. Sieve tubes contribute to the transport of food — mainly sucrose — within the plant.
The companion cell
Beside each sieve tube lies an elongated cell called a companion cell. It matters because of something missing from the sieve tube cell: a sieve tube cell has no nucleus. The nucleus of the companion cell controls the activity of the sieve tube cell next to it. Sieve tube cells, companion cells and phloem parenchyma are all living; the phloem fibres scattered through the tissue are not.
A living cell with no nucleus of its own
Grade 10 established that the nucleus controls the cell. A sieve tube cell has given its nucleus up — which clears the tube for the food moving through it — and borrows the control it still needs from the cell next door. Two cells, one working as a unit, is why phloem has to be a complex tissue.
The two tubes compared
Xylem vessel
Cross walls completely dissolved — a clear open pipe. The cells are dead.
Sieve tube
Cross walls incompletely dissolved — a sieve plate at every join. The cells are alive.
Why the sieve plate is left in place
An open pipe would be faster. But food has to be delivered to particular places, not simply poured through, and a plant that was damaged would lose the contents of an open tube. The sieve plates let the tube be sealed off, section by section, and they let the living cells control what passes. The holes are a compromise between flow and control.