The food pipes

The idea

Phloem, mostly living cells, transports food: its sieve tubes, long cells joined end to end by perforated walls, carry food from leaves to other parts, and companion cells regulate them, monitoring sugar loading and unloading.

Sweet sap from a leaf to a fruit

Leaves make sugar by photosynthesis. A mango far from the leaves still gets sweet.

How does the food get from the leaf to the fruit?

• Through living tubes in the plant
• It cannot; fruits make their own food from nothing
• It flows up from the roots

Food flows down

Let the leaf make food. Then look at the sieve tube, the perforated wall and the companion cell.

Phloem

Phloem is mostly made of living cells. It consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Sieve tubes are long tubular cells joined end to end by perforated walls; they transport food from the leaves to other parts of the plant. Companion cells are specialised parenchyma cells that regulate the sieve tube cells: they monitor the loading and unloading of sugars.

Phloem parenchyma stores food, resin, tannins and latex. Phloem fibres are sclerenchymatous and give strength.

Notes

Phloem (mostly living) carries food from the leaves. Sieve tubes are cells joined end to end by perforated walls; companion cells monitor sugar loading and unloading.

Check yourself

Phloem transports…

Sieve tubes are…

Which cells regulate the sieve tube cells?

Unlike xylem, phloem is mostly made of…

• water from the roots to the leaves. That is xylem.
• only air. Phloem carries food.
• food from the leaves to other parts — correct. Yes.
• long cells joined end to end by perforated walls — correct. Yes.
• hairs on the root surface. Those are root hairs.
• hard dead cells forming wood. That describes sclerenchyma.
• Tracheids. Tracheids are in xylem.
• Companion cells — correct. Yes. They monitor loading and unloading of sugars.
• Phloem fibres. Fibres give strength.
• dead cells. Xylem is mostly dead; phloem is mostly living.
• air spaces. Air spaces belong to parenchyma of aquatic plants.
• living cells — correct. Yes.