Water goes up through the xylem; food comes out of the leaves through the phloem. Both are complex permanent tissues, made of more than one type of cell working together.
| Xylem | Phloem | |
|---|---|---|
| Cells present | Tracheids, vessels, xylem fibres, xylem parenchyma | Sieve tubes, companion cells, phloem parenchyma, phloem fibres |
| Living or dead | Mostly dead and sclerenchymatous; xylem parenchyma is the only living component | Mostly living |
| Carries | Water and minerals, root → rest of the plant | Food, leaves → rest of the plant |
| Also does | Gives strength to the plant | Phloem parenchyma stores food, resin, tannins and latex |
Water upwards. Tracheids and vessels are tubular and thick-walled, and being dead they are hollow — an unbroken pipe from root to leaf. Water evaporates from the leaf through the stomata (transpiration); this pulls on the continuous column of water in the xylem, and the whole column is dragged upwards. The thick lignified walls stop the tube from collapsing under that pull.
Food outwards. Sieve tube cells are long and tubular, joined end to end by perforated walls so that the contents of one flow into the next. Each sieve tube is looked after by a companion cell, a specialised parenchyma cell whose main job is to monitor the loading and unloading of sugars into and out of the sieve tube. Phloem fibres, being sclerenchymatous, hold the soft sieve tubes in place.