Flowering plants use two main types of transport system:

• xylem which transports water and minerals from the roots to the stem and leaves
• phloem which carries dissolved sugars from the leaves to the rest of the plant

Xylem

Xylem vessels are dead hollow tubes. The cell pathways are made from a substance called lignin which makes them waterproof. It also means that the vessels are very strong and stops them from collapsing under high pressure.

Xylem vessels also contain bordered pits. This allows water to move into them from the roots. The opening, or lumen, is very wide so that a lot of water can travel through it at once.

Transpiration stream

The movement of water from the roots through the xylem and into the leaves is called the transpiration stream.

Water enters the root cells through osmosis. The root cells also absorb mineral ions but they achieve this through active transport because the concentration of minerals in the soil is very low.

Water is transported through the root cells, moving with the concentration gradient. It’s able to enter the xylem vessels via the pits in the vessel wall.

In the leaves water evaporates through open stomata. On dry, hot or windy days the stomata tend to close and so the transpiration rate decreases to prevent too much water loss.

In order to prevent wilting it’s important that the leaves continuously receive water and this is achieved through the xylem.

When water moves into a leaf from the top of a xylem vessel the pressure here becomes less than at the bottom. This pushes water up the vessel. The thin lumen, the attraction of the water molecules to the vessel wall (adhesion) and the surface tension of the water molecules ensure that the column of water stays intact and keeps flowing.

Phloem

There are four types of phloem cells in flowering plants all of which are living, unlike xylem cells. They can be found next to xylem in leaves. Phloem carries substances around the plant in a process called translocation.