Vessel element

A vessel element (trachea) is one of the cell types found in xylem, the water conducting tissue of plants.

A cross-section of a stem. The holes with dark red outlines are xylem vessels

SEM image (top) and light microscope image (bottom) of vessel elements in Oak

Vessel elements are found in flowering plants (angiosperms) but not in most gymnosperms such as conifers. Vessel elements are the main feature distinguishing the "hardwood" of angiosperms from the "softwood" of conifers.

Structure

Xylem vessels are a long straight chain made of tough long dead cells known as vessel elements. The vessel have no cytoplasm. They are not living, but are made by living cells. The cells are arranged end to end and the cell walls have disappeared. This makes a tube.

Vessels are made of a substance called lignin. They have a lignified cell wall and a central cavity. Vessel members are interconnected through perforations in their common walls. Lignin is a hard organic polymer. It makes the cell walls rigid and is very long-lasting. It is the lignin in xylem vessels which holds trees up.

Function

Water flows up the xylem vessels. The evaporation of water from the stomata cells in the leaves pulls up the water in a transpiration stream. This is called capillary action, because it depends on the way water molecules stick to the walls of the xylem (adhesion).

There is also some root pressure, because water enters the roots by osmosis. This is important so water gets upwards during the night when transpiration is low.

As the water goes up, it carries some minerals with it, so actually the xylem brings some basic nutrients to the plant. The phloem mostly distributes the more complex organic molecules.