How does xylem transport water

Name: Julian Gutberlet and Felix Günther, 06/17/2013
HHopemaster, photos, 2020


Water transport and vascular bundles of the plant

Water is essential for the photosynthesis process. To do this, it has to be transported against gravity from the roots upwards into the leaves of the plant.

Water absorption

Water is absorbed through the roots of the plant. This happens through osmotic processes, caused by a difference in concentration between the soil and the cell plasma of the root cells in the fine root hairs.

germinating radish with root and root hairs (click to enlarge)

The roots and the outgoing root hairs in enlargement

The roots have a graduated concentration, which enables water to gradually diffuse into the interior of the root. From there it gets to the so-called Casparian strip, a barrier that can only be overcome through active transport (i.e. with the supply of energy). From here the water gets over thatXylem (water conducting tissue) transported to the leaves of the plant, where it is needed for the light reaction of photosynthesis.

The ability ("work / pressure") of the plant to extract water from the soil is called Water potential designated. As long as the water potential of the plant is lower than that of the soil, it absorbs water passively. (For the formula, see Wikipedia: https://de.wikipedia.org/wiki/Wasserpotential)

The water potential is made up of the Sum of matrix potential and osmotic potential together.

The matrix potential (also capillary potential) describes all forces with which the soil "holds" water. The greater the fine grain of the soil, the greater its matrix potential. In the case of drought, this naturally increases again until there is only "dead water" in the fine soil pores, which can no longer be used.

The osmotic potential (solution potential) is the pressure that is necessary to absorb a certain amount of water from the soil through a selectively permeable membrane. The amount of water and the salt content in the soil influence this. Because the following applies: the larger the amount of water and the lower the salt content, the lower the osmotic potential.

Transport via the vascular bundles of the plant

Liquids in higher plants (monocotyledonous and dicotyledonous plants, mosses and ferns) are conveyed through vessels called vascular bundles transported.

These are in Xylem (responsible for transporting water) and Phloem (responsible for the transport of photosynthetic products) differentiated. The xylem is located further inside the vascular bundle, while the phloem is further outside. In between, with many plants, that lies Cambium (educational tissue), which creates new vascular bundles.

The Xylem has a high strength, which is why it is often referred to as the wooden part of the conductive fabric. It consists of three different types of fabric (Tracheids, trachea and sclerenchyma fibers), which passively (i.e. without using energy) transport water.

Tracheids are elongated individual cells with thick, lignified cell walls that are connected to one another. The transverse walls between two cells are not completely dissolved. They also contain a thin tissue "Spot" to connect.

Tracheaare vascular members with pores at the cell ends. They form a continuous tube.

Sclerenchymal fibersserve for mechanical stability. They are specialized tracheids with strongly thickened and lignified cell walls that hardly conduct water.

Xylem tissue have died in fully grown plants, which is why they can really fulfill their function of transporting water and the mineral salts dissolved in it.

About the Phloem (also called bast part) the photosynthesis products are actively transported (i.e. with energy consumption) in the direction of the roots. It is divided into three different types of fabric: Sieve tube, sieve cells and escort cells. In contrast to the xylem, phloem cells are still alive (due to the active transport of the Photosynthesis products / assimilates).