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Vascular system in plants

The vascular system in plants is a network of tube-like vessels that transport substances around the plant.

The plant vascular system is similar to the circulatory system in animals.

The vascular system has two tissue types:

  • Xylem vessels transport water and dissolved minerals from the roots to the rest of the plant.
  • Phloem vessels transport manufactured food (sugars, amino acids) from photosynthesising areas to the rest of the plant.

In phloem vessels, substances move from a source (where the substance is produced or absorbed) to a sink (where the substance is needed).

Sucrose produced in leaves is delivered to the roots to provide energy.

Having a way of transporting material enables plants (such as trees) to grow very tall.
Having a way of transporting material enables plants (such as trees) to grow very tall.

Xylem vessels transport water and dissolved minerals from the roots of the plant up the stem to the leaves.

Xylem are made from dead cells. These dead cells are arranged with their walls end-to-end to form hollow tubes.

Xylem vessels are adapted for water transport:

  • The cell walls contain a strong, waterproof substance called lignin.
  • Lignin provides structural support for the stem, allowing it to stay upright.

  • Xylem have no cell membrane and no end walls. This allows water to be transported in a continuous column.

Transport through xylem is unidirectional and passive.

The plant uses no energy to transport water through xylem.

Lignin often forms a coiled structure inside the xylem vessel.
Lignin often forms a coiled structure inside the xylem vessel.

Phloem vessels transport dissolved nutrients such as sugars and amino acids around the plant.

Sucrose is transported from the leaves to growing buds that need a lot of energy.

Translocation is the movement of organic nutrients around the plant. Translocation mainly occurs in the phloem vessels.

Transport within the phloem is bidirectional. Nutrients can move up or down phloem vessels. It is also an active process; it uses energy.

Phloem vessels are composed of two cell types:

  • Sieve tube elements are connected vertically end-to-end. The connected end walls (sieve plates) have pores which allow for continuous transport. Sieve tube elements have a cytoplasm but no nucleus or organelles.
  • Companion cells keep the sieve tube elements alive. This compensates for sieve tube elements not being able to carry out their own cellular functions.
Phloem vessels are the major vessels in the vascular system responsible for translocation.
Phloem vessels are the major vessels in the vascular system responsible for translocation.

There are some important differences between the two transport vessels in the plant vascular system: xylem and phloem.

Xylem Phloem
Cells Dead cells Sieve tube elements

Companion cells

Substance carried Water and mineral salts Sugars, amino acids, hormones
Direction Unidirectional (upwards) Bidirectional
Mechanism of transport Passive process (capillary action, transpiration) Active process (uses energy)

Xylem and phloem vessels form a structure called the vascular bundle.

Xylem can be identified as being larger and having thicker walls than phloem vessels.

In the leaf, vascular bundles form the vein network. Xylem vessels are above the phloem vessels:

In the stem, vascular bundles are arranged in a ring. Xylem vessels are on the inside, phloem on the outside:

In the roots, there is a single vascular bundle on the inside: