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Transportation in Plants: Xylem Tissue

A revision bite about xylem, and their function in plants. This article covers their function, how they are made, what different cells are present in xylem tissue and what their individual function is. This article was written to be a short revision bite suitable for students or High School/Senior School Level.

The function of the xylem in a plant is transport up the plant from the roots to an aerial region of the plant. It can only transport materials in one direction, and this transportation is passive and requires no energy. Xylem are used to transport:

  • Water
  • Dissolved mineral
  • Organic nitrogen
  • Hormones

Xylem tissues are made of xylem vessels and parenchyma cells and fibres. These tissues become specialised to fit the function of the xylem in the ways listed below:

  • Xylem vessels are formed from meristem cells. These small cells undergo elongation and the walls are lignified; this means the walls become waterproof and stronger. Having the ligin present in the walls means the cell will not collapse when negative pressure is created by movement of water up the plant. The lignification also kills the cell.
  • Xylem can be lignification can occur in 5 main ways;
    • Annular- Random rings of lignin with a diameter less than that of the lumen
    • Rings- Rings form up the xylem with a diameter approximately equal to that of the lumen
    • Spiral- Lignin forms in spiral patters up the xylem
    • Sclariform
    • Reticulate
    • The ends of each cell disintegrate and long continuous vessels are formed which have a wide lumen, allowing maximum efficiency of transport of water and minerals.
    • As the walls are lignified xylem also help to provide support for the plant.

Xylem tissue is formed from 3 types of cells;

  1. Vessels- As mentioned above, these are hollow, dead cells with wide lumen that form a tube by linking end to end. They have lignified walls and are the waterproof. In flowering plants, or angiosperms vessels provide the best mode of water transport, as they have large leaf surfaces, so lose a large volume of water. The wide lumen in the xylem means more water can be transported up the plant. The Vessels have small holes or unlignified pits which allow the lateral movement of water through the xylem tissue. The main function of these is alternative routes of water transportation; i.e. if one xylem get blocked by an air bubble water flow is not impeded, but can continue in an adjacent vessel.
  2. Tracheid- This is very similar to a xylem vessel, and is believed to be a previous evolution of a xylem. They are found in some trees, such as pine trees, which require narrower vessels. They are still dead lignified cells, but they have a narrower lumen, and form either a pentagonal or hexagonal cross section. They still have end walls, but have multiple bordered pits in to help with flow of water. They are only really found in plants with needle leaves, as they do not lose as much water due to transpiration.
  3. Fibres- These have very thick lignified walls, and so have a structural role to provide strength. Parenchyma- These cells have a storage role, and are used to store starch. They are living, but are unspecialised. Their function within the xylem tissue is to conduct materials into the xylem vessels.

Water Flow- Water flow in the xylem is not halted because of the lack of end walls, there is no content within the cell, including cytoplasm or nucleus, and the thick reinforced walls prevent wall collapse.

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