Xylem is a tissue system consisting of specialized cells with lignified secondary cell walls responsible for water movement through the plant.

Photo of plant tissue cross section showing xylem.

What is lignified?

Cells with secondary cell wall thickening typically contain lignin. Lignin is a chemical substance that resists breakdown. Along with cellulose, lignin gives wood its strength. This allows trees to attain great size.

here are 2 types of xylem cells:

  1. Tracheids
  2. Vessel Elements

Their primary function is for water transport.

Photo of a tree trunk.

A tracheid is a xylem element that does not have end wall openings.

Water moves through side wall pits only.

They are common in gymnosperms and function to conduct water.

Photo showing tracheid.

A vessel element is a xylem cell with overlapping end wall perforations. They are more evolutionarily advanced than the tracheid.

Their function is to conduct water.

Vessel elements overlap to connect end-to-end to support water conduction.

They are non-living at maturity.

Photo showing a vessel element.

Primary xylem is an early stage of xylem development.

It is made of protoxylem and metaxylem.

Photo showing the protoxylem and metaxylem of primary xylem.

In the protoxylem, the secondary walls form first as annular and then as helical side walls.

This is followed by scalariform and then pitted side walls in metaxylem.

Illustration showing structural appearance of secondary walls; annular, helical, scalariform, and pitted.

Protoxylem with annular and helical secondary wall thickening.

Illustration of protoxylem showing annular and helical secondary wall thickening.

Altered color photo showing actual plant protoxylem with annular and helical secondary wall thickening in comparison to the preceding illustration.

The final stages of metaxylem formation is scalariform thickening that eventually leads to pitted sidewalls.

Illustration showing scalariform thickening in final stages of metaxylem formation.

Color altered photo of plant material showing the same scalariform thickening as in the preceding illustration.

Notice how there are connections (arrow 1) between xylem elements through the sidewall pits that will allow water to flow between cells.

Image showing how sidewall pits form connections between xylem elements.


Secondary xylem is xylem tissue formed from the vascular cambium that is responsible for lateral growth in a stem.

Secondary xylem equals wood formation.

Wood is made of mature xylem with full secondary wall thickening. Except for the ray tissue, secondary xylem is non-living at maturity. (Rays are described under parenchyma cells).

Photo of plant material showing secondary xylem.

Annual rings are the amount of xylem growth that takes place during one season in the secondary xylem.

Xylem can remain functional for many years. However, phloem is crushed by the expanding xylem and there is only a small outer ring of functional phloem elements made every year.

Photo showing annual rings of xylem growth in a plant cross section.

Counting annual rings is used to discover the age of large trees.

Photo showing the cross section of a large tree trunk with centuries of annual ring formation.