Xylem. Word that derives from the Greek “xylon”, literally “wood”, is also the woody tissue present in the upper vegetables that carries water and inorganic salts upwards throughout the plant and also provides mechanical support.

The vascular system of the plant is composed of xylem and phloem. Xylem is a woody tissue present in higher plants that conducts water and inorganic salts upward throughout the plant and also provides mechanical support. In the leaves, flowers and young stems, the xylem appears combined with phloem in the form of conductive vascular bundles. The roots have a central xylem cylinder. The xylem formed from the growth points of stems and roots is called primary. But in addition, the division of the cells of the cambium, located between the xylem and the phloem, can produce a new xylem or secondary xylem; This division gives rise to new xylem cells inwards at the roots and outwards in almost all the stems. Some plants have little or no secondary xylem, in contrast to woody species;

The xylem of the oldest species from the point of view of evolution, such as ferns and conifers, is made up of tracheids. In almost all angiosperms (flowering plants), the xylem also contains well-developed vessels and fibers.


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  • 1 Components of xylem
    • 1 Vascular or conductive elements
    • 2 Nonvascular elements
      • 2.1 Features
    • 3 Support elements
  • 2 Primary xylem
  • 3 Secondary xylem
  • 4 Structure of the secondary xylem in gymnosperms
  • 5 Structure of the secondary xylem in dicotyledons
  • 6 Evolution of the xylem
  • 7 Sources

Components of the xylem

Vascular or conductive elements

Tracheas or woody vessels: They result from the union of cylindrical cells through the basal walls. These cells are wider than the elements of the tracheids. Each element of the vessel is connected to neighboring cells by pits and is connected by perforations with cells of the same row through the basal walls, although perforations may also be in the lateral walls. The part of the wall that has perforations is called the perforated plate.

Tracheids: Present in all vascular plants as well as in vascular cryptogams and gymnosperms and, in the ranal order, are the only vascular elements. The elements of the tracheids are elongated, narrow and have bordered pits, both on the transverse and lateral walls. Among the tracheids, the pits are usually bordered and, in contact with parenchymal cells, are simple. Tracheids do not have perforations in the transverse walls, this is what differentiates them from the tracheas. Perforations are literally holes in the cell wall.

Both tracheas and tracheids have a supporting function.

Nonvascular elements

Parenchymal cells: They carry out active exchanges with the vascular elements with which they communicate by means of semi-circular pits.


  • They provide the xylem with solutes (amino acids, hormones, mineral salts).
  • They have abundant RER and synthesize proteins.
  • Other parenchymal cells store reserve substances such as starch and fat.

Support elements

  • Sclereids
  • Fibers

Primary xylem

It originates from the procambium at the beginning of the differentiation of the primary body of the plant. The primary xylem begins with the formation of the protoxylem consisting of tracheas and tracheids. There are few tracheas and a lot of parenchyma. Within the primary growth, the development of the xylem is completed with the formation of the metaxylem. It is also made up of parenchyma and fibers. The metaxylem generally begins to form while the plant’s primary body is still growing. But it matures, especially when growth ends. In plants with secondary growth the xylem grows by formation of the secondary xylem and the metaxylem stops being functional. In plants without secondary growth, the metaxylem remains active.

Secondary xylem

It is formed from the vascular cambium. The most characteristic structure of this xylem is the existence of two cell systems that differ in the orientation of their longitudinal axes. One is vertical and consists of vessels, fibers, and axial parenchyma. Another is horizontal, made up of a radiomedullary parenchyma. The outermost part of the secondary xylem contains living parenchymal cells and is the part responsible for transporting water. This part is called sapwood. In most trees, the inner part of the secondary xylem ceases its activity and its parenchymal cells are dead. This part is called heartwood.

Structure of the secondary xylem in gymnosperms

It presents only tracheids as conductive elements and a very small amount of axial parenchyma. The tracheids are very long and the adjacent ones are connected by bordered pits. When the axial parenchyma appears, it is usually arranged in bands of homogeneous distribution throughout the secondary xylem. The rays of gymnosperms can only be formed by parenchymal cells, they are called homocellular rays, or by parenchymal and tracheid cells, called heterocellular rays. The radial parenchyma cells contain living protoplasm in the algura and almost always colored resins in the heartwood. Radial tracheids have all secondary lignified membranes. Certain gymnosperms have resin ducts that develop in the axial system or both (axial and medullary radius). They originate from resin-producing parenchyma cells. They can also occur as a result of injury, pressure, or friction.

Structure of the secondary xylem in dicotyledons

The secondary xylem is more complex, thus, the disposition of the tracheas in the secondary xylem is a characteristic that is used for the identification of the species. When all the tracheas have a more or less similar thickness and are distributed homogeneously in the xylem, it is called scattered porosity wood (eg, eucalyptus or acacia). When the tracheas are of different sizes depending on the period in which they were formed, we speak of ring-pored wood (they are the noble woods: oak). With respect to the distribution of the tracheas in cross sections, these can be observed isolated (eg: in the eucalyptus or oak) or, present in groups (eg: acacia). The amount of axial parenchyma varies according to the species. Two types of axial parenchyma are distinguished:

  1. Apotracheal:independent of the tracheas.
  2. Paratracheal:clearly associated with the tracheas.

In the radial parenchyma, the rays are usually made up of parenchymal cells only. When the rays are made up only of precumbent cells they are called homogeneous. When they also have vertical cells, they are called heterogeneous. These last cells can be uniseriate or multiseriate. In the xylem of dicotyledons there are secretory channels similar to the resin ducts of gymnosperms. These channels can form naturally or occur as a result of injury. They contain various substances such as resins, oils, gums and mucilages. Differences in the growth of the secondary xylem during seasonal periods of place to the growth rings. Cells produced at the end of the growth period (in autumn) are narrower, especially in the radial direction, and sound to have thicker walls. The early cells ( spring ) are relatively wide and have the largest lumen. At first glance, growth rings are distinguished by the color difference between early (light) and late (dark) wood.

Xylem evolution

In the evolution of the vascular components, the tracheids are considered more primitive than the tracheas. Within the tracheas, the phylogenetic development of the lateral walls precedes the perforations of the basal walls. As for the side walls, the envelope would be from greater to less lignification. Regarding perforated plates, the simple ones are considered the most evolved.


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