Complex permanent tissues in plants (Structure, types and functions)

  • A group of more than one type of cells having common origin and performing different but closely related functions as a unit is called complex permanent tissues.
  • The cells may be living or dead.
  • The important permanent tissues in vascular plants are: Xylem and Phloem
  • Both of these tissues are commonly known as vascular tissues as they are conductive in function.

1. Xylem:

  • It is the chief conducting tissue of vascular plants responsible for conduction of water and inorganic solutes.
  • It consists of four components; tracheids, vessels (tracheae), xylem fibers (wood fibers) and xylem parenchyma.

a. Tracheids:

  • They are elongated tube-like dead cells (without protoplasm) having tapering ends.
  • The walls are hard and lignified but not much thick and enclose a wide empty lumen.
  • In the beginning, these cells possess living protoplasm but due to lignification and deposition of thickening materials in wall, they become dead at maturity.
  • Tracheids possess various types of wall thickenings; annular (ring like), spiral (helical), scalariform (ladder like), reticulate (network) and pitted. The pits may be simple or bordered.
  • The end walls are perforated by the presence of bordered pits which permit flow of water from one cell to another.


  • They conduct water and dissolved mineral elements from roots to the leaves.
  • They also provide mechanical support due to the presence of hard and firm secondary walls.

b. Vessels or tracheae:

  • They are long tubes consisting of a series of drum-shaped cells placed one above the other with their walls perforated or dissolved.
  • Vessels are syncytes formed by the fusion of cells.
  • Each cell appears circular, oval or sometimes polygonal with a very wide lumen which later becomes lignified and dead.
  • Vessels like tracheids possess various types of thickenings like scalariform, reticulate and


  • They are the chief conducting tissues of vascular plants, particularly in angiosperms. They translocate water and minerals from roots to the leaves.
  • They also provide mechanical support to the plants.

c. Xylem fibers or wood fibers:

  • The xylem fibers develop from the same meristematic tissues as the other xylem cells.
  • They have lignified secondary walls and narrow cell lumen.
  • They are usually longer than the tracheids of the same plant and present both in primary as well as secondary xylem.


  • It conducts water and minerals from root to leaves and also provides mechanical support to the plant.

d. Xylem parenchyma:

  • These are living parenchymatous cells present as component of the xylem, both in primary and secondary xylem.
  • Wood parenchyma and ray parenchyma are two types of parenchyma present in secondary xylem.
  • The wood parenchyma is formed from fusiform cambium initials whereas ray parenchyma is formed from ray initials of the cambium.
  • Both of them have thin walls and living protoplasm.


  • Their main function is storage of food in the form of starch or fats.
  • They also help in conduction of water and minerals.
                                                        Components of Xylem

2. Phloem:

  • It is the chief food-conducting tissue of vascular plants responsible for translocation of organic solutes.
  • The phloem is composed of four components; sieve elements, companion cells, phloem parenchyma and phloem fibers.
  • A fifth kind of cell type, the transfer cell has recently been reported from the phloem.

a. Sieve elements:

  • These are the main components of the phloem which are placed one above the other forming sieve tubes.
  • They are long tubular structures which consist of living cells without nucleus, endoplasmic reticulum, mitochondria, plastids etc.
  • Cytoplasm occurs in the form of thin lining enclosing a big central vacuole which is filled with albuminous substance.
  • There is presence of sieve areas (group of pores present in walls) and sieve plates (the portion of cross wall with sieve areas) in sieve elements.
  • The sieve plates may be simple or compound.
  • The sieve tubes are syncytes (formed by fusion of cells) and allow free diffusion of organic substances.


  • They help in translocation of organic solutes (prepared food) from leaves to different body parts.

b. Companion cells:

  • These are living cells, each cell always associated with one sieve tube or sometimes more.
  • The cell consists of thin cellulose cell wall and active protoplast with all important cellular components; nucleus, plastid, endoplasmic reticulum, ribosomes etc.
  • The common wall between companion cell and sieve tube shows presence of fine pits which are traversed by plasmodesmata.
  • They are present in angiosperms (both monocots and dicots) but are absent in pteridophytes and gymnosperms.


  • They assist the sieve tubes in the process of translocation of solutes.

c. Phloem parenchyma:

  • They are living parenchymatous cells which are elongated with rounded ends and primary cellulose cell walls.
  • They are present in most of the dicots and absent in monocots.
  • Phloem ray cells are present in secondary phloem which become lignified as soon as the sieve tubes cease to function.


  • The cells store food in the form of starch and fats. Sometimes they contain resins and tannins.
  • They also help in the translocation of organic solutes.

d. Phloem fibers:

  • They are non-living cells also called bast fibers and mostly occur in secondary phloem.
  • These fibers have both cellulose and lignified thickenings.
  • Secondary phloem consists of elongated lignified cells with simple pits.
  • The ends of these cells may be pointed needle-like or blunt.


  • They provide mechanical support and give strength and rigidity to the plant parts.
                                               Components of phloem

Complex permanent tissues in plants (Structure, types and functions)