Xylem and Phloem


Xylems are non-living conducting tissues. They conduct water and dissolved salts from root to different parts of plant. Xylem also forms \S ood in plants. It supports plant body. Xylem tissues are present  only in tracheophytes. Composition of XylemXylem is composed of thllow Mg types of tissues

only in tracheophytes.

Composition of Xylem

Xylem is composed of thllow Mg types of tissues:

I. Tracheary elements: The specialized water anti salt conducting cells of xylem are called tracheary elements

These cells are elongated and lignified. They have thick secondary wall with various types of pits. These cells are non­living at maturity. Tracheal,: elements are of two types:

(a) Vessel members: The tracheary elements which are

short, wide and with perforated end walls are called vessel members. These vessel members are united end to end to form long xylem vessels. The length of xylem vessels is variable in different plants. It varies from 2-15 Ii. I he perforated end plats of vessels arc called perforation plates. The perfiwation plates are simple or multiple.. Simple plates have only single aperture. Multiple plates !me many apertures. Different .vessels are connected to each other through pitted walls. Water c; n move through these pitted walls from one vessel to other. The secondary walls of

. vessels have different thicknesse. These may he annular (ringed), spiral. scalariform wwl reticulate. These thicknesses are found in newly forred xylem. The vessels of mature xylem have uniform waits. Vessels are prusent only in dicot angiosperms.

(h) Tracheids: The tracheary elements which are elongated tube like with tapering ends are called tracheids. They lack perforated plates in the end wall. Water moves from one tracheid to adjacent tracheids through pit membranes. Tracheids are present in all the vascular plants. The mature tracheids have characteristics thickenings like annular. spiral. reticulate and pitted. The two adjacent traeheids have bordered pits in the common wall. But the tracheal walls have simple pits.

2. Fibers: Fibers commonly _occur in xylem. The fibers are

elongated thick \vaned structures. They perform the supporting functions. Fibers have thick wall. Mature fibers are dead cells. The fibers in the xylem are dix ided into two ty pes:

(a) Fiber tracheids: These are like tracheids. But they have %cry thick wall. These walls ha e some remnant of bordered pit.

(b) Libriform tracheids: These fibers are narrower. They have remnant of simple pits in their %A ails.

3. Parenchyma: Parenchyma is present in both primary and secondary xylems. They form vertical rows in primary xylem. They are parallel to tracheary elements. Parenchymal cells are present in both vertical rims and transverse rows in secondary xylem. The xylem parenchymal cells are living. They retain protoplast. The parenchyma cells store food in the form of

starch. The all of parenchyma cells may remain thin.
Sometimes. they develop secondary wall with simple pits. The amount of parenchyma determines the softness and hardness of wood. Soft wood has a large number of parenchyma. Hard wood have tin+ er parenchyma. ‘Hie old tracheal elements become Wil­la net iona L The contents of the adjecent parenchyma In ‘grate into the tracheal elements through pits forming

Arrangement of xylem

Arrangement of xylem

Types of xylem

There are two types of xylems:

I. Primar xylem: The xylcms produced ns a result of primary growth are called primary xylem. The xylem components are arranged vertically only in prim’ y xylem. In this case, xylem elements are arranged to parallel axis of plant and plant form the . axial system. Primary items are produced procambium during primary growth. Tiere are two forms of primary xylems:

  • Protoxylem: The early primary xylems are called protoxylem. Protoxylem appears at the beginning of differentiation. Protoxylem mostly becomes mature before elongation phase. They have annular and spiral thickenings. Protoxylems have few treachery elements. But they have a large amount of parenchyma cells. They are mostly present near the pith in stern. In root, they are present away from centre.

  • Metaxylem: The xylems appear later during differentiation are called metaxylem, Metaxylem matures after elongation phase. Metaxylem has spiral. reticulate and pitted walls. Metaxylems are composed of tracheids. vessel. parenchyma and fibers. Metaxylems remians functional only in plants in which secondary growth does not occur (grasses). In other plants they become non-functiolial.

    2. Secondary Xylem: The xylems which are produced as a result of secondary growth are called secondary xylem. Secondary xylems are produced by the activity of cambium. They are found in only those plants in which secondary growth occurs. Secondary xylems form axial system and ray systems. Secondary xylems are composed of two types of systems:

    • Axial or vertical systems: In this case, xylem elements are present parallel to vertical axis. They mostly composed of dead tracheary elements (tracheid and vessels), fibers and parenchyma.

    • Radial or transverse system: This system is composed of parenchyma cells only. Their long axes are at right angles to the long axis of organs. They form xylem rays. They are mostly composed of living cell.




      Phloem are living conducting tissues. They conduct prepared food form leaves to different part of the plant. They are also involved in storage of food and mechanical support. They are always associated with xylem to form aseular Ii .

      Components of phloem

      Phloem is composed of sieve elements. companion cells, phloem parenchyma and phloem litters.

      I. Sieve elements: The elongated cells with characteristic sieve

      areas in their walls are called sieve elements. Sieve areas arc lbrmed by modified pits. I he protoplasts of the adjoining cells are continuous through the pores of sieve areas. this elements have primary thin walls. The sieve areas have pores cytoplasmic connecting strands. These strands may be thin like plasmodesmata: These strands are surrounded In callose carbohydrate. The cal lose layer becomes thick in mature sieve elements. The cadose completely closes the dormant sieve elements like a pad. The callose disappears from the acticale sieve elements. ‘Some sie e elements arc oblique. Such sieve elements have sescrai prominent  areas. Some sieve elements bine trans’, ertu end walls. Such sieve elements only single sic

      area with large pores. The sieve elements are divided into sieve tube members and sieve cells.

      1. Sieve cells: The sieve areas are not specialized in sieve cells.

      These areas are not restricted to some specific part of the wall of the cell. The sieve cells are arranged in longitudinal files. The sieve areas are present mostly in the adjacent walls of the sieve cells. The phloem of gymnosperms and lower vascular plants contain sieve cell only.

      1. Sieve tube members: Sieve areas are well developed in

      sieve tuber members. Sieve areas are confined only to end walls of cells. They form sieve plates at end walls. The sieve tube members join end to end to form long tubes called sieve tubes. The phloem of angiosperms is composed of mainly sieve tube members.

      1. Companion cells: The thin walled parenchyma cells closely associated with the sieve elements are called companion cells. Companion cells cut off from the cells initial which later form sieve element. Companion cells are living. They arc physiologically active. They have a prominent nucleus. The siese tube element and companion cell have close contact. Companion cells are absent in gymnosperms and loser vascular plants. But certain parenchyma cells called albuminous cells are closely’ associated with sieve elements in these plants. These albuminous cells are similar 10 the companion cells in their function.

      2. Sclerenchyma: Sclerenchymatous fibers are commonly Ibund in both primary and secondary phloem. The fibers occupy the outer portion in the primary phloem. They are used as commercial Fibers. e.g. Hibiscus. Phloem fibers are elongated. They have thick secondary wall. They may be living or dead at maturity. the phloem fibers may be septate in certain cases. The phloem% are called bast due to presence of these fibers. The thick ssall of fibers is not lignified. It is composed of only cellulose. The fibers are also arraiiged axially (vertically ) in secondary ph loem s.

      3. Parenchyma: Small parenchymas are present in both primary and secondary’ phloems. These parenchyma cells are arranged vertically in primary phloem. But these are arranged venically and radially in secondary phloem. These cells become thick wall in older portion of phloem and change into sclerenchyma cells. Phloem parenchyma cells are thin walled. They are living and physiologically active. They store different compounds like starch, tannins, and different crystals.

        Types of phloem

        1. Phloems may he -primary or secondary.

          1.  Primary phloem: The phloem funned as a result of primary crmult is called primary phloem. Primary phloem is initiated in the embryo. It develops Man procambium. Primary phloem has two types.
          • Protophloem: Protophloem is composed of elongated sieve elements only. They lack companion cells. Sieve tube elements lack nucleus. So they remain active only for a short time. Thus they soon disappear.

          • Metaphloem: Metaphloem mature later than protophloem. So they remain active for longer time. Secondary growth does not occur. in monocot plants. So they remain active in

        2. Secondary phloem: The phloem formed as a result of secondary growth is called secondary phloem. It has two types:

        1. Vertical system: Vertical system is composed of sieve elements, companion cells, phloem fibers and phloem parenchyma.

        2. Ray system: Ray system is composed of ray parenchyma only.

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