Difference Between Xylem And Phloem

Difference Between Xylem And Phloem : The complex tissues comprise more than one type of cell, which work together as a unit. Complex tissues are also known as vascular tissues or conducting tissues. There are two types of Complex tissues-Xylem and Phloem.

Xylem

• Nageli coined the term xylem.
• Its main function is to conduct water and minerals upwards from roots to stems and leaves. It also provides mechanical support to plant parts.
• For more efficient water conduction, the death of protoplasm is necessary. So, dead cells are more developed in water-scarce conditions.
• Xylem is poorly developed in hydrophytes and very well developed in xerophytes.
• Based on their origin, the xylem is classified as primary and secondary.

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Primary Xylem

• Primary xylem originates from procambium during vascularisation.
• It is formed early in the life of the plant.
• Based on development, primary xylem is divided into two parts- protoxylem and metaxylem.
• Protoxylem is the first formed primary xylem and consists of small cells.
• Metaxylem is later formed as a primary xylem, and its cells are larger than protoxylem cells.

Secondary Xylem

• It originates from vascular cambium during secondary growth.
• So, xylem formed at the time of secondary growth is known as secondary xylem.
• This xylem is not differentiated into protoxylem and metaxylem.

• Tracheids
• Vessels
• Xylem fibres
• Xylem parenchyma

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Tracheids

• They are primitive conducting elements of the xylem.
• A single tracheid comprises tube-like cells with thick lignified cell walls. They possess narrow lumen.
• The ends of the tracheids are tapering, or they can be chisel-like.
• Tracheids are separated from each other by a cross wall or end wall which bears bordered pits.
• They lack protoplasm, so they are dead.
• They are unicellular.
• The deposition of lignin on the cell wall is responsible for forming different types of thickenings: annular, spiral, scalariform, pitted and reticulate.
• Annular and spiral types of thickenings are found in protoxylem.
• Reticulate and pitted types of thickenings are found in the metaxylem.
• Scalariform thickenings are found in metaxylem tracheids or pteridophytes and in metaxylem tracheids of Cycas (Gymnosperm).
• Pits are non-lignified areas on lignified walls.

Vessels

• They are advanced conducting elements of the xylem.
• They are long cylindrical, tube-like structures with lignified walls and wide central cavities or lumen.
• They are multicellular and composed of many cells called vessel members or vessel elements.
• They are also dead cells as they are devoid of protoplasm.
• Their end walls are perforated, which helps them work like pipelines during water conduction. They are better adapted for the conduction of water than tracheids.
• Presence of vessels is a characteristic feature of flowering plants or Angiosperms. They are absent in Gymnosperms. But there are some exceptions- Ephedra, Gnetum , and Welwitschia (they have the presence of vessels in their xylem).
• Vessels are absent in some Angiosperms, also Dracaena, Yucca, Dagenaria, and Drimys .
• Some plant families, such as Tetracentraceae, Trochodendraceae, and Winteraceae, also lack vessels.
• Tracheids and vessels are known as tracheary elements.

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Xylem Fibers

• Also known as wood fibres.
• They can be septate or aseptate.
• They have thick walls with obliterated lumens.
• They are most abundant in the secondary xylem.
• Absent in Gymnosperm wood also. Gymnosperms are also known as softwood spermatophytes.

Xylem Parenchyma

• They are the only living element of the xylem and have thin cellulose cell walls.
• They store food materials in the form of starch or fat and also store other substances such as tannins.

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Phloem

• Nageli coined the term Phloem.
• Main function of the phloem is to conduct or transport food material from leaves to other parts of the plant.
• Based on origin, phloem is classified as primary and secondary.
• Primary phloem originates from procambium at the time of vascularisation, and secondary phloem originates from vascular cambium at the time of secondary growth.
• Based on development, the primary phloem is classified as protophloem and metaphloem. The first formed phloem is the protophloem. It has narrow sieve tubes.
• The later formed phloem is known as the metaphloem and has bigger sieve tubes.

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Phloem consists of 4 parts-

• Sieve tube elements
• Companion cells.
• Phlonucleinchyma
• Phloem fibres

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Sieve Cell/Sieve Tube Element

• The Sieve element was discovered by Hartig.
• Sieve tube elements are joined in Angiosperms and form a sieve tube. Their end walls are perforated in a sieve-like manner to form sieve plates. These pores help in the translocation of food material.
• The Sieve tube is an example of living syncytia.
• Sieve tube elements are long, tube-like structures longitudinally arranged along with companion cells.
• Sieve tube elements and sieve cells are living and thin-walled.
• But a mature sieve tube element possesses a peripheral cytoplasm and large vacuole without a nucleus.
• Sieve tube element functions are controlled by companion cells.
• Callose deposition occurs on the radius of sieve pores during autumn to form a thick layer. This is known as a callus pad/callose pad. Found in deciduous plants.
• Callose dissolved during the spring season.
• Sieve elements contain a special type of protein known as P protein.
• P protein functions as a sealing mechanism on wounding along with callose and is also related to the conduction of food material.

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Companion cells

• They are living, specialised parenchymatous cells that remain closely associated with sieve tube elements.
• Pit fields in common longitudinal walls connect companion cells and sieve tube elements.
• Both companion cells and sieve tube elements originate from single mother cells. So, they are known as sister cells.
• Companion cells are living with large elongated nuclei.
• Companion cells are only found in Angiosperms
• Special types of sieve cells in Gymnosperms and Pteridophytes are present in place of companion cells. They are known as albuminous cells or Strasburger cells.

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Phloem Fibres

• Also known as bast fibres.
• They are made up of sclerenchymatous cells.
• They are elongated, unbranched, and have needle-like apices.
• Phloem fibres are absent in the primary phloem and are found in the secondary phloem.
• They provide mechanical support to sieve elements.

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Phloem Parenchyma

• Its cells are elongated, tapering, cylindrical, and living. They have dense cytoplasm and nuclei.
• Cell wall is made up of cellulose and has a pitted field.
• It stores food material and other substances such as resins, latex, mucilage, etc.
• Main function of phloem parenchyma is the storage of food material, and the function of phloem rays is the conduction of food in the radial direction.
• Phloem parenchyma is absent in monocotyledons.

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Difference Between Xylem and Phloem