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.

1. 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.

1. 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.

Xylem comprised of four different types of elements-

1. Tracheids
2. Vessels
3. Xylem fibres
4. Xylem parenchyma 

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.

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.

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.

Phloem consists of 4 parts-

1. Sieve tube elements
2. Companion cells.
3. Phlonucleinchyma
4. Phloem fibres

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.

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.

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.

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.

Difference between Xylem and Phloem 

FAQs Difference Between Xylem And Phloem

1. What are syncytia?

The structure formed by the fusion of cells is known as a syncytium.

2. What is leptome?

The conducting element of the phloem is known as the leptome. Haberlandt gave Leptome term.

3. What is hadrome?

Tracheids and vessels together are known as water-conducting elements or hadrome. The Hadrome term was proposed by Haberlandt.