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Protoxylem and Metaxylem

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An Introduction to Protoxylem and Metaxylem

Xylem is a water-conducting vascular tissue, which helps in the upward movement of water and minerals from roots to other parts of the plant. The xylem tissue consists of four elements – tracheids, xylem parenchyma, vessel elements and fibres. The vascular tissue undergoes primary and secondary growth.

The procambium provides the xylem with protoxylem and metaxylem during primary growth. One of the two categories of vascular tissues in plants is the xylem. Primary and secondary xylems are two different types of xylems. The primary xylem develops during primary growth.

Protoxylem and metaxylem are the two components of primary xylem. Protoxylem is formed before the plant organs complete their elongation while metaxylem formed later after primary xylem matures, once completion of the growth of the plant organs. The protoxylem has smaller cells and metaxylem contains larger cells.

What is Protoxylem and Metaxylem?

Two primary xylem kinds that emerged during the primary growth of vascular plants are protoxylem and metaxylem. Metaxylem is the later created primary xylem that matures after the expansion of the plant organs, whereas protoxylem is the first formed primary xylem that matures before the plant organs finish their elongation. Protoxylem also contains smaller cells.

The metaxylem, however, has bigger cells. Additionally, protoxylem has a limited amount of lignification, which makes it less effective in conducting water. Due to its increased lignification, the metaxylem is very effective at conducting water. This encapsulates how protoxylem and metaxylem differ from one another.

Protoxylem

  • Proto means ‘first’ and Xylem means ‘wood’; the first primary xylem

  • The primary xylem that forms initially during primary growth is called the protoxylem.

  • Prior to plant organ elongation, protoxylem develops.

  • Protoxylem is found toward the outside of a stem.

  • It comprises more compact cells.

  • In other words, it has tracheids and components of thin vessels. As a result, the cells' lumen is small.

  • In the protoxylem cells, lignification is also not very extensive.

  • The secondary cell walls of protoxylem vessels exhibit annular and spiral thickenings.

  • Additionally, protoxylem lacks xylem fibres and has a significant quantity of parenchyma.

  • Protoxylem is less effective in conducting water than metaxylem.

Metaxylem

  • Meta means ‘last’ and Xylem means ‘wood’; the last primary xylem.

  • The primary xylem's metaxylem is the portion that forms the following protoxylem.

  • After the plant organs have finished growing, the metaxylem develops.

  • It can be found near the stem's interior.

  • Larger cells like tracheids and broader arteries are found in metaxylem.

  • The lignification of metaxylem cells is also quite widespread.

  • The secondary cell walls of metaxylem vessels have scalariform, reticulate, and pitted thickenings.

  • Metaxylem also has a small amount of parenchyma cells and xylem fibres. Protoxylem is less effective than metaxylem for conducting water and minerals.

  • Therefore, compared to the lumen of protoxylem cells, the lumen of cells is bigger.

Protoxylem and Metaxylem Difference

The distinctions between protoxylem and metaxylem is provided in the table below.

Protoxylem

Metaxylem

It represents the first formed xylem element.

It represents the later formed element of primary xylem.

Founds in developing organs.

Founds in developed organs.

Founds closest to the centre of the stem.

Found closest to the periphery of the stem.

Occurs closest to the periphery of the root.

It is found closest to the root's surface.

Cells are comparatively smaller.

Cells are comparatively larger.

Lignification occurs later.

Lignification occurs earlier.

Tracheary elements are small and narrow lumen.

The tracheary elements are large and wider lumen.

Vessels are narrow in diameter.

Vessels are broader in diameter.

More parenchyma and less fibres.

More fibres and less xylem parenchyma.

Matures before the organs of plants grow and vary.

Matures during the development and differentiation of plant organs.

Lacks tyloses.

Contains tyloses.

Subjected to stress and strain.

Not subjected to stress and strain

Less efficient in conducting water.

More efficient in conducting water.

Exhibit annular and spiral thickening in vessels.

Exhibit reticulate scalariform pitted thickening.



Similarities Between Protoxylem and Metaxylem

  • Two primary xylem kinds that conduct water and minerals are protoxylem and metaxylem.

  • Protoxylem and metaxylem both include both living and dead cells.

  • Additionally, both are created throughout a plant's primary development.

  • Protoxylem and metaxylem both include both living and dead cells

  • In vascular plants, they exist.



Protoxylem and Metaxylem in Stem

  • The stem bears conspicuous nodes and internodes.

  • In the stem, primary xylem and phloem lie side by side in the same radius i.e. they are conjoint collateral.

  • The xylem of the stem is Exarch, Endarch or Mesarch.

  • The exarch condition is more common in present day plants.

Protoxylem and Metaxylem Diagram

T. S. showing protoxylem and metaxylem


T. S. Showing Protoxylem and Metaxylem

T. S. of stem and roots showing protoxylem and metaxylem


T. S. of Stem and Roots Showing Protoxylem and Metaxylem

Protoxylem and Metaxylem in Root

  • Roots are Generally of Two Types: primary roots which originate from embryo and adventitious roots which arise secondarily from stem, leaf or other tissue.

  • In roots, the strand of primary xylem and phloem lies in different radii, separate from one another.

  • The xylem of the root is always in exarch condition.

Interesting Facts

  • As metaxylem is formed after protoxylem, it is more mature than protoxylem. metaxylem is a differentiated part of protoxylem.

  • Protoxylem has deposition of lignin in the form of rings in an annular pattern.

Important Questions

1. How to identify protoxylem?

Ans: The protoxylem is characterised by smaller cells with the narrow lumen. It has lesser lignifications, and the vessels contain simple thickenings.

2. Name the two main groups in which secondary xylem can be found.

Ans: The two groups in which secondary xylem are observed are Coniferae and Angiospermae. In conifers, all species have secondary xylem and as they grow tall secondary xylem is marketed as soft wood while in angiosperms secondary xylem is rarely found in monocot and in non monocot plants it is marketed as hard wood.

Key Features

  • Two primary xylem kinds that conduct water and minerals are protoxylem and metaxylem.

  • In vascular plants, they exist.

  • Additionally, both are created throughout a plant's primary development.

  • Additionally, both come from procambium.

  • Protoxylem and metaxylem both include both living and dead cells.

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FAQs on Protoxylem and Metaxylem

1. The xylem, in which the protoxylem faces the centre and the metaxylem the periphery is called?

The primary water-conducting tissue in plants is called xylem. The metaxylem is located towards the centre while the protoxylem is located towards the periphery in the xylem. This condition is known as Exarch. The roots of plants contain this ailment.

2. Which protoxylem-containing vascular bundle is located at the periphery?

In roots, the metaxylem is found in the core and the protoxylem is found in the periphery. Exarch is the name of this configuration.

3. Explain the differences between endarch and exarch xylem.

The two forms of xylem are determined by how the protoxylem and metaxylem are arranged:

  • Endarch xylem

  • Endarch xylem

 Endarch is the name given to this configuration. While in roots, the metaxylem is located in the middle and the protoxylem is located in the periphery. Exarch is the name of this configuration.


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