What is the Difference Between Protoxylem and Metaxylem in Structure and Function
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.
Similarities Between Protoxylem and Metaxylem
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. 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?
2. Name the two main groups in which secondary xylem can be found.
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.
FAQs on Protoxylem and Metaxylem in Plants
1. What is protoxylem and metaxylem?
Protoxylem and metaxylem are the two types of primary xylem that differ in their time of formation and structure.
- Protoxylem is the first-formed xylem and develops during early growth when plant organs are still elongating.
- Metaxylem forms later, after elongation, and has larger, more developed vessels.
- Both are involved in the conduction of water and minerals from roots to other parts of the plant.
2. What is the main difference between protoxylem and metaxylem?
The main difference between protoxylem and metaxylem is that protoxylem forms first and has narrow vessels, while metaxylem forms later and has wider vessels.
- Time of formation: Protoxylem is formed early; metaxylem develops later.
- Vessel size: Protoxylem has small, narrow vessels; metaxylem has large, wide vessels.
- Wall thickening: Protoxylem shows annular or spiral thickening; metaxylem shows scalariform, reticulate, or pitted thickening.
3. What is the function of protoxylem?
The function of protoxylem is to conduct water and minerals during the early stages of plant growth.
- It supplies water and dissolved salts to rapidly elongating regions.
- Its annular and spiral thickenings allow flexibility during growth.
- It supports young tissues before full maturation.
4. What is the function of metaxylem?
The function of metaxylem is to provide efficient water conduction and mechanical support after the plant organ has matured.
- It has wider vessels for better water transport.
- Its thick, lignified walls give structural strength.
- It replaces protoxylem as the main conducting tissue in mature regions.
5. Where are protoxylem and metaxylem located in roots and stems?
Protoxylem and metaxylem are arranged differently in roots and stems based on the type of primary xylem development.
- In roots, protoxylem lies towards the periphery and metaxylem towards the center (exarch condition).
- In stems, protoxylem lies towards the center and metaxylem towards the periphery (endarch condition).
- This arrangement helps in proper growth and conduction.
6. Why does protoxylem have annular and spiral thickening?
Protoxylem has annular and spiral thickening to allow flexibility during organ elongation.
- Annular thickening forms ring-like bands.
- Spiral thickening forms helical bands.
- These patterns prevent vessel collapse while allowing stretching in growing regions.
7. What type of cell wall thickening is found in metaxylem?
Metaxylem typically shows scalariform, reticulate, or pitted secondary wall thickening.
- Scalariform thickening looks like a ladder.
- Reticulate thickening forms a net-like pattern.
- Pitted thickening has pits that allow lateral water movement.
8. What is meant by exarch and endarch xylem?
Exarch and endarch refer to the position of protoxylem in relation to metaxylem in primary xylem.
- Exarch: Protoxylem is towards the outside and metaxylem towards the center (common in roots).
- Endarch: Protoxylem is towards the center and metaxylem towards the outside (common in stems).
9. Are protoxylem and metaxylem part of primary or secondary growth?
Protoxylem and metaxylem are components of primary growth in plants.
- They develop from the apical meristem.
- They form the primary xylem.
- Secondary growth, in contrast, produces secondary xylem from the vascular cambium.
10. Can you give an example of protoxylem and metaxylem in a plant?
An example of protoxylem and metaxylem can be seen in the vascular bundles of maize stem or a dicot root like sunflower.
- In a maize stem, protoxylem lies toward the inner side of each vascular bundle and metaxylem toward the outer side.
- In a sunflower root, protoxylem is found at the tips of xylem arms and metaxylem occupies the central region.
- These examples clearly show the structural and positional differences.
