Difference Between Protoxylem and Metaxylem | Protoxylem vs Metaxylem


The key difference between protoxylem and metaxylem is that protoxylem is the first formed primary xylem which has smaller cells, especially narrow vessels and tracheids while metaxylem is the later formed primary xylem which has larger cells, especially wider vessels, and tracheids.

Xylem is one of the two types of vascular tissues in plants. Xylem conducts upward movements of water and minerals from roots to the other parts of the plant. It is a tube-like structure containing different types of specialized cells including vessel elements, tracheids, parenchyma, and fibers. There are two types of xylems as primary xylem and secondary xylem. Primary xylem develops during primary growth. Furthermore, primary xylem has two parts: protoxylem and metaxylem. Both protoxylem and metaxylem originate from the pro-cambium during the primary growth. Protoxylem originates first, and metaxylem originates after the protoxylem.

CONTENTS

1. Overview and Key Difference
2. What is Protoxylem 
3. What is Metaxylem
4. Similarities Between Protoxylem and Metaxylem
5. Side by Side Comparison – Protoxylem vs Metaxylem in Tabular Form
6. Summary

What is Protoxylem?

Protoxylem is the primary xylem that develops first during primary growth. Protoxylem matures before plant organs complete their elongation. In a stem, protoxylem is located towards the outside. It consists of smaller cells. In other words, it contains narrow vessel elements and tracheids. Hence, the lumen of the cells is narrow.

Difference Between Protoxylem and Metaxylem | Protoxylem vs Metaxylem

Figure 01: Protoxylem

Furthermore, lignification is not extensive in the protoxylem cells. Protoxylem vessels show annular and spiral thickenings in their secondary cell walls. In addition, protoxylem contains large amounts of parenchyma, and it does not contain xylem fibres. Compared to metaxylem, protoxylem is less efficient in water conduction.

What is Metaxylem?

Metaxylem is the part of the primary xylem that develops after protoxylem. Metaxylem matures after the completion of the growth of the plant organs. It is present towards the inside of the stem. Metaxylem has larger cells such as wider vessels and tracheids. Hence, the lumen of cells is larger in comparison to the lumen of the protoxylem cells.

Difference Between Protoxylem and Metaxylem | Protoxylem vs Metaxylem

Figure 02: Metaxylem

Furthermore, metaxylem cells have extensive lignification. Metaxylem vessels show scalariform, reticulate and pitted thickenings in their secondary cell walls. In addition, metaxylem contains xylem fibres and low amount of parenchyma cells. Metaxylem is more efficient in the conduction of water and minerals than protoxylem.

What are the Similarities Between Protoxylem and Metaxylem?

  • Protoxylem and metaxylem are two types of primary xylem that conduct water and minerals.
  • They occur in vascular plants.
  • Also, both are formed during the primary growth of the plants.
  • Moreover, both originate from procambium.
  • Furthermore, both protoxylem and metaxylem consist of living and dead cells.

What is the Difference Between Protoxylem and Metaxylem?

Primary xylem has two parts as protoxylem and metaxylem. Both develop during primary growth. However, protoxylem is the first formed part of the primary xylem while metaxylem is the later formed part of the primary xylem. Therefore, we can consider this as the key difference between protoxylem and metaxylem. Generally, protoxylem consists of smaller cells; hence it is less prominent. Whereas, metaxylem consists of larger cells and is more prominent. So, this is also a difference between protoxylem and metaxylem.

Moreover, a further difference between protoxylem and metaxylem is that the protoxylem has more parenchyma cells and does not have xylem fibres, but metaxylem has xylem fibres and contains few parenchyma cells. One more difference between protoxylem and metaxylem is the efficiency of water conduction. Protoxylem is less efficient while metaxylem is more efficient in water conduction. Also, the lignification is not extensive in protoxylem cells while the lignification is extensive in metaxylem cells.

The below infographic presents more details on the difference between protoxylem and metaxylem.

Difference Between Protoxylem and Metaxylem | Protoxylem vs Metaxylem

Summary – Protoxylem vs Metaxylem

Protoxylem and metaxylem are two types of primary xylems developed during the primary growth of the vascular plants. Protoxylem is the first formed primary xylem that matures before the plant organs complete their elongation while metaxylem is the later formed primary xylem that matures after the completion of the growth of the plant organs. Moreover, protoxylem has smaller cells. Whereas, metaxylem contains larger cells. Furthermore, lignification is not extensive in protoxylem; hence, it is less efficient in water conduction. Metaxylem shows a higher lignification; hence it is highly efficient in water conduction. Thus, this summarizes the difference between protoxylem and metaxylem.

Reference:

1. Gilmore, Micheal. “Xylem Development”, Study Notes: Trickster, Available here.
2. “Xylem.” Wikipedia, Wikimedia Foundation, 2 Mar. 2019, Available here.

Image Courtesy:

1. “Helical Protoxylem of Celery” By Berkshire Community College Bioscience Image Library (Public Domain) via Flickr
2. “Woody Dicot Stem: Proto and Metaxylem in One Year Liriodendron” By Tmatheus – Own work (CC BY 4.0) via Commons Wikimedia