Metaxylem vs. Protoxylem

What's the Difference?

Metaxylem and protoxylem are two types of xylem tissues found in plants. Metaxylem is the mature and fully developed xylem tissue, while protoxylem is the immature and developing xylem tissue. Metaxylem is characterized by larger and more mature tracheary elements, such as vessel elements or tracheids, which are responsible for water and mineral transport in plants. On the other hand, protoxylem consists of smaller and less developed tracheary elements. It is usually found towards the outer region of the stem or root, and its primary function is to provide support during the early stages of plant growth. Overall, metaxylem and protoxylem play different roles in plant development, with metaxylem being the fully functional xylem tissue and protoxylem serving as the precursor to metaxylem.


LocationIn the center of the vascular bundleAt the periphery of the vascular bundle
MaturationMature xylem tissueImmature xylem tissue
Cell wall thicknessThicker cell wallsThinner cell walls
FunctionConducts water and mineralsProvides support
Vessel elementsPresentAbsent

Further Detail


Within the complex vascular system of plants, xylem plays a crucial role in transporting water and minerals from the roots to the rest of the plant. Xylem tissue is composed of various cell types, including metaxylem and protoxylem. While both metaxylem and protoxylem are involved in water conduction, they possess distinct attributes that contribute to their specific functions and developmental stages. In this article, we will explore and compare the characteristics of metaxylem and protoxylem, shedding light on their roles within the plant's vascular system.


Metaxylem is a type of xylem tissue that develops later during the plant's growth. It is found towards the center of the vascular bundle, closer to the pith. Metaxylem cells are larger in size compared to protoxylem cells and possess a more mature and specialized structure. These cells have thickened secondary cell walls, which provide strength and support to the plant. The secondary cell walls contain lignin, a complex polymer that enhances the rigidity and waterproofing properties of the cell walls. This lignification process in metaxylem cells allows them to withstand the high pressure generated during water transport.

Metaxylem vessels are characterized by their wider diameter, allowing for efficient water conduction. These vessels are interconnected, forming a continuous network throughout the plant. The presence of perforation plates, which are areas of cell wall degradation, facilitates the movement of water between adjacent metaxylem vessels. This interconnectedness ensures a steady flow of water and minerals from the roots to the upper parts of the plant.

Another notable attribute of metaxylem is its ability to undergo secondary growth. Secondary growth refers to the increase in girth or thickness of plant organs, such as stems and roots. Metaxylem cells contribute to this process by dividing and differentiating into specialized cell types, such as fibers and parenchyma cells. These cells provide mechanical support and storage capacity to the plant, further enhancing its structural integrity.


Protoxylem, in contrast to metaxylem, is the first-formed xylem tissue during plant development. It is located towards the periphery of the vascular bundle, closer to the cortex. Protoxylem cells are smaller in size and have thinner primary cell walls compared to metaxylem cells. The primary cell walls of protoxylem lack lignin, making them more flexible and less resistant to mechanical stress.

Protoxylem vessels are characterized by their narrower diameter, which allows for a controlled and regulated flow of water. These vessels are arranged in a spiral or helical pattern, providing additional strength and flexibility to the plant. The spiral arrangement of protoxylem vessels allows for elongation and growth of the plant, accommodating the expansion of tissues without compromising the water transport system.

Unlike metaxylem, protoxylem cells do not undergo secondary growth. Instead, they retain their primary structure throughout the plant's lifespan. This lack of secondary growth enables protoxylem vessels to maintain their flexibility and adaptability, ensuring efficient water conduction even in rapidly growing plant parts.


While metaxylem and protoxylem share the common function of water conduction, they differ in several key attributes. Metaxylem cells are larger and possess thickened secondary cell walls with lignin, providing strength and rigidity. In contrast, protoxylem cells are smaller and have thinner primary cell walls without lignin, offering flexibility and adaptability.

The diameter of metaxylem vessels is wider, allowing for a higher volume of water transport. On the other hand, protoxylem vessels have a narrower diameter, ensuring controlled and regulated water flow. The interconnectedness of metaxylem vessels through perforation plates enables efficient water movement, while the spiral arrangement of protoxylem vessels provides additional strength and flexibility.

Metaxylem cells contribute to secondary growth, allowing for an increase in girth and thickness of plant organs. In contrast, protoxylem cells retain their primary structure and do not undergo secondary growth, maintaining their flexibility and adaptability throughout the plant's lifespan.


Metaxylem and protoxylem are two distinct types of xylem tissue with unique attributes and functions within the plant's vascular system. Metaxylem, located towards the center of the vascular bundle, possesses larger cells with thickened secondary cell walls containing lignin. These cells contribute to secondary growth and provide strength and support to the plant. Protoxylem, found towards the periphery of the vascular bundle, consists of smaller cells with thinner primary cell walls. These cells do not undergo secondary growth and offer flexibility and adaptability to the plant.

Understanding the characteristics and roles of metaxylem and protoxylem enhances our knowledge of plant physiology and the intricate mechanisms involved in water conduction. By unraveling the complexities of xylem tissue, scientists and researchers can further explore ways to improve water transport efficiency in crops, leading to advancements in agriculture and sustainable plant growth.

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