Difference Between Primary Xylem and Secondary Xylem

The key difference between primary xylem and secondary xylem is that the procambium forms the primary xylem during the primary growth, while the vascular cambium forms the secondary xylem during the secondary growth.

Xylem and phloem are the main types of complex tissues in plants. The xylem tissue in higher vascular plants transport water and dissolved minerals across the plant through osmosis and simple diffusion. The development of xylem takes place in two stages. Moreover, the two main types of xylems, the primary xylem and secondary xylem, get their names depending on their growth stage.

CONTENTS

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

What is Primary Xylem?

The formation of the primary xylem takes place during the primary growth in flowering and non-flowering vascular plants. The main function of the xylem tissue is the transport of water and dissolved minerals in the plant for its metabolic activities. Primary xylem forms from the procambium of apical meristems during the primary growth.

There are four types of xylem cells in the primary xylem. They are the xylem tracheids, xylem vessels, xylem parenchyma and the xylem fibres. The xylem tracheids and vessels form the hollow tube that conducts water up to the plant. The xylem parenchyma performs photosynthesis, and xylem fibres provide strength to the xylem tissues.

Figure 01: Primary Xylem

The primary xylem can be further divided as the protoxylem and metaxylem. During the primary growth, the primary xylem first differentiates into the protoxylem, followed by the differentiation to the metaxylem. The metaxylem of the primary xylem has four main forms. They are endarch, exarch, centrarch and mesarch. They differentiate based on the site where the differentiation of the primary xylem takes place. Thus, endarch xylem differentiates from the centre to the periphery, exarch xylem differentiates from the periphery to the centre, the centrarch xylem differentiates in a cylindrical fashion from the centre and the mesarch differentiates from the middle to the centre and the periphery.

What is Secondary Xylem?

Secondary xylem develops during the secondary growth of the plant. Thus, the vascular cambium of the plant is responsible for the growth of the secondary xylem. Similar to the primary xylem, the secondary xylem also conducts water. However, there are structural dissimilarities between the two types of xylem. Secondary xylem originates from the vascular cambium, which is a lateral meristem during the secondary growth. It occurs along the sides of the stem and the root.

Figure 02: Secondary Xylem

Secondary xylem also comprises xylem tracheids, xylem vessels, xylem fibres and xylem parenchyma. However, the xylem vessels are much shorter and wider in the secondary xylem. Vessels also contain tylose deposits. The xylem vessels form thickenings which form pit fields.

What are the Similarities Between Primary Xylem and Secondary Xylem?

• Both primary and secondary xylem conduct water and dissolved minerals in a plant.
• Moreover, both consist of xylem vessels, xylem tracheids, xylem parenchyma and xylem fibres.
• Both are living tissues.
• Furthermore, they are a type of complex tissues in plants.
• Both are present only in the vascular plants.
• Moreover, both conduct water from the root hairs to the shoots (upper part of the plant)
• Also, they provide structural strength to the plant.

What is the Difference Between Primary Xylem and Secondary Xylem?

The key difference between primary xylem and secondary xylem tissue in plants is their development. The primary xylem develops from the apical meristem during the primary growth, while the secondary xylem develops from the lateral meristem during the secondary growth. Moreover, the primary xylem originates from the procambium, while the secondary xylem originates from the vascular cambium. So, this is another signficant difference between primary xylem and secondary xylem.

Furthermore, primary xylem contains long and thin tracheids and vessels, while secondary xylem contains short and wide tracheids and vessels. Besides, a further difference between primary xylem and secondary xylem is that the primary xylem constitutes of few xylem fibres, while the secondary xylem contains numerous xylem fibres.

The below info-graphic presents more information regarding the difference between primary xylem and secondary xylem.

Summary – Primary Xylem vs Secondary Xylem

Primary and secondary xylem are present in vascular plants. They conduct water from the root tips to the upper parts of the plant body for various metabolic functions. The key difference between primary xylem and secondary xylem depends on the growth phase that they develop; primary xylem develops during the primary growth of the plant, while secondary xylem develops during the secondary growth of the plant. Furthermore, the tracheids and vessels of the primary xylem are thin and long, while the tracheids and vessels are short and wide in the secondary xylem. Moreover, secondary xylem has a lot of xylem fibres, unlike the primary xylem. Thus, this is the summary of the difference between primary xylem and secondary xylem.

Reference:

1. Wiley, Suzanne S. “What Is the Difference in the Formation of Primary Xylem & Phloem vs. Secondary Xylem & Phloem?” Home Guides | SF Gate, 21 Nov. 2017, Available here.

Image Courtesy:

1. “Herbaceous Dicot Stem: Primary Xylem in Young Helianthus” (Public Domain) via Flickr
2. “Gymnosperm Stem Secondary Phloem and Xylem in Five Year Pinus (35498751834)” By Berkshire Community College Bioscience Image Library – Gymnosperm Stem: Secondary Phloem and Xylem in Five Year Pinus (CC0) via Commons Wikimedia

ncG1vNJzZmivp6x7pbXFn5yrnZ6YsqOx07CcnqZemLyue8OinZ%2Bdopq7pLGMm5ytr5Wau2680aKkmqqpYsW6uMSmZJqmlGLApq%2FOp5uaqqlixbq4xKZm