What is the Difference Between Primary Xylem and Secondary Xylem

The main difference between primary xylem and secondary xylem is that primary xylem is formed by the primary growth of the procambium whereas secondary xylem is formed by the secondary growth of the vascular cambium. 

Primary xylem and secondary xylem are two growth stages of the xylem in angiosperms and some gymnosperms. The main function of the xylem tissue in plants is to conduct water and minerals from root to the leaf. Furthermore, the primary xylem consists of protoxylem and metaxylem while secondary xylem occurs inside the bark, which is the outermost protective layer of a plant that has undergone secondary growth. 

Key Areas Covered 

1. What is Primary Xylem
     – Definition, Components, Differentiation
2. What is Secondary Xylem
     – Definition, Components, Differentiation
3. What are the Similarities Between Primary Xylem and Secondary Xylem
     – Outline of Common Features
4. What is the Difference Between Primary Xylem and Secondary Xylem
     – Comparison of Key Differences

Key Terms 

Primary Xylem, Procambium, Secondary Xylem, Tracheids, Vascular Cambium, Vessels 

What is Primary Xylem 

Primary xylem is the xylem produced during the primary growth of all vascular plants, both angiosperms and gymnosperms. Here, the apical meristem is responsible for the primary growth of plants. Generally, this type of meristem occurs in the tips of the shoot and the root. It helps the plant to grow in length. However, the medullary rays of the apical meristem give rise to the procambium, which in turn gives rise to cells that differentiate into the primary xylem. The four types of cells in the primary xylem are tracheids, vessels, fibers, and parenchyma. Both tracheids and vessels are long and narrow. Moreover, primary xylem contains fewer xylem fibers. 

Figure 1: Woody Dicot Stem: Primary Xylem

Furthermore, the two main types of xylem differentiation in the primary xylem are protoxylem and metaxylem. protoxylem is the first type of xylem differentiated while metaxylem is differentiated later. According to the form of differentiation of the metaxylem, there are four types of primary xylem formations: endarch, exarch, centrarch, and mesarch. Here, endarch is a form of primary xylem differentiation which occurs from the center towards the periphery. In exarch, the differentiation occurs from the periphery to the center. Meanwhile, centrarch is the differentiation of the xylem from the center to the periphery in the form of a cylinder while mesarch is a type of differentiation in which the primary xylem differentiates from the middle to both center and periphery. 

What is Secondary Xylem 

Secondary xylem is the type of xylem produced during the secondary growth of angiosperms and some gymnosperms including conifers, Gnetophyta, Gingkophyta, and to a lesser extent in Cycadophyta. Here, the lateral meristem is responsible for the secondary growth of plants. It occurs along the sides of the stem and root. Also, the two components of the lateral meristem are vascular cambium and cork cambium. Generally, the secondary medullary rays of the vascular cambium give rise to the conducting tissues including the secondary xylem to the outside and the secondary phloem to the inside. However, the cork cambium gives rise to the secondary dermal tissues of the plant. Therefore, the lateral meristem is responsible for growing the plant by width. 

Figure 2: Gymnosperm Stem: Secondary Growth

Moreover, the secondary xylem also contains the four types of cells including tracheids, vessels, fibers, and parenchyma. Significantly, the tracheids and vessels of the secondary xylem are wider and shorter. In addition, vessels contain blockage by the deposition of tyloses. Also, tracheids contain thickenings which form pits. However, a number of xylem fibers occur in the secondary xylem. On the other hand, large woody plants with secondary xylem show a clear distinction between sapwood and heartwood. Also, these trees contain annual rings. For instance, the secondary xylem of confers is identified as softwood while that of the non-monocot angiosperms is identified as hardwood. Generally, secondary xylem is rare in monocots. 

Similarities Between Primary Xylem and Secondary Xylem 

• Primary xylem and secondary xylem are two growth stages of xylem. 
• Both occur in angiosperms and some gymnosperms. 
• Both differentiate from the cells produced by the cambium. 
• The main function of both xylem tissues is to conduct water from the root to the upper part of the plant. 
• Apart from that, both types of xylem tissue provide structural strength to the plant. 
• Both contain tracheids, vessels, xylem fibers, and xylem parenchyma. 

Difference Between Primary Xylem and Secondary Xylem 

Definition 

Primary xylem refers to the xylem formed during the primary growth from the procambium of the apical meristem while secondary xylem refers to the xylem formed as a result of the secondary growth from the vascular cambium of the lateral meristem.  This is the main difference between primary xylem and secondary xylem.

Occurrence 

Primary xylem occurs in both monocots and dicots while secondary xylem generally occurs in dicots. 

Occurrence in Gymnosperms 

Primary xylem occurs in all types of gymnosperms while secondary xylem occurs in some of the gymnosperms including conifers, Gnetophyta, Gingkophyta, and to a lesser extent in Cycadophyta.  

Type of Growth 

Moreover, primary xylem develops during the primary growth while secondary xylem develops during secondary growth. 

Derived from 

Another major difference between primary xylem and secondary xylem is that primary xylem is derived from the procambium while secondary xylem is derived from the vascular cambium. 

Type of Meristem 

Procambium, which gives rise to the primary xylem, derives from the medullary rays produced by the apical meristem. Meanwhile, vascular cambium, which gives rise to the secondary xylem, derives from the medullary rays of the lateral meristem. 

Type of Differentiation 

The four types of primary xylem differentiation are endarch, exarch, centrarch, and mesarch while there is no special type of secondary xylem differentiation. 

Significance 

Another difference between primary xylem and secondary xylem is that primary xylem differentiates into protoxylem and metaxylem while secondary xylem occurs inside the bark. 

Sapwood and Heartwood 

In addition, primary xylem does not differentiate into sapwood and heartwood while secondary xylem differentiates into sapwood and heartwood.  This is another difference between primary xylem and secondary xylem.

Annual Ring Formation 

Primary xylem is not involved in annual ring formation while secondary xylem involves annual ring formation. This is another difference between primary xylem and secondary xylem.

Tracheids and Vessels 

Both tracheids and vessels are narrower and longer in the primary xylem while both tracheids and vessels are wider and shorter in the secondary xylem. This is also a difference between primary xylem and secondary xylem.

Pits 

Tracheids of the primary xylem do not contain pits while tracheids of the secondary xylem contain thickenings, which form pits. 

Tyloses 

Vessels of the primary xylem are not blocked by tyloses while vessels of the secondary xylem are blocked by tyloses. 

Xylem Fibers 

Another difference between primary xylem and secondary xylem is that primary xylem contains fewer xylem fibers while secondary xylem contains a large number of xylem fibers. 

Conclusion 

Primary xylem is the type of xylem formed during the primary growth of both angiosperms and gymnosperms. The cells derived from the procambium differentiate into the cells in the primary xylem including tracheids and vessels, which are narrow and long. Also, primary xylem contains fewer xylem fibers. In comparison, secondary xylem is the type of xylem formed during the secondary growth of angiosperms and some gymnosperms. The cells of the vascular cambium give rise to the secondary xylem. The tracheids and vessels of the secondary xylem are shorter and wider. Especially, the vessels contain tyloses and a large number of xylem fibers occur in the secondary xylem. Therefore, the main difference between primary xylem and secondary xylem is the type of growth of plants, which gives rise to each type of xylem. 

References:

1. “Xylem – Definition, Types and Function.” Biology Dictionary, Biology Dictionary, 29 Apr. 2017, Available Here.

Image Courtesy:

1. “Woody Dicot Stem: Primary Xylem in Late One Year Quercus” By Berkshire Community College Bioscience Image Library (Public Domain) via Flickr
2. “Gymnosperm Stem: Secondary Phloem and Xylem in Five Year Pinus” By Berkshire Community College Bioscience Image Library (Public Domain) via Flickr

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