Difference Between Tracheids and Vessels

Main Difference – Tracheids vs Vessels

Tracheids and vessels are two types of conducting elements found in the xylem of plants. Both tracheids and vessels are involved in providing the mechanical support to the plant as well. Both these conducting elements consist of tubular structures. The main difference between tracheids and vessels is that tracheids are narrow and less efficient in water conduction whereas vessels are wide and highly efficient in water conduction. Tracheids are the major conducting elements in ferns and gymnosperms. In contrast, vessels are the major conducting elements in angiosperms. The other two cellular components of the xylem are xylem fibers and xylem parenchyma.  

Key Areas Covered

1. What are Tracheids
      – Definition, Characteristics, Function
2. What are Vessels
      – Definition, Characteristics, Function
3. What are the Similarities Between Tracheids and Vessels
      – Outline of Common Features
4. What is the Difference Between Tracheids and Vessels
      – Comparison of Key Differences

Key Terms: Angiosperms, Ferns, Gymnosperms, Lignin, Perforation Plate, Pits, Tracheids, Vessels, Water Conduction, Xylem

What are Tracheids

Tracheids are one of the two conducting elements in the xylem of angiosperms. Tracheids are also present in ferns and gymnosperms as their major conducting element. Angiosperms have both vessels and tracheids; hence tracheids are only secondary conduction elements in angiosperms.

Figure 1: Tracheid of Oak

Tracheids consist of a single cell with pointed ends. The diameter of tracheids is around 30 μm. During secondary cell wall thickening, the tracheids are highly lignified, forming a polygonal cross section. Only the regions of the pits are not lignified. After lignification, tracheids become dead cells. Their protoplast becomes empty with the maturation. In addition to water conduction, tracheids are capable of providing mechanical strength to plants. They provide mechanical strength to softwood plants. Due to the high surface to volume ratio, tracheids can hold water against the gravity. A tracheid of oak is shown in figure 1. 

What are Vessels

Vessels are the other type of conducting elements found only in angiosperms. Vessels do not contain a protoplasm at maturity, and secondary cell wall thickening by lignification produces a dead, tubular cell for the conduction of water. These lignified vessels are also involved in providing mechanical strength to plants. Wood is mainly made up of vessels.

Figure 2: Vessels of Oak

Vessels are shorter cells with a wider lumen than tracheids. Due to the increased diameter, vessels conduct water more efficiently than tracheids. Moreover, vessels are arranged end to end, forming a tube. The ends of the vessels are made up of perforation plates. Some perforation plates consist of a single opening. Some may consist of several elongated openings. Another type of perforation plates may consist of several round openings or net-like openings. The presence of a perforation plate increases the efficiency of water conduction. The vessel elements of oak are shown in figure 2. 

Similarities Between Tracheids and Vessels

• Tracheids and vessels are two components of the xylem.
• Both tracheids and vessels are tubular cells
• Both tracheids and vessels are dead at maturity since they possess secondary lignification.
• Both tracheids and vessels are found in both primary and secondary xylem.
• Both tracheids and vessels are involved in water conduction along the stem as well as providing mechanical support to the plant.

Difference Between Tracheids and Vessels

Definition

Tracheids: Tracheids are tubular cells in the xylem of vascular plants, involving in the water conduction from roots to the leaves.

Vessels: Vessels are elongated dead cells found in the xylem of flowering plants, consisting of perforated cell walls through which the water flows.

Found in

Tracheids: Tracheids are present in all vascular plants.

Vessels: Vessels are only present in angiosperms.

Origination

Tracheids: Tracheids are originated from a single cell.

Vessels: Vessels are originated from a longitudinal file of cells. Therefore, they produce continuous tubes.

Diameter of the Lumen

Tracheids: The tracheids contain a narrow lumen.

Vessels: Vessels contain a wide lumen.

Pits

Tracheids: Tracheids consist of less number of large pits.

Vessels: Vessels contains a large number of small pits.

Perforated/Imperforated Cells

Tracheids: Tracheids are imperforated cells.

Vessels: Vessels are perforated cells.

Efficiency of Conduction of Water

Tracheids: Tracheids are inefficient in water conduction since they are imperforated cells.

Vessels: Vessels are efficient in water conduction.

Cell Wall Thickness

Tracheids: Tracheids contain thin cell walls.

Vessels: Vessels contain highly thickened cell walls.

Cross Section

Tracheids: Tracheids contain polygonal cross sections.

Vessels: Vessels contain circular cross sections.

Average Length

Tracheids: Tracheids are shorter cells (about 1 mm long).

Vessels: Vessels are longer cells (about 10 cm long).

Ends

Tracheids: Tracheids contain tapering end walls.

Vessels: Vessels contain diagonal or transverse end walls.

End to End Connection

Tracheids: Tracheids are laterally connected.

Vessels: Vessels are connected by end to end.

Surface to Volume Ratio

Tracheids: Tracheids consist of a high surface to volume ratio.

Vessels: Vessels consist of a low surface to volume ratio.

Prevention of Air Embolism

Tracheids: Tracheids prevent air embolism due to its high adhesion force in the narrow tube.

Vessels: Vessels do not prevent the air embolism.

Conclusion

Tracheids and vessels are the two water conducting elements found in the xylem. Tracheids are the major conducting element in ferns and gymnosperms. Vessels are only present in angiosperms. The diameter of tracheids is lesser than that of vessels. Furthermore, vessels consist of perforation plates at the ends of the cells. Therefore, the efficiency of water conduction is high in vessels than in tracheids. Both tracheids and vessels are involved in providing mechanical strength to the plants. The main difference between tracheids and vessels is their diameter and the efficiency in water conduction.

Reference:

1. “Tracheid.” Encyclopædia Britannica, Encyclopædia Britannica, inc.,Available here. Accessed 24 Aug. 2017.
2. Nandkeolyar, Anya. “Xylem Vessels.” Prezi.com, 18 Dec. 2013, Available here. Accessed 24 Aug. 2017.

Image Courtesy:

1. “Tracheid of oak (from Marshall Ward)” By Harry Marshall Ward (1854–1906) – Marshall Ward H. The Oak: A Popular Introduction to Forest-Botany. – London: Kegan Paul, Trench, Trübner and Co. Ltd., 1892. – Figure 16, (Public Domain) via Commons Wikimedia
2. “Hardwood Pores” By McKDandy at English Wikipedia (CC BY 2.5) via Commons Wikimedia