What is the Difference Between Sieve Cells and Sieve Tubes

The main difference between sieve cells and sieve tubes is that sieve cells are long cells with narrow pores whereas sieve tubes are shorter cells with wide pores. Furthermore, sieve cells lack sieve plates while sieve tubes have sieve plates.

Sieve cells and sieve plates are the two types of sieve elements and are the main type of conducting elements of the phloem. Sieve cells are the main conducting elements of the phloem in lower plants while sieve tubes are present in the phloem of angiosperms.

Key Areas Covered

1. What are Sieve Cells
     – Definition, Occurrence, Structure
2. What are Sieve Tubes
     – Definition, Occurrence, Structure
3. What are the Similarities Between Sieve Cells and Sieve Tubes
     – Outline of Common Features
4. What is the Difference Between Sieve Cells and Sieve Tubes
     – Comparison of Key Differences

Key Terms

Albuminous cells, Companion Cells, Phloem, Sieve Cells, Sieve Elements, Sieve Plate, Sieve Tubes

What are Sieve Cells

Sieve cells are a type of sieve elements that occur in the phloem of flowering plants, gymnosperms including Gnetum and Ephedra, and pteridophytes including selaginella and Pteridium. They are elongated cells with tapered ends. Therefore, they do not form a sieve tube. They also lack a sieve plate. On the other hand, the diameter of the sieve cells is low. But, a large number of sieve pores are involved in the effective translocation of food materials along the phloem. Sieve cells are evenly distributed along the phloem tissue.

Figure 1: Phloem

Albuminous cells are the associated type of cells with the sieve cells. These cells have a long, overlapping area with the sieve cell. Also, this area is unspecialized and contains nutrients required by the phloem. The albuminous cells connect the sieve cells to the parenchyma cells for the stabilization and obtaining nutrients for the tissue.

What are Sieve Tubes

Sieve tubes are the most advanced type of sieve elements only present in the phloem of angiosperms. They are short cells with wide pores and are responsible for the transport of carbohydrates throughout the plant. Their end walls are horizontal and broad. Also, this forms sieve plates between two sieve tubes. Furthermore, due to the presence of horizontal cell walls, sieve tubes can arrange from end to end in a longitudinal manner, forming a tube. Thus, each sieve tube cell in this tube is called a sieve-tube member. Since the sieve pores are broad, the sieve tubes transport food materials with a minimum resistance.

Figure 2: Sieve Tube

Each sieve-tube member associates with a companion cell, which supplies ATP, nutrients to the sieve tubes and facilitates signalling. Since sieve-tube members do not contain either a nucleus or ribosomes, they require the assistance of companion cells for the functioning.

Similarities Between sieve Cells and Sieve Tubes

• Sieve cells and sieve tubes are two types of sieve elements in the phloem.
• Both are responsible for the translocation of food throughout the plant.
• Also, both are living cells with a primary cell wall, and secondary cell wall thickening is absent.
• Furthermore, both their protoplasm is dense and granular.
• Moreover, both lack a nucleus.
• They occur in the primary and the secondary phloem in angiosperms.

Difference Between Sieve Cells and Sieve Tubes

Definition

Sieve cells refer to a type of sieve elements of a primitive type present in ferns and gymnosperms, with narrow pores and no sieve plate. Sieve tubes refer to a series of sieve tube elements placed end to end to form a continuous tube. Thus, these definitions explain the underlying difference between sieve cells and sieve tubes.

Occurrence

Moreover, sieve cells occur in angiosperms, gymnosperms, and pteridophytes while sieve tubes only occur in angiosperms.

Specialization

Specialization is another difference between sieve cells and sieve tubes. Sieve cells are less specialized cells for the translocation of food while sieve tubes are more specialized cells.

Shape

Furthermore, sieve cells are long cells with narrow pores while sieve tubes are short cells with wide pores.

End Wall

Also, the end wall of the sieve cells has tapered ends while the end wall of the sieve cells is broad.

Sieve Plates

Another difference between sieve cells and sieve tubes is that sieve cells lack sieve plates while sieve tubes have sieve plates.

Tube Formation

In addition, sieve cells are individual cells and they do not form a tube while sieve tubes are cell aggregations, which arrange end to end, forming tubes.

Sieve Pores

The sieve pores of the sieve cells occur on both end walls and lateral walls while the sieve pores of the sieve tubes occur on the sieve plates.

Sieve Pores Grouping

The sieve pores of sieve cells occur in many groups on the end wall while the sieve pores of the sieve cells occur in a single group. Hence, this is another difference between sieve cells and sieve tubes.

Companion Cells

Sieve cells lack companion cells, but they are associated with less specialized, albuminous cells while sieve tubes contain companion cells. Furthermore, the albuminous cells are not ontogenically-related to the sieve cells while companion cells are ontogenically-related to the sieve tubes.

Conclusion

Sieve cells are the type of sieve elements found in both higher and lower plants. They have tapered ends; hence, they do not form a tube. Moreover, they occur individually in the phloem. On the other hand, sieve tubes area more specialized type of sieve elements found in angiosperms. Their end wall is broad; hence, these cells form a tube. They have companion cells as well. Therefore, the main difference between sieve cells and sieve tubes is their occurrence and structure.

Reference:

1. Sengbusch, Peter v. “The Phloem.” Botany Online. Available Here

Image Courtesy:

1. “Phloem cells” By Kelvinsong – Own work (CC BY-SA 3.0) via Commons Wikimedia  
2. “Figure 30 05 06” By CNX OpenStax –  (CC BY 4.0) via Commons Wikimedia