Question

Protoplast is absent in all except (a) Tracheids (b) Vessels (c) Xylem sclerenchyma (d) Xylem parenchyma

Short answer

Xylem parenchyma retains protoplast even at its mature state thus, it is the correct answer.

Step-by-step solution

Identify the presence of protoplast in Tracheids

Tracheids are part of the xylem, that transports water and dissolved nutrients. These cells, once mature, are devoid of protoplast. So, Tracheids are not the correct answer.

Identify the presence of protoplast in Vessels

Vessels, like Tracheids, are also part of the xylem, and their main function is transport. At maturity, they lose their protoplast, making it incorrect.

Identify the presence of protoplast in Xylem sclerenchyma

Xylem sclerenchyma provides support to the plants. At maturity, these cells also lose their protoplast, hence, this option is also not correct.

Identify the presence of protoplast in Xylem parenchyma

Xylem parenchyma is a living tissue of the xylem, that also functions in storage and can help in the conduction of water. Being a living cell type, even at maturity, xylem parenchyma retains its protoplast. This makes it our correct answer.

Key concepts

Tracheids

Tracheids are elongated cells found in the xylem of vascular plants. They play a critical role in transporting water and dissolved minerals. These cells are part of the plant's vascular system and are most often found in gymnosperms, such as pine and fir trees. Unlike other xylem components, tracheids are unique due to their lengthy and tapering form. When young, tracheids contain a protoplast, which is the living part of the cell enclosed by the cell membrane. As they mature, however, they lose their protoplast and become hollow tubes to efficiently conduct water throughout the plant.

Key features of tracheids include:

• Thickened cell walls made of lignin, providing structural support.
• Overlapping ends that allow water passage between cells.
• Found primarily in wood and are crucial for both support and transport.

Despite the absence of protoplast in mature tracheids, their structure is essential for the plant's ability to move water against gravity.

Vessels

Vessels, or vessel elements, are another important component of the xylem tissues in plants. They differ from tracheids primarily in their shape and arrangement. Vessels form long tubes through which water can travel more efficiently than through tracheids. This is due to their larger diameter and the end-to-end connection without end walls, allowing for quicker water movement. Vessels are more common in angiosperms, like flowering plants, which allows them to support fast growth and the development of wider plant structures.

Here are the main characteristics of vessel elements:

• Short and wide compared to tracheids, making them efficient conduits for water.
• Have perforated end walls, which permit continuous flow of water and nutrients.
• Mature vessels are dead and hollow, having lost their protoplasts during their development.

This absence of living material in vessels means that while they are vital for water transport, they cannot store nutrients or heal themselves if damaged.

Xylem sclerenchyma

Xylem sclerenchyma is a type of supportive tissue found within the xylem. Its primary role is to provide mechanical strength to the plant, helping it to remain upright and resist various stresses like wind and rain. Unlike tracheids and vessels, which are more about transportation, sclerenchyma focuses on maintaining the plant's structure. During their development, sclerenchyma cells heavily lignify, which means they become impregnated with lignin, a complex polymer that fortifies the cell wall.

Key aspects of xylem sclerenchyma include:

• Very thick and strong secondary cell walls.
• Cells become completely lignified and lose their protoplast upon maturity.
• Essential for plant rigidity and integrity as they provide a framework.

In their mature state, xylem sclerenchyma cells are no longer living, due to the absence of protoplast, allowing them to efficiently support structural integrity.

Xylem parenchyma

Xylem parenchyma is unique among the xylem cells because it retains its protoplast even at maturity. This feature enables it to carry out multiple roles that other xylem components cannot. As living cells, they assist in water storage and may even aid in some transport within the plant. Xylem parenchyma forms a crucial part of both vascular and supportive systems, and is often found around vessels and tracheids.

Some important characteristics of xylem parenchyma include:

• Retains a living protoplast at maturity, unlike other xylem cells.
• Involved in storage of nutrients and radial transport within the plant.
• Can also help in repairing and replacing damaged tissues.

Being a living tissue, xylem parenchyma plays an important role in plant growth and survival by allowing flexibility in response to environmental changes.