Problem 39 - Objective Biology for NEET

Question

Gymnosperm lacks in their xylem. (a) Tracheids (b) Vessels (c) Xylem fibres (d) Xylem parenchyma

Step-by-Step Solution

Answer

Vessels are absent in the xylem of gymnosperms. So, the answer is (b) Vessels.

Step 1: Understanding Gymnosperm Xylem

Gymnosperms are vascular plants that lack flowers and fruits. The vascular tissue in gymnosperms includes xylem, which is crucial for water and nutrient transportation from the roots to the leaves. The main types of xylem cells include tracheids, xylem fibres, and xylem parenchyma.

Step 2: Identification of Absent Structure

Vessels are typically found in the xylem of angiosperms, also known as flowering plants. These structures are absent in gymnosperms. The other three options, tracheids, xylem fibres, and xylem parenchyma, are present in the xylem of gymnosperms.

Step 3: Final Conclusion

Therefore, among the given options, gymnosperms lack vessels in their xylem.

Key Concepts

Vascular Plants

Vascular plants, often known as tracheophytes, form a large group of plants that are defined by the presence of specialized tissue for conducting water and nutrients. This group includes familiar plants such as ferns, gymnosperms, and angiosperms.

Two primary types of vascular tissue exist: xylem and phloem. The xylem is responsible for transporting water and dissolved minerals from the roots to the rest of the plant, while the phloem transports sugars and other metabolic products downward from the leaves. This system allows vascular plants to grow to larger sizes and adapt to a wide range of environments.

Vascular plants are distinguished from non-vascular plants, like mosses, which do not contain these specialized conducting tissues and are typically smaller and simpler in structure.

Xylem Cells

Xylem cells are the building blocks of the xylem tissue in vascular plants. They perform the critical function of water and nutrient transportation throughout the plant. Xylem cells come in various forms, primarily comprising tracheids, vessel elements, xylem fibres, and xylem parenchyma.

Tracheids and vessel elements are specialized for water conduction and support. Tracheids are long, thin cells that contribute to the shape and strength of the plant. Vessel elements, found predominantly in angiosperms, connect end to end to form long tubes called vessels, which are efficient at conducting water vertically. Xylem fibres provide additional structural support, while xylem parenchyma cells are involved in the storage and lateral transport of water and nutrients.

Tracheids

Tracheids are a type of xylem cell that are elongated with thick, lignified cell walls and tapering ends. They are one of the two primary cells responsible for water transport in vascular plants. Tracheids have pits, small areas where the wall is thinner, allowing water to move laterally between cells.

In gymnosperms, tracheids are the main water-conducting cells in the xylem since they do not have vessels. They also confer structural support to the plant due to their strength. The absence of vessels in gymnosperms results in slower water transport, but tracheids are remarkably efficient in cold climates or environments with limited water supply.

Vessels

Vessels are another principal type of xylem cell found predominantly in angiosperms, the flowering plants. These cells link together to form long continuous tubes known as vessel elements. The end walls of vessel elements are either completely missing or perforated to form a vessel, increasing the efficiency of water transport through the plant.

This adaptation allows angiosperms to transport water more rapidly than gymnosperms, supporting higher metabolic rates and the ability to occupy a wider variety of habitats including those that are drier or have nutrient-poor soils. The presence of vessels is a key evolutionary feature that differentiates most angiosperms from gymnosperms.