Question

are spherical, oval, cylindrical, highly thickened dead cell with very narrow cavities. (a) Sieve tube (b) Companion cell (c) Fibres (d) Sclereids

Short answer

The correct answer is (d) Sclereids. Sclereids are the type of cells that match the description being 'spherical, oval, cylindrical, highly thickened dead cell with very narrow cavities'.

Step-by-step solution

Analyze the characteristics of each cell type

Look through various resources to gain an understanding of the characteristics of the Sieve tube, Companion cell, Fibres, and Sclereids. Sieve tubes are elongated cells found in phloem, and they lack normal cellular features like a nucleus and ribosomes. Companion cells are closely associated with sieve tube and provide metabolic support for sieve tubes. Fibres are elongated cells with a thick cell wall designed to provide support. Sclereids are thickened dead cells with varied shapes such as spherical or cylindrical and have narrow cavities.

Match the description to the cell type

Match the description given in the question with the characteristics of the cell types studied in the previous step. The description 'spherical, oval, cylindrical with very narrow cavities' matches the characteristics of Sclereids.

Key concepts

Sieve Tube Elements

Sieve tube elements are specialized cells that play a crucial role in the transport of sugars and nutrients throughout a plant. They form long columns called sieve tubes that make up the phloem, one of the two types of vascular tissue in plants.

Sieve tubes have a unique structure; they lack a nucleus, ribosomes, and other typical organelles found in living cells which allows for a more efficient flow of sap. Instead of being isolated cells, sieve tubes are connected end-to-end by sieve plates, which have pores that allow the movement of sap between cells.

These elements are crucial for plant health as they distribute the products of photosynthesis, such as glucose, from leaves to other parts of the plant for growth, storage, or immediate use in respiration.

Companion Cells

Companion cells serve an essential support function for sieve tube elements. Each sieve tube element is paired with at least one companion cell. Unlike sieve tubes, companion cells have a full complement of organelles, including a nucleus and dense cytoplasm.

The primary role of companion cells is to manage the metabolic processes necessary for sieve tube function. They facilitate the loading of sugars and other nutrients into the sieve tubes for transport. Moreover, companion cells help in maintaining the pressure gradient necessary for the movement of sap.

Companion cells and sieve tube elements originate from the same parent cell, ensuring a tight physical and functional relationship between them, which is critical for the efficient functioning of the phloem.

Plant Fibres

Plant fibers are elongated cells with thickened cell walls that provide structural support to plants. They help in maintaining the rigidity and shape of the plant and can also be found associated with vascular bundles. Plant fibers can vary in length, and their walls are typically composed of cellulose, hemicellulose, and lignin.

Fibers are commonly categorized into two types: xylary fibers, found within the xylem, and extraxylary fibers, found outside the xylem. The strength and flexibility of plant fibers have made them valuable in commercial use, such as in the production of paper, textiles, and ropes.

Due to their mechanical properties, fibers make an essential contribution to the plant's ability to withstand various stresses, ranging from wind to the physical load of fruits or seed pods.

Sclereids

Sclereids are a type of sclerenchyma cell known for their extreme thickening and hardness. They are present in various shapes - spherical, oval, or cylindrical - and contribute to the mechanical support of the plant. Unlike flexible fibers, sclereids are often star-shaped and create the 'gritty' texture found in some fruits, like pears.

Sclereids are dead at maturity and characterized by their thick, lignified cell walls which can create very narrow cavities inside the cell. These cells function to reinforce and protect the plant tissue. They are found in different plant parts, including seeds, bark, and the hard shells of nuts.

This diverse group of cells also includes stone cells, which contribute to the hardness of some fruits and can influence the quality of crops and their resistance to pests and pathogens.