Question

The xylem fibres are classified into
a. Protoxylem and metaxylem
b. primary and secondary fibers
c. Fibre tracheids and fibers
d. Long and short fibers

Answer 1

Hint: In plants, the various varieties of tissues include the meristematic tissues, the permanent tissues, and also the reproductive tissues. The permanent tissues are further classified into fundamental tissues and therefore the complex permanent tissues. The complex permanent tissues include the vascular tissues, xylem and phloem.

Complete answer: Xylem may be a form of plant tissue in plants primarily involved in transporting water and nutrient (from the roots to the shoot and leaves) and providing structural support Xylem is comprised of the xylem parenchyma, xylem fiber (or sclerenchyma), tracheids, and vessels. It's the first component of wood in plants. Based on the stage and origin of growth, a xylem is also classified as primary or secondary. Primary xylem is created during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem. Secondly, xylem is created during secondary growth from vascular cambium. Although secondary xylem is additionally found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta, the 2 main groups during which secondary xylem are often found are: conifers (Coniferae) and angiosperms (Angiospermae).

Xylem consists of 4 different forms of elements:

> Tracheids: Dead, tube-like cells with a tapering end. They’re present mostly in gymnosperm and lower angiosperm. They need a thick lignified wall and lack protoplasm. Their main function is water and mineral transportation. Vessels: they're present in angiosperms. These are a protracted cylindrical structure having tube-like appearance. The walls are lignified and have an oversized central cavity. They are also dead and lack protoplasm. They need many cells called vessel members which are interconnected through a perforation in common walls. Mostly involved within the conduction of water, minerals and provides mechanical strength to the plant.

> Xylem Fibre: Dead cell with lignified walls and a central lumen. Involved in water transportation and providing mechanical support.

> Xylem Parenchyma: Only living cells of xylem and store starch and fat. They assist within the short distance transportation of water.

> Xylem fibers: they're non-living sclerenchyma cells as they lose their protoplast at maturity. These cells are found in between the tracheids and xylem vessels of the xylem tissue. Sclerenchyma cells are narrow and elongated cells with tapering ends. They’re former parenchyma cells that developed storage cell walls. Their cell walls become lignified. Their elasticity and great strength make them a necessary component within the xylem as they protect and supply mechanical support to the key water conducting tissues of the xylem.

Two major kinds of xylem fibers: (1) fiber tracheids and (2) libriform fibers. Fiber tracheids are rather involved in providing mechanical support than in conduction.

> Fiber tracheids: They are rather involved in providing mechanical support than in conduction. Tracheid: it's the primitive element of xylem (fluid-conducting tissues), consisting of one elongated cell with pointed ends and a secondary, cellulosic wall thickened with lignin (a chemical binding substance) containing numerous pits but having no perforations within the cell wall. At functional maturity, the cell is dead and empty; its former protoplast is represented, if at all, by a warty layer on the wall. Tracheids serve for support and for upward conduction of water and dissolved minerals altogether vascular plants and are the sole such elements in conifers and ferns.

> Libriform Fibers: They're elongated wood cells with narrowed ends that make wood’s durability and hardness. The walls of libriform cells are lignified, with occasional oblique pores. In hardwoods (oak, ash), the walls are greatly thickened; this is often less true in softwoods (linden, poplar). In some species (maple, elder), the cell walls are thin, holding live protoplast for long periods and containing nutrients (fats, starches). Sometimes, libriform cells are divided by thin transverse septa that retain their living contents and nutrients for long periods (septal libriform). Libriform fibers developed evolutionarily from tracheids during the method of specialization of wood cells.

Therefore, the answer is c, fibre tracheids and fibres.

Note: The xylem, vessels and tracheids of the roots, stems and leaves are interconnected to make a nonstop system of water-conducting channels reaching all parts of the plants. The system transports water and soluble mineral nutrients from the roots throughout the plant. It’s also accustomed to replace water lost during transpiration and photosynthesis. Xylem sap consists mainly of water and inorganic ions, although it also can contain a variety of organic chemicals in addition. Transporting sap upwards becomes tougher because the height of a plant increases and upwards transport of water by xylem is taken into account to limit the utmost height of trees. Three phenomena cause xylem sap to flow:
> Pressure flow hypothesis
> Transpirational pull
> Root pressure