Xylem, along with phloem, is one of the conductive vascular tissues in plants and is responsible for the conduction of water and minerals from the roots to the leaves.
The term "Xylem" was coined by Carl Nageli, it’s a Greek word "xylon" which means wood. Xylem is a type of vascular tissue found in plants that mainly carries water and nutrients from roots to stems and leaves. They provide plants with mechanical power, too.
There are two types of xylem cells, based on origin:
1. Primary xylem: It is derived from procambium, further divided into protoxylem (primary xylem, first formed) and metaxylem (primary xylem, later formed). There are two forms of main xylem organization:
> Endarch is present in the stem, a condition where the protoxylem is pith-facing and metaxylem is peripheral.
> Exarch is present in the roots, condition where metaxylem at the middle and protoxylem at the periphery.
2. Secondary xylem: It derives from the vascular change and is responsible for an increase in plant girth. The cells which are cut off to the pith develop into secondary xylem. An annual ring is created by two forms of secondary xylem produced during spring and autumn.
Xylem consists of four distinct elements:
> Tracheids are dead cells, tube-like with a tapering end. They are mainly found in gymnosperm and lower angiosperm. They have a lignified thick wall and lack protoplasm. Their principal role is the transport of water and minerals.
> Vessels are a long cylindrical structure that has the appearance of a tunnel. It signifies the walls and has a wide central cavity. They are dead and lack protoplasm, too. They have several cells called vessel members that are entangled in common walls via a perforation. Often involved in water conduction, minerals and giving the plant mechanical power. They exist in the angiosperms.
> Xylem Fibre is a dead cell with walls lignified and central lumen. They help in transporting water and supplying mechanical assistance.
> Xylem Parenchyma is the only living xylem cell and stores starch and fat. They help in water transportation to a short distance.
Note: The xylem main function is to transport water from the roots to the other parts of the plant. Xylem gives mechanical strength, too. Tracheids and vessels are the largest water conductive components. Xylem fibers provide mechanical support and xylem parenchyma stores food materials and tannins, as well as radially conduct water.