Xylem | Definition, Types, Location and Functions

Definition of Xylem

“Xylem is the part of a plant that transports water from the roots to the leaves and stems, transporting various nutrients along with it”.

It is the chief water-conducting tissue of the plants. Xylem functions as a conducting tissue for water and minerals from the roots to the top of plants, i.e., to the stems and leaves.

Alongwith acting like a conducting tissue, xylem also provides the mechanical strength to the plant parts. Mechanical strength is the ability to tolerate stress, pulling forces, compressive forces etc. without breaking off or tearing off.

Types of Xylem:

On the basis of origin, xylem tissue can be (a) primary xylem, (b) secondary xylem.

A. Primary xylem:

The xylem which arises early, i.e., during primary growth of plant body is called primary xylem. It is further of two types on the basis of origin (i.e., relative state of maturity): (i) Protoxylem, (ii) Metaxylem.

The first formed primary xylem is called protoxylem and the later formed primary xylem is called metaxylem. The metaxylem is more mature than the protoxylem. Protoxylem has vessels with narrow diameter and metaxylem has vessels with broad diameter.

Arrangement of primary xylem: Now on the basis of relative position of protoxylem and metaxylem in an organ, the arrangement of xylem can be: (i) Endarch, (ii) Exarch.

In endarch type of arrangement of primary xylem, the protoxylem or the first-formed primary xylem lies towards the centre (pith) and the metaxylem or later-formed primary xylem lies towards the periphery of the organ. Endarch type of primary xylem is seen in the stems.

In exarch type of arrangement of primary xylem, protoxylem lies towards the periphery and metaxylem lies towards the centre (pith) of the organ. Exarch type of primary xylem is seen in roots.

B. Secondary xylem:

Secondary xylem is the xylem that is formed during the secondary growth. It is formed by the vascular cambial ring (a lateral meristem).

Elements of xylem:

Xylem is a complex tissue which is composed of following four different kinds of elements: A. Tracheids, B. Vessels, C. Xylem fibres, D. Xylem parenchyma.

A. Tracheids

Structure of Tracheids: Tracheids are the elongated cells with tapering ends. These are the tube-like cells (one tube is one cell) whose both ends taper slowly to give an appearance of pointed ends.

• (a) The tracheids have thick and lignified walls (having a deposition of lignin in the cell walls).
• (b) These are the dead cells and are without protoplasm.
• (c) When a tracheid is cut and seen, the inner layers of its cell wall show many types of thickenings. These thickenings are of lignin and vary in form.
• (d) Even after the presence of thickenings, a cavity (lumen) is always present inside the tracheid which is meant for the transport of water and minerals through it.
• (e) The tracheids form a long row, placed one above the other and form a continuous ‘channel’ for the conduction of water and minerals from roots to the stems and leaves

Occurrence of Tracheids: Tracheids are found in all categories of vascular plants, ie., Pteridophyles, Gymnosperms and Angiosperms (flowering plants).

Functions of Tracheids:

1. Main purpose of tracheids is to transport water and minerals from roots to stems and leaves.
2. Due to the presence of thickened and hard wall, they also provide the mechanical support to the plant body.

Did You Know?

• Various types of thickenings in inner walls of tracheids: (i) Annular type: When lignin is deposited in a ring-like manner. (ii) Spiral type: When lignin is deposited in the form of a helix. Fig.: Lignified thickenings in xylem tracheids and tracheae : A. Annular, B. Spiral, C. Scalariform, D. Reticulate, E. Bordered pit (iii) Scalariform type: When lignin is deposited in a ladder-like manner. (iv) Reticulate type: When lignin is deposited in a net-like manner. (v) Pitted type: If the entire inner surface is thickened leaving unthickened circular areas the thickening is described as pitted.

B. Vessels

Structure of Vessels: Vessels are also called the tracheae.

(a) Vessel is a long cylindrical tube-like structure made up of many cells (one tube has many cells). These cells which constitute the vessel, are called vessel members or vessel elements.

(b) Vessels differ from tracheids in being cell fusions because tracheid is a single long cell while vessel is a long tube-like structure made up of many cells which are fused together to form it. So, a vessel is composed of row of cells placed one above the other in which the intervening walls get dissolved.

(c) Each vessel member has lignified cell wall and a large central cavity (space) for facilitating the water transport.

(d) Vessel cells are also devoid of protoplasm and are dead.

(e) The end walls of vessel cells are generally oblique and are called the perforation plates. The vessel members are interconnected through the perforations (openings) present in these perforation plates.

Occurrence of Vessels: The presence of vessels is a characteristic feature of angiosperms. Gymnosperms and pteridophytes lack vessels in their xylem, although, other xylary elements are present in them to permit the conduction of water and minerals. Primitive angiosperm families like Tetracentraceae, Trochodendraceae and Winteraceae do not have vessels.

Functions of Vessels

1. Longitudinal water and mineral transport.
2. Mechanical support.

C. Xylem fibres

They are the sclerenchymatous fibres. They are long, narrow and tapering at both ends. They are also the dead elements like vessels and tracheids.

• (a) They have highly thickened lignified walls.
• (b) Xylem fibres have obliterated central lumens. They are obliterated due to very thick walls which leaves no space inside them.
• (c) The fibres may either be septate or aseptate. The fibres which are septate have internal septa or cross walls and those which are aseptate do not have internal septa.

Occurrence of Xylem Fibres: Xylem fibres are the components of xylem in all categories of vascular plants.

Functions of Xylem Fibres: They are mechanical in function and provide support to the plant organs.

D. Xylem parenchyma

Cells of xylem parenchyma are living and thin-walled. Their cell walls are made up of cellulose. They have a prominent nucleus and dense cytoplasm.

Occurrence of Xylem parenchyma: Xylem parenchyma is present in the xylem of all vascular plants.

Functions of Xylem parenchyma:

1. The xylem parenchyma cells, as usual, store food materials in the form of reserve foods like starch or fat.
2. They also store other substances like tannins.
3. These cells help in the radial conduction of water, .e., in the radial directions.

Parenchymatous cells which help in radial conduction of the water are called ray Parenchyma cells.