Xylem – Definition, Types and Function

This article we will discuss about Xylem – Definition, elements of xylem, types of xylem and functions of xylem

Xylem – Definition

• The term xylem was introduced by nageli (1858)
• Xylem is conducting tissue which conducts water and mineral nutrients upwards from the root to the leaves
• Xylem besides conduction also provide mechanical strength
• They are composed of four different types of cells
• The elements of xylem are xylem tracheids, xylem vessels, xylem fibres and xylem parenchyma

Elements of xylem

i) Tracheids

• Tracheids are dead and lignified cells
• Tracheids are elongated cell, thick walls with tapering end
• They are placed one above other and all separated by cross walls which bear bordered pits
• The deposition of lignin on cell wall is responsible for formation of different types of thickenings
• Types of thickening in tracheids are annular, spiral, scalariform, reticulate and border pitted
• Tracheids are found in pteridophytes, gymnosperms and xylem of dicotyledons
• Function: Conduct water, dissolved salts and also give mechanical support

ii) Vessels

• Vessels are dead and lignified cells
• Vessels are short cells, pipe like structure and thick walled with wide lumen
• Usually they have simple pits
• The deposition of lignin on cell wall is responsible for formation of different types of thickenings
• Types of thickening in vessels are annular, spiral, reticulate and pitted thickening
• The end walls of vessel elements are called perforation plates
• The perforation may be simple or multiple
• Function: Conduct water, dissolved salts and also give mechanical support

iii) Xylem fibres

• Xylem fibres are dead and lignified cells
• Fibres are long, thick walled and tapering ends
• Lumen is highly reduced
• They are generally not fount in gymnosperm wood
• Function: They give support to vessels and tracheids

iv) Xylem parenchyma

• Xylem parenchyma are living and thin walled cells
• Cell walls are made up of cellulose
• Function: Store food materials and radial conduction of water

Types of xylem

Basis of size and time of development

I) Protoxylem

• First formed xylem
• Consists of smaller elements
• Contain large amount of parenchyma
• Developed directly from procambium

II) Metaxylem

• Later formed xylem
• Consists of larger sized elements
• Contain less amount of parenchyma
• Differentiated part of the protoxylem

Based on position of primary xylem

I) Endarch

• Protoxylem elements are situated closer to centre of axis
• Metaxylem develops periphery
• Centrifugal development
• Feature of stems

II) Exarch

• Protoxylem elements are directed away from centre of axis
• Metaxylem elements develop towards centre of axis
• Centripetal development
• Feature of root

III) Mesarch

• Protoxylem elements are in centre surrounded by metaxylem elements
• Feature of ferns

Function of xylem

• The main function of xylem is to carry water and mineral salts upward from the root to different parts of shoots.
• Since walls of tracheids, vessels and sclerenchyma of xylem are lignified, they give mechanical strength to the plant body

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