Complex Permanent Tissue Notes

This article we will discuss about Complex Permanent Tissue:- Xylem and phloem

• Complex tissues are made of more than one type of cells and they are heterogenous
• They are also known as vascular tissue or conducting tissue
• Complex tissues are absent in gametophytes
• Complex tissues are of two types:- Xylem and Phloem

Xylem

• 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

Phloem

• The term phloem was introduced by nageli
• Phloem is conducting tissue which conducts food materials from the leaves to other parts of the plants
• First formed phloem is protophloem and later formed phloem is called metaphloem
• They are composed of four different types elements
• The elements of Phloem are sieve elements, companion cells, phloem fibres and phloem parenchyma

Elements of phloem

I) Sieve Elements

• Sieve elements are living cells with protoplasm
• They are food conducting cells
• Nucleus absent in the sieve elements
• Two types of sieve elements are found in the phloem

a) Sieve tubes

• Sieve tube occurs in angiosperms
• They are rows of cells arranged one above other to form long pipes
• In sieve tubes the sieve area is located on end wall called sieve plate
• During unfavourable conditions the cytoplasmic strand is ensheathed by the callose material in the sieve area and transport of food material is stopped

b) Sieve Cells

• Sieve cells occur in phloem of ferns & gymnosperms
• These are primitive type of sieve elements
• Sieve cells are elongated with tapering end
• Sieve pores are present all over sieve cells

II) Companion cells

• Companion cells are living cells associated with the sieve tubes
• They are present only in angiosperms and are absent in pteridophytes & gymnosperms
• Companion cell and sieve tube are the sister cells because both of them originated from same mother cell
• They are contains both nucleus and cytoplasm
• Special type of cells are attached with sieve cells in gymnosperm and pteridophytes in place of companion cells.
• These cells are called Albuminous cells or Strasburger cells

III) Phloem parenchyma

• They are living cells with thin walled
• Phloem parenchyma cells in generally store up food materials and help in conduction of food
• Phloem parenchyma is totally absent in monocot plants
• Two types of parenchyma occurs in phloem
• Axial parenchyma:- Present in primary phloem and they store starch
• Ray parenchyma:- Present in secondary phloem and useful for radial conduction of food materials

IV) Phloem fibres

• This is only the dead component of phloem
• They possess lignified cell wall with pits on their wall
• Phloem fibres are very long, soft and silky in appearance, hence have immense economic importance
• These fibres provide mechanical support to the plants

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