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  Phloem Complex Tissue

Introduction to Phloem Complex Tissue

Such as xylem, phloem is as well a complex tissue. It conducts food materials to several parts of the plant.  The phloem elements that are made up from the procambium of apical meristem are termed as primary phloem. The phloem elements that are generated through the vascular cambium are termed as secondary phloem. The primary phloem elements which develop first from the procambium are smaller in size and termed as the protophloem, while those develop later are larger in size is known as metaphloem.  The protophloem is short lived.  It is crushed through the developing metaphloem.

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Diagram:  Phloem tissue

Phloem is formed of four types of cells:

1. Sieve elements,

2. Companion cells,

3. Phloem parenchyma and

4. Phloem fibres.

Companion cells are exists only in angiosperms. Companion cells are not present in gymnosperms and pteridophytes.  Phloem fibres are not present in the primary phloem of most of the angiosperms. But they are generally exists in the secondary phloem.

Sieve elements

Sieve elements are the phloem's conducting elements. They comprise thick primary walls.  Their end walls are transverse / oblique.  The end wall comprises some pores and it act like a sieve.  Thus it is termed as a sieve plate. The sieve elements are arranged in the order that one element is above another and create vertical sieve tubes. In matured sieve tube, nucleus is not present. It consists of a lining layer of cytoplasm. This is a significant characteristic of sieve elements. A particular protein termed as slime body is seen in it. The conduction of food material occurs by cytoplasmic strands. They are differentiated into sieve cells and sieve tubes. Sieve cells take place in pteridophytes and gymnosperms, when sieve tubes take place in angiosperms.

Sieve cells comprise sieve areas only on their lateral walls and are not arranged one above another in linear rows. They are not related with companion cells. Sieve tubes that are arranged in the order that is one above another in linear rows and comprises sieve plates on their end walls. They are related with the companion cells.  In mature sieve elements, occasionally the pores in the sieve plate are blocked via a substance termed as callose.

Companion cells

The thin-walled, elongated, specialised parenchyma cells that are related with the sieve elements are termed as companion cells.  In difference to sieve elements, the companion cells comprise cytoplasm and a prominent nucleus.  They are associated to the sieve tubes by pits found in the lateral walls.  The companion cells are available only in angiosperms and not present in gymnosperms and pteridophytes.  They help the sieve tubes in the conduction of food materials.

Phloem parenchyma

The parenchyma cells related with the phloem are termed as phloem parenchyma.  These are living cells.  They store fats and starch.  They also consist of resins and tannins in some plants.  They are present in all, pteridophytes, gymnosperms and dicots.  In monocots, generally phloem parenchyma is not present.

Phloem fibres

The fibres of sclerenchyma related with phloem are termed phloem fibres / bast fibres.  They are narrow, vertically elongated cells along with very thick walls and a small lumen (the cell cavity).   Between the four types of phloem elements, phloem fibres are only the dead tissue.  These are the supporting and strengthening cells.

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