Phloem - Complex permanent tissue in plants

Phloem is the food conducting complex tissues of vascular plants. which is associated with xylem in the vascular system also termed as bast or laptome.
The term phloem was coined by C. Nageli (1858)
Like xylem phloem is also consists of living and dead cells of primary and secondary origin.
The Phloem which is derived from procambium is called primary phloem and the phloem which is derived from vascular cambium is called secondary phloem.
Early formed primary phloem elements are called protophloem whereas the later formed primary phloem elements are called metaphloem.
Protophloem is short lived. It gets crushed by the developing metaphloem.

1. Plant Anatomy Complex Permanent Tissue- Phloem By: Dr. Sunita Sangwan Assistant Prof. Deptt. of Botany MNSGC Bhiwani

2. Complex permanent Tissue- Phloem Dr. Sunita Sangwan Botany paper1- Plant Anatomy

3. Phloem

4. Phloem Phloem is the food conducting complex tissues of vascular plants. which is associated with xylem in the vascular system also termed as bast or laptome . The term phloem was coined by C. Nageli (1858 ) Like xylem phloem is also consists of living and dead cells of primary and secondary origin . The Phloem which is derived from procambium is called primary phloem and the phloem which is derived from vascular cambium is called secondary phloem. Early formed primary phloem elements are called protophloem whereas the later formed primary phloem elements are called metaphloem . Protophloem is short lived . It gets crushed by the developing metaphloem .

5. Phloem Phloem consists of four components as below : Sieve elements, Companion cells, Parenchyma cells Fibers.

6. Sieve elements Sieve elements are the conducting elements of the phloem. Sieve elements consist of two types of cells, the less specialized sieve cells and more specialized sieve tube cells.

7. Sieve Cell They are primitive type of conducting tissue. These are present in members of lower vascular plants and gymnosperms . These are narrow, elongated, tapering ends . Sieve cell have sieve area only in their lateral walls and absent or rarely present in end walls. They are not associated with companion cells . Sieve cells have acted as precursor cell and give rise of sieve tubes.

8. Sieve tubes Sieve tubes are long tube like conducting elements in the phloem. These are formed from a series of cells called sieve tube elements . The sieve tube elements are arranged one above the other and form vertical sieve tube. The end wall contains a number of pores and it looks like a sieve . So it is called as sieve plate. The sieve elements show nacreous thickenings on their lateral walls. They may possess simple or compound sieve plates.

9. Sieve tubes The function of sieve tubes are believed to be controlled by companion cells. In mature sieve tube, Nucleus is absent. It contains a lining layer of cytoplasm. A special protein ( P. Protein = Phloem Protein ) called slime body is seen in it. In mature sieve tubes, the pores in the sieve plate are blocked by a substance called callose (callose plug ). The conduction of food material takes place through cytoplasmic strands. Sieve tubes occur only in Angiosperms.

10. Functions Longitudinal translocation of prepared food materials from leaves to the storage organs. Upward translocation to growing regions. Food mainly moves in the form of sucrose through sieve tubes.

11. Complex permanent Tissue- Phloem Part-2 Dr. Sunita Sangwan Botany paper1- Plant Anatomy

12. Companion Cells The thin walled, elongated, specialized parenchyma cells, which are associated with the sieve elements, are called companion cells . These cells are living and they have cytoplasm and a prominent nucleus . They are connected to the sieve tubes through pits found in the lateral walls. Through these pits cytoplasmic connections are maintained between these elements. These cells are helpful in maintaining the pressure gradient in the sieve tubes. Usually the nuclei of the companion cells serve for the nuclei of sieve tubes as they lack them .

13. Companion Cells The companion cells are present only in Angiosperms and absent in Gymnosperms and Pteridophytes. In pteridophytes and gymnosperms , other type of specialized cell is associated with sieve tube cell instead of companion cells called albuminous cell.

14. Functions of Companion Cells Maintain the pressure gradient in the sieve tube cell. Control the passage of materials. Provide energy to sieve tube cell for loading and unloading of assimilate.

15. Phloem Parenchyma Phloem parenchyma is living cell with dense cytoplasm, nucleus and has thin cell walls. Cells of phloem parenchyma are elongated, tapering to cylindrical, sub-spherical or polyhedral in shape. The phloem parenchyma are mainly associated with storage of organic food materials and other substances such as resins, tennins, mucilage, latex etc. Phloem parenchyma cells, called transfer cells. Border parenchyma cells, are present near the finest branches and terminations of sieve tubes in leaf veinlets, where they also function in the transport of foods. Phloem parenchyma are not found in monocots.

16. Phloem fibres or bast fibres Sclerenchymatous fibres are found in both primary and secondary phloem and commonly called as phloem fibres or bast fibres . They are screnchymatous cells and among the four phloem elements phloem fibers are only dead tissue. These are typically elongated cells with lignified (Hemp) or non-lignified (Linum) walls with only simple pits . The fibres may be septate or non-septate. They support the conductive cells of the phloem and provide strength to the stem. Bast fibres are classified as soft fibres, and are flexible.

17. Phloem Parenchyma and phloem fiber

18. Primary phloem This is classified into protophloem and metaphloem . In protophloem, sieve tube members are without companion cells . Sieve tubes function for a brief period and soon they get crushed by the surrounding pressure . The crushed cells may disappear. Metaphloem tissue survives for a longer period. Its elements are longer and wider. Usually fibers are absent in dicotyledons , whereas in monocotyledons and herbaceous dicots, parenchyma cells are absent .

19. Secondary phloem Similar to secondary xylem, there are two systems of arrangements in secondary phloem: Axial system producing sieve elements, phloem parenchyma and phloem fibers. Transverse system producing ray parenchyma cells.

20. Difference between Primary phloem and secondary phloem Primary phloem Secondary phloem It originates from procambium of apical Meristem. First formed protophloem and later formed metaphloem are distinct from each other.  Phloem fibres are found in the outer part of phloem.  Sieve tubes are usually longer with narrow lumen.  Phloem parenchyma is poorly developed.  Sclereids are usually not found.  Medullary rays found in this region are arise from the activity of apical Meristem.  Medullary rays are homocellular. Callose formation around the sieve pore is either very little or absent. It arises from the vascular cambium that is a Lateral Meristem.  There is no distinction found in protophloem and metaphloem.  They are found among the phloem parenchyma cell.  Sieve tubes are shorter with broad lumen. Phloem parenchyma is highly developed and act as a storage for food material or may contain crystal, latex etc.  Sclereids are found in secondary phloem of several plants. Medullary rays found in this region are developed from the activity of ray initials of cambium. Medullary rays may be homocellular or heterocellular. Callose formation is abundant .

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