Complex Permanent tissue - Phloem and its Components

• Phloem or bast is a  complex permanent tissue which conducts the transportation of organic food. Its elements were discovered by Hartig.

• The term was coined by Nagali. It has four types of elements named - sieve tube or sieve cells,  companion or albuminous cell, phloem parenchyma and phloem fiber.

• Sieve tube or sieve cells - They are the conducting element of the phloem.

Remember👌👌 In non flowering plants, sieve tubes are called sieve cells. In a flowering plant. Sieve tubes are known as sieve tube elements. 

• Sieve tube elements are arranged end to end to form a long tube channel called a sieve tube.

• The Sieve tube also has a sieve pore or sieve pit. The sieve Pores connect to protoplasm of adjacent sieve tube element through a protoplasmic strand called connecting strand.

• Each sieve pore also contains callose outside the connecting strand.

• In non-flowering plants, the sieve cells have pointed and tapering ends.

• The cells are  quite long as compared to the sieve tube elements of flowering  plants.

• Sieve cells are narrower than  sieve tube elements. Sieve área has a number of small pore or sieve pore. 

Companion cells or Albuminous cells -

• Their  cells are associated with the sieve elements.

• Companion cells are found in flowering plants and developed from the same mother cell which form a sieve tube element.

• The cells are nucleated and contain abundant reserve foods with ribosome mitochondria plastid endoplasmic reticulum.

• The companion cells are connected with a sieve tube element with the plasmodesmata. 

• The companion cells are believed to control the functioning of the sieve tube elements.

• They also help in maintaining pressure gradient in the sieve tube. 

• A sieve tube element is associated with one or sometimes more than one companion cell. 

• In non  flowering plant sieve cells are  associated with albuminous cells.

• Albuminous cells are  also called  cells. The albuminous cells are stained  with cytoplasm dye and  devoid of starch.

• They are nucleated like companion cells and control the functioning of sieve cells. 

• Phloem Parenchyma- Their cells are with dense  protoplasm. They are   elongated in the region  of phloem rays but more vertically elongated in the rest of phloem.

• Their cells  store food resin mucilage and látex.

• It also take part in lateral conduction of the food.

• In older Phloem, their cells  get  to produce fiber and sclereids.

• Phloem Parenchyma is absent in most monocot and some herbaceous dicot. 

• Phloem or bast fiber - They are made up of sclerenchyma present inside the phloem.

• They are  absent in primary phloem but are found in large numbers in secondary phloem.

• They are elongated dead and empty cells.  The cell wall is thick and lignified.

• They  provide mechanical strength. They are separated through renting and economically exploited. Corchorus or jute, Linum or Flax and cannabis or Hemp are phloem fiber.