2. Phloem (bast):
Phloem is another kind of complex permanent tissue.
The main function is to conduct prepared organic food material to different parts of the plant body.
It is also known as bast.
It is composed of four kinds of cells.
i) Sieve cells or sieve tube elements
ii) Companion cells
iii) Phloem parenchyma,
iv) Phloem fibres.
i) Sieve cells or sieve tube elements:
Sieve cells are narrow, elongated cells with tapering ends.
The sieve areas located laterally.
They are present in Pteridophytes and Gymnosperms.
In Angiosperms, sieve tubes are present.
They are elongated; tubular conducting channels of phloem.
They are arranged one above the other in distinct linear rows.
They have sieve plates (oblique or transverse perforated septa) at their end walls.
Sieve tubes are associated with companion cells.
Sieve tube is a living cell with a layer of cytoplasm but without nucleus.
The cytoplasm of adjoining sieve tubes is continuous through sieve pores found on sieve plates.
ii) Companion cells:
These are elongated living cells of phloem containing protoplasm and a large nucleus.
Companion cells are usually attached to the lateral side of sieve tube,(hence the name).
They originate from the same meristematic cells that give rise to the sieve tube element.
The cells are thin-walled and communicate with the sieve tubes by means of simple pits.
These are found in Angiosperms only.
They are absent in Gymnosperms and Pteridophytes.
These cells help in transport of food along with sieve tubes.
The nucleus of companion cells control the functioning of sieve tube.
iii) Phloem parenchyma:
These are simple parenchyma cells found in association with sieve tubes and companion cells.
They are absent in monocots.
They store food material and also help in transport of food.
iv) Phloem fibres (bast fibres):
These are sclerenchyma cells associated with phloem.
Their cell walls are lignified with simple pits.
They provide mechanical support to the plant body.
They are very long and used for making ropes and rough clothes.
