Phloem (Bast) and its elements

March 6, 2019

It is a living, non-lignified tissue which is also called bast.
Its main function is to transport organic food inside the plant body from the leaves to the storage organs and from the storage organs to various parts according to the need of the organ.
Thus, the translocation of food by the phloem is in both the directions.
It consists of four elements. They are as follows:

a)Sieve tubes

Sieve tubes are slender, tube like structures composed of elongated cells, placed end to end.
Their walls are thin and made of cellulose.
Each transverse wall is, however, perforated by a number of pores and looks like a sieve and is called sieve plate.
The most important characteristics feature of the sieve element is that it lacks a nucleus.
Sieve tube has a lining layer of cytoplasm which is continuous through the pores.
They carry prepared food materials from the leaves to the storage organs and from the storage organs to the growing regions of the plant body.

b) Companion cells

They are thin walled, elongated, living cells containing protoplasm.
Each cell contains a large elongated nucleus.
These cells are associated with sieve tube and also connected with it by simple pits.
The companion cell is present only in angiosperms.
Usually the nuclei of the companion cells serve for the nuclei of sieve tubes as they lack them.
The companion cells do not contain starch.
These cells help the sieve tube in the conduction of food.

c) Phloem parenchyma

These are living parenchymatous cells which are associated with phloem.
These cells are concerned with many activities characteristics of living parenchyma cells, such as storage of starch, fat, and other organic substances.
The tannins and resins are also found in these cells.
Phloem parenchyma is mostly absent in most of monocotyledons and some herbaceous dicots.
They store food materials and serve to conduct them.

d) Bast fibres or phloem fibres

Sclerenchymatous cells found in the phloem or bast are known as bast fibres.
In many flowering plants, fibres form a prominent part of both the primary and secondary phloem.
The phloem fibre is rarely found or absent in phloem of Pteridophytes.
They are also not found in some gymnosperms and angiosperms.
Only simple pits are found on the walls of phloem fib res.
Because of the strength of strands of phloem fibres, they have been used for long time in the manufacture of ropes, mats, and cloths.
The fibres used in this way, has been known since early times as bast.
Hence, phloem fibres are also known as bast fibres.
They serve to the mechanical strength of phloem and the plant body as a whole.