Phloem or bast is another permanent complex tissue. It is also known as bast leptome. It is composed of four elements.
(i) Sieve-tubes (ii) Companion cells (iii) Phloem parenchyma (iv) Phloem fibers.
Sieve-tubes occur as long, slender tube-like structures composed of elongated cells, placed end to end. Their walls are thin and made of cellulose. Their transverse walls are oblique and are perforated (sieve pores) in a sieve like manner to form the sieve plate.
The protoplasmic strands of one sieve tube pass to the adjacent sieve plate. In winter, the sieve-plates get impregnated with callus pads made up of cellulose. On arrival of spring i.e. under the favorable conditions the callus gets dissolved.
A young sieve tube contains nucleus, cytoplasm and other organelles. In old sieve tubes the callus forms a permanent deposit. Its nucleus gets disintegrated but a layer of cytoplasm prevails which is continuous through the pores.
Sieve tubes conduct food materials like carbohydrates and proteins from leaves to the storage organs and later from the storage organs to the growing regions of the plant body.
(ii) Companion cells:
These are specialized parenchymatons cells which are closely associated with sieve tube elements in their origin, position and function.
It is living, containing dense granular cytoplasm and large elongated nucleus. Companion cells are connected with sieve tubes by pits present in their common longitudinal wall.
The companion cells play an important role in the maintenance of pressure gradient in the sieve tubes. They occur only in angiosperms.
(III) Phloem parenchyma:
These are thin walled living parenchymatous cells associated with phloem tissue. They store organic food materials but are absent in monocotyledons.
(iv) Bast fibers or phloem fibres:
Elongated sclerenchymatons cells having pointed, needle like apices occurring in the phloem are known as phloem fibers or bast fibers.
The walls of these fibers are usually thickened due to the deposit of cellulose or lignin. At maturity the bast fibers lose their protoplasts and become dead. Phloem fibers of many plants like Corchorus, Cannabis etc. are the sources of commercial fibers.