Phloem is a food conducting tissue and is also called bast. Phloem is composed of four elements.
(a) sieve elements (b) Companion cells © Phloem parenchyma (d) Phloem fibres.
(a) Sieve elements : Sieve elements in lower vascular plants (Pteridophytes and gymnosperms) are referred to as 'sieve cells' whereas in higher plants (Angiosperms) they are referred to as 'sieve tubes'.
Sieve cells are elongated, thin walled, and arranged one above the other , and show thin protoplasmic layer within. Sieve cells posses perforated lateral walls and they are not associated with companion cells, but with the cells albuminous cells.
sieve tubes are much longer than sieve cells and are arranged one above the other longitudinally with perforated transverse end walls, called 'sieve plates' . Protoplasmic strands maintain the continuity through these perforations with the adjoining sieve tubes, sieve tubes are living cells but they are enucleate.
At the end of the growing season , sieve pores are covered by deposits of callose.
(b) Companion cells : These are thin walled living and narrow cells running parallel to sieve tubes in angiosperms. Sieve tubes and companion cells running parallel to sieve tubes in angiosperms. Sieve tubes and companion cells are sister cells derived from the same mother cell. These are known to regulate the functions of sieve tubes, sieve tubes are living cells but they are enucleate.
At the end of the growing season , pores are covered by deposits of callose.
(b) Companion cells : These are thin walled living and narrow cells running parallel to sieve tubes .
(c) Phloem parenchyma : Parenchyma cells associated with phloem are referred to as phloem parenchyma . They are storage in function . Phloem parenchyma is absent in monocot stem.
(d) Phloem fibres : These are sclerenchyma fibres associated with phloem. They are also called bast fibres. They provide mechanical support to the plant body.
