Phloem is a complex tissue composed of sieve tubes, companion cells, phloem fibres
and phloem parenchyma
Sieve Tubes
: A sieve tube, like xylem vessels, is a series of cells
(sieve elements) joined end to end. The cross walls between successive cells
(sieve elements) become perforated forming sieve plates. The cell walls are thin.
Although the cell contents are living, the nucleus disintegrates and disappears.
The lumen is filled with a slimy sap which is composed mainly of protein.
Companion Cells
: Companion Cells are specialized parenchyma cells
which always appear with the sieve tube element. They are also elongated,
thin-walled and there is a distinct nucleus in the cytoplasm of the companion
cell. Companion cells are linked with the sieve tubes by small canals filled with
cytoplasm, which are smaller than pits.
Phloem Fibres
: These cells are elongated tapering cells, found
particular in the stem. They have thickened walls.
Phloem Parenchyma
: Phloem Parenchyma is living and has thin cell
walls. These cells form the packing tissue between all the other types of
cells.
Functions:
sieve tubes transport organic compounds,
companion cells helps to regulate the metabolic activities of the sieve tube
elements,
the phloem fibres give the plant mechanical strength,
the phloem parenchyma stores compounds such as starch.
