Phloem tissue is the living tissue responsible for transporting organic nutrients produced during photosynthesis (mainly as the carbohydrate sucrose) to all parts of the plant where these are required. The phloem tissue is made up of the following major types of cells:
- sieve elements: these are conducting cells which transport sucrose.
- parenchyma cells: which store food for transport in phloem.
- companion cells: are associated with parenchyma cells and control the activities of sieve tube elements, since the latter have no nuclei. Companion cells are responsible for providing energy to the sieve elements to allow for the transport of sucrose. Companion cells play an important role in loading sieve tubes with sucrose produced during photosynthesis. Companion cells and sieve tube elements are connected via connecting strands of cytoplasm called plasmodesmata.
- fibres: unspecialised cells and supportive cells.
In the table below, the key structural features of the phloem are related to their function.
|Contain large number of ribosomes and mitochondria.||Due to absence of organelles or nucleus in sieve tube, companion cells perform cellular functions of sieve tube.|
|Has many plasmodesmata (intercellular connections) in the wall attached to the sieve tube.||Allows transfer of sucrose-containing sap over a large area.|
|Sieve tube elements are long conducting cells with cellulose cell walls.||Form good conducting tubes over long distances. Allows for transfer over a large area.|
|They are living cells with no nucleus or organelles such as vacuoles or ribosomes.||Allows for more space to transport sap. It is also why sieve elements need companion cells to carry out all cellular functions.|