Phloem tissue consists of: conducting cells, generally called sieve elements; parenchyma cells, including both specialized companion cells or albuminous cells and unspecialized cells; and supportive cells, such as fibresand sclereids).
Conducting cells (Sieve elements)Sieve elements are the group of cells that are responsible for actually moving sugar-laden fluids through the plant.At maturity they lack a nucleus and have very few organelles, so they rely on companion cells or albuminous cells for most of their metabolic needs. Sieve tube cells do contain vacuoles and other organelles, such as ribosomes, before they mature, but these generally migrate to the cell wall and dissolve at maturity; this ensures there is little to impede the movement of fluids. One of the few organelles they do contain at maturity is the smooth endoplasmic reticulum, which can be found at the plasma membrane, often nearby the plasmodesmata that connect them to their companion or albuminous cells. All sieve cells have clusters of pores at their ends that grow from modified and enlarged plasmodesmata, called sieve areas; the 'sieve' part of sieve cell is from these groups of pores having a sieve-like appearance. The pores are reinforced by platelets of a polysaccharide called callose.
Sieve cellsSieve cells are the more primitive of the two main conducting cell types in phloem, and are found in most seedless vascular plants (e.g., ferns, club mosses, horsetails) and gymnosperms (conifers, Ginkgo, etc.). Sieve cells have relatively narrow, uniformly-sized pores in the sieve areas.Sieve tube cellsThe sieve-tube cells, also known as sieve-tube members, are the more advanced type of conducting cell, and are the only sieve element found in the phloem of angiosperms.
The sieve tube is an elongated rank of individual cells, arranged end to end, and functioning to conduct food materials throughout the plant. The sieve areas of these cells are called sieve plates; the pores in sieve plates are generally larger and more variable in size than those in sieve cells.Companion cellsThe survival of sieve-tube members depends on a close association with the companion cells, a specialized form of parenchyma cell. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell; a typical plant cell except the companion cell usually has a larger number of ribosomes and mitochondria. This is because the companion cell is more metabolically active than a 'typical' plant cell. The cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.
There are three types of companion cell.1. Ordinary companions cells, which have smooth walls and few or no plasmodesmata connections to cells other than the sieve tube. 2.
Transfer cells, which have much-folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer. They are specialised in scavenging solutes from those in the cell walls that are actively pumped requiring energy. 3. Intermediary cells, which have smooth walls and numerous plasmodesmata connecting them to other cells.
The first two types of cell collect solutes through apoplastic (cell wall) transfers, whilst the third type can collect solutes via the symplast through the plasmodesmata connections. Albuminous cellsAlbuminous cells have a similar role to companion cells, but are associated with sieve cells only and are therefore found only in seedless vascular plants and gymnosperms. Other parenchyma cells Other parenchyma cells within the phloem are generally undifferentiated and used for food storage. Supportive cells Although its primary function is fluid transport, phloem may also contain supportive cells. These generally fall into two categories: fibres and sclereids.
Both cell types have a secondary cell wall and are therefore dead at maturity. The secondary cell wall makes them particularly rigid and resistant to damage.FibresFibres are the long, narrow supportive cells that provide tension strength without limiting flexibility. They are also found in xylem, and are the main component of many textiles such as paper, linen, and cotton.
SclereidsSclereids are irregularly-shaped cells that add compression strength but may reduce flexibility to some extent. They also serve as anti-herbivory structures, as their irregular shape and hardness will increase wear on teeth as the herbivores chew. For example, they are responsible for the gritty texture in pears.
