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Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), in particular, sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλόος (phloos) "bark". The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation.
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Structure
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 cells
Sieve cells are the more primitive of the two main conducting cell types in phloem, and are found in most seedless vascular plants (e.g.,
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), in particular, sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλόος (phloos) "bark". The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation.
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Structure
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 cells
Sieve cells are the more primitive of the two main conducting cell types in phloem, and are found in most seedless vascular plants (e.g.,