noun, plural: sieve tube elements
The phloem is the vascular tissue responsible for the translocation of photosynthate materials throughout the plant. It is comprised of sieve elements, companion cells, phloem sclerenchyma, and phloem parenchyma cells. In angiosperms, the sieve element is referred to as a sieve tube, which is a tube comprised of sieve-tube elements joined end-to-end. A sieve-tube element is a specialized type of sclerenchyma. However, unlike the phloem sclerenchyma cells that have lignified cell walls (and therefore are more associated with providing support), the sieve-tube elements are not lignified. The sieve-tube elements have primary cells walls only. They are connected to one another at their end walls, which may be inclined or transverse. The end walls also have perforations. These pores allow cytoplasmic streaming of the protoplasts in between sieve-tube elements. A set of perforations in the wall area of a sieve tube is called a sieve area.
The sieve tube is closely associated with a companion cell. It has an ontogenic relationship with the companion cell as they arise from a common progenitor cell. At maturity, the sieve-tube element would only have retained fewer types of organelles, particularly plastids and mitochondria.
- sieve-tube member