subject: Elements Of The Phloem [print this page] Elements of driving or sieve elements Elements of driving or sieve elements
Are the most specialized, are characterized by their protoplasts modified and special cellular connections. They were discovered by Hartig in 1837. There are two types: sieve cells and members of sieve tubes . Their common features are:
Cell wall: cellulose and primary. Its thickness is variable, some primitive families of angiosperms (Magnolia, Persea) have side walls with pearly swellings. These thickenings are composed of many layers of densely arranged cellulose microfibrils and pectins. In some species is so marked thickening almost occludes the lumen. Apparently the function of these walls would be to facilitate radial transport apoplast. Some gymnosperms, according to Esau, nacreous thickenings present secondary in nature, other authors disagree, consider that it is primary walls.
Intercellular communication: the sieve elements communicate with each other through sieve areas. These are depressed areas of the wall provided with pores through which connect neighboring elements protoplasts through cytoplasmic cords. Differ from the primary fields of scores for two features: 1) the pore size, usually much greater than the plasmodesmata, can be observed by optical microscope, and 2) the presence of a visible callose cylinder, which surrounds the cord cytoplasm and may also appear on the surface of the area cribrosa.
Sieve cells: found in Pteridophytes and Gymnosperms . Communicate with each other by areas sieve, which are scattered throughout the cell surface.
In the Pteridophytes sieve cells are long, sharpened and enucleated with poorly differentiated sieve areas. They usually have beads, protein bodies limited by a membrane. Sieve cells of gymnosperms are are long and slender, with sharp edges that overlap. Sequoia sieve areas are located on the radial walls.
Sieve tube elements: found in angiosperms . Are longitudinal series of cells called "sieve tubes members" interconnected by means of sieve plates simple or compound. The side walls have sieve areas more or less specialized, usually difficult to see.
Cells escorts
Parenchyma cells are highly specialized, ontogenetically associated with members of the sieve tubes and phloem in the secondary metaphloem Angiosperms. Some are differentiated as transfer cells . Primary wall are primary fields with plasmodesmata branched scores, faced with the pores of the sieve areas of sieve elements. During ontogeny, callose is deposited sieve element side, but not the passenger side of the cell, where the primary fields permanencen scores. His protoplast is characteristic of metabolically active cells: often polyploid with large nuclei, large nucleoli, small vacuoles, endoplasmic reticulum well developed, large mitochondria, dictyosomes, abundant ribosomes. They may have chloroplasts and leucoplasts, but do not form starch. They assume the nuclear functions of the sieve elements, die when they cease to be functional. Serve the function of loading and unloading of the sieve elements, transporting assimilates laterally.
Location: there may be no companion cells in the protophloem of angiosperms. In the Gramineae is a very regular arrangement of sieve tubes and companion cells. It has been found that this arrangement is correlated with advanced types of vascular bundles, while the irregular disposition occurs in more primitive types of beams.
Ontogeny: is formed from the same cell meristematic members of the sieve tubes. This is divided longitudinally one or more times, giving cells of different size. The larger cell will differentiate into sieve tube member, and the remaining cells form companion cells prior transverse division may not occur. In summary, a sieve tube member may have a variable number of associated companion cells, arranged in longitudinal series.
parenchyma cells
There are varying amounts, and are less specialized companion cells or albuminous cells. In the phloem are elongated parallel to the tubes in the phloem secondary system are presented in vertical and horizontal. In the vertical are two basic types: spindle cells or rows of cells. In the horizontal spokes are the phloem, composed of two types of cells: procumbent, elongated in the radial direction and erect, usually marginal, elongated vertically. They can be differentiated into transfer cells with labyrinthine walls. Its functions are involved in loading and unloading of sugars transporting sieve elements to companion cells. Store starch, fats, tannins and crystals.
The movement of nutrients into the phloem, mainly sucrose, is unidirectional and slower: only reaches 2.5 cm per minute. Then be stored in fruits, seeds or even the root.
Phloem tubes transport the substances produced by photosynthesis, the substance transported is called sap elaborated.
