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Phloem (/ ˈ f l oʊ. əm /, FLOH-əm) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, [1] to the rest of the plant. This transport process is called translocation. [2]
In the stems of some Asterales dicots, there may be phloem located inwardly from the xylem as well. Between the xylem and phloem is a meristem called the vascular cambium. This tissue divides off cells that will become additional xylem and phloem. This growth increases the girth of the plant, rather than its length.
The cambium present between primary xylem and primary phloem is called the intrafascicular cambium (within vascular bundles). During secondary growth, cells of medullary rays, in a line (as seen in section; in three dimensions, it is a sheet) between neighbouring vascular bundles, become meristematic and form new interfascicular cambium ...
Sieve elements are specialized cells that are important for the function of phloem, which is a highly organized tissue that transports organic compounds made during photosynthesis. Sieve elements are the major conducting cells in phloem. Conducting cells aid in transport of molecules especially for long-distance signaling.
According to the hypothesis, the high concentration of organic substances, particularly sugar, inside the phloem at a source such as a leaf creates a diffusion gradient (osmotic gradient) that draws water into the cells from the adjacent xylem. This creates turgor pressure, also called hydrostatic pressure, in the phloem. The hypothesis states ...
The siphonostele shown on the left may also be called an amphiphloic siphonostele. The eustele shown on the right is collateral, i.e. with all the phloem on one side of the xylem. Siphonosteles have a central region of ground tissue called the pith, with the vascular strand comprising a hollow cylinder surrounding the pith. [9]
The process for this is retting, and can be performed by micro-organisms either on land (nowadays the most important) or in water, or by chemicals (for instance high pH and chelating agents), or by pectinolytic enzymes. In the phloem, bast fibres occur in bundles that are glued together by pectin and calcium ions. More intense retting separates ...
A vein is made up of a vascular bundle. At the core of each bundle are clusters of two distinct types of conducting cells: Xylem Cells that bring water and minerals from the roots into the leaf. Phloem Cells that usually move sap, with dissolved sucrose (glucose to sucrose) produced by photosynthesis in the leaf, out of the leaf.