At the behest of the Botany, is called Liberian phloem, tube or glasses, to the conductive tissue of a plant that deals with the transport of organic nutrients, mainly, the sugars produced by the photosynthetic and autotrophic aerial part to the subterranean, non-photosynthetic, basal parts; that is to say, it is that part of the central cylinder of dicotyledonous angiosperm plants, which is made up especially of bundles or bundles of sieve vessels that transport the descending sap.
There are two types of phloem: primary and secondary. The first constitutes the vascular bundles and matures in the parts of the plant that are still growing in extension, their sieving elements becoming inactive very soon. It should be noted that in those plants that do not have secondary growth, it constitutes the functional phloem of the adult organs. And the secondary phloem, has its origin in the cambium (specific plant tissue of woody plants), located towards the periphery of the stem or root. It has an axial system and a radial system.
Meanwhile, the elements that make up the phloem are: screening elements (They are the most specialized, of variable thickness and lateral walls with pearly thickenings. Their function is to facilitate radial transport by apoplast. They can be observed through the light microscope); companion cells (highly specialized cells associated with the sieve tubes. They assume the nuclear functions of the sieve elements and die once the latter cease to be functional. They take care of the loading and unloading of the sieve elements); and parenchymal cells (They appear in a variable quantity, are less specialized than the previous ones and present different appearances in the primary and secondary phloem. They are responsible for the loading and unloading of the sieving elements, taking the sugar to the companion cells; they are starch storage , tannins, fats and crystals).
