Phloem (or book) is a complex of tissues typical of vascular plants with a triple function: in fact the phloem performs transport tasks of soluble, reserve and support compounds of the plant. The term phloem comes from the Greek word of bark, φλοιός (floios) and was used for the first time by Nägeli in 1858.
Phloem may have primary or secondary origin.
– It has primary origin when it derives from the differentiation of the procambio (primary meristem) and can be further distinguished in protofloema and metafloema.
– It has secondary origin when it comes from the differentiation of the cribro-vascular exchange (secondary meristem) and is found in the secondary structures of the organs.
Phloem cells, unlike xylematic cells, are also alive at maturity (with the exception of sclerenchymatous elements); these cells, however, lack the nucleus, the Golgi apparatus, the cytoskeleton and ribosomes. Another peculiarity of these cells is that they are not lignified and have porous areas coated with callose so as to allow a cytoplasmic connection between adjacent cells.
The phloem is in turn made up of:
– conductive cells;
– cribrosi elements;
– parenchymal cells;
– other cellular elements (eg parenchyma, fibers).
The conductive cells are responsible for transporting sugars in the plant. The mature ones, to facilitate the liquid movement, are devoid of nucleus and have very few organelles, so they use complementary or albuminose cells for most of their metabolic needs. However, there is a smooth endoplasmic reticulum, often close to the plasma membrane, close to plasmodesms, which are used to communicate these cells with companion cells or with albumin cells.
The cribrosi elements are endowed with pores at the ends reinforced by a polysaccharide called callose and differ according to the systematic groups of the plants in which they are found:
– vascular Cryptogams and Gymnosperms possess elongated cribrose cells with pectic cellulose walls;
– the Angiosperms have the cribroso tubes which are more specialized cells stacked longitudinally and which have a life of a few years and with a completely reabsorbed nucleus.
Parenchymal cells are of two typesː
– companion cells (Angiosperms);
– albuminose cells (Gymnosperms and other non-flowering plants).
Within the phloem, some parenchymal cells are undifferentiated and perform energy reserve function.
The companion cells are cells associated with the elements of the cribroso tubes but, unlike these, they maintain all the cellular organelles with a role of primary importance in the active transport of the lymph.
Phloem also consists of supporting cells that play a mechanical role, which are fibers and sclereids; these cells have a secondary cell wall and died at maturity.
In most plants the phloem vessels are found outside the xylem, so certain trees and plants, if you remove a ring of bark on the trunk or stem can die.
However, this process can be used for agricultural production purposes, to produce large fruits and vegetables. With this technique a ring of bark is engraved at the base of a large branch, from which all but one fruit are removed. In this way the sugars produced by the leaves of that branch can no longer accumulate in other areas of the plant, contributing to grow the fruit beyond normal size.