Conceptual Overview

Phloem, like xylem, is a complex conducting tissue of vascular plants. Its main function is the long distance transport of sugars and other photosynthates from the source (mature leaves), or reserves (the cotyledons of germinating seedlings) towards the sinks,e.g. roots, developing reproductive structures (flowers, fruits and seeds), meristems and young leaves. Phloem is also the primary trafficking pathway in vascular plants for the movement of plant growth regulators and other compounds that signal and direct accumulations or responses at sites remote from their origin. Phloem almost always accompanies xylem, and their elements run parallel to each other in vascular bundles. Phloem consists of several types of cells, including conducting and parenchyma cells, phloem (or bast) fibers, sclereids and, in some plants, secretory ducts and laticifers.

The most important components of phloem tissue are the cells conducting photosynthates, represented in flowering plants by the sieve elements. This term is explained by the presence of areas within the cells that contain specialized pores "sieve pores" in their walls. Reminiscent of sieves, "sieve areas" are modified for the mass intercellular flow of phloem sap, i.e. the aqueous solution of photosynthates. Unlike dead tracheary elements of xylem, sieve elements are typically living cells with an intact, osmotically active plasmalemma, some mitochondria, modified leucoplasts and endoplasmic reticulum. However, in the conducting state they lack cytosol, ribosomes, Golgi apparatus, a nucleus and the central vacuole with its tonoplast. Thus, the cell lumen is mostly free for phloem sap movement.

A unique filamentous protein termed P-protein ("P" derived from "phloem") is formed in the sieve elements of many plants during differentiation. It has filaments which are anchored to the periphery of the mature cell, and which permeate the sieve element lumen. After injury to the phloem, P-protein is released from its anchoring sites and accumulates at sieve pores by hydrostatic pressure of the sieve tube sap, blocks the pores and prevents assimilate loss at the injury site. Such aggregations of P-protein, sometimes also called "slime plugs" are usually formed during the processing of phloem for light microscopy. Sieve elements, unlike tracheary elements, do not usually possess lignified secondary walls.

Two types of sieve elements are recognized: sieve cells and sieve tube elements, which may be considered analogous to tracheids and vessel members of tracheary elements in xylem. Sieve pores are modified plasmodesmata and, like plasmodesmata, they interconnect sieve cells. Sieve tube members possess two types of specialized intercellular communications -sieve areas in their lateral walls, and sieve plates in their end walls. A series of sieve elements arranged end-to-end and interconnected through sieve plates forms a sieve tube, analogous to a vessel in the xylem. Sieve plates may be simple, with the whole end walls modified into a sieve area, or compound, consisting of several sieve areas interrupted by intact end wall areas. Simple straight sieve plates are considered most derived and are adapted for intercellular flow, like straight perforation plates in xylem vessels. On the other hand, inclined compound scalariform sieve plates are considered ancestral, like inclined perforation plates in xylem vessels. Gymnosperms and lower vascular plants have sieve cells, while sieve tubes are characteristic of flowering plants (angiosperms).

During phloem development, plasmodesmata in the walls between future sieve elements are converted into sieve pores usually 0.2-0.4 ╡m in diameter (but up to 1 ╡m in cucurbits). This facilitates the unimpeded flow of the translocation stream between adjacent sieve elements. Like plasmodesmata, the sieve pores are delimited from the cell wall by a plasmalemma which is continuous between the adjacent sieve elements, and also continuous throughout the length of the sieve tube. This, in effect, makes the sieve tube a continuous pathway. Development of sieve pores involves the deposition of an amorphous polysaccharide callose in the cell wall around each plasmodesma that displaces cellulose microfibrils. Due to the subsequent enzymatic digestion of the centers of these cylinders of callose, sieve pores will replace them.

Each sieve tube element in angiosperms is accompanied by one or more usually dense-cytoplasmic cells called companion cells. The companion cells lie along the sieve element and, in fact, are the product of longitudinal division from a common mother cell. Companion cells are thought to play an important part in loading sieve tubes with photosynthates in source area and unloading in sink areas. They also provide ribosome-less sieve elements with essential proteins and ATP. Companion cells and sieve elements together constitute a functional unit in food conductance. They possess specific intercellular communications in their common walls in which several plasmodesmata on the companion cell side merge at the site of the middle lamella into the sieve pore on the sieve tube side. In gymnosperms, typical companion cells are lacking in the phloem, but have specialized cells called albuminous cells that are thought to serve the same purpose. Pteridophytes lack both companion cells and albuminous cells, and have only sieve cells and phloem parenchyma.

As in xylem, two sequential states of phloem development are identified in gymnosperms and dicotyledons: primary phloem and secondary phloem. The primary phloem is differentiated from procambium, whereas secondary phloem is produced by vascular cambium. In stems and roots of plants with secondary growth, primary phloem is very short-lived and, except in leaves, it is replaced by secondary phloem. A common component of both types of phloem are bast fibers that often develop thick, heavily lignified secondary walls. In primary phloem they typically form "caps" over vascular bundles. In secondary phloem they are usually aggregated and variously arranged among other elements of the axial system. As in the wood, there are two types of parenchyma in the secondary phloem, axial parenchyma and ray parenchyma. In gymnosperms lacking companion cells, the marginal cells of phloem rays, termed albuminous cells, are structurally (through numerous plasmodesmata) and functionally associated with sieve cells.