In vascular plants (all plants except mosses and their relatives), phloem is the living tissue that carries sugar and organic nutrients throughout the plant. The other type of transport tissue in plants, xylem, transports water. In trees, the phloem and other tissues make up the bark, hence its name, derived from the Greek word for "bark."
Commonly, an analogy is drawn between the vascular system and sap of plants and the blood vessels and blood of the human body. Similar to the network of veins and arteries, the phloem and xylem of a vascular plant comprises an extensive network of tubes that transport essential fluids from one part of a plant to another area. This view also aligns with the theological concept that human beings "are microcosms of creation" (Burns 2006), encapsulating the structure, elements, and qualities of the macrocosm.
Sap, the watery fluid with dissolved substances that travels through vascular tissues (both xylem and phloem), is transported through phloem in elongated tubes, called sieve tubes, formed by chains of living cells called sieve tube members. The sieve-tube cells lack a nucleus, ribosomes, and a distinct vacuole. In angiosperms, at the end wall of sieve-tube members are pores, called sieve plates, through which phloem sap flows.
Beside each sieve-tube member is a companion cell, which connects to sieve-tube cells by many channels, or plasmodesmata, in the cell wall. Companion cells carry out all of the cellular functions of a sieve-tube element, and the nucleus and ribosomes of a companion cell may serve one or more adjacent sieve-tube cells.
In addition to typical phloem elements, fibers, sclereids (small bundles of supporting tissue in plants that form durable layers), and albuminous cells (similar in function to companion cells and found in gymnosperms) can also be found in phloem.
Unlike xylem, which is composed primarily of dead cells, the phloem is composed of living cells that transport sap. Phloem sap is rich in sugar and is made in photosynthetic areas of the plant. The sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, like tubers or bulbs.
The movement in phloem is variable, whereas in xylem cells movement is unidirectional (upward). Bulk flow moves phloem sap from a sugar source to sugar sink by means of pressure. A sugar source is any part of the plant that produces sugar by photosynthesis or releases sugar by breaking down starch. Leaves are the main source of sugar. Sugar sinks are storage organs that consume water or sugar. Developing seed-bearing organs (such as fruit) are always sinks. Storage organs, including tubers and bulbs, can be a source or a sink depending on the time of year. During the plant's growth period, usually in the spring, storage organs break down, providing sugar for sinks in the plant's many growing areas. After the growth period, storage organs store carbohydrates, becoming sinks. Because of this multi-directional flow, coupled with the fact that sap cannot move easily between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.
In 1930, German plant physiologist Ernst Munch proposed the Pressure flow hypothesis to explain the mechanism of phloem translocation (the transport of food in a plant by phloem). This process is accomplished by a process called phloem loading at a source and unloading at a sink, which causes a pressure gradient that drives the contents of the phloem up or down the sieve tubes from source to sink. In leaves, the sugar source, the xylem, and the phloem are located close to the photosynthetic tissue, which takes water from the xylem and, through active transport, loads sugar (and other products of photosynthesis) into the phloem for transport to the sink. As the organic nutrients accumulate in the phloem, water moves into the sieve-tube element by osmosis, creating pressure that pushes the sap down or up the tube. At the sink, the concentration of free sugar is lower than in the sieve tube. This sugar concentration gradient causes cells to actively transport solutes out of the sieve-tube elements into sink tissue. Water follows by osmosis, maintaining the gradient.
Movement of sap through the phloem is driven by positive hydrostatic pressures; transport of water and minerals through the xylem is driven by negative pressures (tension) most of the time.
Phloem cells are of meristematic origin. A meristem is a tissue in plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place: the roots and shoots. Phloem is produced in phases. Primary and secondary growth occurs simultaneously in different parts of the stem.
Primary phloem is laid down by the apical meristem, which aims to elongate the stem. Meristematic cells divide longitudinally and then elongate, differentiating into sieve elements and companion cells.
The girth, or diameter, of stems and roots increases by secondary growth, which occurs in all gymnosperms and most dicot species among angiosperms. Secondary phloem is laid down by the vascular cambium, a continuous cylinder of meristematic cells that forms the secondary vascular tissue. The vascular cambium forms in a layer between the primary phloem and primary xylem, giving rise to secondary xylem on the inside and secondary phloem on the outside. Every time a cambium cell divides, one daughter cell remains a cambium cell while the other differentiates into either a phloem or a xylem cell. Cambium cells give rise to secondary phloem to the inside of the established layer(s) of phloem during secondary growth.
A cross section of a stem after secondary growth would show concentric circles of pith (the center), primary xylem, secondary xylem, vascular cambium, secondary phloem, primary phloem, cork cambium, cork, and periderm (the outermost layer). Bark consists of tissues exterior to the vascular cambium.
Only the youngest secondary phloem is involved in sap transport. With time, older secondary phloem dies, protecting the stem until it is sloughed off as part of the bark during later secondary growth seasons. The tree's diameter increases as layers of xylem are added, producing wood.
Phloem of pine trees has been used in Finland as a substitute food in times of famine, and even in good years in the northeast, where supplies of phloem from earlier years helped stave off starvation somewhat in the great famine of the 1860s. Phloem is dried and milled to flour (pettu in Finnish) and mixed with rye to form a hard dark bread (Vanharanta 2002). Since the late 1990s, pettu has again become available as a curiosity, and some have made claims of health benefits (Mursu 2005; Vanharanta 1999).
Although the phloem is the principle pathway for the movement of sugar from the leaf to other plant parts, maple sap, used to produce maple syrup, actually derives from the xylem, not the phloem. (See xylem.)
Because phloem tubes sit on the outside of the xylem in most plants, a tree or other plant can be effectively killed by stripping away the bark in a ring on the trunk or stem. With the phloem destroyed, nutrients cannot reach the roots and the tree/plant will die. Trees located in areas with animals such as beavers are vulnerable. The beavers chew off the bark at a fairly precise height. This process is known as girdling, and is used in agricultural purposes. For example, enormous fruits and vegetables seen at fairs and carnivals are produced via girdling. A farmer would place a girdle at base of a large branch, and remove all but one fruit/vegetable from that branch. Thus, all the sugars manufactured by leaves on that branch have no sinks to go to but the one fruit/vegetable, which thus expands to many times normal size.
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