WikiWorks at 05:19, 6 April 2007

2007-04-06T05:19:38Z

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In vascular [[plant]]s, '''phloem''' is the living [[Biological_tissue|tissue]] that carries organic nutrients, particularly [[sucrose]], a sugar, to all parts of the plant where needed. In [[tree]]s, the phloem is underneath and difficult to distinguish from [[bark]], hence the name, derived from the [[Greek language|Greek]] word ''phloios'' for "bark". The phloem is mainly concerned with the transport of glucose and starch made during [[photosynthesis]].

== Structure ==
Phloem tissue consists of less specialised and nucleate ''[[parenchyma]]'' cells, '''''sieve-tube cells''''', and '''''companion cells''''' (in addition ''albuminous'' cells, fibers and ''sclereids''). The sieve-tube cells lack a nucleus, have very few vacuoles, but contain other organelles such as ribosomes. The endoplasmic reticulum is concentrated at the lateral walls. Sieve-tube members are joined end to end to form a tube that conducts food materials throughout the plant. The end walls of these cells have many small pores and are called sieve plates and have enlarged [[plasmodesmata]].

The survival of sieve-tube members depends on a close association with the ''companion cells''. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical [[plant cell]], except the companion cell usually has a larger number of [[ribosomes]] and [[mitochondria]]. This is because the companion cell is more metabollically active than a 'typical' plant cell. The [[cytoplasm]] of a companion cell is connected to the sieve-tube element by plasmodesmata.

==Function==
Unlike [[xylem]] (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport [[Sap (plant)|sap]]. The sap is a water-based solution, but rich in [[sugar]]s made by the photosynthetic areas. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as [[tuber]]s or bulbs.

The '''Pressure flow hypothesis''' was a hypothesis proposed by [[Ernst Munch]] in 1930 that explained the mechanism of phloem [[translocation]]. A high concentration of organic substance inside [[Cell (biology)|cell]]s of the phloem at a source, such as a [[leaf]], creates a [[diffusion|diffusion gradient]] that draws water into the cells. Movement occurs by bulk flow; phloem sap moves from ''sugar source''s to ''sugar sink''s by means of [[turgor]] pressure. A sugar source is any part of the plant that is producing or releasing sugar. During the plant's growth period, usually during the spring, storage organs such as the [[root]]s are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward).

After the growth period, when the [[meristem]]s are dormant, the [[leaf|leaves]] are sources, and storage organs are sinks. Developing [[seed]]-bearing organs (such as [[fruit]]) are always sinks. Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.

While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive [[hydrostatic pressure]]s. This process is termed ''translocation'', and is accomplished by a process called ''phloem loading'' and ''unloading''. Cells in a sugar source "load" a sieve-tube element by [[active transport|actively transporting]] solute molecules into it. This causes water to move into the sieve-tube element by [[osmosis]], creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes ''out'' of the sieve-tube elements, producing the exactly opposite effect.

Organic [[molecule]]s such as sugars, [[amino acid]]s, certain [[hormone]]s, and even [[messenger RNA]]s are transported in the phloem through [[sieve tube element]]s.

==Origin==
The phloem originates
, and grows outwards from, [[meristem]]atic cells in the [[vascular cambium]]. Phloem is produced in phases. ''Primary'' phloem is laid down by the [[apical meristem]]. ''Secondary'' phloem is laid down by the [[vascular cambium]] to the inside of the established layer(s) of phloem.

==Nutritional use==
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 [[Finnish famine of 1866-1868|great famine of the 1860s]]. Phloem is dried and milled to flour (''pettu'' in [[Finnish language|Finnish]]) and mixed with [[rye]] to form a hard dark bread. Recently, ''pettu'' has again become available as a curiosity, and some have made claims of health benefits.

==Girdling==
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 since 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 [[sink|sinks]] to go to but the one fruit/vegetable which thus expands to many times normal size.

==See also==
*[[Xylem]]
*[[Apical dominance]]

[[Category:Plant anatomy]]
[[Category:Plant physiology]]
[[Category:Tissues]]

Phloem - Revision history

WikiWorks at 05:19, 6 April 2007