Phloem (/ˈfloʊ.əm/,FLOH-əm) is the livingtissue invascular plants that transports the solubleorganic compounds made duringphotosynthesis and known asphotosynthates, in particular the sugarsucrose,[1] to the rest of the plant. This transport process is called translocation.[2] Intrees, the phloem is the innermost layer of thebark, hence the name, derived from theAncient Greek wordφλοιός (phloiós), meaning "bark".[3][4] The term was introduced byCarl Nägeli in 1858.[5][6] Different types of phloem can be distinguished. The early phloem formed in the growth apices is called protophloem. Protophloem eventually becomes obliterated once it connects to the durable phloem in mature organs, the metaphloem.[7][8] Further, secondary phloem is formed during the thickening of stem structures.[9]
Phloem tissue consists of conductingcells, generally called sieve elements,parenchyma cells, including both specialized companion cells or albuminous cells and unspecialized cells and supportive cells, such asfibres andsclereids.[citation needed]
Sieve tube elements are the type of cell that are responsible for transporting sugars throughout the plant.[10] At maturity they lack anucleus and have very feworganelles, so they rely on companion cells or albuminous cells for most of their metabolic needs. Sieve tube cells do containvacuoles and other organelles, such asribosomes, before they mature, but these generally migrate to the cell wall and dissolve at maturity; this ensures there is little to impede the movement of fluids. One of the few organelles they do contain at maturity is the roughendoplasmic reticulum, which can be found at the plasma membrane, often nearby theplasmodesmata that connect them to their companion or albuminous cells. All sieve cells have groups of pores at their ends that grow from modified and enlarged plasmodesmata, calledsieve areas. The pores are reinforced by platelets of apolysaccharide calledcallose.[10]
The metabolic functioning of sieve-tube members depends on a close association with thecompanion cells, a specialized form of parenchyma cell. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical nucleateplant cell except the companion cell usually has a larger number of ribosomes andmitochondria. The dense cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.[10] The common sidewall shared by a sieve tube element and a companion cell has large numbers of plasmodesmata.
There are three types of companion cells.
Ordinary companion cells, which have smooth walls and few or no plasmodesmatal connections to cells other than the sieve tube.
Transfer cells, which have much-folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer. They are specialized in scavenging solutes from those in the cell walls that are actively pumped requiring energy.
Intermediary cells, which possess many vacuoles and plasmodesmata and synthesize raffinose familyoligosaccharides.[11][12]
Albuminous cells have a similar role to companion cells, but are associated with sieve cells only and are hence found only in seedless vascular plants andgymnosperms.[10]
Although its primary function is transport of sugars, phloem may also contain cells that have a mechanical support function. These are sclerenchyma cells which generally fall into two categories: fibres and sclereids. Both cell types have asecondary cell wall and are dead at maturity. The secondary cell wall increases their rigidity and tensile strength, especially because they containlignin.[citation needed]
Bast fibres are the long, narrow supportive cells that providetension strength without limiting flexibility. They are also found inxylem, and are the main component of many textiles such as paper, linen, and cotton.[10]
Sclereids are irregularly shaped cells that add compression strength[10] but may reduce flexibility to some extent. They also serve as anti-herbivory structures, as their irregular shape and hardness will increase wear on teeth as the herbivores chew. For example, they are responsible for the gritty texture in pears, and in winter pears.[13]
Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transportsap. The sap is a water-based solution, but rich insugars made by photosynthesis. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such astubers or bulbs.[14]
During the plant's growth period, usually during the spring, storage organs such as theroots are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is multidirectional, whereas, in xylem cells, it is unidirectional (upward).[citation needed][15]
After the growth period, when themeristems are dormant, theleaves are sources, and storage organs are sinks. Developingseed-bearing organs (such asfruit) 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.[16]
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 positivehydrostatic pressures. This process is termedtranslocation, and is accomplished by a process calledphloem loading andunloading.[citation needed]
Phloem sap is also thought to play a role in sending informational signals throughout vascular plants. "Loading and unloading patterns are largely determined by theconductivity and number of plasmodesmata and the position-dependent function ofsolute-specific,plasma membranetransport proteins. Recent evidence indicates that mobile proteins andRNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting ofmacromolecules as they pass through plasmodesmata."[17]
Phloem is also used as a popular site for oviposition and breeding of insects belonging to the order Diptera, including the fruit flyDrosophila montana.[18]
Because phloem tubes are located outside the xylem in most plants, a tree or other plant can be 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, or ring-barking, and can be used for agricultural purposes. For example, enormous fruits and vegetables seen at fairs and carnivals are produced via girdling. A farmer would place a girdle at the 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 nosinks to go to but the one fruit/vegetable, which thus expands to many times its normal size.[citation needed]
When the plant is an embryo, vascular tissue emerges from procambium tissue, which is at the center of the embryo. Protophloem itself appears in the mid-vein extending into the cotyledonary node, which constitutes the first appearance of a leaf in angiosperms, where it forms continuous strands. The hormoneauxin, transported by the protein PIN1 is responsible for the growth of those protophloem strands, signaling the final identity of those tissues.SHORTROOT (SHR), andmicroRNA165/166 also participate in that process, whileCallose Synthase 3 inhibits the locations where SHR, and microRNA165 can go. Additionally, the expression of NAC45/86 genes during phloem differentiation functions to enucleate specific cells in the plants to produce the sieve elements.[19]
In the embryo, root phloem develops independently in the upper hypocotyl, which lies between the embryonic root, and the cotyledon.[20]
In an adult, the phloem originates, and grows outwards from,meristematic cells in thevascular cambium. Phloem is produced in phases. Primary phloem is laid down by theapical meristem and develops from theprocambium.Secondary phloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem. The molecular control of phloem development from stem cell to mature sieve element is best understood for the primary root of the model plantArabidopsis thaliana.[21]
In some eudicot families (Apocynaceae,Convolvulaceae,Cucurbitaceae,Solanaceae,Myrtaceae,Asteraceae,Thymelaeaceae), phloem also develops on the inner side of the vascular cambium; in this case, a distinction betweenexternal andinternal orintraxylary phloem is made. Internal phloem is mostly primary, and begins differentiation later than the external phloem and protoxylem, though it is not without exceptions. In some other families (Amaranthaceae,Nyctaginaceae,Salvadoraceae), the cambium also periodically forms inward strands or layers of phloem, embedded in the xylem: Such phloem strands are calledincluded orinterxylary phloem.[22]Transportation:They can be taken up through two processes, either an active process or a passive process of absorption, the water is that the absorption of the movement of water without a lot of energy, and do not require any energy within the process, but in active transporting the water and mineral are only taken up by the use of ATP energy, through a more active process.
Phloem ofpine trees has been used inFinland andScandinavia as a substitute food in times offamine and even in good years in the northeast. Supplies of phloem from previous years helped stave off starvation in the great famine of the 1860s which hit bothFinland andSweden. Phloem is dried and milled to flour (pettu inFinnish) and mixed withrye to form a hard dark bread,bark bread. The least appreciated wassilkko, a bread made only frombuttermilk andpettu without any real rye or cereal flour. Recently,pettu has again become available as a curiosity, and some have made claims of health benefits.[23]
Phloem fromsilver birch has been also used to make flour in the past.[24]
^Nägeli, Carl (1858)."Das Wachstum des Stammes und der Wurzel bei den Gefäßpflanzen und die Anordnung der Gefäßstränge im Stengel" [The growth of the stem and of the root among vascular plants and the arrangement of the vascular strands in the stalk].Beiträge zur Wissenschaftlichen Botanik (Contributions to Scientific Botany) (in German).1:1–156. From p. 9:"Ich will die beiden Partien Dauergewebe, welche von dem Cambium nach aussen und nach innen gebildet werden, Phloëm und Xylem nennen." (I will call the two parts of the permanent tissue, which are formed by the cambium outwardly and inwardly, "phloëm" and "xylem".)
^Ciffroy, P. & Tanaka, T. "Modelling the Fate of Chemicals in Plants".Modelling the Fate of Chemicals in the Environment and the Human Body 57 (2017): 167.: 174.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Lucas, William; et al. (2013). ""The Plant Vascular System " Evolution, Development and Functions".Journal of Integrative Plant Biology.55 (4):294–388.doi:10.1111/jipb.12041.hdl:10261/76903.PMID23462277.
