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Phloem (, ) is the living
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
in
vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large group of plants ( 300,000 ...
s that transports the soluble
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s made during
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
and known as ''photosynthates'', in particular the
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosacc ...

sugar
sucrose Sucrose is a type of sugar Sugar is the generic name for , soluble s, many of which are used in food. Simple sugars, also called s, include , , and . Compound sugars, also called s or double sugars, are molecules made of two monosacchari ...

sucrose
, to parts of the plant where needed. This transport process is called translocation. In
tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including only wood plants with se ...

tree
s, the phloem is the innermost layer of the
bark Bark may refer to: * Bark (botany), an outer layer of a woody plant * Bark (sound), a vocalization of some animals Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, entertainment, and media * ''Bark'' (Jefferson Airp ...
, hence the name, derived from the
Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the diale ...
word (''phloiós''), meaning "bark". The term was introduced by
Carl Nägeli Carl Wilhelm von Nägeli (26 or 27 March 1817 – 10 May 1891) was a Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other use ...
in 1858.


Structure

Phloem tissue consists of conducting
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
, generally called sieve elements,
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biolo ...

parenchyma
cells, including both specialized companion cells or albuminous cells and unspecialized cells and supportive cells, such as
fibres Fiber or fibre (from la, fibra, links=no) is a natural or man-made substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate fi ...

fibres
and
sclereid Sclereids are a reduced form of sclerenchyma The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primary ...
s.


Conducting cells (sieve elements)

Sieve elements are the type of cell that are responsible for transporting sugars throughout the plant. At maturity they lack a
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
and have very few
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s, so they rely on companion cells or albuminous cells for most of their metabolic needs. Sieve tube cells do contain
vacuole A vacuole () is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts o ...
s and other organelles, such as
ribosome Ribosomes ( ), also called Palade granules, are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (mRNA translation). Ribosomes link amino acids ...

ribosome
s, 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 rough
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
, which can be found at the plasma membrane, often nearby the
plasmodesmata Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and speci ...

plasmodesmata
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 Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and speci ...

plasmodesmata
, called ''sieve areas''. The pores are reinforced by platelets of a
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as catalyst, which produces constituent sugars ...
called
callose Callose is a plant polysaccharide , a beta-glucan is an example of a (1→4)-β-D-glucan composed of glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide ...

callose
.


Parenchyma cells

Other
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biolo ...

parenchyma
cells within the phloem are generally undifferentiated and used for food storage.


Companion cells

The metabolic functioning of sieve-tube members depends on a close association with the ''companion cells'', a specialized form of
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biolo ...

parenchyma
cell. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical nucleate
plant cell Plant cells are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are class ...

plant cell
except the companion cell usually has a larger number of
ribosomes Ribosomes () are molecular machine, macromolecular machines, found within all living cell (biology), cells, that perform Translation (biology), biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order spec ...

ribosomes
and
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
. The dense cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata. 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 posses many vacuoles and plasmodesmata and synthesize raffinose family oligosaccharides.


Albuminous cells

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 and gymnosperms.


Supportive cells

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 Fiber or fibre (from la, fibra, links=no) is a natural or man-made substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate fi ...

fibres
and
sclereid Sclereids are a reduced form of sclerenchyma The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primary ...
s. Both cell types have a
secondary cell wallThe secondary cell wall is a structure found in many plant cells, located between the primary cell wall and the plasma membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, an ...
and are dead at maturity. The secondary cell wall increases their rigidity and tensile strength, especially because they contain
lignin Lignin is a class of complex organic polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its s ...

lignin
.


Fibres

Bast fibre epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regula ...
s are the long, narrow supportive cells that provide
tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to the stretching of an object (the opposite of compression) * Tension (geology), a stress which stretches rocks in two opposite directions * Voltage or elect ...
strength without limiting flexibility. They are also found in xylem, and are the main component of many textiles such as paper, linen, and cotton.


Sclereids

Sclereid Sclereids are a reduced form of sclerenchyma The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primary ...
s are irregularly shaped cells that add compression strength 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 chews. For example, they are responsible for the gritty texture in pears, and in winter pears.


Function

Unlike
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
(which is composed primarily of dead cells), the phloem is composed of still-living cells that transport
sap SAP SE () is a German multinational software corporation based in Walldorf, Baden-Württemberg Baden-Württemberg (; ) is a States of Germany, state (''Land'') in southwest Germany, east of the Rhine, which forms the southern part of Ge ...
. The sap is a water-based solution, but rich in
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosacc ...

sugar
s made by photosynthesis. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as
tuber Tubers are enlarged structures used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing season, and ...
s or bulbs. During the plant's growth period, usually during the spring, storage organs such as the
root In vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large grou ...

root
s 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). After the growth period, when the
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
s are dormant, the
leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together form the shoot system. Leaves are ...

leaves
are sources, and storage organs are sinks. Developing
seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released to generally positi ...

seed
-bearing organs (such as
fruit In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the ...

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 Fluid statics or hydrostatics is the branch of fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical object ...

hydrostatic pressure
s. This process is termed ''translocation'', and is accomplished by a process called phloem loading and ''unloading''. Phloem sap is also thought to play a role in sending informational signals throughout vascular plants. "Loading and unloading patterns are largely determined by the conductivity and number of
plasmodesmata Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and speci ...

plasmodesmata
and the position-dependent function of
solute upMaking a table salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mineral is known ...
-specific,
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
transport proteins Transport (commonly used in the U.K.), or transportation (used in the U.S.), is the movement of humans, animals and goods In economics Economics () is the social science that studies how people interact with value; in particular, t ...
. Recent evidence indicates that mobile proteins and
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of
macromolecules macromolecule A macromolecule is a very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neu ...
as they pass through plasmodesmata." Organic
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
s such as sugars,
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s, certain
hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...

hormone
s, and even
messenger RNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA i ...
s are transported in the phloem through sieve tube elements. Phloem is also used as a popular site for oviposition and breeding of insects belonging to the order Diptera, including the fruit fly '' Drosophila montana''.


Girdling

Because phloem tubes are located outside the
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

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, 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 no
sinks A sink – also known by other names including sinker, washbowl, hand basin, wash basin, and simply basin – is a bowl-shaped plumbing fixture used for washing hands, dishwashing, and other purposes. Sinks have Tap (valve), taps (faucets) that ...
to go to but the one fruit/vegetable, which thus expands to many times its normal size.


Origin

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 hormone
auxin Auxins (plural of auxin ) are a class of plant hormone Plant hormones (or phytohormones) are signal molecule In biology, cell signaling (cell signalling in British English), or cell-cell communication, governs the basic activities of cell (bio ...

auxin
, transported by the protein PIN1 is responsible for the growth of those protophloem strands, signaling the final identity of those tissues. SHORTROOT(SHR), and microRNA165/ 166 also participate in that process, while Callose Synthase 3( CALS3), inhibits the locations where SHORTROOT(SHR), and microRNA165 can go. In the embryo, root phloem develops independently in the upper hypocotyl, which lies between the embryonic root, and the cotyledon. In an adult, the phloem originates, and grows outwards from,
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
atic cells in the
vascular cambium The vascular cambium is the main growth tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ...
. Phloem is produced in phases. ''Primary'' phloem is laid down by the
apical meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dub ...
and develops from the
procambium The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dub ...
. ''Secondary'' phloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem. In some eudicot families (
Apocynaceae Apocynaceae (from ''Apocynum ''Apocynum'', commonly known as dogbane or Indian hemp, is a small genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, includi ...
,
Convolvulaceae Convolvulaceae (), known commonly as the bindweed or morning glory family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relation ...

Convolvulaceae
,
Cucurbitaceae The Cucurbitaceae, also called cucurbits or the gourd family, are a plant family (biology), family consisting of about 965 species in around 95 genera, of which the most important to humans are: *''Cucurbita'' – Squash (plant), squash, pumpkin, ...

Cucurbitaceae
,
Solanaceae The Solanaceae , or nightshades, are a family (biology), family of flowering plants that ranges from annual and perennial herbs to vines, lianas, epiphytes, shrubs, and trees, and includes a number of agricultural crops, medicinal plants, spice ...
,
Myrtaceae Myrtaceae or the myrtle family is a family of dicotyledonous plants placed within the order Myrtales. Myrtus, Myrtle, Metrosideros, pōhutukawa, Pimenta racemosa, bay rum tree, clove, guava, Acca (plant), acca (feijoa), allspice, and eucalyptus a ...
,
Asteraceae The family (biology), family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the Order (biology), order Asterales. Commonly referred to as the aster, daisy, composite, o ...

Asteraceae
,
Thymelaeaceae The Thymelaeaceae are a cosmopolitan family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families ...
), phloem also develops on the inner side of the vascular cambium; in this case, a distinction between ''external'' and ''internal'' or ''intraxylary'' 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 Amaranthaceae is a family of flowering plants commonly known as the amaranth family, in reference to its type (biology), type genus ''Amaranthus''. It includes the former goosefoot family Chenopodiaceae and contains about 165 genera and 2,040 spe ...
,
Nyctaginaceae Nyctaginaceae, the four o'clock family, is a family of around 33 genera and 290 species of flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 6 ...

Nyctaginaceae
,
Salvadoraceae Salvadoraceae is a family in the plant order Brassicales The Brassicales (or Cruciales) are an order of flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land ...
), the cambium also periodically forms inward strands or layers of phloem, embedded in the xylem: Such phloem strands are called ''included'' or ''interxylary'' phloem.Evert, Ray F. ''Esau's Plant Anatomy''. John Wiley & Sons, Inc, 2006, pp. 357–358, .


Nutritional use

Phloem of
pine A pine is any conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The div ...

pine
trees has been used in
Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the west, Russia to the east, Norway to the north, and is defined by the Gulf of B ...

Finland
and
Scandinavia Scandinavia; Sami Places * Sápmi, a cultural region in Northern Europe * Sami, Burkina Faso, a district of the Banwa Province * Sami District, Gambia * Sami, Cephalonia, a municipality in Greece * Sami (ancient city), in Elis, Greece * Sa ...

Scandinavia
as a substitute food in times of
famine A famine is a widespread scarcity of food Food is any substance consumed to provide nutritional support for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual con ...

famine
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 both Finland and Sweden ( Finnish famine of 1866-1868 and Swedish famine of 1867–1869). Phloem is dried and milled to flour (''pettu'' in
Finnish Finnish may refer to: * Something or someone from, or related to Finland * Finnish culture * Finnish people or Finns, the primary ethnic group in Finland * Finnish language, the national language of the Finnish people * Finnish cuisine See also

...
) and mixed with
rye Rye (''Secale cereale'') is a grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants known as grasses. It includes the cereal grasses, bamboo Bamboos are a diverse group of ev ...

rye
to form a hard dark bread, bark bread. The least appreciated was ''silkko'', a bread made only from
buttermilk Buttermilk is a Fermented milk products, fermented dairy drink. Traditionally, it was the liquid left behind after churning butter out of Microbial food cultures, cultured cream. As most modern butter is not made with cultured cream but uncult ...

buttermilk
and ''pettu'' without any real rye or cereal flour. Recently, ''pettu'' has again become available as a curiosity, and some have made claims of health benefits. However, its food energy content is low relative to rye or other cereals. Phloem from
silver birch ''Betula pendula'', commonly known as silver birch, warty birch, European white birch, or East Asian white birch, is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of a ...
has been also used to make flour in the past.


See also

*
Apical dominance In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Anc ...
* Phloem sap


References


External links

{{Authority control Plant anatomy Plant physiology Tissues (biology)