Phloem (, ) is the living tissue
in vascular plant
s that transports the soluble organic compounds made during photosynthesis
and known as ''photosynthates'', in particular the sugar sucrose
, to parts of the plant where needed. This transport process is called translocation. In tree
s, the phloem is the innermost layer of the bark
, hence the name, derived from the Greek
word (''phloios'') meaning "bark". The term was introduced by Carl Nägeli
Phloem tissue consists of conducting cells, generally called sieve elements, parenchyma
cells, including both specialized companion cells or albuminous cells and unspecialized cells and supportive cells, such as fibres
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
and have very few organelles, so they rely on companion cells or albuminous cells for most of their metabolic needs. Sieve tube cells do contain vacuole
s and other organelles, such as 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
, which can be found at the plasma membrane, often nearby the 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
, called ''sieve areas''. The pores are reinforced by platelets of a polysaccharide
cells within the phloem are generally undifferentiated and used for food storage.
The metabolic functioning of sieve-tube members depends on a close association with the ''companion 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 nucleate plant cell
except the companion cell usually has a larger number of ribosomes
. 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 two types of companion cells.
#''Ordinary companion cells'', which have smooth walls and few or no plasmodesmatal connections to cells other than the sieve tube.
'', 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.
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.
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
s. Both cell types have a secondary cell wall
and are therefore dead at maturity. The secondary cell wall increases their rigidity and tensile strength, especially because they contain lignin
s are the long, narrow supportive cells that provide tension
strength without limiting flexibility. They are also found in xylem, and are the main component of many textiles such as paper, linen, and cotton.
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 bears.
(which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap
. The sap is a water-based solution, but rich in 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
s or bulbs.
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 multidirectional, whereas, in xylem cells, it is unidirectional (upward).
After the growth period, when the meristem
s are dormant, the 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
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
and the position-dependent function of solute
-specific, plasma membrane transport proteins
. Recent evidence indicates that mobile proteins and RNA
are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of macromolecules
as they pass through plasmodesmata."
s such as sugars, amino acid
s, certain hormone
s, and even messenger RNA
s are transported in the phloem through sieve tube element
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
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, 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
to go to but the one fruit/vegetable, which thus expands to many times its normal size.
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
, 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
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
atic cells in the vascular cambium
. Phloem is produced in phases. ''Primary'' phloem is laid down by the apical meristem
and develops from the procambium
. ''Secondary'' phloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem.
In some eudicot families (Apocynaceae
), 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
), 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, .]
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. 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
) and mixed with rye
to form a hard dark bread, bark bread
. The least appreciated was ''silkko'', a bread made only from 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
has been also used to make flour in the past.