Phloem is living tissue in vascular plants that transports soluble organic compounds made during photosynthesis and known as photosynthates , especially the sugar sucrose , [1] to parts of the plant where needed Put it on This transport process is called transfer. [2] In trees , the innermost layer of the phloem is the bark , hence the name, derived from the Greek word( phloios ) meaning “bark”. This word is Carl Nagelliwas introduced in 1858.



Phloem tissue consists of conducting cells , commonly called sieve elements, parenchyma cells, which include specialized companion cells or albuminous cells, and nonspecific cells and supporting cells, such as fibers and sclerids .


Conducting cells (sieve element)

Sieve elements are cell types responsible for transporting sugars throughout the plant. [5] At maturity they lack a nucleus and have very few organelles , so they depend on companion cells or albuminous cells for most of their metabolic needs. Sieve tube cells contain vacuoles and other organelles , such as ribosomes , before maturation, but these usually migrate to the cell wall and dissolve upon maturity; This ensures that there is little to impede the movement of liquids. The rough endoplasmic reticulum is one of the few organelles present in them at maturity., which can be found in the plasma membrane, often near the plasmodesmata that connect them to their partner or albuminous cells. At the ends of all sieve cells are clusters of pores that grow from modified and enlarged plasmodesmata , called sieve regions . The pores are strengthened by platelets of a polysaccharide called callose .

Parenchyma cells

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

Supporting cells

The metabolic function of sieve-tube members depends on close association with fellow cells , which are a specialized form of parenchyma cell. All cellular functions of a sieve-tube element are performed by the (very small) companion cell, except for a typical nucleated plant cell , the companion cell usually containing a large number of ribosomes and mitochondria. The dense cytoplasm of the companion cell is attached to the sieve-tube element by plasmodesmata. [5] A sieve tube element and a common sidewall shared by a companion cell contain a large number of plasmodesmata.

There are two types of companion cells.

  1. Simple companion cells , which have smooth walls and few or no plasmodemetal connections to cells other than the sieve tube.
  2. Transfer chambers , which have highly folded walls adjacent to non-sieve cells, allowing large areas of transfer. They are specialized in scavenging the solutes in cell walls that require energy to actively.

albuminous cells

Albuminous cells have a similar role as companion cells, but are attached only to sieve cells and are therefore found only in seedless vascular plants and gymnosperms. [5]

supporting cells

Although its primary function is the transport of sugars, the phloem may also contain cells that have a mechanical support function. These are sclerenchyma cells that generally fall into two categories: filaments and sclerids. Both types of cells have a secondary cell wall and hence they die at maturity. Secondary cell walls increase their rigidity and tensile strength, especially because they contain lignin.


Bast fibers are long, narrow supporting cells that provide tension strength without limiting flexibility. They are also found in the xylem, and are the main components of many textiles such as paper, linen and cotton.

Simplified Phloem and Companion Cells:

  • xylem
  • phloem
  • cambium
  • marrow
  • supporting cells


Sclerids are irregularly shaped cells that add compressive strength [5] but may reduce ductility to some extent. They also serve as anti-herbivory structures, as their irregular shape and hardness will increase wear on the teeth as herbivores chew. For example, they are responsible for the gritty texture in pears and winter bears. 


Unlike xylem (which is mainly composed of dead cells), phloem is still composed of living cells that transport sap. The juice is a water-based solution, but is rich in sugars created by photosynthesis. These sugars are transported to non-photosynthetic parts of the plant, such as roots, or to storage structures, such as tubers or bulbs.

During the plant’s growth period, usually spring, storage organs such as roots are sugar sources, and many growing areas of the plant are sugar sinks. Movement in phloem is multidirectional, whereas in xylem cells it is unidirectional (upward). 

After a period of growth, when the meristems are dormant, the leaves are the source, and the storage organs are the sink. The developing seed-bearing organs (such as the fruit) are always drowned. Due to this multi-directional flow, along with the fact that sap cannot easily move between adjacent sieve-tubes, it is not uncommon for sap to flow in opposite directions in adjacent sieve-tubes. [6]

While the movement of water and minerals through the xylem is most of the time driven by negative pressures (stress), movement through the phloem is driven by positive hydrostatic pressures. This process is called transfer , 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 and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNAs are important for plant long-distance communication.” are part of the signaling system. Evidence also exists for directed transport and sorting of macromolecules as they pass through plasmodesmata.” [7]

Organic molecules such as sugars, amino acids, some hormones and even messenger RNA are transported through sieve tube elements into the phloem. [7]

The phloem is also used as a popular site for ovulation and breeding of insects belonging to the order Diptera, including the fruit fly Drosophila montana . [8]

Is surrounded

Since the phloem tubes in most plants are located outside the xylem, a tree or other plant can be killed by severing the bark into a ring on the trunk or stem. Due to the destruction of phloem, nutrients cannot reach the roots and the tree/plants die. Trees located in areas with beaver-like animals are vulnerable because beavers chew the bark at a fairly precise height. This process is known as a waistband, and can be used for agricultural purposes. For example, the huge fruits and vegetables seen in fairs and carnivals are produced through the waistband. A farmer places a waistband at the base of a large branch, and removes all except a fruit/vegetable from that branch. Thus, all sugars that are not produced by the foliage on the branch go on to sink, but on a fruit/vegetable, which thus expands to several times its normal size.


When the plant is an embryo, the vascular tissue emerges from the procambium tissue, which is in the center of the embryo. The protophloem manifests itself in a mid-vein that extends into the cotyledonary node, which constitutes the first appearance of a leaf in angiosperms, where it forms continuous strands. The hormone auxin, carried by the protein Pin1, is responsible for the growth of protophloem strands, indicating 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, the root phloem develops independently into the upper hypocotyl, which is located between the embryonic root and cotyledon. [9]

In an adult, phloem arises, and grows outward from the meristematic cells in the vascular cambium. Phloem is produced in stages. The primary phloem is determined by the apical meristem and develops from the procambium. The secondary phloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem.

In some eudicot families (Apocynaceae, Stag family, Cucurbitaceae, Solanaceae, Myrtaceae, Asteraceae, Thymelaeaceae), the phloem also develops on the inner side of the vascular cambium; In this case, a distinction is made between external and internal or intercellular phloem. The inner phloem is mostly primary, and differentiation begins later than the outer phloem and protoxylem, although 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 called or interxylary phloem . [10]

Nutritional uses

The phloem of pine trees in Finland and Scandinavia has been used as a substitute food in times of famine and even in good years in the North East. The supply of phloem from previous years helped stave off starvation in the Great Famine of 1860, which affected both Finland and Sweden (the Finnish famine of 1866–1868 and the Swedish famine of 1867–1869). The phloem is dried and ground into flour ( gourmet in Finnish ) and mixed with rye to make a hard dark bread, bark bread. The least appreciated was silkko , a bread made only from buttermilk and pettu without any real rye or buckwheat flour. recently gourmetHas become available again as a curiosity, and some have made claims of health benefits. However, its food energy content is lower than that of rye or other grains. citation needed ]

The phloem from silver birch has also been used to make flour in the past.