Phloem: Definition and Components | Complex Tissue

In this article we will discuss about the definition and components of phloem tissue in plants.

Definition of Phloem:

Phloem is a complex tissue or heterogeneous vascular tissue that stores and conducts principally the products of photosynthesis in vascular plants and sometimes adds mechanical strength.

Phloem in association with xylem constitutes the vascular bundle and forms the conducting tissue system in plants. In roots phloem occurs as isolated patches alternating with xylem (radial vascular bundle). Phloem in the stems is usually external to xylem. A cylinder of phloem may surround a central core of xylem (e.g. haplostele) or discrete strands of xylem are surrounded by phloem (ex. mixed protostele).

Two cylinders of phloem may occur on the external and inner side of xylem (ex. amphiphloic siphonostele). In amphivasal vascular bundle a central strand of phloem is encircled by xylem (ex. Dracaena).

In most dicotyledonous stem phloem strand occurs external to xylem (e.g. collateral vascular bundle). In some species of the families Cucurbitaceae, Asclepiadaceae, Apocynaceae, Solanaceae etc. in addition to outer phloem, it occurs on the inner side of xylem.

These two parts of phloem, according to position, are designated as outer or external phloem and inner or internal phloem (Fig. 31.2B). Esau (1965) suggested the terms abaxial and adaxial phloem to designate outer and inner phloem respectively. The internal phloem is also termed as intraxylary phloem.

Sometimes phloem strands or layers are formed embedded in the secondary xylem. This phloem is termed as included or interxylary phloem. The interxylary phloem is called concentric when it arises as layers alternating with xylem layers, foraminate when it arises as strands encircled by xylem.

Components of Phloem:

(A) Parenchyma termed phloem parenchyma,

(B) Specialized parenchyma cells known as companion cell and albuminous cell,

(C) Phloem fibre, and

(D) Sieve cell and sieve tube. In addition to the above elements, sclereids, laticifers and resin ducts are also present in phloem tissue of some species.

In dicots phloem parenchyma, sieve tubes, companion cells and phloem fibres are present. Monocots and a few ranunculaceous genera do not possess phloem parenchyma in phloem elements. Sieve cells and albuminous cells are present in gymnosperm and vascular cryptogams.

(A) Phloem parenchyma:

The parenchyma cells, other than albuminous and companion cells, which occur in association with phloem, are referred to as phloem parenchyma.

Distribution:

Phloem parenchyma occurs in both primary and secondary phloem. In the latter case they are present in two systems, the axial and the ray system, and accordingly they are termed as axial phloem parenchyma and phloem rays.

Shape, structure, contents and arrangement:

Phloem parenchyma is more or less rectangular or rounded in cross section. In longitudinal section it appears as elongated cell with both ends rounded or pointed; it may also appear as rectangular or cylindrical.

Active parenchyma cells have thin walls that are primary and not lignified. The wall of inactive parenchyma, in some cases, becomes sclerified and thickened. Numerous pit fields occur on the cell wall. Through these pit fields protoplasmic connections are established between the axial and ray parenchyma, between the parenchyma and companion cells or sieve elements.

In some plants crystal- containing parenchyma cells occur. The phloem parenchyma cells of Sherardia arvensis leaf may develop wall ingrowths and these cells are known as phloem transfer cell. Phloem parenchyma cells possess living contents. These cells have nucleus, cytoplasm and may store starch, fats, resins tannins etc.

The parenchyma cells of primary phloem are oriented parallel to the long axis of xylem. In the secondary phloem, the axial and ray parenchyma lie parallel and perpendicular respectively to the long axis of xylem in which they are associated. The ray parenchyma cells are radially elongated. The phloem parenchyma cells that commonly occur at ray margins may be vertically elongated.

i. Function:

(1) Active phloem parenchyma stores fat, starch etc.;

(2) Accumulates resins, tannins etc.;

(3) Phloem transfer cells help in short distance transport of solutes;

(4) In many plants phloem parenchyma gives rise to phellogen that forms the protective tissue – periderm; and

(5) The cell walls of some inactive phloem become thickened by lignin deposition and add mechanical strength of the organ in which they occur.

(B) Companion cell:

The densely cytoplasmic nucleated parenchyma associated with sieve tube with which it has common origin from the same mother cell and plays some role in the functioning of sieve tube in angiosperms, is referred to as companion cell.

Shape, structure, content and arrangement:

The companion cells are vertically elongated and somewhat angular in cross section. In length they may be as long as the associated sieve tube or may be shorter.

The cell wall of companion cell is uniformly thick and possesses many depressed areas. These areas may be either sieve areas that occur on the side of sieve tube or primary pit field that occurs on the side of other companion cell or parenchyma. In the sieve areas and primary pit fields there exist the plasmodesmata, which are branched on the companion cell side. In some companion cells, wall materials deposit on the inner side of the primary wall to form transfer cell.

In contrast to sieve tubes, companion cells have prominent nuclei at maturity. The nucleus may be elongated or lobed and bounded by a normal double membrane. The cytoplasm is very dense due to the presence of abundant organelles that are dictyosomes, endoplasmic reticulum, mitochondria with well- marked cristae, ribosomes, plastids like leucoplasts or chloroplasts etc. In some companion cells P-proteins are found.

A single meristematic cell gives rise to companion cell and sieve tube. They remain strongly attached to each other. They are so tightly appressed that they cannot be separated by the usual maceration technique. Companion cells vary in number in relation to a single sieve tube. Usually the number is one or two and occasionally up to five (Calycanthaceae) or several.

These cells may be developed on one side of sieve tube only or formed on all sides. Companion cells are present in most dicots and monocots. They are absent in some primitive woody dicotyledons and primary phloem (protophloem). They are also absent in gymnosperms and pteridophytes. In gymnosperm the associated parenchyma with sieve cells are termed as albuminous cell.

i. Function:

(1) Companion cell and sieve tube are ontogenetically related and so it is thought that there exists a physiological and functional relationship between them. It is observed that in nonfunctional sieve tube the associated companion cell dies;

(2) They are the active site of protein synthesis;

(3) The endoplasmic reticulum, plastids and plasmodesmata form a route through which sucrose is transferred to neighbouring cells; and

(4) Cutter (1978) is of opinion that there exists a complex functional relationship in between the sieve tube and companion cells or other nucleated cells for effective transport of solutes.

Albuminous cell (Fig. 9.5):

The densely cytoplasmic nucleated parenchyma, which is associated with the sieve cells of gymnosperm and plays some functioning role of it, is referred to as albuminous cell.

Shape, structure, content and arrangement:

Albuminous cells are vertically elongated and may be of same length of the sieve cells or shorter. The end walls may be oblique or tapered. The cell wall is thin and there is connection with the associated sieve cells. Sieve cells have sieve areas on the walls facing the albuminous cell. The cells contain protein rich cytoplasm and stain deeply with cytoplasmic stains, and so these are designated as albuminous cells.

In these cells starch is usually absent. These cells contain nucleus and slime body. Albuminous cells occur in rays and among the axial parenchyma cells. They can be differentiated from neighbouring parenchyma cells by their usual connection with the sieve cells and absence of starch. Albuminous cells occur at the margin of rays and these cells form erect ray cells.

Origin:

Albuminous cell is present in primary and secondary phloem; accordingly its origin differs. In contrast to companion cell it is not ontogenetically related to sieve cells, i.e. albuminous cell originates from individual mother cell. These cells develop either from phloem rays or from phloem parenchyma, which are the derivatives of procambium. In the secondary phloem of Ephedra albuminous cells originate from the fusiform initials of vascular cambium.

i. Function:

(1) Though there is no ontogenetic relation between albuminous cells and sieve cells, there exist morphological and functional relationship between them. It is evident from the fact that in the nonfunctional sieve cells the associated albuminous cells die. Moreover the attachment between the sieve cells and albuminous cells is such that they remain attached even after maceration; and

(2) Albuminous cells possibly helps in the conduction of protein.

(C) Phloem fibre:

The fibre, which occurs in association with phloem, is referred to as phloem fibre.

Sometimes the terms bast fibre or bass fibre or basswood or bast wood fibre are synonymously used to mean phloem fibre.

Shape, structure, content and arrangement:

The fibres are elongated cells and may be very long. The two ends of a fibre are usually tapering to a wedge-shape and interlocked with other fibres. The cell wall is thick and lignified. But the wall of Linum phloem fibre is made up of cellulose. Gelatinous fibre also occurs in phloem. The cell wall contains simple pits with linear or round apertures. Sometimes slightly bordered pits occur.

The phloem fibre of Vitis is septate. Phloem fibres are considered as dead cells and contain no living protoplast at maturity. The septate fibres contain starch, oils, resins, calcium oxalate crystals etc.

The fibres are arranged in parallel to the long axis of the organ in which they occur. They may occur as isolated or scattered strands, as continuous or irregular bands, as clusters over the phloem strand and may form cylinders of tangential sheets encircling the inner tissues.

i. Function:

(1) Phloem fibres with their interlocked ends form a strong strand and provide mechanical strength to the organ in which they occur;

(2) They protect the inner tissues like cambium when occur as cylinders;

(3) Mitchell and Worley (1964) suggested that fibres play a role in the transport of solutes; and

(4) Septate fibres may store starch, oils etc.

(D) Sieve elements:

The conducting elements of phloem are referred to as sieve elements that are characterized by the presence of sieve areas and absence of nuclei from mature protoplasts.

Sieve element is the collective term of sieve cell and sieve tube (or sieve tube member or sieve tube element), which are distinguished on the basis of sieve areas and sieve plates. The sieve cells do not contain sieve plates. They characterize Pteridophyta and Gymnosperm. Exceptions are noted in the four species of Equisetum (E. arvense, E. giganteum, E. hyemale, E. telmateia) and the fern Cyathea gigantea.)

Long sieve element in secondary phloem is considered as primitive. It is to be noted that short sieve tube element does not always indicate an advanced condition because sieve tube element is frequently formed from cambium initial following transverse division.

Sieve tube (Figs. 9.6 & 9.7):

The thin walled, living, enucleate, longitudinally arranged conducting elements of angiospermic phloem with sieve plates and sieve areas on their transverse end walls are designated as sieve tube.

The sieve tube consists of longitudinal files of cells that are connected with each other through sieve areas on their transverse end walls.

Each cell is a sieve tube member and is composed of:

(i) Cell wall,

(ii) Protoplast, and

(iii) Sieve plate.

i. Cell wall:

The cell wall of sieve tube may be thin or thick and is usually primary. The wall is composed of mainly cellulose and pectin. The thin walls are one micron thick. The thick wall may almost fill the cell lumen. The thick wall consists of two layers the outer thin and inner thick layer. The outer thin layer lies towards the middle lamella and the inner layer is towards the cytoplasm.

The inner layers may have glistening properties and therefore the thick wall is termed as nacreous wall (Ex. Magnolia, Cucurbita etc.). The nacreous wall is polylamellate in Cucurbita and is composed of microfibrils. The microfibrils are oriented perpendicular to the long axis of the sieve tube. Nacreous wall is absent from the region of sieve plate.

ii. Protoplast:

Sieve tubes are unique in the fact that they are the only living cells where the nuclei are absent from the mature protoplast, though there are reports of their occurrence in monocotyledons and dicotyledons (Neptunia oleracea, Shah and James, 1968). Ribosomes and dictyosomes are also absent from mature protoplast.

The young sieve tube contains prominent nucleus, abundant dictyosomes, ribosomes, endoplasmic reticulum, plastids, mitochondria and other cell organelles. Sieve tubes accumulate starch of low molecular weight. This starch stains brownish red with iodine in contrast to normal starch that with iodine stains blue.

Discrete substances are observed in young sieve tubes, termed slime bodies. Slime is proteinaceous in nature and found only in the sieve elements. They appear as compact structure in the form of filament, tubule, granule or crystal. Slime bodies are also known as P-protein. It occurs in all dicotyledonous species so far investigated and is rare in monocotyledons.

They are absent in gymnosperms (except Ephedra) and pteridophytes. P-proteins are synthesized in the cytoplasm and occupy the peripheral position. In the stained preparation of sieve tubes P-proteins accumulate at the transverse end walls of tubes and plug sieve plate pores. This plug is termed as slime plug.

Plastids occur in the protoplast of sieve tubes. Two major types of plastid are distinguished on the basis of accumulation of protein and/or starch. Sieve element plastids accumulating only starch are defined as S-type plastids while those containing protein accumulation are called P-type plastids. S-type plastids are found in Bataceae (Batis maritima), Polygonaceae (Polygonum bistortum, Rumex patientia), Plumbaginaceae (Plumbago europaea), Gyrostemonaceae etc. P-type plastids can accumulate starch in addition to protein.

In P-type plastids a central crystalloid remains surrounded by a ring shaped bundle of protein filament. This type is specific for Caryophyllales. In some cases crystalloids may be absent, only the ring shaped bundle of filaments are present. The crystalloids may be globular or polygonal.

Therefore, three types of P-type plastids are represented, i.e. with globular crystalloid, with polygonal crystalloid and without crystalloid (Fig. 9.8). P-type plastids with globular crystalloid occur in Aizoaceae, Basellaceae, Cactaceae, Didiereaceae, Halophytaceae, Hectorellaceae, MoIIuginaceae, Nyctaginaceae, Tetragoniaceae and most of the genera of Phytolaccaceae.

Stegnosperma (Stegnospermataceae), Limeum (Phytolaccaceae) and Caryophyllaceae have P-type plastid with polygonal crystalloid, whereas Chenopodiaceae and Amaranthaceae are characterized by not having any crystalloid. These ultrastructural details of sieve-element-plastids are, now a days, applied to characterize some higher taxa like Magnoliophyta, Caryophyllidae etc.

iii. Sieve plate:

Sieve plate is the region where sieve areas occur. The plate lies at the end walls of sieve tube member and is usually horizontal or oblique to the longitudinal axis of them.

Sieve areas appear as depressed region in the wall where pores occur. In the pores there exist the connecting strands that connect the protoplast of one sieve tube member to the neighbouring member. Thus there is continuity between the sieve tube members, which form the sieve tube.

There may be one or several sieve areas in each sieve plate (Fig. 9.9) and accordingly they are termed as:

(i) Simple sieve plate-where there is one sieve area only (Fig. 9.9A) on the plate (ex. Cucurbita) and

(ii) Compound sieve plate —where there are more than one sieve areas (Fig. 9.9D) in a plate (ex. Vitis, Pyrus etc.). The pores in sieve areas vary in size. They may be less than one micron (e.g. Spiraea) or may be more than ten microns (e.g. Cucurbita, Ailanthus etc.) in diameter.

Sieve pores contain callose (Fig. 9.6C). Callose is carbohydrate and is composed of β-1,3-linked glucan. Callose forms a sheath around the connecting strand in a pore. In mature sieve areas callose also deposits on the surfaces present between the pores. As more and more callose deposit the connecting strands in the sieve pore gradually become thin and ultimately disappear.

The callose masses of the neighbouring sieve areas may coalesce to a single mass that forms callose pad. As a result the sieve pores are plugged when the sieve tube becomes nonfunctional. The ma jure sieve areas appear as raised portions instead of depressions, which occur in young sieve tube member. Callose may not be present in the pores and may be apparently absent in some palms.

Callose may deposit seasonally or permanently. Seasonal accumulation of callose on the sieve areas is usually referred to as dormancy callose. In Vitis the callose accumulates in the autumn when sieve tubes become nonfunctional. In spring callose gets dissolved and the sieve tubes start to function again.

Permanent accumulation of large masses of callose on the sieve areas causing the cessation of function of sieve tubes is designated as definitive callose. Usually sieve tubes remain active during a single growing season, but in Suaeda, Tilia, etc. sieve tube may remain active for two or more years.

Callose is studied by staining it with aniline blue. When viewed with a microscope using ultraviolet light it fluoresces lemon yellow colour.

Sieve cell (Fig. 9.10):

It is the phloem element of pteridophyta and gymnosperm. It is arranged longitudinally, but the cells are not arranged one above the other in a series. The cells are single, elongate and tapered at both ends. They often do not have a distinct end wall.

When present the end walls are either tapered or very oblique. The end walls may overlap one another. The cell wall is thin and made up of cellulose. In Pinus the sieve cells are thick walled. Perforations are present on the walls and these regions are called sieve areas.

Sieve areas are less specialized in sieve cells in comparison to those of sieve tubes. In contrast to sieve plate no wall parts can be distinguished in sieve areas. Sieve plates are reported in four species of Equisetum (Equisetum aruense, E. hyemale, E. giganteum, E. telmateia) and the fern Cyathea gigantea. Sieve areas are present on lateral walls and sometimes also occur on terminal wall. They are more numerous in those positions where the sieve cells overlap each other. Callose deposits in the perforations of sieve areas.

Sieve cells are living cell where nucleus is absent at maturity. But the nucleus is present in the mature sieve cells of Pinus strobus and in the family Taxaceae. A large central vacuole is present. Cytoplasm is present surrounding the peripheral layer of vacuole. Mitochondria, plastids and slime bodies are present. Starch grains are absent in sieve cells.

In contrast to sieve tube, sieve cells are devoid of companion cell. They are associated with albuminous cell and they are not ontogenetically related.

Functions of sieve elements:

(1) Translocation of photosynthetic products and other organic solutes occur through sieve elements. It is suggested that P-protein is involved in the process.

(2) It stores carbohydrates, proteins etc.

Phloem forms a tubular network from leaf to root. In the leaf the mesophyll cells are interconnected through plasmodesmata. There exists connection between mesophyll cells and sieve tubes.

Sugars and other metabolites are produced in the mesophyll, move from cell to cell and finally dumped into sieve tube. The leaf is the source from where the sieve tubes are loaded. At sink, i.e. at root the sugars and other metabolites are unloaded and here they are utilized.

When loading occurs at source osmotic potential and water potential in the mesophyll cells become more negative. As a result water enters in the mesophyll and ultimately in the sieve tube. Sieve tube becomes turgid and it pushes the solution to the next sieve tube.

This process continues and as a result the solution moves from source to sink. Mesophyll cells continuously produce and dump sugar and other metabolites to the sieve tube. Accordingly the loading at source and unloading at sinks continue.

It is now certain that phloem transport is dependent upon its loading at source (Fig. 9.11). Phloem loading is a continuous process and so the solute concentration in the sieve tube is double or almost double than the mesophyll cells. All substances are not readily loaded in the sieve tube.

Amino acids, ions, non- reducing sugars etc. are readily loaded by molecular pump in the plasmalemma. Malic acid, citric acid etc. are not readily loaded. However, once loaded all are translocated with same speed. This selectivity of loading indicates that movement in the phloem is a dynamic process.

Phloem cells are living and they can select the loading materials. By selective loading they can direct nutrients to the organs according to their requirements. It is now certain that phloem loading is both symplastic and apoplastic. In favour of the former the existence of plasmodesmata between sieve tube and neighbours has been cited.

As evidence of the latter the occurrence of sucrose in the apoplast of vascular bundle in maize has been cited. The unloading may be apoplastic or symplastic. The molecular pumps of plasmalemma, which help in loading, also help in unloading. In this context it is to be mentioned that apoplastic loading is advantageous due to the fact that it occurs in response to drought, high temperature etc.

Although the source is constant, i.e. the photosynthesizing cells, but the sink, i.e. the utilizing cells are different. The upper leaves load assimilates for the shoot apex while the lower leaves for the roots. The middle leaves supply to both leaves and roots.

Loading at source and unloading at sink cause a difference in water potential, which must be maintained for effective translocation. The unloaded materials must be utilized to maintain the difference in potential. If the sink fails to utilize, the rate of loading will be reduced. So the functioning of sink will determine the rate of loading at source.

E. Munch hypothesized Source-Sink concept and Phloem Loading and unloading as early as 1930. This hypothesis is also known as Mass flow hypothesis, Pressure flow hypothesis or Munch’s hypothesis. Though there are strong arguments against this hypothesis still this is dominant in elucidating the concept regarding phloem loading and unloading.

Phylogenetic specialization of sieve element:

Sieve elements originated from parenchyma cells, which modified themselves as to the function.

The modification and specialization occurred in the following way:

(i) Sieve cells are considered as primitive and it is assumed that they appeared in pteridophytes and gymnosperms though they may be present in angiosperm (Austrobaileya scandens and Sorbus aucuparia of Rosaceae). They are not arranged in axial files.

(ii) Sieve tube members originated in angiosperms.

(iii) Disappearance of nuclei from sieve elements.

(iv) Association with nucleated parenchyma like albuminous cells in pteridophyta and gymnosperm, and companion cells in angiosperm for metabolic activities.

(v) Reduction of sieve areas on the side walls.

(vi) Localization of sieve areas on the end walls.

(vii) Orientation of end walls from oblique to transverse.

(viii) Development of simple sieve plate from compound sieve plate.

(ix) Development of thick from thin connecting strands that are present in pteridophyta and gymnosperm.

(x) The diametre of pores increased and thus the percentage of pore-area occupied in sieve areas was also increased.

(xi) In monocots the development and specialization of sieve tube members occurred from leaves to roots.

(xii) In angiosperm sieve tube members became shorter in length during evolution.

Protophloem and metaphloem:

Protophloem and metaphloem together constitute the primary phloem. The first elements of primary phloem formed from the procambium strands are designated as protophloem. The lately differentiated elements of primary phloem are known as metaphloem.

Metaphloem is the completely developed primary phloem and consists of sieve elements, phloem fibre and phloem parenchyma. They usually occur in the mature parts of a plant and may remain active for several years. In contrast, protophloem occurs in developing organs and differ markedly from metaphloem.

In gymnosperm, the sieve areas of sieve cells are not prominent in protophloem elements. In some angiosperms the sieve tube elements of protophloem are long, narrow and devoid of companion cells. In these sieve tube elements sieve areas are not easily distinguishable. The sieve tubes of protophloem function for a short period and disappear. The parenchyma in nonfunctional phloem may form fibres or collenchyma.