In this essay we will discuss about the simple and complex permanent tissues found in plants.
Essay # 1. Simple Tissues:
Simple tissues are composed of same type of cells, and thus are homogeneous in nature. The complex tissues, on the other hand, are heterogeneous, being composed of different types of cells. The third group of permanent tissue is the (c) secretory or special tissue. In higher plants, tissues are very seldom homogeneous from morphological or physiological point of view.
There are three types of simple permanent tissues in plants.
(ii) Collenchyma and
The term parenchyma refers to tissues which shows little specialisation and concerned with various physiological functions of the plant. The cells have the power of wound recovery and regeneration. A small group of parenchyma cells or even a single cell may be cultured and differentiated into a whole flowering plant in synthetic medium.
Phylogenetically, the tissue is considered to be primitive as the multicellular plants of the lower groups consist only of parenchyma. Ontogenetically, parenchyma is also considered primitive as its cells are morphologically alike to those of meristems.
It is the most common simple tissue of the plants and occupies major portions of the plant body. Plant life begins with the formation of the parenchymatous tissue by the embryo. The tissue is composed usually of isodiametric cells with intercellular spaces (Fig. 5.51 A).
The cells have functional protoplasts. This tissue is ubiquitous in all plants and generally occupies the softer portions of the plant body like epidermis, cortex, pith, mesophyll tissue, pulp of the fleshy fruits, embryo and endosperm of the seeds etc.
Parenchyma tissue is the fundamental tissue of the plant body as it provides the ground for other tissues. Bodies of lower plants are made up of parenchyma cells. The meristems are also parenchymatous in nature. Thus parenchyma tissue is the precursor of all other tissues. So, the parenchyma tissue is considered to be the most primitive tissue, both phylogenetically and ontogenetically.
Collenchyma is another primary permanent simple tissue consisting of elongated living cells with uneven cellulosic walls. It is the supporting tissue of the plant. Ontogenetically, it develops from certain elongated cells resembling procarnbium which are formed in the very early stages of differentiation of the meristem.
Younger cells of tissue show more extensibility and plasticity than the older ones which are less plastic, harder and more brittle. Collenchyma cells may also contain chloroplasts and carry out photosynthesis. In some cases collenchyma cells store tannins as secondary metabolites. The tissue is actually thick-walled parenchyma specialised to give mechanical support to the plants.
Sclerenchyma is the third type of simple permanent tissue. Cells of this tissue are longer. Cell walls of this tissue are very thick, hard and lignified with very little water due to secondary deposition. The lumen is almost obliterated. At maturity, cells of the sclerenchyma tissue become dead. Simple pits or slightly bordered pits are found over the walls.
Cells of the sclerenchyma tissue differ in shape, structure, origin and development. Different transitional stages are found between the various cell shapes. For that reason, though it is difficult to classify them, but, in general, sclerenchyma is divided into fibres and sclereids.
Fibres are generally elongated cells and sclereids as short cells. The former originate from merisematic cells whereas the latter develop from parenchyma cells due to secondary deposition of the wall materials.
These are very long and narrow cells with tapered and sometimes branched ends. Walls are uniformly thickened and highly lignified. The pits are small, round or slit-like in outline. The cell lumen is small due to much thickening of the secondary wall and may be continuous or septate.
The ends may be blunt or branched with interlocked arrangements. The fibres are always dead at maturity. They are angular in outline after cross section. In certain cases the fibre walls are cellulosic and non- lignified. Some fibres have mucilagenous walls.
Fibres remain distributed in different organs of the plant body. In the leaflets of Cycas they occur singly as idioblasts. They may occur in separate strands in the cortex or as bundle caps associated with vascular bundles or in the xylem and the phloem as wood and bast fibres.
Fibres are classified into two groups based on their positions in the plant body:
(a) Xylary fibres and
(b) Extraxylary fibres.
(a) Xylary Fibres:
These are also known as intraxylary fibres or wood fibres. These are an integral part of the xylem and originate from the same meristematic tissues. Xylary fibres show variations in their shape, size, wall thickness and pitting pattern.
The pits are simple or bordered in nature. These fibres possess lignified secondary walls. There are two main types of xylary fibres – libriform fibres and fibre-tracheids, based on the wall-thickness, type and amount of pits.
The term is derived from the word liber meaning inner bark as they resemble phloem fibres and are usually longer than the tracheids. Walls of these fibres are extremely thick with reduced simple pits. Sometimes in libriform fibres the pit canal becomes elongated and the inner pit aperture becomes slit-like. The inner pit apertures of a pit-pair are usually at right angles to each other.
These are intermediate between libriform fibres and tracheids. Cell walls of the fibre-tracheids are of medium thickness with bordered pits. The inner pit opening is slitlike. These structures are regarded as reduced tracheids Like libriform fibres the fibre-tracheids may be septate (septate fibres).
Gelatinous or mucilaginous fibre is observed in the secondary xylem of dicotyledonous plants. The innermost layer of the secondary wall of such fibres is made of β-cellulose called G-layer which after absorbing water almost covers the entire cell lumen. This less compact and porous G-layer irreversibly shrinks on drying.
Some elongated cells with thin secondary walls and living protoplasts occur in the secondary xylem, which may be confused with the living libriform fibres and fibre-tracheids. These elongated cells are called substitute fibres.
(b) Extraxylary Fibres:
The fibres present anywhere in the plant body other than xylem tissue are called extraxylary fibres or bast fibres. They may remain distributed in the cortex, pericycle and phloem. These fibres are usually long with tapered, blunt or branched ends. The cell walls of these fibres are thick, lignified or non-lignified with simple or slightly bordered pits.
Extraxylary fibres usually may form isolated strands or continuous bands in the cortex and pericycle. They may remain as caps above the vascular bundles. These fibres usually occur as patches in mono- cotyledonous leaves.
There are certain fibres termed septate fibres which are found in the xylem and the phloem even of the same species (e.g. Vitis). These fibres are characterized by the presence of partition walls (septa) and protoplasts with plasmodesmatal connections. The septum consists of a middle lamella and two primary wall-like layers and does not fuse with the fibre wall.
In longitudinal section the fibre tips are viewed as pointed. These fibres may store starch, oil droplets, resins and some times calcium oxalate. Secondary xylem of many dicotyledons possesses septate fibres. Non-vascular septate fibres are found in some monocotyledons.
Presence of the fibres in the different parts of the plant body is mainly to give mechanical strength and rigidity to the plant body as well as to withstand strains and stresses.
Ontogenetically, fibres originate from different meristems such as procambium, cambium, ground meristem and protoderm. Cambium derived fibres develop from the elongated fusiform initials and do not elongate further during maturation. But those derived from the short initials elongate greatly at the time of maturation. Of course, the elongation .s very gradual and may take a few months.
Evolutionary, fibres have developed from tracheids as many transitional forms between these two types of elements are found in some angiosperms. During the course of evolution the tracheid wall has become thickened, the number of pits and the size of the pit chamber have been reduced and ultimately the bordered pits disappeared.
Commercially there are two types of fibres – hard fibres and soft fibres. The hard fibres are stiff and lignified as found in the leaves of Agave, Yucca, Musa textilis, etc. Soft fibres are extraxylary fibres.
They are soft and flexible and may be lignified or non-lignified as found in jute, flax, hemp, ramie etc. The cotton fibres which are also known as surface fibres as they are produced from the testa of seeds represent the most important commercial fibres.
According to their use the fibres may be grouped as:
(a) Textile fibres (used in the manufacture of cloths; e.g., cotton, flax, ramie and hemp),
(b) Cordage fibres (used in making different types of cords or ropes; e.g., jute, cotton, hemp, flax, Musa, Agave etc.),
(c) Brush fibres (used in the manufacture of brushes and brooms; e.g., Agave, fibres from the Palmae and the inflorescences of Sorghum vulgare etc.), and
(d) Filling fibres (used in stuffing furnitures, mattresses, life-belts etc.; e.g., Ceiba pentandra, cotton, jute, Tillandsia usneoides).
Sclereids or sclerotic cells are non-prosenchymatous, isodiametric or irregular in shape. They normally become dead at maturity. They occur as hard masses of cells within soft parenchyma tissue in many different places in the plant body and are much shorter than true fibres in length. They are the major components in the shell of walnuts and seed coats of pea and many other plants.
In many cases, sclereids may be readily distinguished from the surrounding cells by their shape, size and wall thickness which are called idioblasts. In some cases the sclereids are of very peculiar shapes. The sclereid walls generally possess reniform simple pits with branched canals. The hard and thick walls are lignified and also may be cutinised or suberised.
Sclereids are abundantly present in cortex, phloem, pith, mesophyll tissue etc. as idioblasts or as cluster of cells. They are found in fruit pericarp of Pyrus, Psidium etc. They also occur in the seed coats of many plants either singly or in groups.
According to the shape, size and nature of wall thickening the sclereids may be of the following types:
(a) Brachysclereids or Stone Cell:
These are more or less isodiametric in appearance. They are also called stone cells or grit cells as they give gritty texture of the fruit flesh of many plants (e.g., Pyrus, Psidium etc.). Brachysclereids are usually found in the phloem, the cortex and the bark of stems (Fig. 5.54A, B).
These are rod-shaped columnar sclereids. They often form a continuous palisade like epidermal layer in the testa of seeds in Leguminosae (Fig. 5.54D, E). Macrosclereids also occur in the pulp of Malus sylvestris.
These are bone- or spool-shaped sclereids. They are also columnar in arrangement. The ends of these sclereids are enlarged, lobed, or somewhat branched. Such sclereids are mainly found in seed coats (e.g., Pisum) and leaves of certain dicotyledons (e.g., Pisum, Hakea, etc.) (Fig. 5.54F).
These are branched and often star-shaped, mainly found in leaves and stems of many dicotyledonous plants like Thea, Nymphaea, Trchodendron etc. (Fig. 5.54G).
This fifth type of sclereids are very elongated, hair-like, and always single branched sclereids.
Sclereids usually develop from parenchymatous small thin-walled initials by secondary thickening of the cell wall. During brachysclereid development the inner surface of the wall decreases and pits start to develop on the outside of the secondary wall In the early stages of development of the branched sclereid, it begins to branch and ultimately acquires the form of the mature sclereid.
The branches of the sclereid penetrate into the intercellular spaces. The secondary wall becomes thickly deposited in numerous concentric layers with the formation of branched pits. The mode of development of all types of sclereids is common excepting the degree of pitting. The probable cause of such sclerification may be physiological disturbances or ageing.
Essay # 2. Complex Tissues:
Complex tissues are composed of two or more types of cells. Xylem and Phloem are complex tissues. They together comprise the vascular tissue system, the main function of which is conduction of water and minerals from the root xylem to the leaves and prepared food from the leaves to the different parts of the plant body.
This complex tissue is composed of both living and non-living cells.
The main components of the tissue are:
(a) Tracheary elements (vessels and tracheids),
(b) Xylem parenchyma, and
(c) Xylary fibres or wood fibres.
Xylem may be primary and secondary depending on the origin. The former one is derived from the procambium whereas the latter is derived from the vascular cambium during secondary growth. The vessel elements of the primary xylem, which differentiate and mature earlier during ontogeny, are known as the protoxylem and those which mature later are called metaxylem.
(a) Tracheary Elements:
Tracheary elements are the main conducting elements.
There are two types of tracheary elements:
(i) Tracheids and
(ii) Vessels or tracheae.
These elements are non-living. Instead of water conduction they give mechanical support to the plant.
The tracheids are elongated thick-walled cells with tapering ends and remain parallel to the long axis of the organ where they occur. The ends may also be obtuse or rounded, and chisel-like or oblique. It is dead and devoid of protoplast at maturity. The cell lumen is comparatively larger. The hard and lignified cell wall contains bordered pits and annular, spiral, scalari- form and reticulate thickenings.
In cross section, the cell outline appears angular or polyhedral, or sometimes round. The tracheids remain in the tissue one above the other with overlapping ends.
The transverse walls are not obliterated, rather persist with many perforations and the thickening is same as that in the lateral walls. They make communications with surrounding cells through pits. The pits remain on the lateral walls. Bordered pits are present between the adjacent tracheids.
The pit pairs between tracheid and xylem parenchyma may commonly be simple, bordered or half-bordered (Fig. 5.55). In case of half bordered pit the border is formed on the tracheid cell wall and simple pitting on the parenchyma cell wall. Tracheids originate from procambium and vascular cambium.
Tracheids are found in the primary and secondary xylems of the vascular plants. They predominantly occur in pteridophytes, gymnosperms and primitive angiosperms.
The functions of tracheid are conduction of water and mechanical support. Very rarely the tracheids store water.
Origin of Tracheid:
In primary xylem tracheids originate from procambium and in secondary xylem they originate from cambium ring. A single fusiform initial forms a tracheid.
Phylogeny of Tracheid:
Generally, vascular cryptogams have very long tracheids and gymnosperms have tracheids of intermediate length. Different types of secondary wall are found in tracheids the annular, helical, scalariform, reticulate and circular bordered pitted. Ontogenetically the annular elements precede the helical elements. The helical bands again became joined at certain areas forming scalariform elements. Phylogenetically pitted elements are most advanced.
Tracheids with annular thickening have been reported from one of the oldest fossils Cooksonia, a leafless vascular plant from upper Silurian deposits (Stewart, 1983). Baragawanathia another very early leafy vascular plant — also contained the annularly thickened tracheids.
Phylogenetically, vessels have evolved from tracheids and are polyphyletic in origin. Vessels are found in pteridophyta (Selaginella, Equisetum and fern like Actinopteris, Pteridium, Regnellidium and Marsilea) and in gymnosperm (Ephedra, Welwitschia and Gnetum). Apart from angiosperms, the vessels found in the above mentioned genera of pteridophyta and gymnosperm are considered as anomalies.
Vessels or tracheae are also thick- walled, lignified non-living members of the xylem tissue. They remain arranged in vertical rows and form a tube-like structure with their perforated end walls. They also have numerous pits on their lateral walls. Water conduction is their main function.
The vessels are dead at maturity. Each vessel is an elongated cylindrical structure. Sometimes their diameter is greater than their length. The perforations on their end walls are large. The vessel elements are parallel with the long axis of the organ in which they occur. The length of the vessel varies and it may be as long as three metres (e.g., Fraxinus).
The perforated end walls of vessel members are called perforation plates, which may also remain on the lateral walls. So their positions are usually terminal, but sometimes sub-terminal or lateral.
The pattern of perforations may be:
(a) Simple with single large pore at the end (e.g., Quercus);
(b) Scalariform with several parallel elongated transverse pores (e.g., Liriodendron);
(c) Foraminate with several more or less circular pores in groups (e.g., Ephedra; and
(d) Reticulate with a network of small pores (e.g., Rhoeo).
Vessels predominate in the vascular tissues of angiosperms except some primitive groups like Ranales and Magnoliales as well as in Trochodendron and Tetracentron. They are absent in pteridophytes except in Selaginella, Equisetum, Pteridium etc. They are also absent in gymnosperms except Gnetales. They are present in the xylem of both primary and secondary bodies of angiosperms.
Origin and Phylogeny of Vessel:
Vessels of primary xylem originate from procambium and that of secondary xylem develop from cambium. The vessels have evolved from usually long and narrow tracheids. So, the long vessels are primitive, while the short and wide vessels are advanced.
Vessels with long inclined ends are considered as primitive as it is present in tracheid-like vessels and those with transverse end walls are advanced. Scalariform perforation plate in a very inclined end wall of long vessel is considered as primitive.
Simple perforation with circular rim in an almost transverse end wall is the most advanced type. This type of vessel obviously facilitates the easy movement of water through it. The scalariform inclined perforation plates give resistance to water flow.
Scalariform pitting on the lateral wall of long vessel is most primitive whereas the alternate pitting on the lateral wall of short vessel is advanced.
Spiral thickening on the secondary wall, characteristic of the ring porous wood, is an advanced character.
Vessels’ outlines in transverse sections have evolved in angular to circular direction. Angular outline is primitive than circular one.
It has been observed that the number of vessels present per square millimetre area of a transverse section of wood is a taxonomic character. High frequency of vessel is an advanced character.
Vessels may be present as single or in groups. These groupings and arrangements are taxonomic characters. The groups may be arranged in radial, oblique, or tangential lines. Solitary vessel represents the primitive condition.
(b) Xylem Parenchyma:
The parenchyma cells that occur as elements of the xylem tissue is termed as xylem parenchyma.
Xylem parenchyma occurs in the primary and secondary xylem. In the latter they are present as axial and ray parenchyma. They are also known as wood parenchyma.
Structure and Arrangement:
These cells may be oval, round, rectangular or square elongated and sometimes irregular in shape. The cell wall usually consists of thin primary wall. Sometimes the wall becomes thick due to lignin deposition over the primary cell wall. Pits when occur in-between two parenchyma cells are simple.
The pit pairs between the parenchyma and tracheary elements may be simple, half- bordered and bordered. Reserve foods in xylem parenchyma are starch and fat. Crystals and tannins are also found in these cells. The presence of chlorophyll is also reported in some herbs and deciduous trees.
The xylem parenchyma cells are oriented vertically or horizontally. The vertical orientation forms parenchyma strands that are more common in secondary xylem. Sometimes parenchyma protrudes into vessels to form tylosis.
Xylem parenchyma cell performs the following functions:
1. It helps in the transport of water and mineral salts.
2. It stores reserve food in the form of starch and fat and ergastic substances e.g., oils, gums, resin, tannins, silica bodies, crystals etc.
3 The thick walled lignified parenchyma gives mechanical support to some extent.
Ontogeny and Phylogeny:
In primary xylem the xylem parenchyma originates from procambium. In secondary xylem the ray parenchyma cells originate from the ray initials of the cambium. The axial parenchyma along with tracheary elements and fibres originate from fusiform initials of cambium. The fusiform initials of cambium normally divide periclinally. In these cells anticlinal division also occur to keep pace with the growth of the stem in girth.
Phylogeny of Ray Parenchyma:
Hetero- cellular multiseriate and uniseriate rays are the characteristics of the primitive wood. The multi- seriate rays have uniseriate wings. In advanced type of wood, however, either multiseriate or uniseriate ray is present (homocellular), that is one has been lost during the course of evolution.
The multiseriate rays are reduced in size and number. During evolution, the erect cells are lost and so there is a tendency for loss o heterogeneity. The advanced woods exhibit extremely short uniseriate wings on multiseriate rays.
In radial longitudinal section the ray parenchyma is observed to consist of square (isodiametric), erect (upright or vertical y elongate) and procumbent (radially elongate) cells. The square cells are morphologically equivalent to erect cells. The ray parenchyma may be homo- cellular and heterocellular.
In the former case rays are composed either of square cells or erect cells or procumbent cells, or erect and square cells’ In the latter case, however, the rays consist of both square and procumbent cells or of erect and procumbent cells. The rays may be uniseriate or biseriate or multiseriate with uniseriate wings (tapered ends) as observed in tangential longitudinal section (TLS). The outlines of rays appear to be fusiform in TLS.
Phylogeny of Axial Parenchyma:
The fusiform initial of cambium gives rise to axial (vertical) parenchyma. Axial parenchyma may occur independently or remain associated with vessels. So, depending on the presence of this parenchyma, the timbers may be apotracheal (vessels without axial parenchyma) and paratracheal (vessels with axial parenchyma).
The common apotracheal timbers may be:
(i) Diffuse (isolated strands of axial parenchyma),
(ii) Diffuse-in-aggregates (axial parenchyma occurs as aggregates),
(iii) Banded (axial parenchyma appears as narrow or wide bands) and
(iv) Marginal (axial parenchyma occurs either at the beginning of growth ring or at the end).
The common paratracheal forms are:
(i) Scanty (discontinuous parenchymatous sheath surrounding a vessel),
(ii) Vasicentric (continuous parenchymatous sheath surrounding the vessels),
(iii) Abaxial (with more vasicentric parenchyma on abaxial side of vessel),
(iv) Adaxial (with more parenchymatous width on adaxial side of vessel)
(v) Aliform (with winged vasicentric parenchyma) and
(vi) Confluent (with continuous band of extended vasicentric parenchyma).
Phylogenetically, the primitive woods are usually devoid of parenchyma. If present, it is of diffuse type. The advanced wood exhibits a tendency towards grouping of axial parenchyma to form diffuse-in-aggregate type. The wide band of apotracheal parenchyma is more advanced than narrow band of parenchyma.
(c) Xylem Fibre:
The fibre components of xylem are known as xylary fibres or wood fibres. These fibres are of two types – libiriform fibre and fibre-tracheid (discussed under sclerenchyma). The libriform fibre is longer and thick- walled with simple pits.
Xylary fibres may be septate. In the tension wood the libiriform fibre and fibre tracheid are of gelatinous type. The xylary fibres are responsible for mechanical support. They may remain living for a longer period and store reserve food.
Phylogeny of Xylem Fibre:
Fibres originate from procambium in primary xylem whereas that of secondary xylem develops from fusiform initial of cambium. Phylogenetically, the fibres have evolved from tracheids. During evolution the wall thickness has increased, the length has decreased and the bordered pits became reduced in size.
In dicotyledonous secondary xylem the evolution has occurred in the sequence of tracheid – fibre-tracheid – libiriform fibre. The tracheids have bordered pits; the pit border is diminished in fibre-tracheids and disappeared in libiriform fibres.
Phloem is a complex vascular tissue through which photosynthates move from green tissues to the different parts of the plant Sometimes it adds mechanical strength to the plant body. It is one of the component tissues of the vascular bundle. It remains alternately arranged with the xylem (radial) in the roots. In the stem, however, the phloem is usually external to the xylem.
It may surround a central core of xylem (i.e., haplostele) or discrete strands of xylem remain surrounded by phloem (i.e., mixed protostele). Two cylinders of phloem may occur on both sides of xylem (i.e., amphiphloic siphonostele).
In some cases the central phloem strand may be encircled by xylem (i.e amphivasal, e.g., Dracaena). In most dicotyledonous stem phloem strand occurs external to xylem (e.g., collateral vascular bundle).
In some species of Cucurbitaceae, Asclepiadaceae, Apocynaceae, Solanaceae etc. there are both external and internal phloem tissues. They may also be termed as abaxial and adaxial phloem respectively. The internal phloem is also termed as intraxylary phloem. Phloem strands embedded in the secondary xylem is termed as included or interxylary phloem.
Phloem is mainly composed of:
(a) Parenchyma termed phloem parenchyma,
(b) Specialized parenchyma cells known as companion cells and albuminous cells,
(c) Phloem fibres,
(d) Sieve elements.
Sclereids, laticifers and resin ducts are also present in phloem tissues of some species. Phloem parenchyma, sieve tubes, companion cells and phloem fibres are the composition of phloem tissue in dicotyledonous plants. Phloem parenchyma is absent in monocots and a few members of Ranunculaceae. Sieve cells and albuminous cells are present in gymnosperm and vascular cryptogams.
(a) Phloem Parenchyma:
The parenchyma other than albuminous and companion cells in phloem is called phloem parenchyma.
Phloem parenchyma is the component of both primary and secondary phloem. In secondary phloem they remain as axial phloem parenchyma and phloem rays.
Structure and Arrangement:
Phloem parenchyma cells are rectangular or rounded in transverse section. It appears oblong with rounded or tapered ends in longitudinal section. The cell walls are thin and non-lignified with numerous pit fields through which plasmodesmatal connections are established with companion cells and sieve elements.
Sometimes sclerified and thick-walled parenchyma cells occur which are inactive. Phloem parenchyma cells with wall ingrowths are known as transfer cells. Phloem parenchyma may store starch, fats, resins, tannins etc.
In primary phloem the parenchyma cells remain parallel to the long axis of xylem, whereas, in secondary phloem, the axial parenchyma are parallel but the ray parenchyma are perpendicular to the long axis of the associated xylem.
Phloem parenchyma cells perform the following functions:
1 Phloem parenchyma cells perform the function of storage of fat, starch, resin tannins etc.
2. Transfer cells are responsible for short distance transport of photosynthates
3. Phellogen derived from phloem parenchyma forms the periderm to protect the inner tissues
4. The inactive lignified cells add mechanical strength
Procambium gives rise to phloem parenchyma of primary phloem. In secondary phloem the axial phloem parenchyma and phloem rays are developed from fusiform initial and ray initial of cambium, respectively. Some parenchymal cells originate from a common mother cell of sieve elements. In many dicotyledonous stems the parenchyma of protophloem often differentiates into fibre in later stages of development, i.e., when the protophloem elements become functionless.
The phloem parenchyma and fibre of the secondary phloem bear no phylogenetic trend in phloem evolution. The distribution and morphology of them may be of comparative value.
(b) Companion Cell:
These cells with dense cytoplasm remain associated with the sieve tubes. Both of them originate from the same mother cell. Companion cells load photosynthates into the sieve tubes.
Structure and Arrangement:
The cells are vertically elongated and somewhat angular in cross-section. They are usually shorter in length or may be as long as the associated sieve tubes. The uniformly thick cell wall possesses many sieve areas on the sieve tube side and primary pit fields on the opposite side. There are well- developed plasmodesmata in the sieve areas and primary pit fields. In some companion cells wall materials deposit on the inner side of the primary wall to form transfer cell.
The companion cells have prominent elongated or lobed nuclei. The cells abundantly contain Golgi apparatus, endoplasmic reticulum, mitochondria, ribosomes, plastids etc. In some companion cells P-proteins are found.
The sieve tube and companion cell remain strongly attached after their formation from the same mother cell to form a complex known as SE/CC complex. 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 or several. Companion cells are absent in primary phloem. They are also absent in some primitive woody dicotyledons, gymnosperms and pteridophytes. In gymnosperm the associated parenchyma with sieve cells are termed as albuminous cells.
Following are the functions of the companion cell:
1. The companion cells are mainly related with the loading of the sieve tubes with sucrose. When the SE/CC complex is symplastically isolated the sucrose is absorbed actively by the companion cells from the apoplast of the minute veins.
From the companion cells the sugars then pass through the plasmodesmata into the sieve elements. But when there is protoplasmic continuity with the mesophyll cells through plasmodesmata sucrose is loaded through polymer trapping mechanism. The sieve tube becomes non-functional when the associated companion cell dies.
2. They are the active sites of protein synthesis.
The companion cell and sieve tube have got the common origin. The mother cell naturally remains in the primary and secondary phloem. The parent cell itself originates either from procambium or from cambium respectively. The mother cell divides by unequal longitudinal division and the smaller cell differentiates into the companion cell.
Companion cells remain associated with sieve tubes.
Three categories of companion cells are observed:
1. Companion cell may be much shorter than the accompanying sieve tube;
2. It may be almost as long as the sieve tube, and
3. It may be as long as the sieve tube, but septate.
According to many workers the first type is the primitive type. The remaining two types are advanced. Reduced number of companion cells in the phloem indicates the advancement.
The parenchyma cells associated with the sieve cells are called albuminous cells. Each albuminous cell has a prominent nucleus and dense cytoplasm.
Structure and Arrangement:
Albuminous cells are vertically oblong and may be of the same length as that of sieve cells or shorter. There are symplastic connections between the two types of cells. The albuminous cells contain starch-free and protein-rich cytoplasm and stain deeply with cytoplasmic stains. These cells occur at the margins of rays.
Albuminous cells are present in both primary and secondary phloem. Therefore, their origin differs. In primary phloem they develop either from procambium derived phloem rays or from phloem parenchyma. In the secondary phloem of Ephedra, the albuminous cells originate from the fusiform initials of vascular cambium.
There is a morphological as well as functional relationship between the albuminous cell and sieve cell even though they differ ontogenetically. When the sieve cell is non-functional the associated albuminous cell becomes dead. The possible function of the albuminous cell is to help in the conduction of proteins.
(c) Phloem Fibre:
The phloem fibres are the extraxylary fibres. They are also called bast fibres or bass fibre or basswood or bast wood fibres.
Structure and Arrangement:
The fibres are elongated cells and may be very long with tapering ends interlocked with other fibres. Its thick cell wall is usually lignified. But in Linum phloem fibre wall is made of cellulose. Simple pits with linear or round apertures are found in the fibre wall. Slightly bordered pits may rarely occur. The phloem fibre of Vitis is septate.
The septate fibres store starch, oils, resins, calcium oxalate crystals etc. The fibres remain parallel to the long axis. In cross section they appear 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.
The phloem fibres perform the following functions:
1. The phloem fibres with their interlocked ends give mechanical strength to the plant.
2. They protect the inner tissues.
3. Septate fibres may store starch, oils, resins etc.
The phloem fibres occur in both primary and secondary phloem and, therefore, their origins differ. The primary phloem fibre originates from procambium whereas the secondary phloem fibre originates from cambium. The fusiform initial of cambium gives rise to fibre.
(d) Sieve Elements:
The conducting elements of the phloem are referred to as sieve elements that are characterized by the presence of sieve areas and absence of nuclei from mature protoplasts. These include sieve cell and sieve tube. The sieve cells do not contain sieve plates. They are found in Pteridophytes and Gymnosperms.
(i) Sieve Tube:
It is the main solute conducting element in angiospermic phloem. It is living but enucleate and arranged longitudinally. Ribosomes and dictyosomes are also absent from mature protoplast. The sieve tubes have sieve plates and sieve areas on their transverse end walls. The sieve tube consists of longitudinally files of cells that are connected with each other through sieve areas on their transverse end walls.
The cell wall of the sieve tube may be thin or thick and is usually primary. The wall is composed mainly of cellulose and pectin. The sieve tubes of Smilax hispida, and Neptuma oleracea contain nucleus.
The young sieve tube contains prominent nucleus, abundant dictyosomes, ribosomes, endoplasmic reticulum, plastids, mitochondria and other cell organelles. Sieve tubes actuate starch which stains brownish red with iodine instead of blue. Discrete proteinaceous slime bodies are found in the young sieve tubes in the form of filament, tubule, granule or crystal, called P-protein.
It occurs in all dicotyledonous species without any exception and is rare in monocotyledons. They are absent in gymnosperms excepting Ephedra and ptendophytes. The P-proteins are synthesised in the cytoplasm and occupies the peripheral position. In the stained preparation of sieve tubes P-proteins accumulate at the transverse end walls of the tubes and plug the sieve plate pores. This plug is termed slime plug.
Plastids occurring in the sieve tube protoplast may be either S-type or P-type depending on the nature of reserve food. Starch accumulating type is called S-type whereas protein accumulating type is called P-type plastid. S-type plastids are found in Bataceae, Polygonaceae Plumbaginaceae etc. The ultrastructure of the plastids of the sieve tubes is a taxonomic characteristic. These ultrastructural details of sieve element-plastids are, now-a-days, applied to characterise some higher taxa.
The sieve plate is the region where the sieve areas occur. It is usually the horizontal or oblique end wall of the sieve tube. Sieve areas appear as depressed region on the wall where pores occur. Through the pores protoplasmic connections are established between the neighbouring members.
There may be one or several sieve areas in each sieve plate and accordingly they are termed as:
(i) Simple sieve plate with only one sieve area (e.g., Cucurbita) and
(ii) Compound sieve plate with more than one sieve areas in the plate (e.g., Vitis, Pyrus etc.).
The sieve plate pores contain callose. It is a carbohydrate and is composed of β-1, 3-linked glucan. It sheaths the connecting strand in the pore. In mature sieve areas, callose deposition may be throughout the plate. With the increase in deposition the connecting strands in the pores gradually become thin and ultimately disappear.
The deposition of the neighbouring sieve areas may coalesce to a single mass and form the callose pad and, ultimately, the sieve tubes become non-functional. Callose deposition may be seasonal or permanent.
The former is usually referred to as dormancy callose (e.g. Vitis) and the latter as definitive callose. Callose is studied by staining it with aniline blue. When viewed with a microscope using ultraviolet light it fluoresces lemon-yellow colouration.
(ii) Sieve Cell:
It is found in pteridophytes and gymnosperms. The sieve cells are arranged longitudinally, but, unlike sieve tubes, not one above the other in a series. The cells are oblong and tapered at their ends. They are often devoid of distinct end walls. When present, the end walls are tapered and oblique and may overlap.
The cell wall is usually thin and cellulosic but, in Pinus, the sieve cells are thick walled. Perforations on the walls form the sieve areas present on lateral walls and sometimes on the end walls. They are more numerous in the overlapping areas. Callose deposition in the perforations of sieve areas is also found. Sieve cells remain anucleate and living at maturity (exception: Pinus strobus and the family Taxaceae).
A large central vacuole is present pushing the protoplast towards the wall side forming the primordial utricle. 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 remain associated with albuminous cell.
Origin and Phylogeny of Sieve Tube:
Sieve tubes in primary phloem originate from procambium and that of the secondary phloem originate from cambium. The mother cells divide longitudinally to form two daughter cells, one of which forms the companion cell and the other develops into sieve tube.
The differentiating sieve tube increases in length and, at maturity, it contains mitochondria, plastids, P-protein and endoplasmic reticulum. The nucleus disappears except in Taxus, Neptunia oleracea etc. Sieve areas originate at the sieve plate, the common transverse wall of the sieve tubes.
Phylogenetically, long sieve tubes with numerous sieve areas are considered as primitive whereas the shorter ones are advanced. The cambial initials, during the course of evolution tend to become shortened and this caused the formation of short sieve tubes.
Usually, the cambial initial undergoes transverse septation before the differentiation of sieve tube leading to the formation of short sieve tubes. Small pores in the sieve plate are regarded as primitive. The simple horizontal sieve plate with single sieve area is an advanced feature.
Origin and Phylogeny of Sieve Cell:
The mother cells of sieve cells are slender, short cylindrical to oblong with tapering ends and numerous primary pit fields on their lateral walls. During differentiation, the mother cells elongate and the cell wall becomes thick. Sieve areas develop in the position of pit-fields. Plasmodesmata appear in the sieve areas and callose develops surrounding them.
Sieve cell predominates in pteridophytes and gymnosperms. It is reported among angiosperms in Austrobaileya scandens and Sorbus aucuparla. Sieve cells are considered as primitive. They are not arranged in axial files. Moreover, the structure of end walls is similar to the lateral walls.
The group of cells usually of different origin concerned in secretion of oils, nectar, resins, gums, mucilage, latex etc. is generally termed as secretory tissues. This categorisation is based on physiological functions. The secretory cells may be any part of the plant body as a solitary cell, or group of cells, or a whole tissue like pith, cortex, xylem phloem etc.
Secretion means the process of release of by-products of metabolism from a cell. The secretory substances may also be stored in insoluble forms within the cell. These substances have either a special physiological function or may be secreted or stored as wastes.
The removal of waste products of metabolism is defined as excretion. Strictly, the secretory products take part in the metabolism like hormones and enzymes. At the same time the process of cell wall deposition, cutinisation, wax deposition, suberisation etc. are also the examples of secretion. On the other hand, examples of excretory products are terpenes, saponins, rubber, tannins, crystals etc. Of course, there is no sharp demarcation between the two terms secretion and excretion.
The secretory bodies vary greatly in structure and position. They are mainly of two types:
(i) The secretory cells in which the secretory substances are formed and exuded cell outside e.g., simple glandular trichromes, nectaries, multicellular glands with vascular tissues etc., and
(ii) The structures in which the secretory substances are stored and released after the breakdown of the cells. The secretory structures may be external and internal.