The Tissues

A group of cells performing a particular function is collectively called as tissue. A tissue may be defined as, 𠇊 group of similar or dissimilar cells having common origin and performing specific function”.

Tissues are mainly divided into three categories : Meristematic tissues or Meristems, Permanent tissues and Secretory tissues

Meristematic tissues or Meristems

The word “Meristem” originated from “Meristos” (Greek = continuous division) and the term meristem was introduced by Nageli (1858). A group of cells which are much active and capable of showing continuous divisions and redivisions, is called as meristematic tissue. The various characteristic features of the meristems are discussed below :

(1) They contain immature and young cells and are capable of repeated divisions.

(2) Intercellular spaces are not present in meristematic tissue.

(3) They contain a homogeneous thin cellulosic wall.

(4) They contain large nuclei associated with abundant cytoplasm.

(5) They are metabolically very active but they do not store food material and further no plastids in them.

(6) Vacuoles are small or absent.

(7) Meristematic cells are isodiametric in shape.

(8) Undifferentiated tissue in which cells divides continuously

Types of meristems

The meristems may be classified on the basis of their mode of origin, position or function :

According to origin and development : On the basis of origin, meristematic tissues are of three types :

(1) Promeristem or Primordial meristem : The promeristem originates from embryo and therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated. A group of initial cells that lay down the foundation of an organ or a plant part, is called promeristem. It occupies a small area at the tips of stem and root. The promeristem gives rise to all other meristems including the primary meristem.

(2) Primary meristem : A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia. Primary meristem gives rise to the primary permanent tissue.

(3) Secondary Meristem : They always arise in permanent tissues and have no typical promeristem. Some living permanent cells may regain the meristematic nature. This process in which permanent tissue regains meristematic nature is called dedifferentiation. The secondary meristems are so called because they originate from permanent cells. The phellogen or cork cambium arising from epidermis, cortex or other cells during secondary growth, is an important example of secondary meristem. The secondary meristems produce secondary tissues in the plant body and add new cells for effective protection and repair.

According to position : On the basis of their position in the plant body meristems are classified into three categories :

(1) Apical meristem : This meristem is located at the growing apices of main and lateral shoots and roots. These cells are responsible for linear growth of an organ. Solitary apical cells occur in ferns and other Pteridophytes while apical initials are found in other vascular plants.

(2) Intercalary meristem : These are the portions of apical meristems which are separated from the apex during the growth of axis and formation of permanent tissues. It is present mostly at the base of node (e.g., Mentha viridis, Mint), base of internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophytes like Equisetum) or at the base of the leaf (e.g., Pinus). The intercalary meristems ultimately disappear and give rise to permanent tissues.

(3) Lateral meristem : These meristems occur laterally in the axis, parallel to the sides of stems and roots. This meristem consists of initials which divide mainly in one plane (periclinal) and result increase in the diameter of an organ. The cambium of vascular bundles (Fascicular, interfascicular and extrastelar cambium) and the cork cambium or phellogen belongs to this category and are found in dicotyledons and gymnosperms.

According to function : Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types :

(1) Protoderm : It is the outermost layer of the apical meristem which develops into the epidermis or epidermal tissue system.

(2) Procambium : It occurs inside the protoderm. Some of the cells of young growing region which by their elongation and differentiation give rise to primary vascular tissue, constitute the procambium.

(3) Ground meristem : It constitutes the major part of the apical meristem which develops ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary rays.

According to plane of cell division : On the basis of their plane of cell division meristem are classified into three categories :

(1) Mass meristem : The cells divide anticlinally in all planes, so mass of cells is formed. e.g., formation of spores, cortex, pith, endosperm.

(2) Plate meristem : The cells divide anticlinally in two planes, so plate like area increased. e.g., formation of epidermis and lamina of leaves.

(3) Rib or File meristem : The cells divide anticlinally in one plane, so row or column of cells is formed. e.g., formation of lateral root.

Structure and organisation of apical meristem

(1) Vegetative shoot apex : Shoot apex was first recognized by Wolff (1759) shoot apex is derived from meristem present in plumule of embryo and occurs at the tip of stem and its branches as terminal bud. It also occurs in the inactive state in the axils of leaves as lateral buds. The tip of the shoot apex is dome-shaped and from its flanks at the base of the dome divide to form one or more leaf primordia. This continues throughout the vegetative phase. Many theories have been put forward to explain shoot apex, such as :

(i) Apical cell theory : This theory was proposed by Nageli (1858). According to this theory, shoot apical meristem consists of single apical cell. This theory is applicable in case of higher algae, bryophytes and in many pteridophytes but not in higher plants (i.e., gymnosperms and angiosperms).

(ii) Histogen theory : It was proposed by Hanstein (1870). According to this theory, the shoot apical meristem consists of three distinct meristematic zones or layers (or histogens).

(a) Dermatogen : Outermost layer and it forms epidermis and epidermal tissue system.

(b) Periblem : It is the middle layer which gives rise to cortex and endodermis.

(c) Plerome : The innermost layer forms pith and stele.

(iii) Tunica corpus theory : This theory was proposed by Schmidt (1924). According to this theory, the shoot apex consists of two distinct zones.

(a) Tunica : It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus. The tunica shows only anticlinal division and it is responsible for surface growth.

(b) Corpus : It represents the central core with larger cells. Corpus shows divisions in all planes and it is responsible for volume growth.

(2) Root apex : A group of initial cells, present at the subterminal region of the growing root tip, which is protected by a root cap, is called root apical meristem or root apex. It is embryonic in origin and formed from the radicle part of embryo. However, in adventitious roots it is produced from derivatives of root apex. According to Hanstein (1870) root apex of most of the dicotyledons also consists of three meristematic zones - plerome, periblem and dermatogen (fourth meristem calyptrogen to form root cap only in monocots). Regarding the apical organisation of root following theories have been put forward.

Korper-Kappe theory : It was proposed by Schuepp (1917). This theory is comparable with the tunica and corpus theory of shoot apex. Korper means body and Kappe means cap.

Quiescent centre theory : It was proposed by Clowes (1961) in maize. According to him, in addition to actively dividing cells, a zone of inactive cells is present in the central part of the root apex called quiescent centre.

The cells in this region have light cytoplasm, small nuclei, lower concentration of DNA, RNA and protein.

(3) Reproductive apex : During reproductive phase, the vegetative apices are converted into reproductive apices. Before conversion, the apex stops producing leaf primordia. The summit of the apex which remained inactive during the vegetative phase, starts dividing. As a result of cell divisions, the apical meristem undergoes change in shape and increase in size. The apex may develop into a flower or an inflorescence.

Permanent tissues

Permanent tissues are made up of mature cells which have lost the capacity to divide and have attained a permanent shape, size and function due to division and differentiation in meristematic tissues. The cells of these tissues are either living or dead, thin-walled or thick-walled. Permanent tissues are of following types :

Simple permanent tissues

Simple tissues are a group of cells which are all alike in origin, form and function. They are further grouped under three categories :

(1) Parenchyma : Parenchyma is most simple and unspecialized tissue which is concerned mainly with the vegetative activities of the plant.

The main characteristics of parenchyma cells are :

(i) The cells are living, thin walled, soft, possess a distinct nucleus, having well developed intercellular spaces, vacuolated cytoplasm and cellulosic cell wall.

(ii) The shape may be oval, spherical, cylindrical, rectangular and stellate (star shaped) in leaf petioles of banana and canna and some hydrophytes.

(iii) This tissue is generally present in roots, stems, leaves, flowers, fruits and seeds.

(iv) If they enclose large air spaces they are called as aerenchyma if they develop chlorophyll, they are called as chlorenchyma and if they are elongated cells with tapering ends, they are called as prosenchyma.

Functions : They perform the following functions :

(i) Storage of food materials. e.g., Carrot, Beetroot etc.

(ii) Chlorenchyma helps in photosynthesis. Aerenchyma helps in floating of the aquatic plants (Hydrophytes) and also helps in gaseous exchange during respiration and photosynthesis. e.g., Hydrilla.

(iii) In turgid state they give rigidity to the plant organs.

(iv) In emergency they behave like meristematic cells and help in healing of the various plant injuries.

(v) Sometimes they store secretory substances (ergastic substance) such as tannins, resins and gums and they are called as idioblasts.

(2) Collenchyma : The term collenchyma was coined by Schleiden (1839). It is the tissue of primary body. The main characteristics of collenchyma are given below :

(i) The cells of this tissue contain protoplasm and are living without intercellular spaces. The cell walls are thickened at the corners and are made up of cellulose, hemicellulose and pectin.

(ii) They are compactly arranged cells, oval, spherical or polygonal in outline. The tissue is elastic, extensible and have capacity to expand.

(iii) Collenchyma occurs chiefly in the hypodermis of dicotyledonous stems (herbaceous, climbers or plants e.g. Cucurbita, Helianthus) and leaves. They are usually absent in monocots and in roots.

Types of collenchyma : Majumdar (1941) divided collenchyma into three types on the basis of thickening :

(i) Angular collenchyma : When the thickening of the cells is confined to the corners of the cells. e.g., Tagetes, Tomato, Datura, Potato, etc.

(ii) Plate or Lamellar collenchyma : When the thickenings are present in the tangential walls. e.g. hypodermis of sunflower stem.

(iii) Lacunar or Tubular collenchyma : If the thickened cell wall is associated with intercellular spaces of the adjacent cells. e.g. leaf petioles of compositae (asteraceae) and malvaceae etc. hypodermis of Cucurbita stem, Salvia, Malva.

Functions

(i) Provide mechanical support to petiole, pedicels, branches of stem, roots and fruits.

(ii) If they contain chlorophyll they help in photosynthesis.

(iii) It is present at the margins of some leaves and resists tearing and bending effect of the wind.

(3) Sclerenchyma : It was discovered and coined by Mettenius (1805).

The main feature of sclerenchyma are :

(i) It consists of thick-walled dead cells.

(ii) The cells vary in shape, size and origin.

(iii) In the beginning the cells are living and have protoplasm but due to deposition of impermeable secondary walls (lignin) they become dead, thick and hard.

Types of sclerenchyma : They are of two types :

(i) Sclerenchymatous fibres : These are greatly elongated and tapering at both the ends. The fully developed fibre cells are always dead. They are polygonal in transverse section and walls are highly lignified. Intercellular spaces are absent and lumen is highly obliterated. The walls show simple and oblique pits. They provide mechanical strength to the plant. Some of the longest fibre yielding plants are Linum usitatissimum (Flax or Alsi), Corchorus, Cannabis, etc. The fibres are present in hypodermis of monocot stem, in pericycle of many dicots, in secondary wood and vascular bundle sheath in monocot stems. There are three different kinds of fibres :

(a) Bast fibres : The fibres present in the pericycle (e.g., Cannabis sativa /Hemp or Bhang), Linum usitatissimum and phloem (e.g., Corchorus capsularis (Jute), Hibiscus cannabinus (Patsan), Calotropis, Nerium, Sunn hemp etc.). These fibre are also known as extraxylary fibres.

(b) Wood fibres : Those fibres which are associated with wood or xylem have bordered pits are known as wood fibres. Thick walled wood fibres having simple pits are called libriform fibres whereas thin walled wood-fibres having bordered pits are called fibre-tracheids. A specific type of wood fibre is produced by Quercus rabra and is called gelatinous or mucilagenous fibres.

(c) Surface fibres : The fibres present over surface of plant organs are called surface fibres. e.g., Cotton fibres found in the testa of seeds, mesocarp fibres of Coconut (Cocus nucifera).

(ii) Stone cells or Sclereids : They are lignified, extremely thick walled so that the lumen of the cells is almost obliterated and may be spherical, oval, cylindrical, T-shaped and even stellate. They are generally found in hard parts of the plant, e.g., endocarp of Walnut and Coconut. The sclereids provide mechanical support and hardness to the soft parts.

Kind of sclereids : They are of five types :

(a) Brachy-sclereids or stone cells : These are small and more or less isodiametric in shape. They occur in the cortex, pith, phloem, and pulp of fruits (e.g., Pyrus).

(b) Macrosclereids or rod cells : These are rod-shaped elongated sclereids usually found in the leaves, cortex of stem and outer seed coats.

(c) Osteosclereids or bone cells : These are bone or barrel-shaped sclereids dilated at their ends. e.g., leaf of Hakea.

(d) Astrosclereids or stellate cells : These are star-shaped sclereids with extreme lobes or arms. e.g., leaf of Nymphaea.

(e) Trichosclereids or internal hairs : These are hair-like sclereids found in the intercellular spaces in the leaves and stem of some hydrophytes.

Complex permanent tissues

A group of more than one type of cells having common origin and working together as a unit, is called complex permanent tissue. The important complex tissues in vascular plants are : xylem and phloem. Both these tissues are together called vascular tissue.

(1) Xylem : The term xylem was introduced by Nageli (1858). Xylem is a conducting tissue which conducts water and mineral nutrients upwards from the root to the leaves.

On the basis of origin xylem is of two types :

(i) Primary xylem : It is derived from procambium during primary growth. It consists of protoxylem and metaxylem.

(ii) Secondary xylem : It is formed from vascular cambium during secondary growth.

Xylem is composed of four types of cells :

(i) Tracheids : Term “Tracheids” was given by Sanio (1863). The tracheids are elongated tubelike cells with tapering or rounded or oval ends with hard and lignified walls.

The cells are without protoplast and are dead on maturity. Tracheids possess bordered pits. Maximum bordered pits are formed in gymnospermous tracheids. They also possess various kinds of thickenings, e.g., annular, spiral, scalariform, reticulate or pitted tracheids. All the vascular plants have tracheids in their xylem. The main function of tracheids is to conduct water and minerals from the root to the leaf. They also provide strength and mechanical support to the plant.

(ii) Xylem vessels or Tracheae : Vessels are rows of elongated tube-like cells, placed end to end with their end walls dissolved. Vessels are multicellular with wide lumen. The vessels may be annular, spiral, scalariform, reticulate or pitted. Vessels are absent in pteridophytes and gymnosperms (except Ephedra, Gnetum, Selaginella, Pteridium). In angiosperms (porous wood) vessels are always present (Vessels are absent in family - Winteraceae, Trochodendraceae and Tepacenpaceae of Angiosperm i.e. Lotus, Wintera, Trochodendron). It also provide mechanical support to the plant and help in conduction. On the basis of distribution and size of vessels, porous wood is of two types :

(a) Diffuse porous wood (Primitive) : Vessels of same size are uniformly distributed throughout the growth or annual ring e.g., Pyrus, Azadirachta, Eucalyptus, Mangifera sp., Betula. They are characteristics of plants growing in tropical region.

(b) Ring porous wood (Advanced) : Large vessels are formed in early wood when the need of water is great and small vessels are formed in late wood e.g. Quercus, Morus, Cassia, Delbergia, Tilea sp.

(iii) Wood (xylem) parenchyma : These are the living parenchymatous cells. As found associated with xylem they are known as wood parenchyma. They serve for the storage of reserve food and also help in conduction of water upwards through tracheids and vessels.

(iv) Wood (xylem) fibres : The long, slender, pointed, dead and sclerenchymatous cells found associated with xylem are termed wood fibres. They aid the mechanical strength of xylem and various organs of plant body.

(2) Phloem (bast) : Term “Phloem” was given by Nageli. Its main function is the transport of organic food materials from leaves to stem and roots in a downward direction.

On the basis of position phloem is of three types :

(i) External phloem : It is normal type and present outside the xylem e.g., Mostly angiosperms and gymnosperms.

(ii) Internal or Intraxylary phloem : It originates from procambium and is primary phloem which occurs on innerside of primary xylem. It is primary anomalous structure. e.g., Members of Apocynaceae, Asclepiadaceae, Convolvulaceae, Solanaceae.

(iii) Induced or Interxylary phloem : It originates from cambium and is secondary phloem which occurs in groups within the secondary xylem. It is secondary anomalous structure. e.g., Leptadaenia, Salvadora, Chenopodium, Boerhaavia, Amaranthus.

On the basis of origin, phloem is of two types :

(i) Primary phloem : It is formed by procambium during primary growth. It may or may not show differentiation in protophloem (consists of sieve elements and parenchyma) and metaphloem (develop after protophloem and consists of sieve elements, parenchyma and fiber).

During the primary growth the protophloem elements are crushed by the surrounding tissues and disappear. This process is known as obliteration.

(ii) Secondary phloem : It is produced during secondary growth by vascular cambium.

It consists of the following elements :

Sieve element

(i) They are long tube-like cells placed end to end, forming a continuous channel in the plant parts.

(ii) Their cell wall is made up of cellulose and transverse wall is perforated like a normal sieve and hence they are called as sieve tubes.

(iii) Nucleus is not found in the mature cells.

(iv) Callus pad may be visible in the winter season.

(v) Their main function is to translocate the food material from one part to the other.

Companion cells

(i) They are thin-walled, more or less elongated cells and which are associated with sieve tubes.

(ii) They are connected with the sieve tube through sieve pore.

(iii) They contain nucleus and are therefore, living in nature.

(iv) They are not found in pteridophytes and gymnosperms but are always present in angiosperms.

Phloem parenchyma : The parenchyma associated with the phloem is called phloem parenchyma. The cells are elongated with rounded ends and possess cellulosic cell walls. These cells are living and store food reserves in the form of starch and fats. They are present in pteridophytes and most of dicotyledonous angiosperms. They are absent in monocots and few dicots like Ranunculus.

Phloem or Bast fibres : The sclerenchymatous fibres associated with the phloem are called as phloem fibres or bast fibres. The fibres are elongated lignified sclerenchymatous cells with tapering ends and with simple pits. They are non-living cells that provide mechanical support to the organs.

Special or Secretory tissues

These tissue perform special function in plants, e.g., secretion of resins gum, oil and latex.

These tissues are of two types :

(1) Laticiferous tissues : They are made up of thin walled, elongated, branched and multinucleate (coenocytic) structures that contain colourless, milky or yellow coloured juice called latex. These occur irregularly distributed in the mass of parenchymatous cells. Latex is contained inside the laticiferous tissue which is of two types :

(i) Latex cells : A laticiferous cell is a highly branched cell with long slender processes ramifying in all directions in the ground tissue of the organ. They do not fuse and do not form network. Plants having such tissues are called simple or non-articulated laticifers. e.g., Calotropis (Asclepiadaceae) Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).

(ii) Latex vessels : They are formed due to fusion of cells and form network like structure in all directions. At maturity, they form a highly ramifying system of channels full of latex inside the organ. Plants having such tissues are called compound or articulated laticifers. e.g., Argemone, Papaver (Papaveraceae), Sonchus (Compositae), Hevea, Manihot (Euphorbiaceae).

(2) Glandular tissue : This is a highly specialized tissue consisting of glands, discharging diverse functions, including secretory and excretory. Glands may be external or internal.

(i) External glands : They generally occur on the epidermis of stem and leaves as glandular hair in Plumbago and Boerhaavia, stinging hair secrete poisonous substance in Urtica dioica, nectar secreting glands in flowers or leaves. e.g., Rutaceae and Euphorbiaceae. Digestive enzyme secreting glands in insectivorous plants e.g., Drosera (Sundew), Nepenthes (Pitcher plant).

(ii) Internal glands : These are present internally and are of several types. e.g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus, mucilage canals in Cycas. Water secreting glands (hydathodes) in Colocasia (present at the tip of leaves), Tropaeoleum (along margin), etc. The glands which secrete essential oil are called osmophores (osmotrophs).