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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 showingcontinuous divisions and redivisions, is called as meristematic tissue. The various characteristic features of the meristems are discussed below :
❒ They contain immature and young cells and are capable of repeated divisions.
❒ Intercellular spaces are not present in meristematic tissue.
❒ They contain a homogeneous thin wall.
❒ They contain large nuclei associated with abundant cytoplasm.
❒ They are metabolically very active but they do not store food material.
❒ Only proto-plastids are present instead of plastids, chloroplast absent.
❒ Dense cytoplasm is present which contains several premature mitochondria.
❒ Vacuoles are absent.
❒ Meristematic cells are isodiametric in shape.
❒ Undifferentiated tissue in which all divides continuously G1 → S → G2 → M.
The meristems may be classified on the basis of their mode of origin, position or function :
(i) According to origin and development : On the basis of origin, meristematic tissues are of three types :
(a) Promeristem or Primordial meristem : Thepromeristem 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 thefoundation of an organ or a plant part, is called promeristem. This group consists of a limited amount of cells, which divide repeatedly to give rise primary meristem. It occupies a small area at the tips of stem and root. The promeristem gives rise to all other meristems including the primary meristem.
(b) 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 tothe primary permanent tissue.
(c) 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 calleddedifferentiation. Thesecondary meristems are so called because they originate from permanent cells. The phellogen orcork 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.
(ii) According to position : On the basis of their position in the plant body meristems are classified into three categories :
(a) Apical meristem : This meristem is located at the growing apices of main and lateral shoots and roots. These cells areresponsible for linear growth of an organ. The initiating cells may be single or in groups. Solitary initial cells are known as apical cells whereas those occurring in groups are called apical initials. Solitary apical cells occur in ferns and other Pteridophytes while apical initials are found in other vascular plants. The apical initials may occur in one or more tiers. Position of apical cells may either be strictly terminal or terminal and subterminal.
(b) 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, Pteridophyts like Equisetum) or at the base of the leaf (e.g., Pinus). The intercalary meristems ultimately disappear and give rise to permanent tissues.
(c) 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) andresult increase in the diameter of an organ. Thecambium of vascular bundles (Fascicular, interfascicular and extrastelar cambium) and thecork cambium or phellogen belong to this category and are found in dicotyledons and gymnosperms.
(iii) According to function : Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types :
(a) Protoderm : It is the outermost layer of the apical meristem which develops into theepidermis or epidermal tissue system.
(b) Procambium : It occurs inside the protoderm. Some of the cells of young growing region which by their elongation and differentiationgive rise to primary vascular tissue, constitute the procambium.
(c) Ground meristem : It constitute the major part of the apical meristem develops ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary rays.
(iv) According to plane of cell division : On the basis of their plane of cell division meristem are classified into three categories :
(a) Mass meristem : The cells divide anticlinally in all planes, so mass of cells is formed. e.g., formation of spores, cortex, pith, endosperm.
(b) Plate meristem : The cells divide anticlinally in two planes, so plate like area increased. e.g., formation of epidermis and lamina of leaves.
(c) 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.
(i) Vegetative shoot apex : Shoot apex was first recognized by Wolff (1759) shoot apex is derived frommeristem 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 :
(a) 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).
(b) Histogen theory : It was proposed byHanstein (1870). According to this theory, theshoot apicalmeristem consists of three distinct meristematic zones or layers (or histogens).
❒ Dermatogen : Outermost layer and it formsepidermis and epidermal tissue system.
❒ Periblem : It is the middle layer gives rise to cortex andendodermis.
❒ Plerome :Innermost layer formspith andstele.
(c) Tunica corpus theory : This theory was proposed bySchmidt (1924). According to this theory, theshoot apex consists of two distinct zones.
❒ 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.
❒ Corpus : It represents the central core with larger cells. Corpus shows divisions in all planes and it is responsible for volume growth.
(ii) 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. The root apex differs from shoot apex as it is short and more or less uniform due to complete absence of lateral appendages (leaves and branches) and differentiation of nodes and internodes. According to Hanstein (1870) root apex of most of the dicotyledons also consists of three meristematic zones - plerome, periblem and dermatogen (fourth meristemcalyptrogen to formroot cap only in monocots). Regarding the apical organisation of root following theories have been put forward.
(a) 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.
(b) Quiescent centre theory : It was proposed by Clowes (1961). According to him, in addition to actively dividing cells,a zone of inactive cells is present in the central part of theroot apex calledquiscent centre.
The cells in this region havelight cytoplasm, small nuclei, lower concentration of DNA, RNA and protein. These cells also contain fewer number of mitochondria, less endoplasmic reticulum and small dictyosomes.
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 three types :
(1) Simple tissues :Simple tissues are a group of cells which are all alike in origin, form and function. They are further grouped under three categories :
(i) 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 :
(a) The cells are thin-walled and soft.
(b) The cells usually are living and possess a distinct nucleus.
(c) The cells containwell-developed intercellular spaces amongst them.
(d) The cytoplasm is vacuolated and cell wall is made up of cellulose.
(e) The shape may be oval, spherical, cylindrical, rectangular and stellate (star shaped) in leaf petioles of banana and canna and some hydrophytes.
(f) Thistissue is generally present in almost all the organs of plants, i.e., roots, stems, leaves, flowers,fruits and seeds.
(g) If they enclose large air spaces they are called asaerenchyma; if they develop chlorophyll, they are called aschlorenchyma and if they are elongated cells with tapering ends, they are called asprosenchyma.
Functions : They perform the following functions :
❒ Storage of food materials. e.g., Carrot, Beetroot etc.
❒ Chlorenchyma helps in photosynthesis.
❒ Aerenchyma helps infloating of the aquatic plants (Hydrophytes) and also help in gaseous exchange during respiration and photosynthesis. e.g., Hydrilla.
❒ In turgid state they give rigidity to the plant organ.
❒ In emergency they behavelike meristematic cells and help in healing of the various plant injuries.
❒ Sometimes they store secretory substances (ergastic substance) such as tannins, resins and gums and they called asidioblasts.
(ii) Collenchyma : The term collenchyma was coined bySchleiden (1839). It is the tissue of primary body. The main characteristics of are given below :
❒ The cells of this tissue containprotoplasm and areliving.
❒ The cell walls arethickened at the corners and are made up of cellulose, hemicellulose andpectin.
❒ They are never lignified but may posses simple pits.
❒ They are compactly arranged cells, oval, spherical or polygonal in outline.
❒ No. intercellular spaces are present.
❒ The tissue is plastic, extensible and have capacity to expand.
❒ They provide mechanical strength to younger part where xylum is less developed.
❒ Collenchyma occurs chiefly in thehypodermis ofdicotyledonous stems (herbaceous, climbers or plants e.g. Cucurbeta, Helianthus) and leaves. They are usually absent in monocots and in roots.
(a) Types of collenchyma : Majumdar (1941) divided collenchyma into three types on the basis of thickening :
❒ Angular collenchyma : Where the thickening of the cells is confined to the corners of the cells. e.g., Tagetes, Tomato, Datura, Potato, etc.
❒ Plate or Lamellar collenchyma : When the thickenings are present in the tangential walls. e.g. hypodermis of sunflower stem.
❒ Lacunar or Tubular collenchyma : If the thickened cell wall is associated with intercellular spaces of the adjacent cells. e.g. leaf petioles of compositae and malvaceae etc. hypodermis of Cucurbita stem, Salvia, Malva.
❒ Provide mechanical support to petiole, pedicels, branches of stem, roots and fruits.
❒ If they contain chlorophyll they help in photosynthesis.
❒ It is present at the margins of some leaves and resists tearing effect of the wind.
(iii) Sclerenchyma : It was discovered and coined by Mettenius (1805).
The main feature of sclerenchyma are :
❒ It consist ofthick-walled dead cells.
❒ The cells vary in shape, size and origin.
❒ They possesshard and extremely thick secondary walls due to uniform deposition oflignin.
❒ In the beginning the cells are living and have protoplasm but due to deposition of impermeable secondary walls they become dead.
Types of sclerenchyma : They are of two types :
(a) 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 :
❒ 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.
❒ Wood fibres : Those fibres which areassociated with wood or xylem have bordered pits are known as wood fibres. Thick walled wood fibres having simple pits are calledlibriform fibres whereas thin walled wood-fibres having bordered pits are calledfibre-tracheids. A specific type of wood fibre is produced by Quercus rabra and is called gelatinous or mucilagenous fibres.
❒ 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).
(b) Stone cells or Sclereids : They arelignified, extremelythick 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. They form part of seed coat in some members of leguminosae. The sclereids provide mechanical support and hardness to the soft parts. Sclereids may be :
❒ 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).
❒ Macrosclereids or rod cells : These arerod-shaped elongated sclereids usually found in the leaves, cortex of stem and outerseed coats.
❒ Osteosclereids or bone cells : These are bone or barrel-shaped sclereids dilated at their ends. e.g., leaf of Hakea.
❒ Astrosclereids or stellate cells : These are star-shaped sclereids with extreme lobes or arms. e.g., leaf of Nymphaea.
❒ Trichosclereids or internal hairs : These are hair-like sclereids found in the intercellular spaces in the leaves and stem of some hydrophytes.
(2) Complex 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 andphloem. Both these tissues are together calledvascular tissue.
(i) Xylem : The term xylem was introduced by Nageli (1858). Xylem is a conducting tissue whichconducts water and mineral nutrients upwards from the root to the leaves.
On the basis of origin xylem is of two types
❒ Primary xylem : It is derived from procambium during primary growth. It consists of protoxylem and metaxylem.
❒ Secondary xylem : It is formed from vascular cambium during secondary growth.
(a) Tracheids : Term “Tracheids” was given by Sanio (1863). The tracheids are elongated tubelike cells with tapering or rounded or oval ends with hard andlignified walls.
The walls are not much thickened. The cells are without protoplast and aredead on maturity. The tracheids of secondary xylem have fewer sides and are more sharply angular than the tracheids of primary xylem. The cell cavity or lumen of a tracheid is large and without any contents. Tracheids possessbordered 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.
(b) 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 classified into several types according to the thickening developed in their wall. They may be annular, spiral, scalariform, reticulate or pitted. Vessels are absent in pteridophytes andgymnosperms (except Ephedra, Gnetum, Selaginella, Pteridium). Inangiosperms (porous wood) vessels are always present (Vessels are absent in family - Winteraceae, Trochodendraceae and Tepacenpaceae of Angiosperm i.e. Lotus, Wintera, Trochodendron). Vessels along with tracheids forms the main tissue of xylem of vascular bundles of the angiosperms and help in conduction. It also provide mechanical support to the plant.
On the basis of distribution and size of vessels, porous wood is of two types :
❒ 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 arecharacteristics of plants growing in tropical region.
❒ Ring porous wood (Advanced) : Large vessels are formed in early wood when theneed of water is great and small vessels are formed in late wood e.g. Quercus, Morus, Cassia, Delbergia, Tilea sp.
(c) Wood (xylem) parenchyma : These are theliving 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.
(d) Wood (xylem) fibres : The long, slender, pointed, dead and sclerenchymatous cells found associated with xylem are termed wood fibres. They possess mostly thickened walls and few small pits. These pits are found abundantly in woody dicotyledons. They aid the mechanical strength of xylem and various organs of plant body.
(ii) Phloem (bast) : Term “Phloem” was given byNageli. 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 :
(a) External phloem : It is normal type and present outside the xylem e.g., Mostly angiosperms and gymnosperms.
(b) Internal or Intraxylary phloem : It originates from procambium and is primary phloem which occurs on innerside of primary xylem. It is primary anamolus structure. e.g., Members of Apocynaceae, Asclepiadaeae, Convolvulaceae, Solanaceae.
(c) Induced or Interxylary phloem : It originates from cambium and is secondary phloem which occurs in groups within the secondary xylem. It issecondary anamolus structure. e.g., Leptadaenia, Salvadora, Chenopodium, Boerhaavia, Amaranthus.
On the basis of origin phloem is of two types
(a) Primary phloem : It is formed by procambium during primary growth. It may or may not show differentiation of 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 curshed by the surrounding tissues and disappear. This process is known as obliteration consists of sieve elements, parenchyma and fibre.
(b) Secondary phloem : It is produced during secondary growth by vascular cambium.
It consists of the following elements :
Phloem fibres or bast fibres
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
(2) Glandular tissues
(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 very 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 are generally occur on the epidermis of stem and leaves asglandular hair in Plumbago and Boerhaavia, stinging hair secrate 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).
The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “Tissue System''. According to Sachs (1975) there are three major tissue systems in plants as follows :
(1) Epidermal tissue system (2) Ground or fundamental tissue system (3) Vascular tissue system
(1) Epidermal tissue system :The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment.
(i) Epidermis : Epidermis is composed of single layer cells. These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some casesepidermis may bemultilayered e.g. Ficus, Nerium, Peperomia, Begonia etc.
The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces.
Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types :
(a) Inaerial roots, the multiple epidermal cells are modified tovelamen, whichabsorb water from the atmosphere (e.g., Orchids).
(b) Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. It is hygroscopic in nature. e.g., Ammophila. They are thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves.
(c) Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells.
(ii) Cuticle and Wax : In aerial parts, epidermis is covered by cuticle.The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all walls. it helps in reducing the loss of water by evaporation. Usually the cuticle is covered with wax which may be deposited in the form of granules, rods, crusts or viscous semiliquid masses. Other substances deposited on the cuticle surface may be oil, resin, silicon and salts (cystoliths are crystals of calcium carbonate, e.g., Ficus.Druse and Raphides, e.g., Pistia) are crystals of calcium oxalate.Thick cuticle are found in leaves ofdry habitats plants.
(iii) Stomata : Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard cells. Stomata are absent in roots. In xerophytes the stomata are sunken in grooves due to which rate of transpiration is greatly reduced (e.g. Nerium). Usually there is a large air cavity below each aperture, it is called substomatal cavity. In some species the guard cells are surrounded by subsidiary cells or accessory cells which differ morphologically from the other epidermal cells. In monocots e.g., Doob, Maize guard cells are dumb bell shape. Stomata are scattered in dicots leaves but they are arranged in rows in monocots.
Depending upon distribution of stomata, the leaves are :
(a) Apple-mulberry type : e.g. Oxalis, Mulberry, Apple.
(b) Potato type : e.g. Bifacial (dorsiventral leaves of pea, bean, tomato).
(c) Oat type : e.g. Suberect (isobillateral) leaves of most grasses and cereals (monocotyledens).
(d) Nymphea type : e.g. Floating leaves of Nelumbo, Nymphia, water lily.
(e) Potamogeton type : e.g. Submerged plants like Hydrilla, Vallisneria, Potamogeton.
(iv) Trichomes : These areepidermal outgrowths present temporarily or permanently on almost all plant parts. They may be unicellular or multicellular and vary in size and shape in different species.
They may be of different types : stellate hair, glandular hair, short glandular hair, floccose hair, urticating hair and stinging hair.
The trichomes serve for checking excess loss of water and for protection.
(v) Root hairs : They are enlargements of special epiblema cells called trichoblasts and occurs in a particular zone of young root called root hair zone. A root hair cell has vacuolated protoplast with nucleus present towards the apical part of hair. They are specialised to absorb water from soil crevices. They also hold soil particles.
(2) Ground or Fundamental tissue system :Ground tissue system includes all the tissues of plant body except epidermal tissue system and vascular tissues. It forms the bulk of body. This tissue system mainly originates from ground meristem. The ground tissues constitute the following parts :
(i) Cortex : It lies between epidermis and the pericycle. The cortex is distinct in dicotyledons but not in monocotyledons where there is no clear demarcation between cortex and pith. It is further differentiated into :
(a) Hypodermis : It is collenchymatous in dicot stem and sclerenchymatous in monocot stem. It provides strength.
(b) General cortex : It consists of parenchymatous cells. Its main function is storage of food.
(c) Endodermis (Starch sheath) : It is mostly single layered and is made up of parenchymatous barrel shaped compactly arranged cells. The inner and radial or transverse wall of endodermal cells havecasparian strips of suberin. In roots thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem pathches. These thin walled endodermal cells are calledpassage cells or transfusion cells. A fully developed endodermis is found in all types of roots. Endodermis with characteristic casparian bands is absent in woody dicot stem, monocot stem and leaves of angiosperms. The young stems of angiosperms show a layer with abundant starch deposition. This layer occurs in the position where endodermis would have been situated which is called as starch sheath. In Selaginella trabeculate endodermis is found due to formation of air spaces between two endodermal cells.
Endodermis behave as water tight dam to check the loss of water and air dam to check the entry of air in xylem elements. Endodermis is internal protective tissue.
(ii) Pericycle : It is a single layered or multilayered cylinder of thin-walled or thick-walled cellspresent between the endodermis and vascular tissues. In some cases, the pericycle is made up of many layers of sclerenchymatous cells (Cucurbita stem) or in the form of alternating bands of thin-walled and thick-walled cells (Sunflower stem). In case of roots, the pericycle is made up of thin-walled parenchymatous cells which later ongives rise to lateral roots. In dicot roots thecork cambium originates in the pericycle which results in the formation of periderm. Pericycle also gives rise to a part of vascular cambium in dicot roots.
(iii) Pith or Medulla : It occupies the central part in dicot stem, and monocot root. It is mostly made up of parenchymatous cells. in dicot root pith is completely obliterated by the metaxylem elements. In dicot stem the pith cells between the vascular bundles become radially elongated and known as primarymedullary rays orpith rays. They help inlateral translocation.
(3) Vascular tissue system :The central cylinder of the shoot or root surrounded by cortex is called stele. The varying number of vascular bundles formed inside the stele constitute vascular tissue system. Xylem, phloem and cambium are the major parts of the vascular bundle. Vascular bundle may be of following types :
(i) Radial : The xylem and phloem strands alternate with each other separated by parenchymatous cells. such kinds of vascular bundles are called radial and found mainly inroots.
(ii) Conjoint : A vascular bundle having both xylem and phloem together, is called conjoint. Normally the xylem and phloem occur in the same radius. They occur in stems. Such vascular bundles are of two types :
(a) Collateral : A vascular bundle in which thephloem lies towards outerside and xylem towards inner side, is calledcollateral, e.g., Sunflower.
Collateral bundle having acambium between xylem and phloem is said to be of theopen type, e.g., Dicot stem.
Collateral bundle lacking a cambium between xylem and phloem is said to be of the closed type, e.g., Monocot stem.
(b) Bicollateral : Avascular bundle having the phloem strandson both outer and inner side of xylem, is called bicollateral. e.g., Cucurbita.
(iii) Concentric : A vascular bundle in which one tissue is completely surrounded by the other, is called concentric. The concentric bundles are of two types :
(a) Amphivasal (Leptocentric) : The phloem lies in the centre and remains completely surrounded by xylem. e.g., Dracaena, Yucca.
(b) Amphicribal (Hadrocentric) : The xylem lies in the centre and remains completely surrounded by phloem. e.g.,Ferns.
The increase in thickness or girth due to the activity of the cambium and the cork cambium is known as secondary growth.
(1) Secondary growth in stem :On the basis of the activities ofcambium and cork-cambium, secondary growth in stem can be discussed under the following heads :
(i) Activity of cambium (ii) Activity of cork-cambium
(i) Activity of cambium : The vascularcambium in between xylem and phloem is called intrafascicular or fascicular cambium which is primary in origin. At the time of secondary growth the parenchymatous cells of medullary rays between the vascular bundles become meristematic and form strip of cambium called as interfascicular cambium which is secondary in origin. Both inter and intrafascicular cambium joins together and form cambium ring which is partly primary and partly secondary in origin. By anticlinal divisions the circumference of the cambium increase. By periclinal division cambium produced thesecondary xylem and phloem tissues on innerside and outerside. The amount of sec. xylem produced is 8-10 times greater than sec. phloem. The cambium has two types of cells :
(a) The fusiform initials which are elongated and form fibres,sieve cells, sieve tubes, tracheids.
(b) Ray initials which produce parenchyma cells of the rays in wood and phloem. Ray initials are much shorter than fusiform initials. Certain cells of cambium form some narrow bands of living parenchyma cells passing through secondary xylem and secondary phloem and are called secondary medullary rays. These provide radial conduction of food from the phloem, and water and mineral salts from the xylem.
❒ Annual rings : Activity of cambium is not uniform in those plants which grow in the regions where favourable climatic conditions (spring or rainy season) alternate regularly with unfavourable climatic conditions (cold water or dry hot summer). In temperate climates, cambium becomes more active in spring and forms greater number of vessels with wider cavities; while in winter it becomes less active and forms narrower and smaller vessels. The wood formed in the spring is known as spring wood and that formed in the dry summer or cold winter autumn wood or late wood. Both autumn and spring wood constitutea growth or annual ring. In one year only one growth ring is formed. Thus by counting the number of annual rings in the main stem at the base we can determine the age of a tree. This branch of science is known as dendrochronology. Age is determined by an instrument increment borer. Growth rings are distinct or sharply demarcated in the plants oftemperate regions where as in tropical climate (near equator) they are not distinct or sharply demarcated in the trees.
(ii) Activity of cork cambium :Cork cambium or phellogen develops from outer layer of cortex. It produces secondary cortex or phelloderm on innerside and cork or phellum on outerside. The cells of phellem are dead, suberized and impervious to water. Cells of phelloderm are thin walled, living and store food.Phellem, phellogen and phelloderm collectively called as periderm. Periderm is secondary protective tissue. Due to pressure of secondary xylem, epidermis raptures and cortex is largely lost after two or three years of secondary growth.
(a) Bark : All dead tissues lying outside the active cork-cambium are collectively known as bark. This includes ruptured epidermis, hypodermis and cork. When cork-cambium appears in the form of a complete ring, it is known as ring bark, e.g., Betula (Bhojpatra). If the cork cambium occurs as separate strips and the resulting bark appears in the form of scales, such a bark is known as scaly bark. e.g., Eucalyptus, Psidium guava. The outermost layer of bark is dead and called as rhytidome.
(b) Lenticels :These are aerating pores formed in the cork through which gaseous exchange takes place.They are formed as a result of the action of phellogen. A lenticel appears as a scar or protrusion on the surface of the stem and consists of a radial row of thin-walled cells, known as complementary cells or filling tissue.They are found in old dicot stem, main function is guttation.
(c) Cork : It consists ofdead cells with thick walls heavily impregnated with suberin. These cells are compactly arranged in radial rows without intercellular spaces. Cork is impervious to water and prevents its loss from the plant surface. It also protects the inner tissues from the attack of fungi and insects. There is no differentiation of bark, sap wood and heart wood of Date palm.
(d) Heart wood and sap wood : In old trees, secondary wood is differentiated into a centrally situated darker and harder wood called the heart wood or duramen which are physiologically inactive (almost dead)and an outerlight-coloured zone called the sap wood or alburnum which arephysiologically active. Dark colour of heart wood is due to the deposition of tannins, resins, gums, essential oils, etc. in the cell walls and cell cavities.The water conduction takes place through sap wood. During the conversion of sap wood into heartwood the most important change is development of tyloses in the heart wood.Tyloses are ballon like structures, develop from xylem parenchyma. These tylosesblock the passage of xylem vessels so also called as tracheal plug. The heart wood is commercially used as wood. When the plant is made hollow, it will not die because the water conduction takes place through sap wood. The heart wood is well developed in Morus alba (Mulberry). The heart wood is absent in Populus and Salix plant. As a tree grows older thickness of heartwood increases and sap wood remains same.
(2) Secondary growth in dicot roots : Vascular bundles in dicot roots are radial, exarch and mostly triarch. Vascular cambium is formed secondarily from conjuctive parenchyma cells lying just below each phloem strand. Thus the number of cambium strips formed equals the number of phloem strands. The cells of pericycle lying outside the protoxylem also become meristematic to form part of strips of cambium. These cambial strips join the first formed cambium strips to form complete but wavy ring of vascular cambium. This cambium ring produced secondary xylem on inner side and secondary phloem on outer side. In roots, the growth rings are not distinct because there is no seasonal variation under the soil. From the outer layers of pericycle arises the phellogen which cuts phellem (cork) on the outer side and secondary cortex or phelloderm toward the inner side.