Tissue : A tissue is defined as a group of similar cells which carry or help to carry a common function and has a common origin.
Tissues are of two types—
1. Meristematic tissue.
2. Permanent tissue.
Various types of tissues combine to form complex plant organs. These organs are roots, stem, fruits, flowers and leaves.
1. Meristematic Tissue : A meristematic tissue or meristem is a group of cells which are in continuous state of division or retain their power of division.
The characterstics of these tissues are :
(a) These tissues have thin walls.
(b) They have dense protoplasts.
(c) Cells are small in size.
(d) Spherical, oval or polygonal in shape.
(e) Every cell has a large nucleus.
Distribution and Occurrence
Meristematic tissues are—
(a) Appical meristem—found at the apices of stems, roots etc.
(b) Lateral meristem—found at the sides of axis.
(c) Intercalary meristem—just above the nodes.
Meristematic tissues are of the following types—
(a) Promeristem meristem : This is also called primordial meristem. It consists of variable shaped young dividing cells. The cells are full of cytoplasm. It does not have vacuole. These tissues are found in the regions of new growth on plants.
(b) Primary meristem : Primary meristem is the first derivative of promeristem. It is found at the apices of stems, roots and leaves.
(c) Secondary meristem : Secondary meristems get developed from mature tissues by redifferentiation.
According to the functions meristems are again of three types namely—
(i) Protoderm : Outer most cell layer of primary meristem.
(ii) Procambium : Intermediate layer of cells of primary meristem.
(iii) Ground meristem : Central core of the primary meristem.
Functions :
(i) Apical meristem produces new buds and leaves.
(ii) The root meristem is responsible for the growth of roots.
(iii) Intercallary meristem produces leaves.
Permanent Tissues : It is a group of mature cells having a definite shape, size and function after complete growth and differentiation and which have lost the power of division temporarily or permanently. These are of two types—
(A) Simple Tissues : A simple tissue may be defined as a permanent tissue having a group of cells that are homogenous in structure and function. They are of three types—
(a) Parenchyma : Parenchyma is fundamental ground tissue; has thin walled living cells with intercellular spaces; serves the functions of assimilation, storage, circulation of air, secretion and radial conduction.
(b) Collenchyma : Collenchyma is living mechanical tissue; occurs in subepidermal region of aerial parts; cell walls show localised thickenings of pectins; performs mechanical and assimilatory functions.
1. Lamellar or plate collenchyma : Cells in rows; tangential walls highly thickened; found in hypodermal layers of dicot stems, e.g. Stem of Raphanus, sambuscus, stem of Lactuca and Leucas.
2. Lacunar collenychma : Walls surrounding intercellular spaces are highly thickened, e.g. Aerial roots of Monstera, Datura, Cucurbita.
3. Angular collenchyma : Cell walls highly thickened at corners, e.g. Stems of Tagetus, Solanum.
(c) Sclerenchyma : It is a mechanical tissue with dead cells. Cells are of two types—fibres and sclereids. Fibres are long dead cells with lignified secondary walls, sclereids are isodiametric with narrow tissues, and vary in shape.
Sclerenchyma is a simple permanent tissue, possessing lignified secondary walls. Cells lack protoplast and show great variation in shape, size, origin and development. Their principal function is to provide mechanical support and help in over coming stress and weight.
Fibres : Provide mechanical support and save the plant from stress and strain of strong winds. They occur in hypodermis, pericycle, xylem and phloem.
Fibres are long, narrow, thick and lignified sclerenchyma cells with pointed or blunt ends.
Hard fibres : They are rigid with lignified walls, e.g. Agave americana (0.0-8 mm), Mus textilis (2-12 mm), Yucca (2-15 mm) Gloriora.
Soft fibres : Lack lignified walls. Soft and tensile, e.g. phloem fibres of Linum (flax) Cannabis (hemp), Corchorus (jute), Boehmeria (ramie). Epidermal hairs of cotton seeds and Ramie fibres are the longest fibres.
Sclereids : Sclereids are also known as stone cells or sclerotic cells.
Brachysclereids (stone cells) : Isodiametric, very common in cortex, pith and phloem of stems, e.g. Pyrus communis, Cocos nucifera.
Macrosclereids (malphigian cells) : Rod shaped; found in seeds and fruits in the form of a layer, e.g. Pyrus malus; Pisum sativum.
Osteosclereids : Bone shaped, e.g. Leaves of Hawckia.
Asterosclereids (stellate sclereids) : Possess star like arms, e.g. Water plant like Nymphaea trochodendron.
Trichosclereids : Hair like; sometimes branched; found in intercellular spaces of stem and leaves of some hydrophytes, e.g. aerial roots of Monstera.
Filiform sclereids : Thin, long, filamentous, e.g. Leaves of Olea.
Idioblasts : Specialised parenchyma, become stony with rigid, lignified walls; contain tannins; some isolated cells bear raphides or crystals, e.g. Provide rigidity to fruits and mechanical strength to leaves and roots. Sclereids are dead cells. Cell walls are thick. Lumen is very narrow. Sclereids carry out mechanical function. They give rigidity to the organs.
(B) Complex Permanent Tissue: A complex permanent tissue is made up of more than one type of cells, it is called a complex permanent tissue.
The main type of complex tissues are—
(i) Xylem and
(ii) Phloem.
(i) Xylem
It is a complex tissue and is also called Hydrome. This is responsible for the conduction of water in the plant.
Cells of xylem are heterogeneous in structure and function. Tracheids, vessels (Tracheary elements), fibres and parenchyma are grouped together in xylem.
Tracheary elements : Tracheary elements are highly specialised cells of xylem. These are elongated cells with lignified secondary walls. Cells are non-living and lack protoplasts at maturity.
Xylem fibres : Xylem fibres are sclerenchymatous cells found in the xylem tissue. They are dead cells with narrow lumen and lignified walls. They develop from tracheids.
Xylem parenchyma : Parenchymatous cells found in the xylem constitute ‘xylem or wood parenchyma’. They are living cells.
Pits : Pits are places in secondary cell wall which is not laid down.
Pitpair : The pits in the walls of adjacent cells lie opposite to each other. These are called pit pair.
Pit membrane : The membrane which separates two pits is called pit membrane.
Pit Aperture : Pit aperture is the opening into the lumen of the cell. Pits are classified into simple pits and bordered pits.
Bordered pit : If the secondary wall partially overarches the pit cavity, it is called bordered pit. This arching takes place on both sides of the pit.
Simple pit : If such overarching is not found over pit cavity, it is called simple pit.
Torus : The pit membrane of a bordered pit pair gets thickened in the central portion. It is called torus.
Margo : Margo is thin pit membrane around the torus.
(ii) Phloem
Phloem is complex tissue, with sieve elements, companion cells, fibres and parenchyma. It is food conducting tissue. Sieve cells originate from single cell.
Sieve elements : Sieve elements include the conducting cells of phloem. They are of two types—sieve cells and sieve tube elements.
Companion cells : Companion cells are parenchymatous cells which are associated with sieve tubes through numerous plasmodesmata present in their common walls. They are characteristic of angiosperms and are absent in gymnosperms and pteridophytes. They are formed from sieve tube mother cells by longitudinal division.
Phloem fibres : Phloem fibres are also called bast fibres. They are abundant in secondary phloem. They are composed of dead cells. The chief function of phloem fibres is to give strength to the organ.
Bast fibres have high commercial value. Secondary phloem of Corchorus capsularis (Jute plant) yields high quality jute. It is extracted from the plant by a process called retting.
Phloem parenchyma : Phloem parenchyma is absent in monocots. These cells have rounded ends and thin cellulose cell walls. Cells possess cytoplasm and nucleus. In secondary phloem, parenchyma is present in vertical and also in radial directions. These cells are concerned with storage of starch, fat and other organic food materials.
Special tissues : They are specialised groups of parenchyma cells.
Gland : A group of cells that are endowed with the capacity to secrete or excrete products, is called a gland.
Laticiferous Tissue : These tissues secrete milky viscous fluid-latex. These are of two types—latex cells and latex vessels. Latex vessels anastamose and fuse to form a network. Latex cell are living, coenocytic and never fuse. Latex is a rich source of rubber.
Latex vessels : They are articulated laticifers. They can be called syncytes, composed of a large number of cells with their transverse wall dissolved to form a long vessel. These are living, coenocytic structures, e.g. Papaver, Luctuca, Hevea, Musa etc.
Latex cells : They are simple laticifers. They are multinucleate cells of innumerable lengths. Nuclei divide many times without wall formation. Latex cells may be branched or unbranched, e.g. Euphorbia, Nerium, Ficus etc.
Secretory cavities : Many plants show the presence of glands. Their secretion is collected in a cavity.
Schizogenous cavity : This cavity is formed by breakdown of cells at the middle lamella. It is lined by intact cells.
Lysigenous cavity : It is formed by lysis of some cells. This has partly disintegrated cells along the periphery. e.g. Oil cavities of Citrus & Eucalyptus.
Digestive glands : These glands secrete proteolytic enzymes which digest insects. e.g. Drosera and Dionaeatentacles.
Tissue System : A tissue system contains two or more than two tissues.
Epidermal Tissue System : This system forms an outermost covering of plant organs. It is exposed to outer environment, so it develops a number of structures to adapt itself to the varying environmental conditions.
Epidermis : It is outermost protective layer with living cells. It is covered by cuticle and has stomata.
Cuticle : It is the layer of waxy substance-cutin, on the outer wall of epidermal cells or aerial parts.
Bulliform cells : These are also called motor cells. They are present in the upper epidermis of grass leaves. They are large, thin walled and loose turgidity in dry weather and bring about rolling of leaves to reduce transpiration.
Stomata : Stomata are very small pores in the epidermis. The aperture is called ‘stoma’.
Trichomes : Epidermal hairs are called trichomes. They may be unicellular or multicellular.
Ground Tissue System : All the tissues except epidermal system and vascular system together constitute the ground tissue system.
Hypodermis : Hypodermis constitutes the outer layers of cortex immediately next to epidermis. It is collenchymatous in dicots and sclerenchymatous in monocots.
General Cortex : This cortex is made up of many layers of parenchyma.
Endodermis : Endodermis is the innermost layer of cortex. Cells are barrel shaped in cross section.
Pericycle : Pericycle is the peripheral zone of stele present beneath the endodermis.
Pith : Pith occupies the centre of vascular cylinder. Pith is not distinct in monocot stem. It may or may not be present in dicot root.
Medullary Rays : Parenchy-matous extensions from pith in between vascular bundles are called medullary rays. They are not seen in monocot stem and root. Their function is radial conduction.
Conjunctive Tissue : This is parenchyma tissue filling the gaps between alternating strands of xylem and phloem in roots.
Mesophyll : Mesophyll constitutes the ground tissue in leaf lying between two epidermal layers except veins. In the parenchymatous with chloroplasts and assimilatory in function, it is differentiated into palisade tissue with narrow elongated closely packed cells on upper side and spongy tissue with loosely arranged cells on lower side in a bifacial leaf.
Vascular Tissue System : Vascular tissue system is also known as fascicular or conducting tissue system.
Types of Vascular Bundles
Conjoint, Collateral : Xylem and phloem are present in the same bundle. They lie on the same radius. Phloem is situated outsde and xylem-inside, e.g. stem or Angiosperms.
Bicollateral : Xylem is situated in the middle of the bundle and phloem is arranged on both sides, e.g. Cucurbita stem.
Concentric : Phloem is situated in the centre of the bundle and surrounded by the other tissue in the form of rings.
Amphicribal or Hadrocentric : Xylem is surrounded by phloem on all sides in the bundle, e.g. Ferns, Selaginella.
Amphivasal or Leptocentric : In this phloem is surrounded by xylem on all sides in the bundle, e.g. Rhizome of Iris.
Radial : Xylem and phloem are present in separate strands and are arranged in different radii, e.g. Roots of angiosperms and gymnosperms.
Open : Cambium is present between xylem and phloem in a collateral bundle, e.g. dicot stem.
Closed : Cambium is absent in a collateral bundle between xylem and phloem, e.g. monocot stem.
Exarch : Protoxylem is towards periphery and metaxylem towards centre (centripetal development), e.g. Root.
Endarch : In this protoxylem is towards centre and metaxylem is towards periphery, e.g. Stem of angiosperms.
Mesarch : Protoxylem is surrounded by metaxylem on all sides e.g. Rachis and pinna of Cycas.
Leaf Trace : It is the lower portion of the vascular supply to the leaf. Part of the vascular strand of leaf present in the stem is called leaf trace.
Branch Trace : Vascular strand that connects the vascular system of the branch with that of the main stem is called branch trace.
Eustele : Vascular bundles appear in a limited number in a ring like fashion. It is called eustele.
Atactostele : Vascular cylinder consists of widely scattered network of numerous vascular bundles. Such a stele is called atactostele.
Transfusion : Conducting tissue is present between palisade and spongy tissues. Cells have thin cellulose cell wall with scelariform, reticulate and pitted thickenings. Cells are dead. They carry out lateral conduction.
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