Internal Structure of Angiospermous Plants

The plant body is made up of cells which organize themselves to form Tissues. Tissues organize to form tissue system. The study of the internal structure of plants is known 'Plant Anatomy'. 1.1 Tissue: -It is a group of similar or dissimilar cells performing a common function and having a common origin. Types of tissues:-Two types -Meristematic and Permanent. 1. Meristematic Tissues(Meristems): -These are a group of cells which are in a continous state of division. These tissues divide to form new cells which differentiate to give rise to permanent tissues. Types of meristems: -  A. On the basis of origin & development, these are of three types:  i. Promeristem: -It is a group of initial cells that lay down the foundation of an organ or plant part. ii. Primary meristem: -It is formed from promeristem. Primary body, tissue differentiation and their organization' results from activity of primary meristem. iii. Secondary meristem: -These are formed secondarily from permanent tissues which acquire the power of division. B. Depending on their position in plant body, these are of three types: i. Apical meristem: -Present at the tips of stem, root and their branches. Responsible for growth in length. It is dome like with outer layer-Tunica& inner mass-Corpus. It has promeristem which differentiates into three regions: - * Dermatogen: -Outermost layer; gives rise to epidermis of the stem. In roots, it forms 'Calyptrogen' which is a mass of tissues which forms root cap. In dicots-it forms epilema of roots. * Periblem: -next to dermatogen. Forms cortex of root and stem. * Plerome: -lies internal to periblem. Cells elongate here and form procambuim which later forms Xylem & Phloem. ii. Intercalary Meristem: -It is located in between the regions of permanent tissues. These cells divide and continue to add new cells. These are responsible for growth in length. Present at bases of node, internodes and leaves. iii. Lateral Meristem:-These occur on sides & increase the girth of plant. 2. Permanent Tissues: -These are the tissues whose cells have lost the capacity to divide and attained a permanent shape, size and function. Types:- 2 main types -Simple tissues & Complex tissues. 1. Simple Tissues: -are made up of structurally and functionally similar cells. They are of three types: -Parenchyma, Collenchyma and Sclerenchyma. Parenchyma: -It is most abundant & common tissue in plants & is made up of similar isodiametric (all sides equal) cells. Cells may be oval, rounded or polygonal enclosing a large central vacuole and peripheral cytoplasm. Adjacent parenchyma cells are connected by Plasmodesmata. It forms cortex, pith, leaf mesophyll and floral parts. Collenchyma: -Cells of this tissue contain protoplasm and are living. The cells are generally elongated with oblique end walls. Cell walls show localized thickenings due to the presence of pectin, cellulose and hemicellulose. These provide mechanical strength and elasticity to the plant. Sclerenchysma: -It consists of thick-walled dead cells which vary in shape, size and origin. They have thick and hard secondary walls due to the deposition of lignin. These can be fibres or Sclereids. Fibres are pointed, needle like and occur in groups (as sheets or cylinders). Sclereids are thick, dead cells with narrow cavities. Cell walls have pits 2. Complex issues: -are the permanent tissues which contain more than one type of cells. These are of two types-Xylem & Phloem. Xylem & phloem, together are called Vascular Tissues. A. Xylem: -It is conducting tissue, conducts water and mineral salts upwards from the root. It is of 4 kinds-Tracheids, Vessels, Wood fibres and Wood parenchyma. Tracheids: -These are elongated dead cells with hard and lignified walls and a large cavity. Ends are tapering, blunt or chisel like. The walls have thickenings which can be annular, Spiral, reticulate, scalariform or pitted. These provide mechanical support and transport water, hormones and solutes from roots to stem, leaves and floral parts. Vessels: -These are much elongated tubes with dead cells, lignified walls and a wide central cavity. End walls have simple (one pore) or multiple (many pores) performations. Found in majority of angiosperms, few pteridophytes & gymnosperms. Provide mechanical support to plant and transport water & minerals. Xylem (wood) Parenchyma: -made of small thin/thick walled parenchyma cells having pits. It stores food, sometimes tannin and helps in lateral conduction of water/sap. Xylem (wood) fibers: -These are sclerenchyma fibres associated with xylem. Can be septate/aseptate and may possess simple/bordered pits. These are mainly mechanical in function. The first formed xylem is called 'Protoxylem'. It lies towards center of stem and has annular, spiral and scalariform vessels. The latter formed Xylem is called 'Metaxylem'. It lies away from center and has tracheids with reticulate and pitted vessels. B. Phloem:-Also known as 'Bast'. It is a conducting tissue which conducts prepared food from the leaf to the storage organs or growing regions of the plant. It consists of four types of cells:-Sieve tubes, campanion cells, phloem parenchyma and phloem fibres. Sieve Tubes: -are long tube like structures associated with companion cells. Their end walls have performations (like sieve) and form a sieve plate. Later callose deposits here. They don't have nucleus but possess cytoplasm and a vacuole. Nucleus of companion cell controls its functioning. 'P-proteins'. (P-phloem) are present in the lumen of sieve tube which help in sealing a wound. Sieve cells replace sieve tubes in lower plants & gymnosperms. Companion cells: -These are narrow, elongated and thin walled living cells lying on the sides of sieve tubes. These contain nucleus & cytoplasm. Its nuclei controls the activities of sieve tubes through plasmodesmata. Death of one results in death of another. Both are derived from same meristematic cells. These maintain pressure gradient in the sieve tube. Phloem Parenchyma: -These are parenchyma cells associated with phloem. Their wall is made of cellulose, has pits, interconnecting axial parenchyma cells and ray cells. It stores food, resins mucilage, latex etc. Phloem fibres: -(Bast fibres) -these are sclerenchyma fibres in phloem, occurs as sheets/cylinders, have pointed apex, thick cell wall with pits. At maturity, they loose protoplasm and die. Protophloem: -It is the outer part of phloem containing narrow tube elements. Metaphloem: -The inner part of phloem made of broader sieve tube elements. 1.2 THE TISSUE SYSTEM: - Plants also have organized systems to perform functions. These are called 'Tissue System'. Sachs (1875) identified three tissue systems in plants based on morphological characters, development and function. These are: A. Epidermal System. B. Ground (fundamental) system. C. Vascular (store/conducting) system. A. Epidermal System: -It consists of: - * Epidermis: -(epi-upon; derma-skin)-Outermost protective layer of plant organs, usually single layered, made up of compactly arranged living cells without intercellular spaces. Each cell has central vacuole surrounded by thin layer of cytoplasm. Root epidermis is called 'Epiblema'/Piliferous layer'. These have root hairs. Outer wall of epidermis is thick & covered by waxy 'Cuticle'. * Stomata: -Epidermis of all green aerial parts of plant have minute openings called 'Stomata'. They are abundant in leaves (more on lower surface). In aquatic plants, these are more on upper surface. These are sunken in pits in xerophytes. Each stoma is surrounded by two live guards cells which have chloroplast. Their outer walls are thin and inner thick. Sometimes, guard cells are surrounded by less modified epidermal cells called 'Subsidiary Cells'. Stomatal aperture, guard cells and subsidiary cells together constitute 'Stomatal Apparatus'. * Epidermal Appendages: -e.g. Trichomes: -these are unicellular/multicellular epidermal outgrowths. Unicellular Trichomes are simple & unbranched. * Root Hair: -These are unicellular structures formed due to elongation of epidermal cells. Found in Root Hair zone. Root hair cells have vacuolated protoplasm, nucleus lies in apical part of hair, wall is thin and pectocellulosic. They are short lived and help the plant in anchoring in the soil. Functions of Epidermal System: - - Provides protection to the plant parts. - Cuticle checks the rate of water loss. - Epidermal hair form insulating layer. - Stomata take part in exchange of gases & transpiration. - Trichomes help in seed/fruit dispersal. B. Ground Tissue System:-It forms the main bulk of plant body, includes all tissues except epidermal and vascular and originates from periblem & plerome.In root and dicot stem, it has following parts: - * Hypodermis: -Lies below epidermis. It is few layered thick madeup of collenchymatous/sclerenchymatous cells. It provides mechanical strength and rigidity. * Cortex: -Lies between Epidermis/hypodermis and endrous. It is made of thin walled parenchyma cells. It is distinct in dicots but not in monocots. It stores food, performs photosynthesis (if chlorenchymatous), helps in retentation of gases (if aerenchymatous) and transfer of absorbed water & minerals to the interior. * Endodermis: -Innermost layer, present between cortex and pericycle. It has tightly packed, barrel shaped cells without intercellular spaces. Waxy material 'Suberin' is deposited on the cells in the form of strips/bands called 'Casparian Strips' or 'Casparian bands'. It acts as check post between cortex and vascular strand. * Pericycle: -It is multilayered & lies between endodermis & vascular bundles. In dicot stem, it encircles vascular bundle and pith like a cylinder. Made of parenchyma & Sclerenchyma cells. Absent in root & stems of aquatic plants. * Pith (Medulla): -It is the central part of ground tissue and is often parenchymatous. It is well-developed in dicot stems & monocot roots. Intercellular spaces may be present or absent Stores food reserves. * Medullary Rays: -Present between adjacent vascular bundles in primary structure of dicot stem. It is made of parenchyma cells and helps in lateral transport. C. Vascular Tissue System:-It is formed by Xylem and phloem and their elements. These are arranged in bundles called 'Vascular Bundles'. In dicots, vascular bundles are open type i.e. Infrafascicular cambium is present between xylem and phloem. In monocots, cambium is absent and so vascular bundles are closed type. Types: -Depending upon the arrangement of xylem and phloem, these are of two types-Radial and Conjoint. 1. Radial bundles: -Xylem and phloem alternate with each other. Mainly found in roots. 2. Conjoint bundles: -A vascular bundles, having xylem & phloem, both together. So both are at same radius. Mainly found in stem and leaves. These are of 3 types: - i. Collateral: -Phloem lies outside & xylem inside. It can be open or closed. ii. Bicollateral: -Phloem occurs on outer side as well as inner side of xylem. Always open type & found in members of family Cucurbitaceae. iii. Conjoint/Concentric:-A vascular bundle in which one type of tissue completely surrounds the another. It is of two types: - Amphicribal:-Xylem is in the centre surrounded by phloem. e.g. Fern. Amphivasal: -phloem lies in the center and is surrounded by xylem. E.g. Dracaena. Position of Protoxylem in relation of metaxylem Exarch: -Protoxylem lies towards periphery, Metaxylem towards center. Endarch: -Protoxylem lies towards center and metaxylem towards periphery. Mesarch: -Protoxylem is surrounded by metaxylem. 1.3 Internal Structure of Dicot and Monocot Plants: -  DICOT ROOTS: -e.g. young root of sunflower, pea or Gram. A transverse section of sunflower root shows: - * Epiblema(Piliferous layer) : -Single layered, made up of living parenchymatous cells. A few cells elongate to form root hair. Cuticle & Stomata absent. * Cortex: -Multilayered, present next to epiblema. It is made up of parenchyma cells with intercellular spaces. * Endodermis: -It is single layered, made up of compactly arranged barrel shaped cells with no intercellular spaces. Radial walls of cells have casparian thickenings. Endodermis is usually considered innermost layer of cortex. * Stele: -All tissues inside the epidermis form stele. These include pericycle, vascular bundle and pith. Pericycle: -It is made up of thick walled parenchyma cells. It forms lateral roots or root branched. Vascular bundles: -Radial type, Xylem has both, Protoxylem and Metaxylem. Xylem is exarch. Number of xylem & phloem bundles may be 2-6 (diarch to hexarch). Phloem & xylem bundles are separated from each other by one/more layers of thin walled cells called 'Conjuctive tissues'. Pith: -Often absent. If present, it is quite small. MONOCOT ROOT: -e.g. Maize. T.S. of a monocot root shows: - * Epiblema(Piliferous layer): -Outermost layer made of single row of thin walled cells. No intercellular spaces, no cuticle. Few cells elongate to from root hairs. * Cortex: -Very wide region, made up of thin walled parenchyma cells with intercellular spaces. In older roots, epiblema dies off and few outer layers of cortex become cutinised and form exodermis. * Endodermis: -Single layered consicting of barsel shaped cells with casparian thickenings. Few cells, present opposite to xylem, remain thin walled and are known as 'Passage Cells'. These transfer water and salts from cortex to xylem. * Stele: -It is made up of one or few layers of parenchymatous cells and gives rise to lateral roots. Vascular bundle: -Radial, Polyarch (many xylem & phloem bundles) and Exarch. Conjuctive tissue present. Pith: -Very well developed. Made of parenchymatous cells. It stores food. DICOT STEM: -e.g. Sunflower. T.S of sunflower stem shows following structure: * Epidermis: -Outermost layer, made up of compactly arranged parenchymatous cells. On its outer side, a thin layer of cuticle is present. Trichomes present. * Cortex: -It has 2 parts-Hypodermis & general cortex. Hypodermis: -just below epidermis. It is made up of 3-4 layers of collenchymatous cells. It has thickenings & chloroplast. Provides mechanical strength. General cortex: -Consists of multilayered parenchymatous cells. Intercellular spaces and oil ducts present. * Endodermis: -Innermost layer made up of barrel shaped cells which contain starch grains. So it is also called as 'Starch Sheath'. * Pericycle: -Present between endodermis and vascular cylinder. It occurs as semilunar patches of sclerenchyma. Patch associated with phloem of vascular bundle is called 'Hard bast'. * Vascular bundles: -Many vascular bundles are arranged in the form of a ring. Each vascular bundle is conjoint, collateral, endarch and open. It has xylem, phloem and cambium. Phloem: -It is made up if thin walled polygonal cells and lies on outerside. It has sieve tube elements, campanion cells, phloem parenchyma & fibres. Xylem: -Lies below phloem. It has vessels, tracheids, xylem parenchyma and fibres. Cambuim: -Present between xylem & phloem. It has 2-3 layered thin walled cells. * Medullary Rays: -These are parenchymatous areas between two vascular bundles. * Pith: -Central part consisting of round parenchymatous cells with lots of intercellular spaces. MONOCOT STEM: -e.g. Maize. T.S. of maize shows following structure. * Epidermis: -is the outermost layer of parenchymatous cells. It has a thick cuticle and few stomata. * Hypodermis: -is present below epidermis and is made of sclerenchymatous cells. * Ground Tissues: -It does not show distinction into cortex, endodermis, pericycle, pith and pith rays. Entire mass from hypodermis to the ventre is called Ground tissue and is made up of parenchymatous cells with inter cellular spaces. * Vascular Bundles: -Many in number, oval in shape, conjoint, collateral, endarch and closed. Lie scattered in the ground tissue and each bundle is surrounded by sheath of sclerenchymatous cells called 'Bundle Sheath'. Xylem is in the form of letter 'Y'. Two big oval metaxylem form divergent arms of Y. Protoxylem forms lower arm of Y. Water containing lysigenous cavity is at the end of protoxylem. Phloem has sieve tube elements and companion cells. Phloem parenchyma is absent. DICOT LEAF: -(Dorsiventral leaf) V.S. (Vertical section) of a dicot leaf shows following: - * Upper Epidermis: -is made of single layer of parenchymatous cells. Thick cuticle is present on outer side. Stomata, if present, are very few in number. * Lower Epidermis: -is made of single layer of parenchyma cells covered by thin cuticle. Large number of stomata are present. * Mesophyll: -Entire tissue between upper and lower epidermis is Mesophyll. It is differentiated into Palisade Parenchyma & Spongy Parenchyma. Palisade Parenchyma :-Consists of 1-3 layers of vertically elongated, closely placed cells with little intercellular spaces. They have a vacuole, chloroplasts and perform photosynthesis. Spongy Parenchyma: -have oval, loosely arranged cells with many air spaces/cavities which open out by stomata. Contain chloroplast. Perform photosynthesis and help in diffusion of gases. * Vascular bundles: -Conjoint, collateral and endarch. Each vascular bundles is surrounded by Bundle Sheath(a layer of thick walled, compactly arranged cells). Xylem lies towards lower epidermis. MONOCOT LEAF: -(Isobilateral leaf) . A section of monocot leaf shows following: - * Epidermis: -Single layered epidermis occurs on both the sides of leaf. It consists of compactly arranged parenchymatous cells. Cuticle is present on outer side. Equally distributed stomata are present on both sides. Bulliform cells (empty, colourless cells) are seen in grasses on upper surface. * Mesophyll: -is made up of thin walled, parenchymatous cells with few intercellular spaces. Not differentiated into palisade & spongy parenchyma. Contains chloroplast. * Vascular Bundles: -Conjoint, collateral, endarch and closed. Many vascular bundles are there, each surrounded by bundle sheath. Xylem lies towards upper side and phloem towards lower side. 1.4 SECONDARY GROWTH: - Primary growth involves growth in length. Secondary growth is the formation of secondary tissues to increase the diameter. Secondary Growth is seen in most dicot stems and roots. In DICOT STEM: -The process of secondary growth and structures associated with secondary growth are following: - * Formation of Cambial ring: -In dicot stems, intrafascicular cambium is present between primary xylem and primary phloem. Secondary growth starts by formation of medullary ray. It is formed by division of parenchymatous cells of each medullary ray which become meristematic. Intrafascicular and interfascicular cambium join to form a complete ring of vascular cambium. * Activity of Cambial ring: -Cells of cambium divide continuously. Out of the two cells produced from single cell, one becomes secondary tisues and other remains cambial cells. * Formation of secondary Xylem & Secondary Phloem: -Vascular cambium has 2 types of cells: -Fusiform initials and Ray initials. Fusiform initials divide and ad new cells, both on inner side as well as outerside,. New cells on outer side form Secondary phloem. New cells on inner side form Secondary xylem. So xylem is formed more rapidly than phloem. Gradually xylem crushes primary & Secondary phloem of earlier years. * Formation of Secondary medullary rays: -Ray initials divide and add new cells. New cells remain meristematic for some time & then differentiate into parenchymatous cells of rays. Secondarymedullary rays are formed by these cells in between secondary xylem & secondary phloem. * Formation of Annual Rings: -Activity of cambium is affected by seasonal variations. During Summer & Spring: -Temperature is high, relative humidity is high, duration of sunshine is long and new leaves supply hormones. These factors favour cambial activity, cells divide rapidly and several layers of undifferentiated cells can be seen. So large volume of xylem is produced which has larger, thin walled, light-staining components. It forms wood known as 'Early or Spring wood'. In Winter & Autumn: -Temp. is low, sunshine is for lesser time. So cambial activity slows down. Lesser xylem is produced which has smaller, thick walled, dark-staining components. It forms wood known as 'Late or Autumn Wood'. Spring wood has light colour and Autumn wood has dark colour. One light and one dark coloured zone forms an Annual Ring/Growth Ring. Each annual ring corresponds to the growth of one year. So age of plant can be easily calculated. * Activity of cork Cambuim(Phellogen): -When secondary growth continues for many years, stem becomes thick. This may crack or break the outer tissues. To avoid this, plants develop a new cambium called 'Cork cambium/Phellogen' on the outer side. Phellogen divides on its outer side and produces dead cells which form 'Phellem'. On inner side, phellogen divides and forms living cells which make 'Phelloderm'. Phellem (cork) + phellogen (cork cambuim) + Phelloderm (Secondary cortex) together form 'Periderm'. Bark: -All the tissues outside vascular cambium form Bark. It includes secondary. phloem, elements of primary phloem, primary & secondary cortex in crushed state. * Lenticels: -These are openings in the cork tissue which look like raised spots on surface of stem. Found in woody trees but not in climbers. There is a pore with loose mass of parenchymatous cells below it. This mass is called as 'Complementary Tissue'. These help in exchange of gases. * Heart wood and Sapwood: -After certain years, older xylem becomes non-functional as it gets filled with oils, resins, gums, tannins, phenols etc. It becomes dead, darker in colour with lignified walled cells. This is called 'Heart Wood or Duramen'. The outer young xylem which is lighter in colour and is functional is known as 'Sapwood or Alburnum'. IN DICOT ROOT: -Secondary growth in dicot root is almost similar to dicot stem but the manner of formation of cambium & tissue differentiation differs. * Formation of Vascular Cambuim: - A part of cojuctive-tissue below phloem bundle becomes meristematic and divides to form brick shaped cells of cambium. This extends between xylem & phloem. A part of pericycle above protoxylem also becomes meristematic and forms cambium. Both cambiums join to form a complete ring. * Activity of Vascular cambium: -Cambial ring below phloem cuts off cells on inner side. Because of this overproduction, cambium & primary phloem are pushed outwards and so cambium ring becomes circular in shape. Entire cambium, now forms cells. Cells formed on inner side form secondary xylem. Cells formed on outer side form secondary phloem. Secondary tissues corresponding to each protoxylem bundle modify into parenchyma and form primary Medullary rays. * Formation and activity of cork cambium: -Cells of pericycle become meristematic & form Phellogen. It divides on outer side to form Pheloem. Cells formed on the innerside consitute phelloderm. Lenticels are also present.