Anatomy of Flowering Plants

Anatomy of Flowering Plants INTRODUCTION A. MERISTEMATIC TISSUES (MERISTEMS):  Anatomy is the branch which deals with the study of  gross internal structure of plant organs as observed after section cutting.  Study of this branch started in 1671, when Marcello Malpighi and Nehemiah Grew independently studied the anatomy of vegetable plants. The term meristem has been derived from a Greek word meristos – which means divisible or having cell division activity, so meristem is a group of cells which has power of continuous division. e.g.; meristem at apex of stem, root and vascular cambium, etc.  Nehemiah Grew is known as 'Father of plant anatomy'.  The term meristem was given by C. Nageli (1858) for group of continuously dividing cells.  K.A. Chaudhary is known as 'Father of Indian plant anatomy'.  In multicellular organisms, growth is limited to specific regions. These areas are called meristems.  The cells of meristematic zone are capable of division until death. TISSUE  A group of cells having a common origin and performing a similar function.  The term tissue was coined by Nehemiah Grew. TYPES OF TISSUES The characteristics of meristematic cells are as follows :  Tissues may be classified into two groups : Based on whether the cells being formed are capable of  dividing or not  A. Meristematic tissues B. Permanent tissues  The cell of meristematic tissue are always living and found in vegetative regions of the plant. They have thin walls of cellulose. Cells are normally isodiametric, oval, polygonal or rectangular. Abundant cytoplasm is present. 44 Anatomy of Flowering Plants  Cells are compactly arranged and lack intercellular spaces.  Cells have the capacity to divide.  Vacuoles are either absent or very small.  Large prominent nucleus is present.  They have no reserve food material and further no ER and plastids in them. CLASSIFICATION OF MERISTEMATIC TISSUE : Meristematic tissues may be classified on the basis of : (a) Origin and development (b) Position in the plant body (c) Plane of division (d) Functions (a) Meristems based on origin and development : On the above basis meristems can be divided into following three types : (i) Promeristem ( = primordial meristem )  A group of cells which represent primary stages of meristematic cells.  They are present in a small region at the apices of shoots and roots.  They give rise to primary meristems. (ii) Primary meristem  The meristematic cells that originate from promeristem are primary meristems.  These cells are always in active state of division and give rise to primary permanent tissues.  In most monocots and herbaceous dicots, only primary meristem is present. (iii) Secondary meristem  They are the meristems developed from primary permanent tissue.  They are not present from the very beginning of the formation of an organ but develop at a later stage and give rise to secondary permanent tissues. Examples :– Cambium of roots, interfascicular cambium of stem and cork cambium. Figure - Apical meristem: (a) Root (b) Shoot  Secondary growth occurs due to the activity of these cells. It increases the thickness of the plant parts.  It is generally found in shrubs and trees. 45 (b) Meristems based on position in plant body : On the basis of position, the meristematic tissues can be divided into the following three types : (i) Apical meristem  It is found at the apex of growing points of root and shoot.  It divides continuously and brings about growth in length of shoot and root.  The apical meristem includes promeristem as well as primary meristem. (ii) Intercalary meristem  It is present away from apical meristem.  It is present at the base of internodes e.g.; in grasses and wheat (Gramineae) or at the base of leaves e.g.; in pinus or at the base of nodes e.g.; mint or Mentha (Labiatae).  It is responsible for increase in length. (iii) Lateral meristem  They are located parallel to the long axis of the plant organs.  Their activity results in increase of the diameter of the plant organs. e.g.; Cork cambium and Vascular cambium. Anatomy of Flowering Plants (c) Meristems based on plane of division : It includes three types of meristems : (i) Mass meristem : In this cell division occur in all planes so that an irregular shaped structure is formed e.g. endosperm. (ii) Plate meristem : It consist of parallel layers of cell which divide anticlinally in two planes so that a plate–like structure is formed. This pattern is seen in the development of leaf lamina. (iii) Rib meristem : In this type, cells divide at right angles or anticlinally to the longitudinal axis. It is found in the development of cortex and pith. (d) Meristems based on function : Haberlandt (1980) recognized three categories of apical meristem. It is as follows :– (i) Protoderm : It is the outermost layer of the young growing region which develops the epidermal tissue system. (ii) Procambium : It is composed of narrow, elongated cells that give rise to the vascular tissue system, that is, xylem and phloem. (iii) Ground meristem : It consists of large, thin– walled cells which develop to form ground tissue system, that is hypodermis, cortex and pith. B. PERMANENT TISSUES :  It is formed due to division and differentiation in meristematic tissue.  The cells of this tissue may be living or dead, thin– walled or thick–walled.  The thin walled tissues are generally living whereas the thick–walled tissues may be living or dead. Permanent tissue can be of following types : (a) Simple tissue Different types of meristems on the basis of position in plant body (b)Complex tissue 46 Anatomy of Flowering Plants (a) Simple tissues Functions These are homogeneous in nature and are composed  of structurally and functionally similar cells. These are of three types :  These cells may have the power of division. (i) Parenchyma  These cells help in wound – healing and in formation of adventitious buds and roots.  In aquatic plants parenchyma cells store air and provide buoyancy to plants. (ii) Collenchyma (iii)Sclerenchyma (i) Parenchyma      Parenchyma cells are the centres of respiration, photosynthesis, storage, secretion. etc. Parenchyma is considered as the precursor of all  Parenchyma cells of xylem and phloem help in conduction of water and food materials. other living tissues. It is also the most primitive tissue from phylogenetic point of view. SPECIALIZED PARENCHYMA Parenchymatous cells are living,thin–walled, Chlorenchyma containing distinct nuclei.  When parenchyma cells contain chloroplasts, it is The cell walls are made up of cellulose, known as chlorenchyma. hemicellulose and pectic materials.  Examples – leaf mesophyll tissue, outer cortex of Cells have small or large intercellular spaces. young stem, outer cortex of xerophytic stem etc. Cells are generally isodiametric (but may also be  Its function is to manufacture food material elongated, lobed and polygonal). (photosynthesis).  Photosynthetic parenchyma contains chloroplasts, while the parenchyma present in underground parts contains leucoplasts.  All meristems are made up of parenchyma.  Parenchyma is usually present in soft and fleshy parts of plants. Chlorenchyma Palisade Parenchyma  Chlorenchyma whose cells are columnar, is called palisade.  Example – Tissue below the upper epidermis in a dorsiventral leaf. 47 Anatomy of Flowering Plants Spongy parenchyma  Chlorenchyma having well defined intercellular spaces.  Example – Tissue above the lower epidermis in a dorsiventral leaf. (ii)        Collenchyma Collenchyma word was given by Schlieden (1839). These are living elongated cells with thick walls. The cell wall is made up of cellulose, hemicellulose and pectic materials. The wall thickening is not uniform. The walls are often provided with simple pits. Sometimes chloroplasts are present in collenchyma cells. Collenchyma is found in many herbaceous dicot stems, petioles and younger regions of woody stems. Collenchyma is absent in roots and monocot stems. Palisade and spongy parenchyma Aerenchyma  In hydrophytes, the parenchyma develop air spaces and such parenchyma with air cavities is known as aerenchyma. Collenchyma  Examples – Hydrilla and Eichhornia etc.  It helps hydrophytes to float and provides O2 for Types of collenchyma respiration. On the basis of thickening on cell wall, collenchyma may be of three types :– A. Angular  The deposition is maximum at the angles (where the two cell walls come in contact).  The cells appear polygonal in transverse section.  It is the most common type, e.g.: – Datura, Cucurbita, Solanum, Ficus, Vitis, Polygonum, Morus, Cannabis, etc. B. Lacunar Aerenchyma  Large intercellular spaces occur between the cells.  The deposition occurs on the walls towards the Mucilaginous parenchyma spaces.  It has large vacuoles and mucilage. e.g.  The hollow thickened components are found. e.g.; Succulents. – Salvia, Malva, Althaea etc.  Its function is storage of water. 48 Anatomy of Flowering Plants C. Lamellar  The deposition occurs on tangential walls.  The cells appear plate like or lamellar. It is also called plate collenchyma e.g.;– Rheum, Eupatorium etc. Sclerenchyma Types of sclerenchyma A. Fibres or sclerenchymatous fibres B. Sclereids or stone cells A. Fibres or sclerenchymatous fibres  Cells long, narrow and thick walls, pointed at both ends and lignified.  Cell wall has simple or bordered pits. These are unthickened areas. Functions  Fibres give necessary strength, rigidity, flexibility, and elasticity to the plant body and thus enable it to withstand strains. Different types of collenchyma B. Sclereids A. Angular B. Lamellar C. Tubular or lacunar  These are not much longer than their breadth. Functions  They have also extremely thick wall of lignin with narrow lumen.  It performs both mechanical as well as vital types of functions.  The cells have no definite shape. The sclereids are spherical, oval or cylindrical, highly  Collenchyma provides tensile strength which gives thickened dead cells with very narrow cavities elasticity and support to the growing organs. (lumen). These are commonly found in the fruit walls  Chloroplast containing collenchyma performs of nuts; pulp of fruits like guava, pear and sapota; photosynthetic function. seed coats of legumes and leaves of tea. Sclerenchyma provides mechanical support to (iii) Sclerenchyma (Greek : Scleras = hard) organs.  They are dead cells, and act as purely mechanical.  The cells are long, narrow and pointed at both ends. Functions  The cell walls are lignified and have simple pits.  Sclereids provides mechanical strength to the part of the plant where they are present. They contribute  The cell walls are very thick with the result that the to the firmness and hardness of the part concerned. cell cavity becomes narrow. Develops resistance in plant against unfavourable conditions. 49 (b) Complex tissues   The complex tissues are made up of living and non – living cells which perform different functions.  The complex tissues act as single units.   The complex tissues are also known as vascular tissues. They are of two types : (i) Xylem (ii) Phloem a. Xylem Anatomy of Flowering Plants The function of xylem is to conduct water and mineral salts upwards from the root to the leaf and to give mechanical strength to the plant body. It is a conducting tissue and is composed of four different kinds of elements :– a. Tracheids b. Vessels c. Wood fibres d. Wood parenchyma Tracheids  A single tracheid is highly elongated or tube – like cell with hard, thick and lignified walls and a large cavity.  They are devoid of protoplast and hence dead.  These are only constituents of the xylem of primitive plants.  The secondary wall layers possess various kinds of thickenings in them and may be distinguished as annular (in the form of rings), spiral, reticulate, scalariform or pitted (simple or bordered).  In a transverse section these appear either circular, polygonal or polyhedral in outline.  Tracheids occur alone in the wood of ferns and gymnosperms, whereas in the wood of angiosperms they occur with the vessels.  Tracheids are the only elements which are found in the fossils of seed plants. Functions  These carry out transport of water, harmones and solutes from the root to the stem, leaves and floral parts.  It gives mechanical support to the plant body. Phloem tissues (i) Xylem or wood  Xylem word was given by Nageli (1858).  It is also called as wood because the major part of stem and root in vascular plants is constituted by xylem. Type of wall thickenings in vessels 50 b. Vessels  A vessel is a long, cylindrical, tube–like structure with lignified walls and a wide central cavity.  The cells are dead and without protoplast.   Anatomy of Flowering Plants Xylem parenchyma cells of primary xylem are thin–walled and made up of cellulose, while those found in secondary xylem are thick–walled and made up of lignin. Functions These are arranged in longitudinal series in which the transverse walls (the end plates) are perforated  It serves for food storage. and as such the entire structure looks like a water  It helps in the conduction of water upwards. pipe.  The perforations may be simple (only one pore) or multiple (several pores).  Vessels also have various type of thickenings similar to tracheids.  Vessels are found only in some pteridophytes (e.g. Pteridium, Selaginella) and gymnosperms (e.g. Ephedra, Gnetum). However they are present in almost all angiosperms. Functions   They serve as a more efficient mode of transport of water and minerals as compared to tracheids due to the presence of perforation plates. A,B–Vessel, C,D–Tracheid, E–Fibre, These also give mechanical support to the plant body. F–Wood parenchyma (ii) Phloem or bast c. Xylem fibres (Wood fibres)  Sclerenchymatous cells associated with xylem are  called xylem fibres.  They are long, narrow, thick and lignified cells; usually pointed at both ends.  Xylem fibres are dead cells.  The term phloem was coined by Nageli (1858). Phloem is another type of conducting tissue like xylem which is responsible for conduction of organic substances. Functions  Xylem fibres provide mechanical strength to the xylem and to the plant body as a whole. d. Xylem parenchyma (Wood parenchyma) :  The parenchymatous cells found in xylem are living and isodiametric.  T.S. of phloem showing its elements Xylem parenchyma cells are more common in primary xylem than secondary xylem. 51 Anatomy of Flowering Plants  A sieve plate is called simple if it has only one sieve area. A sieve plate is called compound if it has many sieve areas.  In lower vascular plants and gymnosperms in place of sieve tube elements, sieve cells are present. Functions  The main function of the sieve tubes is the transport of prepared food materials from leaves to the storage organs in the downward direction and then to growing regions in the upward direction. (ii) Companion cells  These are specialised parenchyma cells which are closely associated with the sieve tube elements in their origin, position and function.  These originate from the same meristematic cells that give rise to the sieve tube elements.  The sieve tube elements and companion cells are connected by pit fields present in their longitudinal walls, which is a common wall for both and with the B. L.S. of phloem death of one, the other cell also dies. The phloem is composed of four elements :  The companion cell has dense cytoplasm and prominent nucleus. (i) Sieve tube elements  Its nucleus also controls the metabolic activities of (ii) Companion cells the sieve tube. (iii) Phloem parenchyma (Bast parenchyma)  Companion cells are characteristic feature of (iv) Phloem fibres (Bast fibres) angiosperms. (i) Sieve tube elements  Sieve elements were discovered by Hartig (1837). Function  Sieve tubes are tube–like structures, composed  The companion cells play an important role in the maintenance of a pressure gradient in the sieve of elongated cells, arranged in longitudinal series tubes. and associated with companion cells.     Their walls are thin and made of cellulose. In a mature sieve tube the nucleus is absent but peripheral cytoplasm as well as large vacuole is present. The uniqueness of the sieve tube is that although without nucleus, it is living and the nucleus of the companion cells control its functional activities. The transverse partition walls are perforated by a number of pores, giving the appearance of sieves. They are called the sieve plates. (iii) Phloem parenchyma (Bast parenchyma)  These are living parenchymatous cells which may be cylindrical, sub–spherical or polyhedral in shape.  The cells have dense cytoplasm and nucleus.  The cell–wall is composed of cellulose. Function  The main function of phloem parenchyma is the storage of food material and other substances such as resin, tanin and mucilage. 52 Anatomy of Flowering Plants A. Epidermal tissue system (iv) Phloem fibres (Bast fibres) It comprises the following :  These are much elongated, unbranched (rarely branched) and have pointed, needle–like apices. (a) Epidermis  Their cell wall is quite thick with simple or slightly  The epidermis (epi : upon ; derma : skin) is the outermost layer of the plant body, which is direct bordered pits. contact with external environment.  At maturity these fibres lose their protoplast and  It is made up of elongated, compactly arranged become dead. cells which constitute a continuous layer without any  These occur in groups e.g.; in Linum usitatissimum intercellular spaces. (flax) and Corchorus capsularis (jute).  The cells have a large, central vacuole surrounded by a thin layer of protoplasm. Functions  The phloem fibres provide mechanical support to the phloem.  The phloem fibres are economically very important as they are used in making cords, gunny bags, and coarse cloth. THE TISSUE SYSTEM  The bodies of multicellular organisms are made up of different types of tissues.  Like animals, division of labour is also found in plants.  Thus different types of tissues perform different functions in plants. Three dimensional structure of leaf showing epidermal tissue system  All the tissues are arranged in three systems.  Each system may consist of only one tissue or a  combination of tissues which may be similar or different in structure, but perform a common function and have the same origin.  Types of tissue systems   It was Sachs, a German scientist, who for the first time in 1875 attempted to classify the tissues on  the basis of their position and morphology. According to him, the following three categories of tissue systems can be distinctly identified: A. The epidermal tissue system. B. The ground or fundamental tissue system. C. The vascular/conducting tissue system.  The epidermis may also be multilayered as in the aerial roots of Orchids and leaves of Nerium and Ficus. The outer wall of epidermis is thick and usually covered by a cuticle formed by the deposition of a waxy material secreted in the epidermal cells. The cuticle is thickest in the xerophytic plants. Some epidermal cells of certain monocots (grasses, maize, sugarcane) are comparatively large, vacuolated and thin–walled. These are called bulliform or motor cells. These cells store water and help in closing and opening of leaves due to changes in turgor. The outermost layer of roots is referred to as epiblema or piliferous layer. There are no stomata and cuticle on the epiblema. 53  In some roots a single or a few layers just below the  epiblema are found. It is called exodermis. It corresponds to the hypodermis of the stem. The cells of exodermis are thick–walled.  Anatomy of Flowering Plants In xerophytes the stomata are sunken in rooves due to which rate of transpiration is greatly reduced. Part of T.S. of Agave leaf showing sunken stomata and substomatal cavity In some monocots e.g.; doob grass, maize the guard cells are dumb–bell shaped. Part of Nerium leaf showing thick cuticle and multilayered epidermis (b) Stomata  Stomata are very small openings found in the epidermis of green aerial parts of the plant especially the leaves.  Stomata are not present in roots and non–green parts of the stem.  Pore of each stoma is surrounded by two kidney shaped (= semilunar) cells, called guard cells.  The guard cells are living and contain chloroplasts. The inner walls are thicker than the outer walls.  The guard cells regulate the opening and closing of the stomatal pores.        Stomata with bean-shaped guard cells  Stomata with dumb-bell shaped guard cell In plants outside the guard cell but attached to them are present some specialized jepidermal cells, which are called subsidiary cells or accessory cells. In dicot leaves the somata are found scattered, while in monocot leaves they are found in rows. Usually stomata remain closed at night but remain open during daytime. When the guard cells become turgid, the stomata are open; when the guard cells become flaccid they are closed. Stomata are most abundant in the lower epidermis of a dorsiventral leaf but none or few are present in the upper epidermis. In submerged leaves no stomata are present. In floating leaves they are present only on the upper epidermis. In xerophytic plants they are situated in pits or grooves. 54 Functions  Stomata are the organs through which evaporation of water (transpiration) takes place.    Anatomy of Flowering Plants The cuticle on epidermal cells retards the rate of transpiration. The chloroplast containing epidermal cells manufacture food material. The interchange of gases (O2 and CO2) between the plant and atmosphere also takes place through B. Ground or fundamental tissue system them.  The ground tissue system forms the main bulk of (c) The epidermal appendages the plant body.  In many plants epidermis bears outgrowths, called  It includes all the tissues except epidermis and trichomes or hairs which vary markedly in their vascular bundles. shape, structure and function.  It is partly derived from the periblem and partly  These may be unicellular or multicellular, simple from the plerome. or branched, soft or stiff.  The primary function of this tissue system is storage  They are found on almost all plant parts and may and manufacture of food material. be temporary or permanent features.  This system has different kinds of tissues such as Functions parenchyma, collenchyma and sclerenchyma; of  Trichomes help in checking excess loss of water these parenchyma is most abundant and carries out (reduction of water loss). a variety of functions.  Trichomes help in protection, dispersal of seeds  In monocotyledonous stem (with scattered and fruits. vascular bundles) the ground tissue is not differentiated into cortex, pericycle and pith. (d) Root hair  In dicot stems and dicot and monocot roots (in which vascular bundles are in a ring), the ground tissues constitute the following parts :  The root hairs are formed due to the elongation of (a) Cortex the epidermal cells and are not protuberances or appendages. (b)Pericycle (c) Medulla or Pith  The thin wall is made up of cellulose and pectic materials. (a) Cortex  The root hairs are ephemeral (short lived) It can be divided into hypodermis, general cortex structures. and endodermis. Functions (i) Hypodermis  It play an important role in anchoring the plant body  It is found just below the epidermis. in the soil besides absorbing water and mineral  It is made up of collenchymatous cells in a dicot solution from it. stem and of sclerenchymatous cells in a monocot stem. Functions of Epidermal tissue system  Epidermis is a protective layer and protects the  Hypodermis remains absent in roots. plant from severe environmental factors and  Hypodermis protects the internal tissues and mechanical injury. gives mechanical support to the peripheral region.  The stiff and sharp hairs on the epidermis protect the plant from grazing by animals.  The epidermis of roots bears root hair in the specialized region – the root hair zone. 55 Anatomy of Flowering Plants (ii) General cortex  This part lies between hypodermis and endodermis.  It is made up of parenchymatous cells. Sometimes, the cell contain chloroplasts.  General cortex provides mechanical support and stores food material. (iii) Endodermis  Endodermis is single layered structure which separates cortex from stele.  Endodermis composed of compactly arranged parenchyma cells.  The cells of endodermis are barrel–shaped, without intercellular spaces, living and containing starch. (Hence it is known as starch sheath).  cells cutitin known as casparian bands or casparian strips (as were discovered by Casperi in 1865). Functions  Endodermis may serve as a passage for water from the cortex to the protoxylem of root.  It may act as a water–tight jacket between the xylem and surrounding tissues.  It may act as a storage tissue.  It may serve as protective layer. (b) Pericycle  Pericycle is situated in between endodermis and vascular bundles.  The cells of pericycle are parenchymatous or sclerenchymatous.  In dicot stem it is multi–layered while it is not distinct in monocot stems.  Pericycle is present in most of the roots except in roots of parasitic plants and hydrophytes.  In dicotyledonous stems it occurs as a cylinder which encircles the vascular bundle and the pith. Functions  In angiosperms, pericycle gives rise to lateral roots.  In dicot roots it gives rise to a portion of the vascular cambium and later the whole of cork cambium.  Its other functions are storage, mechanical and secretion. (c) Pith or Medulla  The pith or medulla is the central core of the stem and the root.  It is usually made up of large parenchyma cells with intercellular spaces.  In the dicot stem the pith is large and well– developed.  In the monocot stem, due to scattered distribution of vascular bundles, it is absent.  In the dicot root pith is either small or absent, while in the monocot root a distinct pith is present.  In the dicot stem the pith extends towards the pericycle between the vascular bundles. The extensions are called pith or medullary rays which are made up of parenchyma cells.  Medullary rays are not present in the roots. Functions  The function of the pith is to store various substances such as starch, mucilage, tanin, etc.  Medullary rays transmit water and food material outwards to the peripheral tissues. C. Vascular tissue system  Central column of axis (root and stem) is called stele, which is made of number of vascular bundles, which constitute Vascular tissue system.  The vascular bundle is having xylem, phloem and cambium (if present).  Xylem may be exarch or endarch.  In roots, xylem is exarch or centripetal, i.e.; protoxylem or first formed xylem is towards periphery.  In stem, xylem is endarch or centrifugal, i.e.; protoxylem is towards centre (pith).  If the cambium is present in between the xylem and phloem are said to be open vascular bundle (e.g.; dicots) .  If the camblum is absent in between the xylem and phloem are said to be closed vascular bundle (e.g.; monocots). 56 Anatomy of Flowering Plants Types of vascular bundle According to the arrangement of xylem and phloem vascular bundles are of three types :– (a) Radial (b) Conjoint (c) Concentric (a) Radial  When xylem and phloem are arranged in an alternate manner on different radii, such vascular bundles are called radial.  All the roots of plants contains radial vascular Various types of vascular bundles : bundle.  The development of xylem in these vascular bundle (a) radial (b) conjoint closed (c) conjoint open is centripetal. Thus, these vascular bundles are called exarch. DICOTYLEDONOUS ROOT (b) Conjoint Internal structure of a typical dicotyledon root shows  When xylem and phloem are present on the same following features : radius, this type of vascular bundles are known as 1. Epidermis : It is uniseriate outermost layer. It conjoint. comprising tubular living components. Cuticle and  Conjoint vascular bundles are the characteristic stomata are absent. Unicellular root hairs are formed feature of stem.  Depending upon the mutual relationship of xylem due to elongation of some cells of epidermis. and phloem, these are divided into two types: 2. Cortex : It is made up of parenchymatous cells with (c) Concentric intercellular space.  When xylem surrounds phloem completely or phloem Note : The cells of outer part of cortex are suberized surrounds xylem completely, such vascular bundles in old root. It is called exodermis. are called concentric. Exodermis found in some dicotyledon roots and most  Concentric vascular bundles are always closed. of the monocotyledon roots. 3. Endodermis : Inner most layer of cortex is known as endodermis. Casparain strips are present on radial and tangential wall of endodermis. These strips are made up of suberin. Casparian strips are discovered by Caspari. 57  The cells of endodermis which are situated in front of protoxylem cells lack of casparain strips. These are called passage cells.  The number of passage cells is equivalent to the protoxylem cells and number of rows of root hair 5. equivalent to protoylem cells.  Passage cells provide path to absorbed water from cortex to pericycle. Note : (1) Root hairs are linearly arranged on root apex. (2) Casparian bands and passage cells are well developed in monocot root.   (3) Endodermis acts as a water tight jacket and prevents  radial conduction of water 6. Anatomy of Flowering Plants Note : Adventitious root are also endogenous. Because these are originated from stellar region. Some part of vascular cambium in root is originated from pericycle. Vascular Bundles : Vascular bundles are radial and exarch. Xylem and phloems are separate and equal in number. The numbers of xylem bundles are usually two to four (diarch to tetrarch upto hexarch). But exceptionally, Ficus (Banyan tree) root is polyarch. Parenchyma which is found between xylem and phloem, called Conjunctive tissue. Vascular cambium is developed from it. Pith : In dicot root pith is small or inconspicous. MONOCOTYLEDONOUS ROOT  The internal structure of a typical monocotyledon root is similar to dicotyledon root. But (1) Number of xylem bundles are more than six (Polyarch) in monocotyledon root (exceptionally the number of xylem bundles are two to six in onion). (2) Pith is well developed in monocotyledon root (3) Only lateral roots are originated from pericycle. Fig: T.S. - Dicot root (Primary) 4. Pericycle : It is few thick layered. It is composed of prosenchyma.  Lateral roots are originated from the part of pericycle which is lying opposite to protoxylem. Thus lateral root are endogenous in origin.  A few mature cells of pericycle usually opposite to protoxylem, become meristematic. these cells divide by periclinal divisions and form some layers of cells. these divisions are followed by anticlinal divisions forming a primordium which grows to form a lateral root. Fig: T.S. - Mononcot root 58 Difference between dicot root and monocot root DICOTYLEDONOUS STEM I. Scattered Vascular Bundles : In some of dicotyledon stem, vascular bundles are not arranged in a ring, they are Scattered in the cortex. Example : - Thalictrum, Nymphaea. II. Phloem on innermost radius : Anomalously in some plants, the position of phloem is towards the inner side of xylem. Such type of phloem is called Internal or intraxylary phloem. Because, this phloem lies towards the pith, so it is also known as medullary phloem. Example : Calotropis, Capsicum, Leptadaenia, etc. Anatomy of Flowering Plants III. Medullary vascular Bundle : In some plants vascular bundles are present in pith. These are found in addition to normal ring of vascular bundles. These are called medulllary vascular bundles. Example : Amaranthus, Boerhaavia, Chenopodium, Mirabilis, Achyranthes, Bougainvillea, Raphanus sativus. IV. Cortical Vascular Bundles : Some of the vascular bundles are also present in the cortex of plants except the ordinary ring of vascular bundles. They are known as cortical vascular bundles. Example : Casuarina, Nyctanthes and Lathyrus etc. V. Polystelic condition : Each vascular bundle is surrounded by a separate endodermis and pericycle in some plants. Hence, each vascular bundle is a stele. It is the normal situation in pteriodphytes but in some dicotyledons it is present abnormally. Example : - Primula, Dianthera VI. Exclusively xylem vascular Bundle : Abnormally, some vascular bundles are only formed by xylem except the normal vascular bundles. Phloem is not present in these vascular bundles. Example : - Paonia VII. Exclusively phloem vascular Bundle : Abnormally, some vascular bundles are only formed by phloem except normal vascular bundles in some plants. Xylem is not present in these vascular bundle. Examples : - Cuscuta & Ricinus communis 59 Anatomy of Flowering Plants MONOCOTYLEDONOUS STEM Vascular bundle situated in Ring : Normally vascular bundles are found in monocotyledon stem in scattered form but in the stem of some monocotyledon plants vascular bundles are arranged in ring. Such as Triticum, Secale, Hordeum, Avena, Oryza etc. Members of family Gramineae. SPECIAL POINT 1. Monarch condition  2. Triarch condition  3. Tetracrch condition  4. 5. 6. 7. 8. in Trapa root in Pisum root in Helianthus annus, Cicer arietenum root Waiting meristem concept : This concept was given by Buvat. According to this, there is an inactive centre in the shoot apex which is known as waiting meristem and it acts as reservoir of active initials and on induction it give rise to reproductive apex. Tannin is found in latex of banana. When it comes in contact with air it gets oxidized and becomes reddish brown in colour. Tannin glands are found in Camellia. These glands are schizogenous in origin. Salt glands are found in Tamarix which secretes sodium chloride. Chalk glands are found in plants of plumbaginaceae family which secretes calcium carbonate. 9. Maltilayered (14 to 15 layers) epidermis is found in Peperomia leaves. 10. The most durable wood is Tectona grandis. 11. Tracheids are the chief water transporting elements in gymnosperms. 12. Phloem is embedded into the secondary xylem in some plants. Such phloem is called included phloem or interxylary phloem. This is secondary anomalous structure. Example : Leptadaenia, Salvadora etc. dicot stem. 13. Pericycle is absent in roots and stem of some aquatic plants. 14. In some monocotyledonae roots, pith is sclerenchymatous. Ex. Canna. 15. A nectar secreting glands cell contains granular cytoplasm and a large conspicuous nucleus. DORSIVENTRAL (DICOTYLEDONOUS) LEAF  Cuticle is present on both surfaces but cuticle of upper surface is thicker.  Dorsiventral leaves are mostly hypostomatic i.e. stomata present on lower surface. Note : In amphistomatic dorsiventral leaves stomata are more on lower surface.  Mesophyll of these leaves is divided into two regions - Palisade tissue and spongy tissue. 60  Palisade tissue is found towards upper surface. These  cells have more chloroplasts. Spongy tissues is found towards lower surface and have large intercellular space. ISOBILATERAL (MONOCOTYLEDONOUS) LEAF  The thickness of cuticle on the both surface is equal.  Distribution of stomata on both surface's are equal. Note : Isobilateral leaves are Amphistomatic i.e. stomata present on both sides.  Mesophyll of isobilateral leaves is not differentiated into palisade and spongy tissues. It is completely made up of spongy tissues. Palisade tissues are absent. Anatomy of Flowering Plants Bulliform cells : Large cells are found in the epidermis of psammophytic (desert) grasses which are filled by liquid or empty (mostly) and colourless are called bulliform cells or motor cells. When the bulliform cells in the leaves have absorb water and become turgid the leaf surface is exposed. When they are flaccid due to water stress they make the leaf curl and minimize water loss. Example : Ammophila, Poa, Empectra and Agropyron etc. are Psammophytic grasses VASCULAR BUNDLES OF LEAVES  Similar types of vascular bundles are found in both dorsiventral and isobilateral leaves. Vascular bundles of leaves are conjoint, collateral and closed.  Protoxylem is situated towards the adaxial surface and protophloem towards the abaxial surface in the vascular bundle.  The sizes of the vascular bundle are dependant on the size of vein. The veins vary in thickness in the reticulate venation. Thus different size vascular bundles are present in dicot while in parallel venation similar size vascular bundle are present.  Vascular bundles are surrounded by a bundle sheath. Bundle sheath is chlorenchymatous in C-4 plants.  Epidermis of Nerium (both upper & lower) and Ficus (only upper epidermis) becomes multilayered. This is an adaptation to reduce transpiration. SPECIAL POINTS 1. Xerophytes with isobilateral leaves contain palisade on both sides of leaf. Examples : Eucalyptus & Nerium. 2. Desert grasses contain palisade like spongy tissue 3. Unifacial or cylindrical leaf : In these leaves there are no differentitation of upper surface and lower surface. Example : Onion, Garlic 4. Albascent leaf : Palisade tissue is restricted in half part of leaf so half part appears more green and other half appears less green. Example : Abutilon 61 Anatomy of Flowering Plants Comparison of the T.S. of monocot stem (A) and dicot stem (B) Differences between monocot and dicot root 62 Anatomy of Flowering Plants Comparison of the T.S. of monocot and dicot roots 63 Anatomy of Flowering Plants Differences between anatomy of root and stem Differences in the anatomy of dicot and monocot leaves 64 Anatomy of Flowering Plants Detailed structure of a part of T.S. of mango leaf SECONDARY GROWTH  "Secondary growth is increase in girth or diameter of axis (root and stem) of the plant by formation of secondary tissue by activity of lateral meristem (vascular cambium and cork cambium)."  Some cells of medullary rays (i.e.; between vascular bundles) also become meristematic and this is called interfascicular cambium.  Both these cambia collectively constitute a complete ring of vascular cambium or intrastelar cambium.  The ring of vascular cambium or true cambium cuts off cells both on outer side and innerside.  The cells cut off on outer side are secondary phloem and on inner side are secondary xylem.  Amount of secondary xylem cut off is more than secondary phloem and thus with the formation of secondary tissue, increase in girth or diameter occurs, which is thus called secondary growth.  Cambium cells are rectangular, thin–walled, full of protoplasm and having meristematic activity. The secondary tissue is of two types :– 1. Secondary tissue formed by true cambium or vascular cambium or intrastelar cambium. e.g.; Secondary xylem and secondary phloem. 2. Secondary tissue formed by cork cambium or phellogen or extrastelar cambium. e.g.; Phellem or cork cells and phelloderm (sec. cortex). (A) SECONDARY GROWTH IN DICOT STEM (a) SECONDARY GROWTH BY VASCULAR CAMBIUM : Xylem and phloem rays (vascular rays)   The vascular bundles in dicot stem are conjoint, collateral and open and are arranged in a ring.  The cambium present between xylem and phloem in vascular bundles is called fascicular or  intrafascicular cambium.  At some place the cambium does not form secondary xylem and secondary phloem but form parenchymatous cells instead of xylem and phloem. Thus these cells form continuous strips from secondary xylem to secondary phloem and are called secondary medullary rays. These rays are arranged radially. Primary and secondary medullary rays conduct food, water and minerals from centre to periphery. 65 Anatomy of Flowering Plants Stages of secondary growth in a woody stem ANNUAL RINGS OR GROWTH RINGS   There is a marked difference in activity of cambium with change in season. In spring, the activity of  cambium is more and hence the wood elements are larger in size with wide lumen. Moreover, the amount is more and the secondary xylem or wood formed during spring is called spring wood.  The activity of cambium is less during winter or autumn and the wood elements are smaller in size with narrow lumen. Moreover, it is lesser in amount and the wood formed during winter or autumn is called winter or autumn wood.  Spring wood + Autumn wood of a year constitute annual ring. Annual rings are conspicious in temperate (cold) regions. The age of tree can be determined by counting annual rings in oldest or basal portion of tree trunk. Calculation of age of the tree by counting annual rings is called Dendrochronology. Diagram representing annual rings in an old stem 66 Anatomy of Flowering Plants HEART WOOD AND SAP WOOD One of the annual rings  In perennial woody trees, the central portion of stem is darker in colour. Further it is hard and tough due to deposition of resins, tanins, gums and formation of tyloses. This central hard, tough and darker region constitutes heart wood or duramen.  The conduction function of heart wood stops due to formation of tyloses in vessels and hence heart wood is mechanical in function. The heart wood is generally used for making furniture.  The outer or peripheral portion of the trunk is lighter in colour and soft which performs the function of conduction of water and minerals and it is known as sap wood or alburnum. This wood is used for making pulp wood, tool–handles etc. POROUS AND NON–POROUS WOOD  The wood of dicotyledonous trees in which vessels are present in the xylem is called porous wood. It is also known as the hard wood.  The wood in which vessels are not found is called non–porous wood, as in gymnosperms. It is also known as the soft wood.  The porous wood is of two types. In some trees large spring vessels are arranged more or less in a ring. This type of wood is said to be ring– porous. In others, the vessels have equal diameter and are uniformly distributed throughout the wood. This wood is said to be diffuse porous. T.S. of stem showing heart wood and sap wood Diffuse–porous and ring–porous wood in transection 67 Anatomy of Flowering Plants Differences between heart wood and sap wood (b) SECONDARY GROWTH BY CORK CAMBIUM : These are ballon–like structures which are In many woody plants further increase in girth takes produced due to ingrowth of adjacent xylem place by formation of new tissue in extrastelar parenchyma cells into the lumen of xylem vessels regions. These new tissues are called periderm. through pits. Periderm is made up of three tissues :– Tyloses are always found in heart wood. (i) Phellogen (=cork cambium)  It is a secondary lateral meristem that may arise from permanent living cells of hypodermis or outer cortex.  It is composed of a single layer of meristematic cells.  In transverse section the cells appear almost rectangular and radially flattened.  Its cell divide in a tangential plane, cutting cells towards its inner as well as outer face. (ii) Phellem (= cork)  These cells are formed as a result of tangential and periclinal divisions of phellogen cells towards the outer face.  These cells are compactly arranged and have thin cellulose walls in the beginning.  As they mature there is a gradual loss of living matter and cells get elongated radially, vertically Tyloses L.S. or tangentially. TYLOSES   68 Anatomy of Flowering Plants  The cell walls become thick because of development BARK of fatty substance called suberin. Suberin is  Bark is a loose term and is used to define all the impervious to water. tissues, outside vascular cambium.  In Quercus suber which yields bottle cork, the Bark = Periderm + Cortex + Pericycle + cavities of cork cells are filled with air which makes Primary and secondary phloem. the cork light in weight. It also provides thermal Bark has two parts :– insulating qualities. (i) Outer bark (iii) Phelloderm (=secondary cortex)  Outer bark is dead.  Layers of thin walled cells cut off towards the inner  All the tissue lies outside the cork cambium are side of the phellogen form phelloderm. called outer bark. It is also known as Rhytidome.  The cells of this layer are living and possess (ii)Inner bark cellulose cell wall.  The region in between the cork cambium and  In some species these cell may contain chloroplasts vascular cambium is called inner bark. and starch. This is also called secondary cortex.  Its most part is living.  The main region of inner bark is the secondary phloem or bast.  Thus bark constitute of both type of tissues – living and non–living (dead).  A plant will die if we remove the complete bark of the plant because maximum loss of water occurs from this.  If a ring of bark removed from the base of the plant, within a few days plant will die because phloem is separated due to this activity and plant comes in the state of deficiency of food. Formation of phellogn (A) and phelloderm Outer and inner bark (B) and phellem (C) 69 Anatomy of Flowering Plants LENTICELS  Lenticels are openings formed in the bark through which exchange of gases takes place.  At each lenticel the cork cambium, instead of producing cork cells, forms oval, spherical or irregular parenchymatous cells which are loosely arranged with abundant intercellular spaces between them. This mass is known as the complementary cells.  Lenticels are characteristics of woody stem. Lenticels help in gaseous exchange and transpiration (i.e.; lenticular transpiration) Lenticel T.S. of stem showing secondary growth 70 (B) SECONDARY GROWTH IN DICOT ROOT:  Secondary growth is essential in roots to provide strength to the growing aerial parts of the plants and fulfill the requirement of water and minerals.   Secondary growth is not found in monocot roots.  It occurs due to the activity of vascular cambium  and cork cambium. (a) ACTIVITY OF VASCULAR CAMBIUM :  The parenchymatous cells on the inner side of the  phloem become meristematic and gives rise to a strip of cambium.  The parenchymatous cells lying in between xylem  and phloem bundles also become meristematic.  After this, the portion of the pericycle lying opposite the protoxylem becomes meristematic Anatomy of Flowering Plants and forms a strip of cambium. Thus, a wavy cambium is formed extending over the xylem and down the phloem. It begins to cut off new cells on both sides but more on the inside. As a result of increased formation of new cells on the inner side the cambium and phloem are pushed outwards. The wavy cambium soon becomes circular. The whole of the cambium ring behaves in the same way as in the stem, giving rise to the secondary xylem on the inside and secondary phloem on the outside. The cambium forms distinct radial bands of parenchyma against the protoxylem. These are the primary medullary rays. Different stages in secondary growth of root 71     Anatomy of Flowering Plants Some medullary rays are also formed by the cambium Functions of secondary meristem (Cambium) along the inner edge of the phloem and are called Healing of wounds secondary medullary rays.  Wounds in plants are formed due to external Medullary rays are larger and more prominent in the roots than in the stems.  The amount of secondary phloem is much less than the secondary xylem. The primary phloem gets crushed. (b) ACTIVITY OF CORK CAMBIUM :   The cork cambium may develop either from the pericycle or the phloem.   The cork cambium produces a few brownish layers of cork (phellem) on the outside and the secondary cortex (phelloderm) on the inside.  The cells of the secondary cortex do not contain chloroplasts.  The bark forms only a thin covering. Here and there few lenticels may be developed.  The cortex and endodermis become compressed and soon slough off. injuries. Healing of wounds is important for plant protection otherwise bacteria, fungi or other microbes may cause diseases. Besides this additional evaporation from the wound area may cause damage to the plants. If the wound is superficial, the exposed cells die and dry up. In case of deep wounds, the uninjured living cells below the wound become meristematic and produce a mass of undifferentiated parenchyma cells. This is known as callus. Knots in a stem A. Surface view, B. Callus formation as seen in T.S. Leaf abscission or leaf fall  Leaf abscission is a common phenomenon in most of the woody angiosperms and gymnosperms (in some herbaceous angiosperms and pteridophytes, the leaves die on the plant without their fall or abscission).  The falling or shedding of leaves from plant without causing any injury, is called leaf abscission or leaf fall.  For separation of leaf, abscission zone is formed at the base of petiole. Secondary grwoth in root 72