CRP 201: Introduction to Agricultural Botany II - Lecture Note 4 - PDF

CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS Page 1 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGAN...

CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS Page 1 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS INTRODUCTION Plant anatomy is the study of the tissue and cell structure of plant organs. The term anatomy, as applied to plants, generally deals with structures that are observed under a high-powered light microscope or electron microscope. Plant anatomy also involves the study of internal structure and functional organisation of higher plants. Comparative anatomy, however, deals with the establishment of evolutionary relationships on the basis of structural similarities and differences. Plants consist of several organs, which in their turn are composed of tissues that are in turn made up of specialised cells. The three main types of vegetative organ are the root, stem and leaf. Primary organs and tissues develop initially from the shoot and root apical meristems and from cell divisions in meristems closely adjacent to them. A tissue is a group of cells having a common origin and usually performing a common function. A plant is made up of different kinds of tissues. Tissues are classified into two main groups, namely, meristematic and permanent tissues based on whether the cells being formed are capable of dividing or not. The meristematic tissues are further divided into primary and secondary or lateral tissues. Permanent tissues, on the other hand, are divided into simple and complex tissues. Permanent tissues having all cells similar in structure and function are called simple tissues. Permanent tissues having many different types of cells are called complex tissues MERISTEMATIC TISSUES Meristems are populations of small, isodiametric (having equal dimensions on all sides) cells with embryonic characteristics. The term meristem is derived from a Greek “meristos” which means divisible or having cell division activity. The term 'meristem' was coined by K. Nageli (1858). Vegetative meristems are self-perpetuating. Not only do they produce the tissues that will form the body of the root or stem, but they also continuously regenerate themselves. Undifferentiated cells that retain the capacity for cell division indefinitely are said to be stem cells. Characteristics of meristematic tissues i) Cells are small in size and isodiametric, cubical or polyhedral in shape. ii) Cells are young and immature iii) Cells are arranged compactly without intercellular spaces. iv) Cell wall is thin and formed of cellulose. v) Dense cytoplasm and abundant vi) Vacuoles are either absent or small vi) Proplastids are present. vii) Ergastic substances absent. viii) Prominent big nucleus is present ix) Cells divide continously and show active metabolism Page 1 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS Classification of meristematic tissues Meristematic tissues are classified on the basis of; (a) Origin and development (b) Position in plant body (c) Plane of division and (d) Function (a) Meristem based on origin and development (i) Promeristem (a primordial meristem). They are group of cells which represents the primary stages of meristematic cells. They are present in small regions of the apices of shoots and roots. (ii) Primary meristems: meristematic cells that originate from pro-meristem. They are always in active state of division. They give rise to primary permanent tissue. They are present as only meristems in most monocots and herbaceous dicots. (iii) Secondary meristem: Meristem from primary permanent tissues. They are not present from the very beginning of the formation of organ but develop at a later stage e.g. rrot cambium, interfascicular cambium of stem and cork cambium. It increases the thickness of the plant parts. Activity of these celss gives rise to secondary growth. It is generally found in shrubs and trees. (b) Meristem based on position in plant body (i) Apical meristem: The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems. It divides continuously and brings about growth in length of shoot and root. They include pro-meristems and primary meristems. (ii) Intercalary meristems; The meristem which occurs between mature tissues is known as intercalary meristem. They occur in grasses and regenerate parts removed by the grazing herbivores. (iii) Lateral meristems are located parallel to the long axis of the plant organs. Their activity results in increase in diameter of the lant organs e.g cork cambium and vascular cambium. Apical meristem of root (c) Meristem based on plane of division (i) Mass meristem: Cell division occurs in planes so that an irregular shaped structure is formed e.g endosperm. (ii) Plate meristem: consist of parallel layer of cells which divide anticlinal in two planes so that a plate-like structure is formed. It can be seen in development of leaf lamina. (iii) Rib meristem or file meristem: Cells divide at right angles or anticlinal in one plane. It is found in the development of lateral roots. (d) Meristem based on function (i) Protoderm: the outermost layer of the young growing region which develops the epidermal tissues. Apical meristem of shoot Page 2 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS (ii) Procambium: This is composed of narrow, elongated cells that give rise to the vascular tissue system of xylem and phloem. (iii0 Ground meristem: consists of large, thin-walled cells which develop to form ground tissue system hypodermis, cortex and pith. PERMANENT TISSUES These tissues consist of group of cells that has become structurally and functionally specialised and loose the ability to divide. The cells may living or dead, thin walled or thich walled. Thin walled tissue are generally living while thick walled tissues are generally living. Permanent tissues can be divided into three types; (a) Simple tissues (b) Complex tissues (c) Special tissues Simple tissues Simple tissues are homogenous in nature i.e tissues consists of cells that are the same structurally and functionally. These include: (i) Parenchyma (ii) Collenchyma (iii) Sclerenchyma (i) Features of parenchyma  They are considered as precursor of all living tissues  It is the most primitive tissue from phylogenetic point of view  They are living, thin walled containing distinct nuclei.  Cell walls are made up of hemicellulose, cellulose and pectic materials.  Cell have small or large intercellular spaces Parenchyma  Cells are generally isodiametric but may also be elongated, lobed, and polygonal.  All meristems are made up of parenchyma. Functions of parenchyma  They are centers or respiration, photosynthesis, storage, secretion etc.  The cells may have the power of division.  These cells help in wound healing and formation of adventitious buds and roots.  Parenchymatous cells store water in succulent plants.  In aquatic plants, cells store air and provide buoyancy to plants.  Parenchymatous cells of xylem and phloem help in conduction of water and food materials. Specialised parenchyma 1) Prosenchyma: cells are elongated; found in pericycle of roots and function to provide strength Page 3 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS 2) Chlorenchyma: When parenchyma cells contain chloroplast e.g leaf mesophyll tissue, outer cortex of young stem, outer cortex of xerophytic stem. It function to manufacture food material. 3) Aerenchyma Parenchyma develop air spaces i.e parenchyma with air cavity. It helps hydrophyte to float and provides oxygen for respiration. Hydrilla and Erchornia. 4) Idioblast: Non-living ergastic subsatnces like tannins, oils, crystals e.t.c are found in stored form. 5) Mucilagenous parenchyma: It has large vacuoles and mucilage e.g succulents (ii) Collenchyma Features  They are living elongated cells with thick walls.  Cell wall is made up of cellulose, hemicellulose and pectic materials.  Wall thickening is not uniform.  Walls are often provide with simple piths.  They may contain chloroplasts.  Found in many herbaceous dicot stems, petiole and younger regions of woody stem.  Collenchyma is absent in roots and monocot stems. Collenchyma Collenchyma provides tensile strength which gives elasticity and support to the growing organs. Chloroplast containing collenchyma performs photosynthesis A. Parenchyma cells, live at maturity, with cellulosic primary cell wall. Note nucleus. B. Collenchyma cells (cross-sectional view) live at maturity, with unevenly thickened, pectic-rich primary cell walls. Types of collenchyma Based on the thickenings of cell walls, collenchyma are of three types: Page 4 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS A. Angular: Deposition is maximum at the angles where two cells come in contact. Cells appear polygonal in transverse section It is the most common type e.g Tagetes, Lycopersicum B. Lacunar Large intercellular space occur between the cells. Deposition occurs on the walls towards the spaces. Hollow thickened components are found. E.g curcbita. C. Lamellar Deposition occurs in tangential walls. Cells appear plate-like or lamella. (iii) Sclerenchyma Greek: Scleras: ‘Hard’ Features They are dead cells and act as purely mechanical Cells are long, narrow and pointed at both ends Cell walls are lignified and have simple pits Cell walls are very thick. Type: A. Fibres or sclerenchymatous fibres B. Sclereids or stone cells A. Fibres  Cells are long, narrow and thick walled.  They are pointed at both ends and lignified. Fibre  Cell wall has simple or bordered pith  Length of fibres is up to 3 mm but in some cases like jute ICorchorus capsularis), flax (Linum), and hemp (Cannabis), fibres are up to 20-550 mm in length. Fibres are of three types: Cortical fibres in cortex Pericyclic fibres in pericycle. Also called perivascular fibers Phloem fibres in phloem. Also called bast fibres. B. Sclereids  These are not much longer than their breadth  They have extremely thick wall of lignin with narrow lumen. Sclereid  Cells have no deficite shape s Page 5 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS  They are commonly found in fruit walls of nuts, pulp of fruit like guava. pear, seed coat of legumes and leaves of Tea. COMPLEX TISSUES Complex tissues are heterogeneous tissue having more than one kind of cells but acting as one unit performing one main function. They are made up of both living and non-living function. The main complex tissues are the xylem and the phloem. These are otherwise referred to as conducting tissues. A. Xylem or wood; Functions of xylem  As a conducting tissue for water and minerals from roots to the stem and leaves.  It also provides mechanical strength to the plant parts. It is composed of four different kinds of elements, namely; (a) Tracheids, (b) Vessels, (c) Xylem fibres and (d) Xylem parenchyma. Tracheids are elongated, spindle-shaped cells that are arranged in overlapping vertical files. Tracheids are elongated or tube like cells with thick and lignified walls and tapering ends. These are dead and are without protoplasm. Secondary wall layers possesses various kinds of thickenings. The inner layers of the cell walls have thickenings which vary in form. Tracheids transport water, hormones, and solutes from the roots to the stem, leaves and floral part. It gives mechanical support. Tracheids occur alone in the wood of ferns and angiosperms but occur together with vessls in angiosperms. Vessel elements tend to be shorter and wider than tracheids and have perforated end walls that form a perforation plate at each end of the cell. Like tracheids, vessel elements have pits on their lateral Page 6 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS walls. Unlike tracheids, the perforated end walls allow vessel members to be stacked end to end to form a larger conduit called a vessel. Vessels vary in length both within and between species. Maximum vessel lengths range from 10 cm to many meters. Because of their open end walls, vessels provide a very efficient low-resistance pathway for water movement. The vessel members found at the extreme ends of a vessel lack perforations at the end walls and communicate with neighbouring vessels via pit pairs. Functions: They serve as most efficient mode of transport of water and minerals as compared to tracheids due to the presence of perforation plates. They also give mechanical support to the plant body. Xylem fibres: These are sclerenchymatous cells that are associated with xylem. They are long, narrow thick and lignified cells usually pointed at both ends. Xylem fibres are dead cells. They also give mechanical support to plant body. Xylem parenchyma: They are thin walled parenchymatous living cells filled with vacuolated and nucleolated cytoplasm. Parenchymatous living cells found in xylem are living and isodiametric. The cell wall is thick and made up of cellulose but non lignified and flexible cells are rectangular in shape. Functions: Xylem parenchyma serves as food storage. They also help in conduction of water upwards. B. Phloem or bast The phloem is a complex tissue that translocates the products of photosynthesis from mature leaves to areas of growth and storage, including the roots. As we will see, the phloem also redistributes water and various compounds throughout the plant body. They are composed of (a) Sieve cells and sieve tubes, (b) Companion cells, (c) Phloem fibres and (d) Phloem parenchyma. Except phloem fibres all are living cells Sieve tubes: They are tube-like in nature. They are composed of elongated cells arranged in longitudinal series and associated with companion cells. Their walls are thin and made of cellulose. In mature sieve tubes, the nucleus is absent but peripheral cytoplasm as well as large vacuole is present. Nucleus of companion cells controls the functional activities of sieve tube element. They have transverse partition walls that are perforated by a number of pores called sieve plates. Sieve plates are of two types: Simple, if it has sieve area and compound if it has one sieve area. At the end of growing seasons sieve plates is covered by callose which bets dissolved during spring. Sieve elements (sieve cells and sieve tube elements) have characteristic sieve areas in their cell walls, where pores interconnect the conducting cells. The sieve area pores range in diameter from less than 1 µm to approximately 15 µm. Unlike sieve areas of gymnosperms, the sieve areas of angiosperms can differentiate into sieve plates. Sieve plates have larger pores than the other sieve areas in the cell and are generally found on the end walls of sieve tube elements, where the individual cells are joined together to form a longitudinal series called a sieve tube. Furthermore, the sieve plate pores of sieve tube elements are open channels that allow transport between cells Companion cells: Specialized parenchyma cells which area closely associated with sieve tube elements in their origin, position and function. Three types exist: the ordinary companion cells; Transfer companion cells, and Intermediary companion cells. Companion cells originate from the same meristematic cells that give rise to the sieve tube. They have dense cytoplasm and prominent nucleus. Its nucleus also controls he metabolic activities of the sieve-tube elements. Page 7 of 18  CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS Schematic drawings of mature sieve elements (sieve tube elements). (A) External view, showing sieve plates and lateral sieve areas. (B) Longitudinal section, showing two sieve tube elements joined together to form a sieve tube. Phloem parenchyma: Phloem parenchyma is made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus. The cell wall is composed of cellulose and has pits through which plasmodesmata connections exist between the cells. The phloem parenchyma stores food material and other substances like resins, latex and mucilage. Phloem parenchyma is absent in most of the monocotyledons. Phloem fibres (bast fibres) are made up of sclerenchymatous cells. These are generally absent in the primary phloem but are found in the secondary phloem. These are much elongated, unbranched and have pointed, needle like apices. The cell wall of phloem fibres is quite thick. At maturity, these fibres lose their protoplasm and become dead. Phloem fibres of jute, flax and hemp are used commercially. Page 8 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS SPECIAL TISSUES OR SECRETORY TISSUES These are tissues that performs special functions in plant e,g resins, gum, oil and latex. They are of two types: The glandular tissue and the laticiferous tissues Glandular tissue Glandular tissue consist of glands (s specialised group of cells capable of secreting some substances. These glands are of two types: External glands and Internal glands External glands generally occurs on the epidermis of stem and leaves as glandular growth e.g glandular hair, nectar secreting and enzyme secreting glands. Internal glands are found inside the plants. They are spherical or tubular. Different types of internal glands include; Oil glands: they are formed due to the dissolution of glandular cells e.g oil glands in leaves and rind of the citrus fruits. Mucilagenous glands: Occurs in lysigenous cavity e.g leaves of Betel vine Resin/Tanin/Gum gland: Glandular cells have schizogenous cavity filled with resin, tannin or gum e.g resin gland in leaves and stem of Pinus, Gum gland. Water secretory glands: Hydathodes; Excretes water in the form of drops found in leaves of some herbaceous angiosperms that generally grow in humid places. Hydathodes are present at the tips of leaves of some plant e.g Colocasia. Exudation of water through water stomata is called guttation. Hydathode consists of vein ends, epithem tissue, chamber and pore (Permanently opened stomata without opening and closing mechanism).Loosely arranged colourless parenchyma in the hydathode is - epithem tissue. Laticiferous tissue Laticiferous tissues are composed of thin walled, elongated, branched and multinucleate tube-like structure. They contain colourless, milky or yellow coloured juice called latex. They are scattered throughout ground tissues of plants and contain stored organic matter in the form of starch, rubber, tannins, alkaloids, enzymes, proteins. Laticeferous tissues are of to types: Latex cells- non-articulated fibres and latex vessels (articulated laticifers). Latex cells differ from latex vessles in that they are not formed due to cell fussins and with other latex cells to form a network. They are branched or unbranched. Latex vessels are composed of large number of cells placed end to end. They are unbranched in the beginning but get branched later. THE TISSUE SYSTEM We were discussing types of tissues based on the types of cells present. Let us now consider how tissues vary depending on their location in the plant body. Their structure and function would also be dependent on location. On the basis of their structure and location, there are three types of tissue systems. These are the: (a) epidermal tissue system, (b) the ground or fundamental tissue system and (c) the vascular or conducting tissue system. Page 9 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS Epidermal tissue system. The epidermal tissue system forms the outer-most covering of the whole plant body and comprises epidermal cells, stomata and the epidermal appendages – the trichomes and hairs. Epidermal cells The epidermis is the outermost layer of the primary plant body. It is made up of elongated, compactly arranged cells, which form a continuous layer. Epidermis is usually single-layered. Epidermal cells are parenchymatous with a small amount of cytoplasm lining the cell wall and a large vacuole. The outside of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water. Cuticle is absent in roots. Stomata are structures present in the epidermis of leaves. Stomata regulate the process of transpiration and gaseous exchange. Each stoma is composed of two bean-shaped cells known as guard cells which enclose stomatal pore. In grasses, the guard cells are dumb-bell shaped. The outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened. The guard cells possess chloroplasts and regulate the opening and closing of stomata. Sometimes, a few epidermal cells, in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells. The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus. Epidermal appendages bears trichmes in many plants. Trichomes help in checking excess loss of water. It also helps in protection and dispersal of seeds and fruits Root hairs formed due to elongation of epidermal cells and are not protruberances. (a) (b) Diagrammatic representation: (a) stomata with bean-shaped guard cells (b) stomata with dumb-bell shaped guard cell together to form a sieve tube. Page 10 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS Ground Tissue System All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. Parenchymatous cells are usually present in cortex, pericycle, pith and medullary rays, in the primary stems and roots. In leaves, the ground tissue consists of thin-walled chloroplast containing cells and is called mesophyll. In monocots, ground tissues are not differentiated into cortex, pericycle, pith but are differentiated into cortex, pericycle, and pith in dicot. Vascular Tissue System The vascular system consists of complex tissues, the phloem and the xylem. The xylem and phloem together constitute vascular bundles. In dicotyledonous stems, cambium is present between phloem and xylem. Such vascular bundles because of the presence of cambium possess the ability to form secondary xylem and phloem tissues, and hence are called open vascular bundles. In the monocotyledons, the vascular bundles have no cambium present in them. Hence, since they do not form secondary tissues they are referred to as closed. When xylem and phloem within a vascular bundle are arranged in an alternate manner along the different radii, the arrangement is called radial such as in roots. In conjoint type of vascular bundles, the xylem and phloem are jointly situated along the same radius of vascular bundles. Such vascular bundles are common in stems and leaves. The conjoint vascular bundles usually have the phloem located only on the outer side of xylem. Various types of vascular bundles : (a) radial (b) conjoint closed (c) conjoint open Page 11 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS ROOT ANATOMY The development of the root system in both monocots and dicots depends on the activity of the root apical meristem and the production of lateral root meristems. The figure shows a generalized diagram of the apical region of a plant root and identifies the three zones of activity: meristematic, elongation, and maturation. In the meristematic zone, cells divide both in the direction of the root base to form cells that will differentiate into the tissues of the functional root and in the direction of the root apex to form the root cap. The root cap protects the delicate meristematic cells as the root moves through the soil. It also secretes a gelatinous material called mucigel, which commonly surrounds the root tip. The precise function of the mucigel is uncertain, but it has been suggested that it lubricates the penetration of the root through the soil, protects the root apex from desiccation, promotes the transfer of nutrients to the root, or affects the interaction between roots and soil microorganisms (Russell 1977). Cell division at the root apex proper is relatively slow; thus this region is called the quiescent center. After a few generations of slow cell divisions, root cells displaced from the apex by about 0.1 mm begin to divide more rapidly. Cell division again tapers off at about 0.4 mm from the apex, and the cells expand equally in all directions. The elongation zone begins 0.7 to 1.5 mm from the apex. In this zone, cells elongate rapidly and undergo a final round of divisions to produce a central ringof cells called the endodermis. Longitudinal section. through plant root The walls of this endodermal cell layer become thickened, and suberin deposited on the radial walls forms the Casparian strip, a hydrophobic structure that prevents the apoplastic movement of water or solutes across the root. The endodermis divides the root into two regions: the cortex toward the outside and the stele toward the inside. The stele contains the vascular elements of the root: the phloem, which transports metabolites from the shoot to the root, and the xylem, which transports water and solutes to the shoot. Phloem develops more rapidly than xylem, attesting to the fact that phloem function is critical near the root apex.. Comparisons of transverse section through Monocot and Dicot roots Dicotyledonous Root The outermost layer is epiblema. Many of the cells of epiblema protrude in the form of unicellular root hairs. The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces. The innermost layer of the cortex is called endodermis. It comprises a single layer of barrel- shaped cells without any intercellular spaces. The tangential as well as radial walls of the endodermal cells have a deposition of water-impermeable, waxy material suberin in the form of casparian strips. Page 12 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS Next to endodermis lies a few layers of thick-walled parenchyomatous cells referred to as pericycle. Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells. The pith is small or inconspicuous. The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue. There are usually two to four xylem and phloem patches. Later, a cambium ring develops between the xylem and phloem. All tissues on the innerside of the endodermis such as pericycle, vascular bundles and pith constitute the stele. Transverse section. through (a) Dicot root (Primary) (b) Monocot root Monocotyledonous Root The anatomy of the monocot root is similar to the dicot root in many respects. It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith. As compared to the dicot root which have fewer xylem bundles, there are usually more than six (polyarch) xylem bundles in the monocot root. Pith is large and well developed. Monocotyledonous roots do not undergo any secondary growth. STEM ANATOMY Dicotyledonous Stem The transverse section of a typical young dicotyledonous stem shows that theepidermis is the outermost protective layer of the stem. Covered with a thin layer of cuticle, it may bear trichomes and a few stomata. The cells arranged in multiple layers between epidermis and pericycle constitute the cortex. It consists of three sub-zones. The outer hypodermis, consists of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem. Cortical layers below hypodermis consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces. The innermost layer of the cortex is called the endodermis. The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath. Pericycle is present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma. In between the vascular bundles there are a few layers of radially placed parenchymatous cells, which constitute medullary rays. A large number of vascular bundles are arranged in a ring ; the ‘ring’ arrangement of vascular bundles is a characteristic Page 13 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS of dicot stem. Each vascular bundle is conjoint, open, and with endarch protoxylem. A large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith Transverse section. through (a) Dicot stem (b) Monocot stem Monocotyledonous Stem The monocot stem has a sclerenchymatous hypodermis, a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue. Vascular bundles are conjoint and closed. Peripheral vascular bundles are generally smaller than the centrally located ones. The phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles. Page 14 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS LEAF ANATOMY Dorsiventral (Dicotyledonous) Leaf The vertical section of a dorsiventral leaf through the lamina shows three main parts, namely, epidermis, mesophyll and vascular system. The epidermis which covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf has a conspicuous cuticle. The abaxial epidermis generally bears more stomata than the adaxial epidermis. The latter may even lack stomata. The tissue between the upper and the lower epidermis is called the mesophyll. Mesophyll, which possesses chloroplasts and carry out photosynthesis, is made up of parenchyma. It has two types of cells – the palisade parenchyma and the spongy parenchyma. The adaxially placed palisade parenchyma is made up of elongated cells, which are arranged vertically and parallel to each other. The oval or round and loosely arranged spongy parenchyma is situated below the palisade cells and extends to the lower epidermis. There are numerous large spaces and air cavities between these cells. Vascular system includes vascular bundles, which can be seen in the veins and the midrib. The size of the vascular bundles are dependent on the size of the veins. The veins vary in thickness in the reticulate venation of the dicot leaves. The vascular bundles are surrounded by a layer of thick walled bundle sheath cells. Look at the figure (a) and find the position of xylem in the vascular bundle. Londitudinal section through plant leaf Isobilateral (Monocotyledonous) Leaf The anatomy of isobilateral leaf is similar to that of the dorsiventral leaf in many ways. It shows the following characteristic differences. In an isobilateral leaf, the stomata are present on both the surfaces of the epidermis; and the mesophyll is not differentiated into palisade and spongy Page 15 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS parenchyma. In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells. These are called bulliform cells. When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss. The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins) as seen in vertical sections of the leaves. Transverse section through leaf : (a) Dicot (b) Monocot Page 16 of 18 CRP 201: INTRODUCTION TO AGRICULTURAL BOTANY II LECTURE NOTE 4 COMPARATIVE ANATOMY OF MAJOR PLANT ORGANS ROOT AND STEM ANATOMY COMPARED THE END OF CRP 201 LECTURE NOTE 4. Page 17 of 18