PLANT ANATOMY. Chapter-1. Anatomy (Gr. ana-to split, tome-cutting): Tissues. Meristematic Tissues (Gr. Meristos : Divide)

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1 Chapter-1 PLANT ANATOMY Anatomy (Gr. ana-to split, tome-cutting): Tissues It deals with internal organisation of plants. There is no difference between term anatomy and histology in plants. Father of plant anatomy : N. Grew Father of modern plant anatomy : Nageli Term tissue was used by N. Grew. The tissues are a group of cells which are similar in origin, structure and function. The complex tissues are, however, composed of different kinds of cells which get aggregate to perform a common function. The tissues are broadly classified into two categories : meristematic and permanent tissues. The former have the ability to divide whereas the latter have lost the ability to divide. Meristematic Tissues (Gr. Meristos : Divide) The term meristem was introduced by Nageli. The mass of undifferentiated cells constituting the shoot and the root apex and capable of dividing and contributing to the growth of the plant as a whole are known as meristems. Such cells which are capable of active cell division are called meristematic cells. They are mostly localized in the apices of the shoot and the root throughout the life of a plant. [1]

2 Characteristics of Meristematic Cells :- (i) Cells are isodiametic, thin walled, non-green and non-vascular. (ii) Absence of Intercellular spaces. (iii) Exhibite power of division. (iv) Possess colourless protoplastids instead of chloroplast. (v) Central vacuole or vacuoles are absent exception cambial cells. (vi) No reserve food material and ergastic substances. (vii) Cell wall (primary) is cellulosic but secondary wall is absent. (viii)cytoplasm is dense having large nucleus. (ix) Nucleocyto plasmic ratio is very high and cells. (x) Meristematic cells are pathogen free (viral free), so these are ideal explants for plant tissue culture. I. MERISTEM ON THE BASIS OF ORIGIN Promeristem : It is also known as embryonic meristem, or primordial meristem. These meristematic cells are the first to arise in the embryonic stage itself. They are therefore, the originator of the primary meristem. They are situated at the tip of the radical and the plumule. Primary meristems : They include the apical meristems, intercalary meristems and the intrafascicular cambium. They give rise to primary permanent tissues for growth, on diffrentiation. Secondary meristems : They arise from permanent tissues as a result of dedifferentiation. They form permanent tissues for secondary growth. They include the interfascicular cambium, vascular cambium in roots, cork-cambium, wound cambium, accessory cambia for secondary growth for monocots. II. MERISTEM ON THE BASIS OF POSITION They are of the following three types : Apical meristem : It is situated in the shoot apex and the root apex. It includes both the promeristem and the primary meristem. Apical meristems give rise to primary permanent tissues which bring about the elongation of the stem and the root. Root apical meristem occupies the tip of a root while the shoot apical meristem occupies the distant most region of the stem axis. During the formation of leaves and elongation of stem, some cells left behind from shoot apical meristem, constitute the axillary bud. Such buds are present in the axils of leaves and are capable of forming a branch or a flower. Fig. Diagrammatic representation of the apical, intercalary and lateral meristems. [2]

3 (A) (B) Fig. Apical meristem : (A) Root apex; (B) Shoot apex. Intercalary meristem : The meristem which occurs between mature tissues is known as intercalary meristem. These are responsible for primary growth and make fallen stems errect in cereals. These are commonly found in above the nodes (e.g., grasses, bamboo), below the nodes (e.g., mint), leaf sheath and stems of all monocots. Once growth (= elongation) in leaves is also due to intercalary meristem. Lateral meristem (Radial Meristem): The meristem that occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis is called lateral meristem. They divide periclinally in tangential plane to form secondary permanent tissues. These are responsible to increase the girth or thickness of the plant body. These are found in vascular cambium, cork cambium (= Phellogen), marginal meristem in same leaves. III. MERISTEM ON THE BASIS OF THE PLANES OF DIVISION Mass meristem : These meristematic cells are capable of dividing in all planes and can, therefore, give rise to structures which have variable shapes like endosperm, cortex, early growth of embryo, formation of sporangia, etc. Plate meristem : These meristematic cells can divide in only two planes i.e., in an anticlinal plane. They produce plate like structures such as a single layered epidermis or multilayered lamina of a leaf eg. Epidermis, Endodermis, Pericycle, roots apex. Rib/File meristem : The cells of this meristem divide at right angles to the length of the stem to form the cortex and pith of the young stem as well as the lateral root. IV. MERISTEM ON THE BASIS OF FUNCTIONS (By Haberlandt, 1914) There are three sub-divisions of the apical meristems on the basis of their functions. Protoderm or Dermatogen : It is the outermost layer and is meant for producing the single layered epidermis e.g., hairs, epidermis, velamen, stomata. Ground meristem or Periblem : It produces hypodermis, cortex and endodermis. Procambium or Plerome : It is the innermost part of the meristem. It gives rise to the stele which comprises primary vascular tissues and ground tissues like pith, medullary rays and the pericycle. [3]

4 THEORIES OF SHOOT APEX ORGANIZATION Several theories have been proposed to explain the structure of the apical meristem. 1. Apical Cell Theory : It was proposed by Nageli (1958). In gametophytes particularly of bryophytes and pteridophytes the growth is due to the activity of apical cell or single pyramidal cell in the shoot apex. Limitation : Not applicable for spermatophytes. 2. Tunica-Corpus Theory : According to tunica-corpus theory of Schmidt (1924) the apical meristem is differentiated into two parts. The outer one or more peripheral layers of small cells is termed the tunica. It produces the epidermis by anticlinal divisions. If tunica is multilayered only the outer layer develops into the epidermis whereas the inner layers give rise to other tissues. The central mass of cells is termed corpus. It is the most widely accepted theory regarding organisation of meristem in shoot apex. Fig. Apical meristem differentiated into Tunica and Corpus. 3. Histogen Theory : It was proposed by Hanstein (1970). According to him the apex consists of three parts: outer dermatogen middle periblem and inner plerome. They give rise to epidermis, ground tissue and vascular tissues respectively. This theory is not accepted in shoot apex because in shoot apex there is no distinction between periblem and plerome. Fig. Apical meristem differentiated into dermatogen, periblem and plerorme. A. Shoot apex; B. Root apex. Haberlandt : Replaced the three terminologies used by Hanstein and coined the terms : protoderm, ground tissue meristem and procambium respectively. [4]

5 THEORIES OF ROOT APEX ORGANIZATION Root apical meristem or root apex is much more simple and less complicated as compared to the shoot apex. Following theories have been given to explain the organization of root apex. 1. Apical cell theory : The root apices of vascular cryptogams such as Dryopteris, Selaginella, etc., show a single apical cell. It gives rise to all the tissues of the root. This observation was the basis for Nageli s theory on the root and shoot apices. This theory is not applicable in phanerogams. 2. Histogen theory : According to this theory, proposed by Hanstein, there are three initiating regions or histogens, viz., dermatogen, periblem and plerome which give rise to epidermis, cortex and vascular cylinder respectively. It is most applicable theory in describing the root apices. 3. Quiescent centre: In some roots, e.g., Zea mays (maize), there is a central region of cells which normally does not divide. This central inactive region was called quiescent centre by F.A.L. Clowes using various techniques he showed that the cells of this region have lesser amounts of RNA and DNA. These cells also have a lower rate of protein synthesis.cells have fewer mitochondria, dictysome, nuclei and nucleolus. Cells of this zone remain inactive and serve as reservoir to meristematics zone. 4. Korper-Kappe theory: It was proposed by Schueep (1917). It is also known as body-cap concept i.e. similar to Tunica-Corpus theory. Korper divides in inverted T-shaped and forms inner mass of cells like corps. Kappe divides T-shaped and form outer convering (cap), similar to Tunica. PERMANENT TISSUES The primary or secondary permanent tissues are derived from the primary or secondary meristematic cells. In course of time they lose the power of division and get differentiated into definite form and shape. They may be living or dead. Permanent tissues are classified into two types-simple and complex. Simple Tissues : Parmanent tissues having all cells similar in structure and function are called are Simple Tissues. Type of Simple Parmanent Tissues : 1. Parenchyma Parenchyma are made up of thin-walled living cells. They are usually isodiametric and appear oval or spherical in shape. Sometimes they become polygonal as a result of pressure of the neighbouring cells. The spherical parenchyma cells have large intercellular spaces. Parenchyma tissue occurs particullarly in all the soft parts of the plant : stems, roots, leaves, flowers, fruits and seeds. The entire mesophyll of the leaf and the cortex and pith of the roots and the stems are parenchymatous in nature. Intercellular spaces Chloroplast Intercellular spaces A B C Fig. : A. Parenchyma, B. Chlorenchyma, C & D. Aerenchyma D [5]

6 Specialized types of parenchyma : Following are the various types of parenchyma. Chlorenchyma or Assimilatory parenchyma : These parenchymatous cells possess abundant chloroplasts. Chlorenchyma is found in leaves and cortex of young stems. Palisade parenchyma : It is a type of chlorenchyma where the cells are elongated. It is mainly found in leaves. Spongy parenchyma : It is also a type of chlorenchyma with cells of irregular shapes and sizes, thus leaving numerous intercellular spaces. It is found in the leaves. Aerenchyma : A parenchyma with well developed intercellular spaces which forms a connected system throughout the entire plant is known as aerenchyma. It is common in hydrophytes. It provide buoyancy to plant. Idioblast : It is a special cell which differs markedly in form, size or contents from other cells in the same tissue; e.g., tannin cells, cells filled with oil, crystals, raphids, etc. Functions :- The following are the major functions of parenchyma :- Parenchyma always retains its meristematic character and is, therefore, may give rise to a potential meristematic tissue. Parenchyma becomes meristematic during wound healing, formation of adventitious roots, grafting, etc. Parenchyma cells play an important role in storage of starch grains, etc. Photosynthesis, respiration, secretion, assimilation, etc., are some of the important processes which occur in parenchymatous cells. Aerenchyma helps hydrophytes to float due to air present in the air spaces formed by them. It also helps in the exchange of vital gases. In succulent xerophytes parenchyma have mucilage. This helps the plant in storing water absorbed by mucilage of the cells. 2. Collenchyma : It is a mechanical tissue that consists of living cells. It retains protoplasm even at maturity. The cells are polygonal, spherical or elongated with pointed or rounded ends. Cell walls thin except at the corners where cellulose and pectin are deposited. The cell wall is generally unevenly thickened. Collenchyma may or may not have intercellular spaces. These may contain chloroplasts and carry out photosynthesis. [6]

7 Thickened corners Protoplasm Vacuole Cell wall A B Fig. A. Collenchyma in T.S., B. Collenchyma in L.S. Collenchyma tissue occurs in the hypodermis of dicot stems and just above and below the vascular bundles of monocot leaves. Types of collenchyma : Following three types of collenchyma are recognised on the basis of thickening or deposition : (1) Angular collenchyma : This is the most common type of collenchyma. The thickening occurs predominantly at the corners or angles of the cells, e.g., stems of Cucurbita, Datura, etc. The intercellular spaces may thus, be altogether absent. (2) Lamellar or plate collenchyma : In this type, thickening is deposited more heavily on the tangential walls than on the radial walls of the cells. The deposition thus, appears stratified or lamellated; e.g., stems of Sambucus, Rhamnus, etc. (3) Lacunar or tubular collenchyma : In these cells thickening is deposited primarily on the walls around the intercellular spaces, e.g., aerial roots of Monstera and stem of Calotropis. Functions : Collenchyma being flexible, provides tensile strength to various plant organs. It has a considerable degree of plasticity and thus functions as supporting tissue for the growing plants. Chloroplast containing collenchymatous cells also perform photosynthesis. 3. Sclerenchyma : Sclerenchyma develops either from the procambium or secondarily from parenchyma due to secondary wall deposition. Sclerenchyma is made up of thick walled dead cells. The thickening is of lignin material. Nucleus and cytoplasm is absent. Lumen is reduced and cells are having pointed ends. [7]

8 A fibre Lumen Thick cell wall A sclereid Lumen Pith Thick cell wall Fig. Sclerenchyma The sclereids are also known as sclerotic cells. Sclereids are short, highly thick-walled hard sclerenchymatous cells. They are strongly lignified. There is so much of thickening in the wall that the lumen gets reduced to the minimum and even disappears. The thickenings appear in the form of lamellae deposited one after the other. A large number of fine branched or unbranched pit canals occur in the cell walls. Sclereids are commonly found in the fruit walls of nuts : Pulp of fruit like guava, pear and sapota; seed coats of legumes and leaves of tea. Phloem, bark, cortex and pith of a variety of plants. On the basis of shape the sclereids are of following types : (i) Brachysclereids (Stone cells or Grit cells). They are oblong in shape. They occur in fruits. (ii) Macrosclereids (Rod cells) : They are long sclereids and appear columnar in shape. They (iii) Osteosclereids (Prop cells) : They are also long and columnar shaped but like bones appear lobed at one or both the ends. (iv) Astrosclereids (Star cells) : They are highly branched irregular star-shaped cells. The branches are of different length. (v) Trachosclereids (Internal hairs) : They are long, hair like branch and commonly found in hydrophytes and aerial roots, e.g., Monstera. Functions : The main function of sclerenchyma is to provide mechanical strength to various plant parts. Fig. Different types of sclereids; A & B Brachysclereids; C & D Macrosclereids; E Osteosclereids; F Astrosclereids. [8]

9 Complex Tissues The complex tissues are made of more than one type of cells and these work together as a unit. 1. Phloem (Bast) : It is a conducting tissue meant for transporting food in both downward and upward directions. The phloem consists of : (i) sieve elements, (ii) companion cells, (iii) phloem parenchyma, and (iv) phloem fibre. 1. Sieve Elements : Sieve elements may be arranged in the form a sieve-tube as in angiosperms or may be alone. They are then called sieve cells. A sieve tube is composed of a vertical row of elongated cells called sieve tube members. They are living and have thin cell walls. Young sievetube members have plenty of protoplasm, nucleus and various organelles. When they matures the protoplast is greatly modified, the organelles are reduced in size and the nucleus disintegrates. A mass of fibres or tubules called slime or P-protein have been observed to be present sieve elements. Mature sieve-tubes have characteristic perforated sieve plates at the end walls. The end walls may be transverse or oblique. The protoplasts of adjacent sieve element are connected with one another by means of cytoplasmic strands called plasmodesmata which pass through the pores of the sieve plate. Fig. Sieve elements and Companion Cell There are two types of sieve plates-simple and compound. In the former sieve pores are scattered all over whereas in the latter the pores occur in clusters. Sieve tube members are believed to be living and functional for only three years and new elements are added every year. A carbohydrate known as callose is deposited around the margins of pores of the sieve-plate during the winter. The deposit is called callus or callose plug. Deposition of P-protein over sieve areas is called slime pluge. Often it plugs the pores completely. In spring, however, it get dissolved with the help of callose enzyme. The callus is permanently deposited in old sieve tubes. Sieve-tubes conduct food materials in longitudinal directions. 2. Companion cells: These are specialised parenchymatous cells and closed associated with sive tube elements. Sieve plates and companion cells are characteristic of angiosperms, the phloem of pteridophytes and gymnosperms have only sieve cells and no companion cells. The companion cells are absent in protophloem usually but absent in Austrabaileya, an angiosperm, exceptionally. The functions of sieve tubes are controlled by the nucleus of companion cells. The sieve tube elements and companion cells are connected by pit fields present between their common longitudinal wall. The companion cells help in maintaining the pressure gradient in the sieve tube. The sieve cells have perforated sieve areas throughout the end walls as well as the lateral walls. [9]

10 3. Phloem parenchyma : It 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 plasmodesmatal connections exist between the cells. Phloem parenchyma stores food material and other substances like resins, latex and mucilage. Phloem parenchyma is absent in most of the monocotyledones. 4. Phloem fibres : (Bast fibres) are made up of sclerenchymatous cells. These are generally absent in the primary phloem but are found in 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. The phloem is usually differentiated into two parts, protophloem and metaphloem. The former has narrow sieve elements and no companion cells. The protophloem degenerates after some time through a phenomenon termed obliteration. The phloem fibres, however, remain on the outer sides of the vascular bundles of many plants as bundle caps. The metaphloem consists of well developed long- lived sieve elements. The fibres are usually absent in the metaphloem of the dicotyledons. The metaphloem becomes inactive and may be partially or completely crushed in plants which produce secondary phloem during secondary growth. II. Xylem (Wood) : Xylem is also a conducting tissue and is concerned with the upward conduction of water and minerals. It is composed of four different kinds of elements, viz., (i) tracheids (iii) xylem parenchyma (ii) vessels, (iv) xylem fibre The first formed xylem is called protoxylem and the later formed xylem is called metaxylem. It consists of tracheids, vessels, xylem fibre and xylem parenchyma. Both tracheids and vessels are thick-walled. E Fig. A. Tracheids, B. Vessels, C and E. Xylem parenchyma, D. Xylem fibres [10]

11 1. Tracheids : These are elongated or tube like cells with thick and lignified walls and tapering ends. These are dead and are without protoplasm. They occur in pteridophytes, gymnosperms and angiosperms. The conduction of water occurs through the lateral walls where they have pit-pairs on their common walls. The inner layers of the cell walls have thickening which very in forms. Fig. Thickenings in the secondary walls of tracheids and vessels. 2. Trachea / Xylem Vessels : It is elongated and tube light structure which is formed from a row of cells placed in to end. The vessels occur in angiosperms. However, vessels are absent in families - Winteraceae, Tetracentraceae and Trochodendraceae. Besides these vessels are also absent in stem and leaves of Yucca and Draceana. There are some non-angiosperms where vessels are present, e.g, species of Selaginella and order Gnetales of gymnosperms (Gnetum, Welwitschia and Ephedra). A vessel is composed of several vessel members. Their end walls are perforated and are termed perforation plates. They help in making continuity between members of a vessel. If there is only one perforation in the plate it is called simple perforation plate and if the plate has several perforations it is called the multiple perforation plate. The secondary wall is deposited with lignin in the form of annular and spiral bands in the protoxylem elements and in a reticulated, scalariform, or pitted manner in the metaxylem (Fig. 1.9). The protoxylem elements are smaller in diameter and the metaxylem elements are large in diameter. Metaxylem has larger proportion of vessels. 3. Xylem parenchyma: These cells are living and thin-walled and their cell walls are made up of cellulose. They store food materials in the form of starch or fat and other substances like tanin. The radial conduction of water takes place by the ray parenchyma cells. 4. Xylem fibres: These have highly thickened walls and obliterated central lumens. These may either be septate or aseptate. [11]

12 In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ. This type of primary xylem is called endarch or centrifugal. However, in roots, the protoxylem lies towards periphery and metaxylem lies towards the centre. Such arrangement of primary xylem is called exarch or centripetal. SPECIAL TYPES OF TISSUES I. Laticiferous Tissue Latex is usually a milky fluid, often watery or brownish. It occurs in some families of angiosperms. It is actually an emulsion with watery background and proteins, gums, resins, alkaloids, etc. This consists of thin walled, greatly elongated and greatly branched ducts, and contains a milky juice known as latex. Laticiferous ducts are of two kinds : latex vessels and latex cells. They contain numerous nuclei which lie embedded in the thin layer of protoplasm lining the cell wall, which is usually thin and made up of cellulose. They are irregularly distributed in the mass of parenchymatous cells, and their function is not clearly understood. They may act as food storage organs or as reservoirs of waste products. They may also act as translocatory tissues. Latex vessels are a result of the fusion of many cells. They are formed from rows of elongated, meristematic cells, the partition walls of which soon dissolve, as in wood vessels. They grow more or less as parallel ducts, and in the mature portion of the plant, they anastomose with one another by the fusion of their branches, forming a network. Latex vessels are found in Papaver (poppy), i.e., opium poppy, garden poppy and prickly poppy of Papaveraceae; Caricaceae; Musaceae and Euphorbiaceae. A. B. Fig. A. Latex vessels; B. Latex cells Latex cells : These are really single celled or independent units. They originate as minute structures and then, with the growth of the plant, elongate and branch, ramifying in all directions through the [12]

13 tissues of the plant, but without fusing together to form a network. They are coenocytic in nature. Latex cells are found in Calotropis (madar) of Asclepiadaceae, Nerium (oleander), Thevetia (yellow oleander), Vinca (periwinkle) of Apocyanaceae; Ficus (e.g., banyan fig., peepal), etc. of Moraceae. II. Glandular Tissue This tissue is made of glands, which are special structures containing some secretory or excretory products. Glands may consist of single, isolated cells or small groups of cells, with or without a central cavity. They are of various kinds and may occur as external glands on the epidermis, or as internal glands lying embedded in other tissues in the interior of the plant body. They are parenchymatous in nature, have large nuclei and contain abundant protoplasm. They contain different substances and have manifold functions. Internal glands are : (i) Oil glands - secreting essential oil, e.g., orange, lemon, Eucalyptus, etc., These glands are lysogenous in nature. (ii) Mucilage secreting glands, e.g., betel leaf. (iii) Glands secreting gum, resin, tanin. In gymnosperm like Pinus, Cedrus, etc., these resin glands are schizogenous in nature. (iv) Water secreting glands or hydathodes or water stomata, i.e., Tropaeolum (garden nasturtium), Colocasia, Pistia and other aroids ad Eichhornia. [13]

14 2. The Tissue Systems The various types of tissues get arranged to form tissue systems. On the basis of structure and location, the tissue systems are of three types : 1. Epidermal Tissue System It includes the outermost protective covering of plant and its parts. Epidermal tissue system consists of epidermis, epidermal appendages (trichomes and hairs) and stomata. The epidermis is usually single layered (mutilayered in Nerium) made up of elongated parenchymatous cells. Intercellular spaces are absent between epidermal cells. Epidermal cells are achlorophyllous. Their outer surface is covered by a waxy layer known as cuticle. In leaves the epidermis is not continuous. It is interrupted by minute pores known as stomata. Stomata helps in gaseous exchange. The stomatal pore is present between two specialized cells known as guard cells. Guard cells are bean shaped in dicots while they are dumbbell shaped in monocots. The guard cell are chlorophyllous, exhibit differential thickening on their wall and regulates the opening and closing of stomata. Fig. Diagrammatic representation : (a) stomata with bean-shaped guard cells (b) stomata with dumb-bell shaped guard cell The epidermal cell gives rise to hairy structures. The epidermal hairs of root are known as root hairs. They helps in absorption of water and are unicellular in nature. The stem hairs are multicellular and protective in nature. They are secretory in nature and are known as trichomes. 2. The Ground Tissue System The ground tissue system includes hypodermis, general cortex and endodermis pericycle and primary medullary rays. Hypodermis occurs in stems and is few layers in thickness. In dicots the hypodermis is collenchymatous while in monocots, it is sclerenchymatous in nature. General cortex is made up of parenchymatous cells which may or may not have intercellular spaces. Endodermis is the innermost layer of cortex. It is well developed in root while in stems it is less developed. It is known as starch sheath. In roots, the endodermis regulates the movement of water. Pericycle is single layered parenchymatous in roots while it is one to many layered sclerenchymatous in stems. In roots pericycle gives rise to lateral branches and part of vascular cambium. Medullary rays : They are parenchymatous in nature. The cells are arranged radially. They connect the cortex with pith and helps in lateral transportation. 3. Vascular Tissue System The Stelar System The concept of stele was given by Van Tiegham and Douliot (1886). Stele is the core of the axis which includes the vascular tissue, the interfascicular portion, the pith and the pericycle. It is classified into following types: [14]

15 Fig. Types of steles. 1. Protostele : Solid column of vascular tissue having no pith. Most primitive one form which the other types have evolved. It is further divided into following types : (a) Haplostele : Smooth core of xylem. e.g., Rhynia, Horneophyton, Selaginella kraussiana, Cooksonia etc. (b) Actinostele : Xylem is in the form of radiating ribs, e.g., Lycopodium serratum, Asteroxylon and Psitotum, etc. (c) Plectostele : Xylem occurs as separate plates usually lying parallel to one another. e.g., Lycopodium clavatum and L. volubile. (d) Mixed protostele : Xylem present in the form of discrete units embedded in phloem. e.g,. Lycopodium cernuum. 2. Siphonostele : Differentiation of pith/medulla in the central region. It is further divided into two types : (a) Ectophloic : Phloem only on outer side of xylem. e.g., Osmunda, Equisetum. (b) Amphipholic : Phloem present on both outer and inner side of xylem. e.g,. Marsillea, Adiantum etc. 3. Solenostele : It is a siphonostele with one leaf gap. e.g., some ferns. 4. Dictyostele : Large and several overlapping leaf gaps. Intervening strands of vascular tissue, each resembling a miniature protostele is called meristele. e.g., Dryopteris, Pteris. 5. Polyclic stele : More than one ring of vascular tissues are present. e.g., Pteridium aquilinum. 6. Eustele : Dissected siphonostele with individual vascular bundles arranged in a ring. e.g,. root and stem of Gymnosperms, roots of Angiosperms and dicot stem. [15]

16 7. Atactostele (without any order) : Vascular bundles remain scattered in the ground tissue. e.g., Monocot stem. Types of Vascular Bundles The vascular bundles are classified into three types on the basis of arrangement of phloem and xylem of a vascular bundle. Fig. Types of Vascular Bundles. (I) Radial; (II)Collateral - A. open, B. closed; (III)Bicollateral; (IV) Concentric - A. xylem central (amphicribal), B. phloem central (amphivasal). 1. Radial : It is the most primitive type of vascular bundle. The phloem and xylem of the vascular bundle lie on different radii. This type of vascular bundle is found in roots. 2. Conjoint : The xylem and phloem of the vascular bundle occur on the same radius. It occurs in stems. It is of two types : (a) Collateral : It has one patch of phloem on the outer side and one patch of xylem on the inner side. If cambium is present in between the phloem and the xylem it is called open (e.g., dicot stems) and if the cambium is absent it is called closed (e.g., monocot stems). (b) Bicollateral : It has an outer and an inner phloem as well as an outer and inner cambium. The xylem occurs in the middle of the vascular bundle (e.g., Cucurbitaceae, Solanaceae and Myrtaceae family). 3. Concentric : It is of two types : (a) Amphicribal or Hadrocentric : The xylem is surrounded by phloem, e.g., Ferns, Staminal bundles. (b) Amphivasal or Leptocentric : The phloem is surrounded by xylem, e.g., Dracaena and Yucca. [16]

17 Dicotyledonous Stem 3. ANATOMY OF DICOTYLEDONOUS & MONOCOTYLEDONOUS PLANTS A young stem of sunflower exhibits typical features of a dicotyledonous plant. Internally it is differentiated into the following parts : 1. Epidermis : It is single layered and consists of thin-walled parenchymatous cells. The cells are flattened and are closely fitted to one another. Epidermis produces a number of multicellular hairs. There is a thick cuticle above the epidermis. 2. Cortex : It is several layered and is differentiated into three regions : an outer hypodermis, middle cortex and an inner endodermis. The hypodermis is composed of 4 or 5 layers of collenchymatous cells. The middle cortex is composed of few layers of oval shaped parenchymatous cells. The endodermis is the innermost layer of the cortex. It is a single layer of barrel-shaped cells. The casparian strips are absent in endodermis of stem. The endodermis in stem is also known as starch sheath because it is rich in starch grains. 3. Pericycle : It is between the cortex and the vascular cylinder. It is several layered in thickness. In sunflower unlike Cucurbita the pericycle is composed of a ring of large patches of sclerenchyma alternating with small masses of parenchyma. The sclerenchymatous patches occur just outside the phloems of vascular bundles and are called the hard bast. 4. Vascular Bundles : The vascular bundles of dicotyledonous stems are always arranged in the form of a ring. They are conjoint, collateral and open. A few layers of small thin-walled rectangular cells of cambium occur in between the phloem and xylem. The structure of the phloem and the xylem is typical. The larger metaxylem is on the outer side and the smaller protoxylem elements point to the inner side i.e. the development of the xylem is centrifugal (endarch). Fig. Outline diagram of T.S. of a young dicot stem. [17]

18 Fig. T.S. of a dicot stem (magnified portion). Monocotyledonous Stem The monocotyledonous stems are characterized by lack of differentiation of the ground tissue into cortex, pericycle and pith and by the scattered arrangement of vascular bundles. The details of its anatomy are as follows: 1. Epidermis : It is a single layer of thin-walled parenchymatous cells. A thick cuticle is present on the outer surface of epidermis. 2. Hypodermis : The hypodermis consists of two or three layers of thick-walled sclerenchyma cells. 3. Ground Tissue : The entire mass of parenchymatous tissue within the hypodermis is called the ground tissue which consists of thin-walled spherical cells. The cells enclose large intercellular spaces. 4. Vascular Bundles : They are scattered all over in the ground tissue. Larger number of the bundles are present in the peripheral part. The vascular bundle are polymorphic outer bundles are smaller in size while inner bundles are larger in size. Every vascular bundle is enclosed within a sclerenchymatous bundle sheath. The sheath is thicker on the outer and inner sides of the vascular bundle. Vascular bundles are collateral and closed. The bundle, therefore, consists of an outer portion of broken phloem, protophloem and an inner portion of metaphloem. The metaphloem consists of sieve tubes and companion cells and few phloem fibres. Phloem parenchyma is completely absent in monocotyledonous stems. The xylem consists of tracheids and four large vessels which are arranged in the form of the letter Y. e.g., Oxalis, Marsilea, grasses, etc. [18]

19 Fig. A. Outline diagram of T.S. of a monocot stem. B. A monocot stem magnified portion. Fig. A vascular bundle of monocot stem. Anatomical similarities and differences between dicotyledonous and monocotyledonous stems Characters Dicotyledonous stems Monocotyledonous stem 1. Hypodermis Collenchymatous Sclerenchymatous 2. Ground Tissue Differentiated into : Undifferentiated (a) Cortex (b) Endodermis (c) Pericycle (d) Pith (e) Medullary rays 3. Vascular Bundles (a) Number not very large (a) Numerous (b) Arranged in a ring (b) Scattered [19]

20 (c) Collateral (conjoint and open) (c) Collateral (conjoint and closed) (d) Oval vascular bundle (d) Wedge shaped vascular bundle (e) Phloem parenchyma is present (e) Phloem parenchyma is usually absent or restricted (f) Xylem vessels arranged (f) Xylem vessels are Y or V in radial rows shaped. (g) Water cavity absent (g) Water cavity is present (h) Bundle sheath is absent (h) Bundle sheath is present Dicotyledonous Roots Internally a dicotyledonous root exhibits the following five zones in a transverse section : 1. Epidermis : It is the outer protective layer of very compact cells. It is single layered. The epidermis of root lacks both the cuticle and the guard cells. Stomata are completely absent. It produces a number of root hairs from its cells. The root hairs are usually contorted within the soil. While old root hairs die after a few days at the posterior end fresh ones are constantly formed at the anterior end. Root hairs are usually absent in aquatic plants. The epidermis of root is also called epiblema or piliferous layer. Fig. T.S. of a young dicot root. 2. Cortex : The cortex is very well developed in the roots. In consists of many layers. The cells of the cortex are parenchymatous with large intercellular spaces. The innermost layer of the cortex is called endodermis. The cells of the endodermis are barrel shaped. They have thickened strips on radial and transverse walls. The thickened strip has a deposit of suberin and is called the casparian strip. It reduces the chances of plasmolysis. There are few thin-walled cells in the endodermal layer. They are called passage cells. They are present opposite the protoxylem elements to permit an easy passage of water. 3. Pericycle : It is one-layered or multilayered and consists of parenchymatous cells. It gives rise to lateral roots as well as cork cambium. It gets lignified in older roots in monocotyledons. The pericycle is absent in some aquatic plants and parasites. [20]

21 4. Vascular bundles : There are two to six radial vascular bundles and the xylem is exarch, i.e., centripetal. Monocotyledonous Roots 1. Epidermis : It is a single layer of thin-walled parenchymatous cells. It has several unicellular hairs. 2. Cortex : It is similar to that of dicot root. 3. Pericycle : It is single layer of thin-walled parenchyma cells. In old roots the cells become lignified. 4. Vascular Bundles : Like the vascular bundles of dicotyledonous roots they are also radial and exarch. The number of vascule bundles, however, is more than six (polyarch). Fig. T.S. of a young monocot root. A comparison between dicotyledonous and monocotydonous root is given below. Differences between Dicot and Monocot roots Characters Dicot Roots Monocot Roots 1. Cambium Develops later and, therefore, Does not develop at all and hence secondary growth takes place there is no secondary growth 2. Pericycle Lateral root formation and part of Only lateral root formation vascular cambium 3. Vascular bundles Radial, xylem exarch, 2-6 in number Radial, xylem exarch, more than 6 4. Pith Reduced or absent Well developed [21]

22 Anatomical differences between stems and roots Characters Stems Roots 1. Hairs Multicellular Unicellular 2. Epidermis Protective Absorptive 3. Cuticle Present Absent 4. Ground Tissue Differentiated in dicot stems only Differentiated in both dicot and monocot roots 5. Endodermis Usually not distinct Always distinct 6. Vascular bundles Conjoint, Numerous Radial, Limited in number 7. Xylem Endarch (centrifugal) Exarch (centripetal) 8. Pith Large Small or absent Anatomy of a Dicot (dorsiventral) Leaf 1. Upper Epidermis : It is single-layered. It is covered with a thick cuticle. 2. Mesophyll : It is differentiated into an upper palisade parenchyma and lower spongy parenchyma. The cells of the palisade parenchyma are cylindrical in shape and are compactly arranged. They have chloroplasts and therefore, are photosynthetic in nature. It also check transpiration. The cells of spongy parenchyma are loosely arranged with large intercellular spaces. They contain chloroplasts and carry on photosynthesis. 3. Vascular Bundle : There is one prominent vascular bundle and several small ones. It has phloem towards the lower (abaxial) side and xylem towards the upper (adaxial) side. The vascular bundle is enclosed by border parenchyma (bundle sheath). The vascular bundle is conjoint collateral closed. 4. Lower Epidermis : It is single layered and has several stomata. Leaves are hypostomatic. Fig. A transverse section of a dorsiventral leaf. [22]

23 Anatomy of Monocot (Isobilateral) Leaf 1. Upper Epidermis : It is composed of large-sized parenchymatous cells. Stomata are also present in this layer. A thick cuticle is present outside the epidermis. Bulliform cells may be present in grasses. 2. Mesophyll : It is composed of round oblong or cylindrical cells. There is no differentiation into palisade and spongy parenchyma. All the cells of the mesophyll are alike. They are loosely arranged and are of the spongy parenchyma type. They have large number of chloroplasts. In sciophytes there may be no spongy parenchyma and the entire mesophyll may consist of only palisade parenchyma. 3. Vascular Bundles : The vascular bundles are of uniform size. Like the dorsiventral leaves they have phloem on the lower side and xylem on the upper side. In maize leaf the xylem consists of two large and two small parenchyma cells. Sclerenchyma patches are usually associated with phloem and xylem. Fig. A transverse section of a monocotyledonous leaf. 4. Lower Epidermis : Like the upper epidermis the cells of the lower epidermis are single-layered and are composed of compactly arranged parenchymatous cells. Stomata are also present in the lower epidermis. Differences between dicot and monocot leaves : Dicot leaf Monocot leaf 1. Dorsiventral. Isobilateral 2. Motor cells or bulliform cells are absent. Present in upper epidermis. 3. Mesophyll is differentiated into palisade and spongy parenchyma. Mesophyll is not differentiated and all cells are alike. 4. Above and below of the vascular bundles, Above and below of the vascular patches of parenchymatous or bundles, patches of sclerenchymatous collenchymatous cells are present. cells are present. [23]

24 4. SECONDARY GROWTH In old dicotyledonous stems and roots two types of meristems are active. While the primary (apical) meristems are active since the beginning and cause the elongation of the stem and the root the secondary (lateral) meristems become active only in the old age and cause increase in the thickness. The lateral meristem comprises the cambium and the cork-cambium. They produce new tissues on the outer and the inner sides and add them to the primary tissues to cause increase in the girth of the stem and the root. The increase in thickness as a result of addition of secondary tissues cut off by the cambium and the cork cambium in the stelar and the extrastelar regions respectively is called secondary growth. Secondary growth in Dicotyledonous Stem The secondary growth in the dicotyledonous stem involves two distinct parts : (a) origin and activity of cambium ring and (b) origin and activity of cork-cambium. I. Origin and Activity of Cambium Ring The cambium is initially limited only to the vascular bundles. Such a cambium is called the fascicular cambium (fascicle = bundle). The parenchymatous cells of the medullary rays lying in line with the fascicular cambium become meristematic and give rise to the interfascicular cambium. The interfascicular and the fascicular cambia join with one another to form a complete cambium ring (Fig. 1.21). All the cells of the cambium ring look uniformly thin-walled and brickshaped in a cross-section. A longitudinal view of the cambium, however, shows the cells to be of two kinds in structure and function. They are known as fusiform initials and ray initials. The fusiform initials are spindle-shaped elongated cells while the ray initials consist of small, isodiametric cells packed in regular groups. The fusiform initials give rise to the secondary tissues viz., vessels, tracheids, fibres, parenchyma, sieve-tube members and companion cells which are all arranged in vertical rows. The ray initials produce the phloem and xylem rays which are arranged in horizontal manner. A fusiform cell of the cambium divides by a tangential wall to form two daughter cells. While the inner daughter cell gets modified into a secondary xylem cell, the outer daughter cell divides again to produce two cells, the outer of which gets modified into secondary phloem cell. The cells of the middle layer remain meristematic and continues the process of addition of secondary xylem and secondary phloem cells on the inner and outer side respectively. Secondary Xylem : The secondary xylem is much larger in amount and constitutes the main bulk of woody stems. It is composed of vertical rows of vessels, tracheids, xylem parenchyma and xylem fibres and radial rows of xylem rays. The vessels are shorter but larger in number in the secondary xylem. Annular and spiral thickenings are absent in the vessels. Pitted vessels are abundant. The secondary xylem elements besides conducting water and dissolved minerals also give mechanical support to the plant. Annual Rings : In sub-tropical and temperate regions where seasonal variations are well pronounced the growth of a dicotyledonous stem is not uniform throughout the year. The cambium is very active during the favourable season, the spring, when it cuts off large-sized secondary xylem elements. This type of secondary xylem is called the spring wood. During the unfavourable season, i.e., late summer or autumn, the cambium is less active and cuts off xylem elements of smaller diameter. This is called the autumn wood. The spring wood and the autumn wood are also termed early wood and late wood. The periodical activity of the cambium, thus results in distinct growth layers or rings of secondary xylem. These are called [24]

25 growth rings. The growth rings of spring wood and autumn wood produced in a year constitute an annual ring. Fig. One of the annual rings If the vessels of the spring wood elements are much larger than those of the autumn wood they are usually arranged in the form of rings. This type of wood is called the ring porous (In temperate region e.g., Quercus), the ring porous wood is specialised and conduct more water as they are formed early during the development when needs for water is great. If all the vessels are of uniform diameter in the wood and there is only a gradual change in the size of the elements the wood is called the diffuse porous (tropical climate e.g., Acer, Betula). [25]

26 . Fig. Stages in the secondary growth of a dicot stem Heartwood and Sapwood : In a very old stem the secondary xylem elements in the inner (central) part turn darker in colour and is called the heartwood or duramen. It consists of dead elements which have a deposit of gums, resins, oils, tannins in their walls and lumens. The heartwood is, therefore, very strong and durable and imparts great amount of mechanical strength to the stem. The cells of the xylem parenchyma and ray parenchyma often produce balloon-like infoldings into the lumen of secondary xylem elements of the heartwood. These infoldings are called tyloses. The tyloses contain resin, starch, etc. They are very large in size and often block the cavities of the xylem elements as a result of which the heartwood is no more capable of conducting water. The heartwood is resistant to attack by micro-organisms and insects. Fig. T.S. of stem showing heart wood and sap wood The sapwood or the alburnum is the light coloured peripheral part of the secondary xylem. It consists of dead tracheids, vessels, and fibres and some living cells. The sapwood elements continue to perform the function of conducting water and solutes, etc. They also give mechanical support to the stem. The gymnospermic wood is called softwood (vessels absent) and the angiospermic wood is called hardwood (vessels present). [26]

27 II. Origin and Activity of the Cork-Cambium The activity of the cambium resulting in addition of a huge mass of secondary tissues in the stelar region causes lot of pressure on the peripheral tissues of the stem. The peripheral tissues including the epidermis, often get ruptured due to the pressure exerted by developing secondary tissue. Some of them get flattened and persist for a long time. The stem develops a new lateral meristem in the peripheral part which cuts off new cells to replace the worn out outer part. It also causes growth in the peripheral part so that it can keep pace with the expanding inner part. Such a secondary meristem is called the cork-cambium or phellogen (phellos = cork ; gen = producing). The place of the origin of the cork-cambium differs in different plants. It can arise in the epidermis, hypodermis or the cortex. One or more layers of cells become meristematic. The meristem is composed of thin-walled living cells. The meristematic cells cut-off new cells on both the inner and the outer sides. The inner cells are modified into the secondary cortex or phelloderm while the outer cells are modified into the cork or phellem. The phellogen, phellem and phelloderm together constitute the periderm. Pit Tyloses Tyloses Fig. Tyloses (A) L.S., (B) T.S. Secondary Cortex : It consists of few layers of thin walled living parenchymatous cells. Sometimes they become thick-walled and have pits. The cells of the secondary cortex often contain chloroplasts and are, therefore, capable of photosynthesising. Cork : The cells of the cork are added on the outer side of the cork-cambium. The cork cells are brown coloured and are large and rectangular. They are arranged in a very compact manner in radial rows. They have no intercellular spaces. They are thick-walled dead cells and have a deposit of suberin. The cork is produced in a continuous manner and is considerably thick in some plants. This is why removal of thick bottle cork from the oak (Quercus suber) plant immediately results in its replacement by the cork-cambium. Bark : The bark is the outer protective portion of the dicotyledonous stem. It constitutes the entire tissues present outside the vascular cambium. The bark includes the cork, the epidermis, hypodermis, a portion of the cortex or even a portion of the secondary phloem. There are two types of bark : the ring-bark and the scaly-bark. If the bark is removed in the form of a complete ring or sheet, it is called the ring-bark, as for example in Betula (birch/bhojpatra). If the bark is removed in the form of small pieces than it is termed the scale-bark. The bark is formed in a ring if the cork-cambium arises in the form of a complete ring. The bark is scaly if the cork cambium arises in strips. [27]

28 Lenticels : The bark often exhibits small raised spots on its external surface. These are the lenticels which are meant for an easy exchange of gases between the inner tissues of the stem and the atmosphere. In a lenticel the cork cambium cuts off on the outer side a mass of cells called complementary cells in place of compact cells of the cork. (A) (B) Fig. (A) Bark and (B) Lenticel Epidermis Complimentary cells Cork cambium Secondary cortex Some dicotyledonous stems like Boerhaavia, Aristolochia, Bougainvillea show anomalous secondary growth. Secondary Growth in Dicotyledonous Roots Dicotyledonous roots like the dicotyledonous stems increase in thickness as a result of addition of secondary tissues cut off by the cambium and the cork-cambium. E Fig. Secondary growth in dicot root. [28]

29 Origin and Activity of the Cambium Ring The first event in secondary growth of a dicolyledonous root is the origin of the cambium. The cells of the conjuctive tissue just inside the phloem and just outside the xylem become meristermatic. In order to make this cambium ring circular the wavy band of cambium becomes active only in those parts which lie on the inner side of the phloem patches. These cambium strips cut off secondary xylem elements only on the inner side as a result of which the primary phloem are gradually pushed on to the outer side. This sort of unequal behaviour of the cambium results in the conversion of its wavy band into a circular ring of cambium. Once the cambium assumes a circular outline it becomes uniformly meristematic and starts cutting off cells both on the inner and the outer sides. Except in the regions of the medullary rays the inner cells get modified into secondary xylem and the outer cells into those of the secondary phloem. Origin and Activity of the Cork-Cambium Addition of secondary tissues in the central part of the root exerts lot of pressure on the peripheral tissues which get ruptured. A new meristem, therefore, known as cork-cambium, develops in the single-layered pericycle to replace the lost peripheral part of the root. The cork-cambium or the phellogen is few layers in thickness and is composed of thin-walled rectangular cells. It cuts off cells on both the outer and the inner sides. The cells on the outer side are modified into brown-coloured suberized cork cells and on the inner side remain thin-walled and parenchymatous. It is termed secondary cortex. The cork and the secondary cortex are also known as phellem and pheloderm respectively. The bark of a root is comparatively thinner. Lenticels are fewer in number. The endodermis, primary phloem and the primary cortex are completely disorganized and lost. Primary Anomalous Structures There are several dicot plants which exhibit abnormal structures right from the seedling stage. Some of them are mentioned here. 1. Bicollateral vascular bundles : Cucurbita stem. Fig. T.S. of Cucurbita stem showing bicollateral vascular bundles. [29]

30 2. Medullary vascular bundles (a) Boerhaavia (b) Mirabilis jalapa Fig. T.S. of stem of Boerhaavia showing medullary vascular bundles Fig. T.S. of stem of Mirabilis Anomalous Secondary Growth 1. Interxylary phloem Leptadaenia stem Salvadora stem and Leptadaenia stem. Fig. T.S. of stem of Salvadora showing inter-xylary phloem. [30]

31 2. Growth rings In several dicot stems the extrastelar cambium rings are formed one after the other resulting in several rings of secondary xylem and secondary phloem. Secondary growth in monocot stems There are only 4 monocots in which secondary growth occurs in the stem as a result of which new amphivasal (leptocentric) vascular bundles are produced by the cambium on the inner side e.g., Draceana, Yucca, Aloe, Agave. Fig. Secondary growth in Monocot stem (Dracaena). CELL INCLUSIONS 1. Calcium oxalate crystals. They are known as raphides. They are needle shaped. They occur either singly or in clusters. They occur in several aroids like Alocasia, Colocasia and Amorphophallus as well as in water hyacinth (Eicchornia crassipes), water lettuce (Pistia), balsam (Impatiens), etc. 2. Cystolith. Cystolith is a crystal of calcium carbonate. They appear as a bunch of grapes suspended from a stalk which is actually an ingrowth of inner cellulosic wall of the upper epidermis Fig. Cystolith in the leaf of Indian rubber plant [31]