Anatomy of Flowering Plants
The branch of Botany dealing with the internal organisation of the plants is called Anatomy. The foundation of Plant Anatomy was laid down by N.Grew. The term tissue was coined by him. He is called the 'Father of Anatomy'.
The Tissues:
A group of similar or dissimilar cells that perform a common function and have a common origin is called a tissue. Tissues are classified into two main groups: Meristematic and Permanant.
(A) Meristematic Tissues:
1. These tissues consist of cells that retain the power of division.
2. The protoplasm within the cell is dense, the vacuole smaller or absent.
3. These cells are isodiametric without intercellular spaces.
4.The nucleus is bigger in size.
5.The cells of the meristimatic tissues have thin cellulosic cell wall.
6. The cells are metabolically highly active with high surface area per unit volume and nucleo- cytoplasmic ratio.
7.Ergastic substances are absent.
8. Colourless protoplastids are present in the cells.
Clasiification of Meristems:
(1) On the basis of origin and Development:
(i) Promeristems: ( primordial meristem) : A group of cells which represent primary stages of meristamatic cells. They are found at the apices of embryonic roots and shoots. They give rise to primary meristems.
(ii) Primary meristems: They originate from promeristems . Found at shoot and root apices, at the apex of leaves and intercalary parts.They give rise to primary permanent tissue.
(iii) Secondary meristems: They are not present from the beginning of the formation of an organ but develop at a later stage.They give rise to secondary permanent tissues. e.g interfascicular cambium , cork, and cambium in dicots.
(2) On the Basis of Position:
(i) Apical Meristem: These cells or tissues are found at the apices of stem and root.Due to continues division the root and stem increase in length. The apical meristem help the plants to grow in length.
(ii) Intercalary meristem: The tissues are intercalated between permanent tissues. These are actually the part of the apical meristem which gets separated from it during the growth of stem and root in length. e.g. Stem of grasses, Equisetum. They are especially responsible for increase in the length of the stems of grasses.
(iii) Lateral Meristem: These meristems are present along the lateral side of stem and roots.They divide in tangential plane , giving rise to the secondary permanent tissues on the inner and outer side and lead to the increase in thickness of girth, of the plant body
e.g. Intrafascicular cambium, interfascicular cambium and cork cambium.
(3)On the basis of Plane of Cell Division:
(i) Mass Meristem: The cells divide anticlinally in all planes, so that mass of cells is formed. e.g. Formation of spores, cortex, pith, endosperm.
(ii) Plate Meristem: The cells divide anticlinally in two planes, so plate like area is increased. e.g. Formation of epidermis and lamina of leaves.
(iii) Rib or File Meristem: The cells divide anticlinally in one plane , so row or colums of cells is formed e.g. Formation of a lateral root.
(4) On the basis of Function :
(i) Protoderm: They are the outermost meristematic cells. They form skin or epidermis of plant and epidermal tissue system.
(ii) Procambium: They are the innermost meristematic cells. They form primary xylem, primary phloem, and cambium.
(iii) Ground meristem: They form ground or fundamental tissue such as hypodermis, cortex, pith, pericycle etc.
Shoot Apex Organisation:
The shoot Apex is present immediately above the youngest leaf primordia. It consists of meristematic cells . Lateral branches of stem and leaves are formed by the activity of shoot apex such as:.
(i) Apical cell theory: It was proposed by Holfmeister and Nageli. According to this theory a single apical cell leads to the development of entire plant body. This theory is applicable to algae, to most of the bryophytes and pteridophytes.
(ii) Histogen theory: It was proposed by Hanstein. According to this theory , shoot apex consist of following histogens:
(a) Dermatogen: Outermost layer. It forms epidermis ( skin) and epidermal tissue system.
(b) Periblem: It gives rise to the tissues between epidermis and stele.
(c) Plerome: Innermost layer. The central mass of cells which gives rise to central stele.
Source:shikshaservices.com
(iii) Tunica Corpus theory : It was proposed by Schmidt (1924) It is based on plane of division of cells . According to this theory, shoot apex consists of two distinct layers as:
(a) Tunica : It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus and divide mostly by anticlinal divisions.
(b) Corpus: It represents central core with larger cells.The cells divide periclinally. Sometimes tunica is multilayered , only outer layer forms epidermis and the remaining layers with corpus form cortex of shoot.
Root Apex Organisation:
Root apex consists of mass of meristematic cells. Root apex is not resposible for the formation of lateral roots. Root cap or calyptrogen is present due to which root meristem becomes subterminal in position. If root cap is independent in origin it arises from the dematogen.
Regarding the organisation of root apex, following theories have been put forward.
(i) Korper-kappe theory: It was proposed by Schuepp: (1917) : This theory is comparable with the tunicacorpus theory of shoot apex , Korper means body and kappe means cap.
(ii) Quiescent center theory: It was proposed by Clowes (1956-58) . According to this theory root apex consist of an inverted cup like structure , the quiescent center. The cells of this region have very low mitotic activity (quiescent). They have low amount of RNA, DNA, and protein. They are surounded by layer of actively dividing cells which is responsible for formation of different structures of roots.
(B) Permanant tissue:
They are composed of living or dead cells which are derived from the meristematic tissue but have lost their ability to divide. These are of three types:
(i) Simple tissue:
They are made up of one kind of cells performing similar function.
(a) Parenchyma (Grew ) These cells are found almost in all parts of plants such as roots, stem, leaves, fruits and seeds. These cells are isodiametric , spherical, oval or polygonal with intercellular spaces. These cells are living and within cellulosic cell wall.
Types:
(a) Prosenchyma: Elongated parenchyma with tapering ends is called prosenchyma.
(b) Aerenchyma: The parenchyma which encloses air cavity is called aerenchyma ( hydrophytes).
(c) Chlorenchyma: The parenchyma containing chloroplasts is called chlorenchyma.
(d) Idioblast: Sometimes they store secretory substances ( ergastric substances) such as tannins , resins, and gum and they called idioblasts.
(e) Storage parenchyma : Fruits , endosperm.
(b) Collenchyma: ( Schleiden) These cells have thickenings on the cell wall and in corners of intercellular spaces. They are not found in roots and monocots. These cells form the hypodermis in stem and petiole. It is living mechanical tissue with high refractive index. The thickening material in the cell wall contains pectocellulose.
Colenchyma is of three types:
1. Angular Collenchyma: Dposition of hemicellulose and pectin occurs only in the angles between the cells. Angular walls thickened e.g. Stem of marigold and tomato.
2.Lamellar collenchyma: Deposition of hemicellulose and pectin occurs only at the crosswalls separating the adjacent cells. Tangential walls thickened e.g stem of Sunflower.
Types of Collenchyma. Source: tutorvista.com
3. Lacunate collenchyma: The deposition of hemicellulose and pectin occurs only along the border of intercellular spaces. Lacunate thickening , intercellular spaces are present e.g. Stem of cucurbita.
Function: They provide mechanical support , flexiblity and elasticity to the organs and due to peripheral position in stems they resist bending and pulling action of wind. It is especially useful for young plants and herbaceous organs where it is an important supporting tissue.
(c) Sclerenchyma: ( Mettenius) These cells have thickened secondary walls due to deposition of lignin At maturity they die. These cells have simple pits. They are of two types:
(i) Sclerids: They may be spherical , oval and cylindrical . They are lignified , extremely thick walled. The lumen of the cells is almost obliterated. They are found in the hard parts of the plants.
Source:studentsguide.in
A: Brachsclereids B: Astrosclereids C & D : Macrosclereids E: Osteosclereid F: Trichosclereid
Types of sclereids:
1. Brachysclereids ( Stone cells) :Grittiness in friuts is due to stone cells . e.g. Pear, Sapota.
2. Macrosclereids ( Rod cells) : Found in seed coat of leguminous plants.
3. Osteosclereids (Prop cells): Found in leaves and seed coat of many monocots and subepidermis of legume seed coats.
4. Astrosclereids (star cells) They are common in stem and leaves of dicots . e.g. Tea leaves. Petiole of lotus.
5. Trichosclereids ( internal hair) Long hair like branched sclereids. They are common in hydrophytes. These are also present in the aerial roots of Monstera and Olive leaves.
(ii) Sclerenchymous fibres: They are long and tapering at the ends. These are thick walled cells which are lignified. The fully developed fiber cells are always dead. The length of fiber varies from 2-550mm in angiosperms and 1-12mm in gymnosperms. The fibres are present in hypodermis of monocot stem , in pericycle of many dicots, in secondary wood and in vascular bundle sheath in monocot stems e.g. Jute, Flax, Hemp. Living fibres are found in Tamarix.
Function: The main function of sclrenchyma to provide mechanical strength.
Complex tissues:
Made up of different types of cells. They include xylem and phloem.
(A) Xylem: (Nageli) or hadrome ( Haberlandt): It is chief water conducting element. Xylem consists of following types of cells.
(i) Tracheids: They are elongated cells with pointed chisel like ends. Their wall is tough , thickened, lignified and thickening may be annular, spiral, reticulate, scariform or pitted. Cells are dead at maturity and have bordered pits. In pteridophytes and tracheids are associated with vessels. The main function is conduction of water. The tracheids are most primitive type of conducting elements in xylem.
(ii) Vessels: They are also elongated tube like, formed from a row of cells placed end. The partition walls are either perforated or disappear altogether resulting in an elongated tube. Walls thickened , lignified, may have annular, spiral, reticulate or scalariform thickening. Vessels are dead at maturity and without nuclei. In pteridophytes and gymnosperms vessels are absent ( Non- porous wood). Sometimes primitive vessels are present in Gnetum and Ephedra ( Gnetales). Vessels are characteristic of angiosperms (Porous wood) . Vesselless angiospermic families are Tetracentraceae, Trochodendraceae, Winteraceae. The main function is conduction of water . Vessels are advanced type of conducting elements.
On the basis of distribution and size of vessels porous wood is of two types:
a) Diffuse porous wood ( primitive): Vessels of same size are uniformly distributed through out the growth e.g. Pyrus, Betula.
b) Ring porous wood ( advanced) : Large vessels are formed in early wood when the need of water is great and small vessels are formed in late wood e.g. Quercus, Morus.
(iii) Wood or xylem fiber: These cells are elongated and pointed at both the ends. Cell wall is highly lignified having simple pits. They are commonly found in scondary xylem. They may be of the folowing types.
a) Fiber tracheids: Fiber like tracheids with bordered pit.
b) Libriform fiber: They have extremely thick walls and simple pits. They provide mechanical support.
(iv) Wood or xylem parenchyma: They are living parenchymatous cells associated with xylem.They may occur as axial parenchyma or ray parenchyma. When the parenchyma ois diffused or not associated with vessels, they are known as apotracheal parenchyma. And when parenchyma surrounds or is associated with vessels , they are called as paratracheal parenchyma.
On the basis of origin , xylem is of two types:
(i) Primary xylem: It is derived from procambium during the formation of primary plant body. It is differentiated into protoxylem ( first formed and consist of tracheary elements and xylem parenchyma) and metaxylem.( later formed and consists of tracheary elements, aylem parenchyma and fiber.) The cells of metaxylem are bigger in size than protoxylem.
(ii) Secondary xylem: It is formed from cambium during secondary growth. It is well dierentiated into two systems.
(a) Axial or vertical system:
(i) Teracheary elements ( Tracheids and vessels) for conduction of water.
(ii) Xylem or wood fiber- for support
(iii) Xylem parenchyma ( for storage of food)
(b) Ray or Horizontal system:
Ray parenchyma: for storage of food.
(B) Phloem or bast or Leptome ( Haberlandt) : Phloem consists of following types of cells.
(i) Sieve element: The sieve elements in angiosperms are sieve tubes which are cylindrical tube like cells with perforated cross walls called sieve plate. Sieve tubes are associated with companion cells and they are without nuclei. In pteridophytes and gymnosperms the sieve elements have sieve plates on their lateral walls and companion cells are absent. They are called as sieve cells. The walls of the sieve tube elements are made up of cellulose and pectic substances. The cytoplasm is confined to a thin peripheral layer. P-Proteins are proteinaceous structures present in sieve tubes and are believed to be responsible for (i) movement of materials and
(ii) sealing of pores after wounding.
At the end of growing season, the old sieve tubes, a callose plug whcih is made up of callose carbohydrate is deposited in sieve plate which inhibits the activity of sieve tubes. In spring season, the callose plug gets dissolved.
(ii) Companion cells: They are elongated living parenchymatous thin walled cells. They are associated laterally to sieve tubes and have dense cytoplasm and nuclei. Companion cells are absent in Pteridophytes and Gymnosperms. Both sieve tubes and companion cells are related ontogenetically since both develop from the same mother cell. They are the sister cells.
(iii) Phloem or bast fiber: These are absent or very few in phloem and abundantly found in secondary phloem. They are schlerenchymatous fibres associated with phloem. Phloem fibers of plants like Jute, flax, and hemp are retted in water and extracted for making ropes and coarse textiles.
(iv) Phloem parenchyma: They are parenchymatous living cells with cellulosic cell wall and nucleus. The main function is storage of food. They are not found in monocotyledonous plants.
Types of Phloem:
(A) On the basis of position:
(i) External phloem: The phloem is of normal type and is present outside the xylem. e.g. Mostly angiosperms and gymnosperms.
(ii) Internal or Intraxylary Phloem: It originates from procambium and is the primary phloem which occurs on innerside of primary xylem in bicollateral bundles. e.g. Members of Apocynaceae, Asclepiadaceae, Convovulaceae, Solanaceae.
(iii) Included or Interxylary phloem: It originates from cambium and is secondary phloem which occurs in groups within the secondary xylem e.g. Savadora, Amaranthus, Chenopodium etc.
(B) On the basis of Origin:
(i) Primary Phloem: It develops from procambium. It does not have radial differentiation. Or rays are absent. It is differentiated into protophloem ( consists of sieve elements and parenchyma) and metaphloem ( develops after protophloem and consists of sieve elements, parenchyma and fiber).
During the primary growth the protophloem elements are crushed by the surrounding tissues and disappear . This process is termed as obiliteration.
(ii) Secondary Phloem: It develops from the cambium during secondary growth. It showa radial differentiation. It consists of two ditinct systems such as :
a) Axial or vertical System:
(i) Sieve elements: Sieve tube and Companion cells. For the conduction of food .
(ii) Bast fiber: For support.
(iii) Bast Parenchyma: For storage of food.
(b) Ray or horizontal system
(i) Ray parenchyma : It is for the storage of food.
(3) Secretory Tissues:
Special modifications of secretory tissues : These tissues are of two types:
(a) Laticiferous tissues : They contain colourless, milky or yellow coloured juice called larex. These tissues are of two types:
(i) Latex cells: They do not fuse and do not form network. Plants having such tissues are called simple or non- articulated laticifers e.g. Calotropis ( Asclepiadaceae) , Nerium , Vinca ( Apocyanaceae), Euphorbia ( Euphorbiaceae) Ficus ( Moraceae).
(ii) Latex vessels: They are formed due to fusion of cells and form network like structure. Plants having such tissues are called compound or articulated laticifers e.g. Argemone, Papaver ( Papaveraceae) Sonchus ( Compositae) Hevea, Manihot ( Euphorbiaceae).
(b) Glandular tissues: They include different types of glands which secrete oils, gums, mucilage, tannins and resins. They may be:
(i) External glands: Present as epidermal outgrowths.
Types:
1. Glandular hair: With a stalk and head e.g. Tobacco,Plumbago, Boerhaavia.
2. Stinging hair : Sereste poisonous substance e.g. Urtica.
3. Nectaries: Sccrete sugary substance , may be extrafloral present on the stem, leaves etc. e.g. Nepenthes, Catheranthus or floral e.g. Corchorus, Thea, Polygonum, Jatropha.
4. Digestive Glands: Present in Insectivoruos plnts. e.g. Drosera, Nepanthes, etc.
(ii) Internal Glands :
(1) Oil Glands: Present in the mesophyll of leaves and cortex of stem fruit e.g. Orange, Lemon etc.
(2) Mucilage secreting glands: e.g. Leaves of Piper betel.
(3) Gum, tanin, and resin secreting glands or ducts present in gymnosperms and angiosperms e.g. Pinus. Resin ducts are scizogenous in origin.
THE TISSUE SYSTEM
The plant body consists of thre major tissue systems.namely:
(A) Epidermal tissue sytem: It consists of epidermis and its associated structure like hairs, trichomes, cuticle, stomata, and bulliform cells. The epidermis is mostly single layered parenchymous but multilayered in Ficus, Nerium. Epidermis is mainly protective in nature. In grasses, motor or bulliform cells are present in upper epidermis. In grasses and Equisetum , silica is present in th epidermal cells. The epidermal cells containing cystoliths are called as lithocycts.
(B) Ground or fundamental Tissue system: It extends from th epidermis to the center of axis. Excluding the vascular tissue. The ground tissue constitute of the following parts:
(i) Cortex: It lies between the epidermis and the pericycle. It is further differentiated into :
(a) Hypodermis: It is collenchymatous in dicot stem and sclerenchymatous in monocot stem. It provides strength.
(b) General cortex: It consists of parenchymatous cells. The main function is the storage of food.
(c) Endodermis: ( Starch sheath) Mostly single layered and is made up of parechymatous barrel shaped compactly arranged cells.
The inner and radial walls of endodermal cells have casparian strips. In roots thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem patches. The thin walled endodermal cells are called passage cells.
This endodermis checks the water loss and entry of air in xylem elements.
(ii) Pericycle: It lies between the endodermis and vascular tissue. It is parenchymatous in roots and sclerenchymatous or mixed with parenchyma in stem. The pericycle cells just opposite the protoxylem are the seat of origin of lateral roots. In dicot roots, pericycle form part of cambium or whole of cork cambium.
(iii) Pith: It occupies the central part in dicot stem and monocot root. Mostly comprised of parenchymatous cells. In dicot root , pith is completely obliterated by the metaxylem elements. In dicot stem the pith cells between the vascular bundles become radially elongated and are known as primary medullary rays or pith rays. They help in lateral translocation.
(C) vascular tissue system:
The vascular bundles found in steler part constitute vascular tissue system. Xylem, phloem and cambium are the major part of the vascular bundle. Vascular bundles may be of the following types:
1) radia: When the xylem and the phloem are arranged on different radii alternating with each other e.g. Roots.
2) Conjoint: When xylem and phloem combine in the same bundles and are present on the same radius e.g. Stem. Conjoint vascular bundles may be:
a) Collateral: Xylem is towards innerside and phloem toward the outerside.
(i) Open: Cambium is present between xylem and phloem . e.g. Dicot stem.
(ii) Closed: Cambium is absent between xylem and phloem e.g. Monocot stem.
b)Bicollateral : When xylem has cambium and phloem on both side e.g. members of Cucurbitaceae, Solanaceae, Apocyanaceae.
3. Concentric: When one vascular tissue surround the other . They are of two types:
(a) Amphicribal or Hadrocentric: The xylem is surrounded on all sides by phloem e.g. ferns.
(b)Amphivasal or Leptocentric: The Phloem is surrounded on all sides by xylem e.g. Yucca, Dracaena.
Collateral closed Bicollateral Collateral open
Amphivasal Amphicibral Radial
Different Types of vascular bundles. Source:www.studentsguide.in
INTERNAL STRUCTURE OF DICOT AND MONOCOT PLANTS
Anatomy of root: The three zones that can be distinguished in a root are:
(i) Epidermis: (Epiblema /Rhizodermis) : It is single layered ( uniseriate) and consists of tightly placed , thin walled uncutinised cells.
This epidermis layer is called as epiblema, piliferous layer or rhizodermis. Epiblema in younger roots bears unicellular root hairs ( water absorbing organs).
(ii) Cortex: It consists of thin walled parencymatous cells with intercellular spaces. In most monocots and some dicots cortex layer below epidermis becomes suberised to form protective tissue called exodermis. The cells of cortex store food material ( e.g. Carrot). The inner most layer of cortex develops into endodermis. It is made up of closely packed living cells characterized by the presence of band like thickening; made up of lignin and suberin on their radial and transverse walls. These bands or strips are called casparian bands or strips . Some cells of endodermis lying opposite to protoxylem remain thin walled and are called passage cells which allow radial diffusion of water.
(iii)Vascular bundles: Vascular bundles are radial and exarch. The center of monocot root is occupied by parenchymatous cells called pith.
Difference between dicot and monocot root
T.S of Monocot Root T.S. Of Dicot Root
Source : tutorvista.com
Anatomy of stem:
1.Primary structure of Dicot stem:
Dicot stem consists of following layers:
(i) Epidermis: It is the outermost layer consisting of single layer of closely arranged cells with cuticle. It bears multicellular hairs.
(ii) Cortex: It is differentiated into hypodermis , general cortex, and endodermis. Hypodermis is collenchymatous. General cortex bundles consisting of phloem and xylem.
(iii) Vascular bundles: Vascular bundles are conjoint , collateral or bicolateral, open and endarch and are arranged in ring. ( Eustele).
(iv) pith: It is the central portion of stem consisting of parenchymatous cells with narrow radially elongated parenchymatous cells extend from pith toward the periphery are called medullary rays. The main function is food storage.
2. Primary structure of monocot stem:
Monocot stem consists of following layers:
(i) Epidermis: It is the outermost layer and consists of sompactly arranged parenchyma cells which are usually covered with cuticle.
(ii) Hypodermis : cells of hypodermis are sclerenchymatous providing mechanical strength to the stem.
(iii) Ground tissue: All the tissue internal to hypodermis represents the ground tissue. It is made up of parenchymatous cells rich in food reserve like starch.
T.S of Monocot Stem T.S. Of Dicot Stem
Source: tutorvista.com
(iv)vascular bundles: they lie scattered in the ground tissue. Each vascular bundle is surrounded by 2-3 layered sclerenchymatous sheath . The vascular bundles are conjoint, collateral, closed and endarch (Atachostele) Vessels arranged in V shaped manner. Schizolysigenous water canals are present below the protoxylem.
Difference between dicot stem and monocot stem
Anatomy of Leaf:
Structure of dorsiventral leaf ( Dicot) :
In the cross section of a leaf the following parts can be seen:
(i) Epidermis: The upper and lower surfaces are covered by the epidermis. Cells of epidermis are parenchymatous and are closely packed together without any intercellular spaces. Mostly the stomata are restricted to lower surface of the leaf which is known as hypostomatic. The outer walls of the epidermal cells are thickened and cutinized which prevents the loss of water.
(ii) Mesophyll : In between the two epidermal layers , there are numerous chlorenchyma cells which constitute the mesophyll. In dicots there are two distinct layers of mesophyll the palisade ( upper layer consisting of closely arranged column shaped cells containing abundant chloroplasts). And spongy tissue ( the lower layer of irregularly shaped cells containing fewer chloroplasts.)
(iii) Vascular bundles: Vascular bundles in the leaf are located in the mid rib and the veins. Vascular bundles are conjoint, collateral, and closed. Bundles are surrounded by a compact layer of parenchymatous cells which is called a bundle sheath. The xylem ( protoxylem) is toward upper epidermis ( adaxial) and the phloem on the lower side (abaxial).
1. Upper Epidermis 2. Cuticle 3. Polisade Parenchyma 4 Sclerenchyma 5. Xylem 6. Border Parenchyma 7. Lower Epidermis 8. Phloem 9. Spongy Parenchyma 10. Stama 12. Respiratary Cavity.
Source: student'sguide.in
Structure of isobilateral leaf ( Monocot)
The monocot leaf is differentitated into three types of tissues:
(i) Epidermis : It consists of upper and lower and lower epidermis, both of which may be interrupted by equal number of stomata.
Both the epidermal layers are cutinized . In some grasses e.g. Poa, Agropyron,Psamma grass epidermal cells are large with thin flexible walls which are called as motor or bulliform cells. These cells help in the rolling of leaves.
(ii) Mesophyll: Mesophyll cells are not differentiated into pallisade and spongy parenchyma. Mesophyll cells are made up of parenchyma cells which have chloroplasts.
(iii) vascular bundles: They are arranged in parallel manner. Vascular bundles are conjoint , collateral, closed and enclosed by a bundle sheath. The xylem is towards the uper side ( adaxial surface) and phloem on the lower side ( abaxial surface)
V.S Monocot Isobilateral Leaf
source: studentsguide.in
1. Xylem 6. Bulliform Cell
2. Phloem 7. Upper Epidermis
3. Sclerenchyma 8. Bundle Sheath
4. Stama 9. Vascular Bundle
5. Sub Stomatal Chamber 10. Mesophyll Tissue
11. Lower Epidermis
Secondary Growth: Increase in girth or thickness or diameter of the axis due to formation of new tissues as a result of joint activity of vascular cambium and extrasteler region respectively. It occurs in the root and stem of gymnosperm and dicots.Secondary growth in dicot stemcompletes in the following steps:
A. Formation of vascular cambium ring:
(i) Intrafascicular cambium: It is primary in origin , present in between primary phloem and primary xylem.
(ii) Interfascicular cambium: It is true secondary meristem. It originates from the parenchyma cells of medullary rays region. It lies in between the vascular bundles .
(iii) Vascular cambium ring: Both intrafascicular and interfascicular cambium joins together and forms cambium ring.
Cells of cambium are of two types:
(a) Fusiform initials: They form tracheids, vessels, fibres and axial parenchyma in secondary xylem and sieve tubes, companion cells , fibres and axial parenchyma in secondary phloem.
(b) Ray initials: These are isodiametric and form ray parenchyma and vascular rays.
(iv) Periclinical division of vascular cambium ring.
(v) Formation of sceondary phloem ( outside the vascular cambium) and secondary xylem produced is 8-10 times grater than secondary phloem.
(vi) Fate of primary phloem and primary xylem being dead and lignified , replaced in the pith region.
Stages of Secondary Growth in a Typical Dicotyledons Stem
source:.inplantphys.info
(vii) Formation of secondary structures i.e. Annual rings: sapwood and heartwood, hardwood and softwood etc.
(a) Annual rings: These are formed by the seasonal activity of vascular cambium. Cambium is not active through out the year. During the summer season or spring the cambium is more active and form large sized xylem elements ( vessels) and constitue autumn wood or late wood. Both autumn and spring wood constitue a growth or annual ring. In a year only one growth ring is formed. In successive years numerous growth rings are formed. Counting the annual rings in the main stem it is easy to ascertain the age of the tree. This branch of science is known as Dendrochronology.
Growth rings are distinct or sharply demarcated in the plants of temperate climate e.g. Shimla, Nainital, Mussourie, due to presence of contrasting seasonal variations. Growth rings are not distinct or sharply demarcated in the trees of troical climate ( near equator) e,g, Clcutta, Bombay, madras due to absence of contrasting seasonal variations.
(b) Heart wood and sap wood: The young elements of secondary xylem in the peripheral region constitute sap wood or alburnum. It is physiologically active and light in colour. The water conduction takes place through sap wood.
Sap wood is converted into heart wood or duramen in the central region. It is darker in colour due to deposition of tannins, gums, resins and physiologically inactive ( dead) and provides only mechanical support. During the conversion of sap wood into heart wood the most important change is development of tyloses in the heart wood. Tyloses are balloon like structures develop from xylem parenchyma. These tyloses block the passage of xylem vessels so also called as tracheal plug. The heart wood is commercially used as wood. When the plant is made hollow , it will not die because the water conduction takes place through sap wood. The heart wood is well developed in Morus alba ( Mulberry) The heart wood is absent in Populus and salix palnt. The wood of Tectona grandis is termite resistant. As a tree grows lder thickness of heartwood increases and sap wood remains the same.
Heart wood is much more durable and resistant to microorganisms , insects and pests etc than sap wood. Wood of dicot trees is called porous or hard wood because it consists of vessels( pores). The wood of gymnosperms do not contain vessels ( pores) and is known as soft or non- porous wood. Such wood consists of 90-95% tracheids and 5-10% of ray cells. Sap wood will decay faster if exposed freely to the air.
B. Formation of cork cambium:
Cork cambium or phellogen develops from outer layer of cortex. It produces secondary cortex or phelloderm on innerside and cork or phellem on the outer side. The cells of phellem are dead, suberized and pimpervious to water. Cork cells are airtight and used as bottle stopper or cork. The bottle cork is prepared from the cork of Quercus suber ( Oak tree). Cells of phelloderm are thin walled , living and store food. Phellem , Phellogen and phelloderm are collctively called periderm. Periderm is secondary protective tissue.
Due to pressure of secondary xylem , epidermis ruptures and cortex is largely lost after two or three years of secondary growth. In the cork layer ( bark) the lenticels are present which re meant for gaseous exchange. In cork, lenticels have loosely arranged cells called complementary cells with intercellular spaces. For bottle corks the cork is processed in such a manner so that lenticels come in vertical direction.
Begining of Lenticel Formation under a Stoma Mature Lenticel f –
Epidermis
a - Epidermis
g – Stoma b - Complementary Cells
h - Complementary Cells c. Phellogen
d - Phellem
Bark includes all the dead and living tissues outside the vascular cambium. It may be:
(i)Scaly bark: When develops in strips e.g. Eucalyptus, Psidium.
(ii) Ring bark: When develops in the form of sheet or ring e.g. Betula ( Bhojpatra)
The outermost layer of bark is dead and is called as rhytidome.
In the ancient times the bark of betula was used as paper for writing the manuscripts.
Secondary Growth in Dicot Root:
Vascular bundles in dicot root are radial , exarch and mostly triarch. Vascular cambium is formed secondary from conjuctive parenchyma cells lying just below each phloem strands. The number of cambium strips formed equals the number of phloem strands. The cells of pericycle lying outside the protoxylem also becomes meristematic to form part of strips of cambium. These cambial strips join the first formed cambium strips to form complete but wavy ring of vascular cambium. This cambium ring produces secondary xylem on inner side and secondary phloem on outer side. In roots, the growth rings are not distinct because there is no seasonal variation under the soil. From the outer layers of pericycle arises the phellogen which cuts phellem ( cork ) on the outer side and secondary cortex or phelloderm toward the innerside.
St ages of Secondary Growth in a Typical Dicotyledons Root
The branch of Botany dealing with the internal organisation of the plants is called Anatomy. The foundation of Plant Anatomy was laid down by N.Grew. The term tissue was coined by him. He is called the 'Father of Anatomy'.
The Tissues:
A group of similar or dissimilar cells that perform a common function and have a common origin is called a tissue. Tissues are classified into two main groups: Meristematic and Permanant.
(A) Meristematic Tissues:
1. These tissues consist of cells that retain the power of division.
2. The protoplasm within the cell is dense, the vacuole smaller or absent.
3. These cells are isodiametric without intercellular spaces.
4.The nucleus is bigger in size.
5.The cells of the meristimatic tissues have thin cellulosic cell wall.
6. The cells are metabolically highly active with high surface area per unit volume and nucleo- cytoplasmic ratio.
7.Ergastic substances are absent.
8. Colourless protoplastids are present in the cells.
Clasiification of Meristems:
(1) On the basis of origin and Development:
(i) Promeristems: ( primordial meristem) : A group of cells which represent primary stages of meristamatic cells. They are found at the apices of embryonic roots and shoots. They give rise to primary meristems.
(ii) Primary meristems: They originate from promeristems . Found at shoot and root apices, at the apex of leaves and intercalary parts.They give rise to primary permanent tissue.
(iii) Secondary meristems: They are not present from the beginning of the formation of an organ but develop at a later stage.They give rise to secondary permanent tissues. e.g interfascicular cambium , cork, and cambium in dicots.
(2) On the Basis of Position:
(i) Apical Meristem: These cells or tissues are found at the apices of stem and root.Due to continues division the root and stem increase in length. The apical meristem help the plants to grow in length.
(ii) Intercalary meristem: The tissues are intercalated between permanent tissues. These are actually the part of the apical meristem which gets separated from it during the growth of stem and root in length. e.g. Stem of grasses, Equisetum. They are especially responsible for increase in the length of the stems of grasses.
(iii) Lateral Meristem: These meristems are present along the lateral side of stem and roots.They divide in tangential plane , giving rise to the secondary permanent tissues on the inner and outer side and lead to the increase in thickness of girth, of the plant body
e.g. Intrafascicular cambium, interfascicular cambium and cork cambium.
(3)On the basis of Plane of Cell Division:
(i) Mass Meristem: The cells divide anticlinally in all planes, so that mass of cells is formed. e.g. Formation of spores, cortex, pith, endosperm.
(ii) Plate Meristem: The cells divide anticlinally in two planes, so plate like area is increased. e.g. Formation of epidermis and lamina of leaves.
(iii) Rib or File Meristem: The cells divide anticlinally in one plane , so row or colums of cells is formed e.g. Formation of a lateral root.
(4) On the basis of Function :
(i) Protoderm: They are the outermost meristematic cells. They form skin or epidermis of plant and epidermal tissue system.
(ii) Procambium: They are the innermost meristematic cells. They form primary xylem, primary phloem, and cambium.
(iii) Ground meristem: They form ground or fundamental tissue such as hypodermis, cortex, pith, pericycle etc.
Shoot Apex Organisation:
The shoot Apex is present immediately above the youngest leaf primordia. It consists of meristematic cells . Lateral branches of stem and leaves are formed by the activity of shoot apex such as:.
(i) Apical cell theory: It was proposed by Holfmeister and Nageli. According to this theory a single apical cell leads to the development of entire plant body. This theory is applicable to algae, to most of the bryophytes and pteridophytes.
(ii) Histogen theory: It was proposed by Hanstein. According to this theory , shoot apex consist of following histogens:
(a) Dermatogen: Outermost layer. It forms epidermis ( skin) and epidermal tissue system.
(b) Periblem: It gives rise to the tissues between epidermis and stele.
(c) Plerome: Innermost layer. The central mass of cells which gives rise to central stele.
Source:shikshaservices.com
(iii) Tunica Corpus theory : It was proposed by Schmidt (1924) It is based on plane of division of cells . According to this theory, shoot apex consists of two distinct layers as:
(a) Tunica : It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus and divide mostly by anticlinal divisions.
(b) Corpus: It represents central core with larger cells.The cells divide periclinally. Sometimes tunica is multilayered , only outer layer forms epidermis and the remaining layers with corpus form cortex of shoot.
Root Apex Organisation:
Root apex consists of mass of meristematic cells. Root apex is not resposible for the formation of lateral roots. Root cap or calyptrogen is present due to which root meristem becomes subterminal in position. If root cap is independent in origin it arises from the dematogen.
Regarding the organisation of root apex, following theories have been put forward.
(i) Korper-kappe theory: It was proposed by Schuepp: (1917) : This theory is comparable with the tunicacorpus theory of shoot apex , Korper means body and kappe means cap.
(ii) Quiescent center theory: It was proposed by Clowes (1956-58) . According to this theory root apex consist of an inverted cup like structure , the quiescent center. The cells of this region have very low mitotic activity (quiescent). They have low amount of RNA, DNA, and protein. They are surounded by layer of actively dividing cells which is responsible for formation of different structures of roots.
(B) Permanant tissue:
They are composed of living or dead cells which are derived from the meristematic tissue but have lost their ability to divide. These are of three types:
(i) Simple tissue:
They are made up of one kind of cells performing similar function.
(a) Parenchyma (Grew ) These cells are found almost in all parts of plants such as roots, stem, leaves, fruits and seeds. These cells are isodiametric , spherical, oval or polygonal with intercellular spaces. These cells are living and within cellulosic cell wall.
Types:
(a) Prosenchyma: Elongated parenchyma with tapering ends is called prosenchyma.
(b) Aerenchyma: The parenchyma which encloses air cavity is called aerenchyma ( hydrophytes).
(c) Chlorenchyma: The parenchyma containing chloroplasts is called chlorenchyma.
(d) Idioblast: Sometimes they store secretory substances ( ergastric substances) such as tannins , resins, and gum and they called idioblasts.
(e) Storage parenchyma : Fruits , endosperm.
(b) Collenchyma: ( Schleiden) These cells have thickenings on the cell wall and in corners of intercellular spaces. They are not found in roots and monocots. These cells form the hypodermis in stem and petiole. It is living mechanical tissue with high refractive index. The thickening material in the cell wall contains pectocellulose.
Colenchyma is of three types:
1. Angular Collenchyma: Dposition of hemicellulose and pectin occurs only in the angles between the cells. Angular walls thickened e.g. Stem of marigold and tomato.
2.Lamellar collenchyma: Deposition of hemicellulose and pectin occurs only at the crosswalls separating the adjacent cells. Tangential walls thickened e.g stem of Sunflower.
Types of Collenchyma. Source: tutorvista.com
3. Lacunate collenchyma: The deposition of hemicellulose and pectin occurs only along the border of intercellular spaces. Lacunate thickening , intercellular spaces are present e.g. Stem of cucurbita.
Function: They provide mechanical support , flexiblity and elasticity to the organs and due to peripheral position in stems they resist bending and pulling action of wind. It is especially useful for young plants and herbaceous organs where it is an important supporting tissue.
(c) Sclerenchyma: ( Mettenius) These cells have thickened secondary walls due to deposition of lignin At maturity they die. These cells have simple pits. They are of two types:
(i) Sclerids: They may be spherical , oval and cylindrical . They are lignified , extremely thick walled. The lumen of the cells is almost obliterated. They are found in the hard parts of the plants.
Source:studentsguide.in
A: Brachsclereids B: Astrosclereids C & D : Macrosclereids E: Osteosclereid F: Trichosclereid
Types of sclereids:
1. Brachysclereids ( Stone cells) :Grittiness in friuts is due to stone cells . e.g. Pear, Sapota.
2. Macrosclereids ( Rod cells) : Found in seed coat of leguminous plants.
3. Osteosclereids (Prop cells): Found in leaves and seed coat of many monocots and subepidermis of legume seed coats.
4. Astrosclereids (star cells) They are common in stem and leaves of dicots . e.g. Tea leaves. Petiole of lotus.
5. Trichosclereids ( internal hair) Long hair like branched sclereids. They are common in hydrophytes. These are also present in the aerial roots of Monstera and Olive leaves.
(ii) Sclerenchymous fibres: They are long and tapering at the ends. These are thick walled cells which are lignified. The fully developed fiber cells are always dead. The length of fiber varies from 2-550mm in angiosperms and 1-12mm in gymnosperms. The fibres are present in hypodermis of monocot stem , in pericycle of many dicots, in secondary wood and in vascular bundle sheath in monocot stems e.g. Jute, Flax, Hemp. Living fibres are found in Tamarix.
Function: The main function of sclrenchyma to provide mechanical strength.
Complex tissues:
Made up of different types of cells. They include xylem and phloem.
(A) Xylem: (Nageli) or hadrome ( Haberlandt): It is chief water conducting element. Xylem consists of following types of cells.
(i) Tracheids: They are elongated cells with pointed chisel like ends. Their wall is tough , thickened, lignified and thickening may be annular, spiral, reticulate, scariform or pitted. Cells are dead at maturity and have bordered pits. In pteridophytes and tracheids are associated with vessels. The main function is conduction of water. The tracheids are most primitive type of conducting elements in xylem.
(ii) Vessels: They are also elongated tube like, formed from a row of cells placed end. The partition walls are either perforated or disappear altogether resulting in an elongated tube. Walls thickened , lignified, may have annular, spiral, reticulate or scalariform thickening. Vessels are dead at maturity and without nuclei. In pteridophytes and gymnosperms vessels are absent ( Non- porous wood). Sometimes primitive vessels are present in Gnetum and Ephedra ( Gnetales). Vessels are characteristic of angiosperms (Porous wood) . Vesselless angiospermic families are Tetracentraceae, Trochodendraceae, Winteraceae. The main function is conduction of water . Vessels are advanced type of conducting elements.
On the basis of distribution and size of vessels porous wood is of two types:
a) Diffuse porous wood ( primitive): Vessels of same size are uniformly distributed through out the growth e.g. Pyrus, Betula.
b) Ring porous wood ( advanced) : Large vessels are formed in early wood when the need of water is great and small vessels are formed in late wood e.g. Quercus, Morus.
(iii) Wood or xylem fiber: These cells are elongated and pointed at both the ends. Cell wall is highly lignified having simple pits. They are commonly found in scondary xylem. They may be of the folowing types.
a) Fiber tracheids: Fiber like tracheids with bordered pit.
b) Libriform fiber: They have extremely thick walls and simple pits. They provide mechanical support.
(iv) Wood or xylem parenchyma: They are living parenchymatous cells associated with xylem.They may occur as axial parenchyma or ray parenchyma. When the parenchyma ois diffused or not associated with vessels, they are known as apotracheal parenchyma. And when parenchyma surrounds or is associated with vessels , they are called as paratracheal parenchyma.
On the basis of origin , xylem is of two types:
(i) Primary xylem: It is derived from procambium during the formation of primary plant body. It is differentiated into protoxylem ( first formed and consist of tracheary elements and xylem parenchyma) and metaxylem.( later formed and consists of tracheary elements, aylem parenchyma and fiber.) The cells of metaxylem are bigger in size than protoxylem.
(ii) Secondary xylem: It is formed from cambium during secondary growth. It is well dierentiated into two systems.
(a) Axial or vertical system:
(i) Teracheary elements ( Tracheids and vessels) for conduction of water.
(ii) Xylem or wood fiber- for support
(iii) Xylem parenchyma ( for storage of food)
(b) Ray or Horizontal system:
Ray parenchyma: for storage of food.
(B) Phloem or bast or Leptome ( Haberlandt) : Phloem consists of following types of cells.
(i) Sieve element: The sieve elements in angiosperms are sieve tubes which are cylindrical tube like cells with perforated cross walls called sieve plate. Sieve tubes are associated with companion cells and they are without nuclei. In pteridophytes and gymnosperms the sieve elements have sieve plates on their lateral walls and companion cells are absent. They are called as sieve cells. The walls of the sieve tube elements are made up of cellulose and pectic substances. The cytoplasm is confined to a thin peripheral layer. P-Proteins are proteinaceous structures present in sieve tubes and are believed to be responsible for (i) movement of materials and
(ii) sealing of pores after wounding.
At the end of growing season, the old sieve tubes, a callose plug whcih is made up of callose carbohydrate is deposited in sieve plate which inhibits the activity of sieve tubes. In spring season, the callose plug gets dissolved.
(ii) Companion cells: They are elongated living parenchymatous thin walled cells. They are associated laterally to sieve tubes and have dense cytoplasm and nuclei. Companion cells are absent in Pteridophytes and Gymnosperms. Both sieve tubes and companion cells are related ontogenetically since both develop from the same mother cell. They are the sister cells.
(iii) Phloem or bast fiber: These are absent or very few in phloem and abundantly found in secondary phloem. They are schlerenchymatous fibres associated with phloem. Phloem fibers of plants like Jute, flax, and hemp are retted in water and extracted for making ropes and coarse textiles.
(iv) Phloem parenchyma: They are parenchymatous living cells with cellulosic cell wall and nucleus. The main function is storage of food. They are not found in monocotyledonous plants.
Types of Phloem:
(A) On the basis of position:
(i) External phloem: The phloem is of normal type and is present outside the xylem. e.g. Mostly angiosperms and gymnosperms.
(ii) Internal or Intraxylary Phloem: It originates from procambium and is the primary phloem which occurs on innerside of primary xylem in bicollateral bundles. e.g. Members of Apocynaceae, Asclepiadaceae, Convovulaceae, Solanaceae.
(iii) Included or Interxylary phloem: It originates from cambium and is secondary phloem which occurs in groups within the secondary xylem e.g. Savadora, Amaranthus, Chenopodium etc.
(B) On the basis of Origin:
(i) Primary Phloem: It develops from procambium. It does not have radial differentiation. Or rays are absent. It is differentiated into protophloem ( consists of sieve elements and parenchyma) and metaphloem ( develops after protophloem and consists of sieve elements, parenchyma and fiber).
During the primary growth the protophloem elements are crushed by the surrounding tissues and disappear . This process is termed as obiliteration.
(ii) Secondary Phloem: It develops from the cambium during secondary growth. It showa radial differentiation. It consists of two ditinct systems such as :
a) Axial or vertical System:
(i) Sieve elements: Sieve tube and Companion cells. For the conduction of food .
(ii) Bast fiber: For support.
(iii) Bast Parenchyma: For storage of food.
(b) Ray or horizontal system
(i) Ray parenchyma : It is for the storage of food.
(3) Secretory Tissues:
Special modifications of secretory tissues : These tissues are of two types:
(a) Laticiferous tissues : They contain colourless, milky or yellow coloured juice called larex. These tissues are of two types:
(i) Latex cells: They do not fuse and do not form network. Plants having such tissues are called simple or non- articulated laticifers e.g. Calotropis ( Asclepiadaceae) , Nerium , Vinca ( Apocyanaceae), Euphorbia ( Euphorbiaceae) Ficus ( Moraceae).
(ii) Latex vessels: They are formed due to fusion of cells and form network like structure. Plants having such tissues are called compound or articulated laticifers e.g. Argemone, Papaver ( Papaveraceae) Sonchus ( Compositae) Hevea, Manihot ( Euphorbiaceae).
(b) Glandular tissues: They include different types of glands which secrete oils, gums, mucilage, tannins and resins. They may be:
(i) External glands: Present as epidermal outgrowths.
Types:
1. Glandular hair: With a stalk and head e.g. Tobacco,Plumbago, Boerhaavia.
2. Stinging hair : Sereste poisonous substance e.g. Urtica.
3. Nectaries: Sccrete sugary substance , may be extrafloral present on the stem, leaves etc. e.g. Nepenthes, Catheranthus or floral e.g. Corchorus, Thea, Polygonum, Jatropha.
4. Digestive Glands: Present in Insectivoruos plnts. e.g. Drosera, Nepanthes, etc.
(ii) Internal Glands :
(1) Oil Glands: Present in the mesophyll of leaves and cortex of stem fruit e.g. Orange, Lemon etc.
(2) Mucilage secreting glands: e.g. Leaves of Piper betel.
(3) Gum, tanin, and resin secreting glands or ducts present in gymnosperms and angiosperms e.g. Pinus. Resin ducts are scizogenous in origin.
THE TISSUE SYSTEM
The plant body consists of thre major tissue systems.namely:
(A) Epidermal tissue sytem: It consists of epidermis and its associated structure like hairs, trichomes, cuticle, stomata, and bulliform cells. The epidermis is mostly single layered parenchymous but multilayered in Ficus, Nerium. Epidermis is mainly protective in nature. In grasses, motor or bulliform cells are present in upper epidermis. In grasses and Equisetum , silica is present in th epidermal cells. The epidermal cells containing cystoliths are called as lithocycts.
(B) Ground or fundamental Tissue system: It extends from th epidermis to the center of axis. Excluding the vascular tissue. The ground tissue constitute of the following parts:
(i) Cortex: It lies between the epidermis and the pericycle. It is further differentiated into :
(a) Hypodermis: It is collenchymatous in dicot stem and sclerenchymatous in monocot stem. It provides strength.
(b) General cortex: It consists of parenchymatous cells. The main function is the storage of food.
(c) Endodermis: ( Starch sheath) Mostly single layered and is made up of parechymatous barrel shaped compactly arranged cells.
The inner and radial walls of endodermal cells have casparian strips. In roots thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem patches. The thin walled endodermal cells are called passage cells.
This endodermis checks the water loss and entry of air in xylem elements.
(ii) Pericycle: It lies between the endodermis and vascular tissue. It is parenchymatous in roots and sclerenchymatous or mixed with parenchyma in stem. The pericycle cells just opposite the protoxylem are the seat of origin of lateral roots. In dicot roots, pericycle form part of cambium or whole of cork cambium.
(iii) Pith: It occupies the central part in dicot stem and monocot root. Mostly comprised of parenchymatous cells. In dicot root , pith is completely obliterated by the metaxylem elements. In dicot stem the pith cells between the vascular bundles become radially elongated and are known as primary medullary rays or pith rays. They help in lateral translocation.
(C) vascular tissue system:
The vascular bundles found in steler part constitute vascular tissue system. Xylem, phloem and cambium are the major part of the vascular bundle. Vascular bundles may be of the following types:
1) radia: When the xylem and the phloem are arranged on different radii alternating with each other e.g. Roots.
2) Conjoint: When xylem and phloem combine in the same bundles and are present on the same radius e.g. Stem. Conjoint vascular bundles may be:
a) Collateral: Xylem is towards innerside and phloem toward the outerside.
(i) Open: Cambium is present between xylem and phloem . e.g. Dicot stem.
(ii) Closed: Cambium is absent between xylem and phloem e.g. Monocot stem.
b)Bicollateral : When xylem has cambium and phloem on both side e.g. members of Cucurbitaceae, Solanaceae, Apocyanaceae.
3. Concentric: When one vascular tissue surround the other . They are of two types:
(a) Amphicribal or Hadrocentric: The xylem is surrounded on all sides by phloem e.g. ferns.
(b)Amphivasal or Leptocentric: The Phloem is surrounded on all sides by xylem e.g. Yucca, Dracaena.
Collateral closed Bicollateral Collateral open
Amphivasal Amphicibral Radial
Different Types of vascular bundles. Source:www.studentsguide.in
INTERNAL STRUCTURE OF DICOT AND MONOCOT PLANTS
Anatomy of root: The three zones that can be distinguished in a root are:
(i) Epidermis: (Epiblema /Rhizodermis) : It is single layered ( uniseriate) and consists of tightly placed , thin walled uncutinised cells.
This epidermis layer is called as epiblema, piliferous layer or rhizodermis. Epiblema in younger roots bears unicellular root hairs ( water absorbing organs).
(ii) Cortex: It consists of thin walled parencymatous cells with intercellular spaces. In most monocots and some dicots cortex layer below epidermis becomes suberised to form protective tissue called exodermis. The cells of cortex store food material ( e.g. Carrot). The inner most layer of cortex develops into endodermis. It is made up of closely packed living cells characterized by the presence of band like thickening; made up of lignin and suberin on their radial and transverse walls. These bands or strips are called casparian bands or strips . Some cells of endodermis lying opposite to protoxylem remain thin walled and are called passage cells which allow radial diffusion of water.
(iii)Vascular bundles: Vascular bundles are radial and exarch. The center of monocot root is occupied by parenchymatous cells called pith.
Difference between dicot and monocot root
T.S of Monocot Root T.S. Of Dicot Root
Source : tutorvista.com
Anatomy of stem:
1.Primary structure of Dicot stem:
Dicot stem consists of following layers:
(i) Epidermis: It is the outermost layer consisting of single layer of closely arranged cells with cuticle. It bears multicellular hairs.
(ii) Cortex: It is differentiated into hypodermis , general cortex, and endodermis. Hypodermis is collenchymatous. General cortex bundles consisting of phloem and xylem.
(iii) Vascular bundles: Vascular bundles are conjoint , collateral or bicolateral, open and endarch and are arranged in ring. ( Eustele).
(iv) pith: It is the central portion of stem consisting of parenchymatous cells with narrow radially elongated parenchymatous cells extend from pith toward the periphery are called medullary rays. The main function is food storage.
2. Primary structure of monocot stem:
Monocot stem consists of following layers:
(i) Epidermis: It is the outermost layer and consists of sompactly arranged parenchyma cells which are usually covered with cuticle.
(ii) Hypodermis : cells of hypodermis are sclerenchymatous providing mechanical strength to the stem.
(iii) Ground tissue: All the tissue internal to hypodermis represents the ground tissue. It is made up of parenchymatous cells rich in food reserve like starch.
T.S of Monocot Stem T.S. Of Dicot Stem
Source: tutorvista.com
(iv)vascular bundles: they lie scattered in the ground tissue. Each vascular bundle is surrounded by 2-3 layered sclerenchymatous sheath . The vascular bundles are conjoint, collateral, closed and endarch (Atachostele) Vessels arranged in V shaped manner. Schizolysigenous water canals are present below the protoxylem.
Difference between dicot stem and monocot stem
Anatomy of Leaf:
Structure of dorsiventral leaf ( Dicot) :
In the cross section of a leaf the following parts can be seen:
(i) Epidermis: The upper and lower surfaces are covered by the epidermis. Cells of epidermis are parenchymatous and are closely packed together without any intercellular spaces. Mostly the stomata are restricted to lower surface of the leaf which is known as hypostomatic. The outer walls of the epidermal cells are thickened and cutinized which prevents the loss of water.
(ii) Mesophyll : In between the two epidermal layers , there are numerous chlorenchyma cells which constitute the mesophyll. In dicots there are two distinct layers of mesophyll the palisade ( upper layer consisting of closely arranged column shaped cells containing abundant chloroplasts). And spongy tissue ( the lower layer of irregularly shaped cells containing fewer chloroplasts.)
(iii) Vascular bundles: Vascular bundles in the leaf are located in the mid rib and the veins. Vascular bundles are conjoint, collateral, and closed. Bundles are surrounded by a compact layer of parenchymatous cells which is called a bundle sheath. The xylem ( protoxylem) is toward upper epidermis ( adaxial) and the phloem on the lower side (abaxial).
1. Upper Epidermis 2. Cuticle 3. Polisade Parenchyma 4 Sclerenchyma 5. Xylem 6. Border Parenchyma 7. Lower Epidermis 8. Phloem 9. Spongy Parenchyma 10. Stama 12. Respiratary Cavity.
Source: student'sguide.in
Structure of isobilateral leaf ( Monocot)
The monocot leaf is differentitated into three types of tissues:
(i) Epidermis : It consists of upper and lower and lower epidermis, both of which may be interrupted by equal number of stomata.
Both the epidermal layers are cutinized . In some grasses e.g. Poa, Agropyron,Psamma grass epidermal cells are large with thin flexible walls which are called as motor or bulliform cells. These cells help in the rolling of leaves.
(ii) Mesophyll: Mesophyll cells are not differentiated into pallisade and spongy parenchyma. Mesophyll cells are made up of parenchyma cells which have chloroplasts.
(iii) vascular bundles: They are arranged in parallel manner. Vascular bundles are conjoint , collateral, closed and enclosed by a bundle sheath. The xylem is towards the uper side ( adaxial surface) and phloem on the lower side ( abaxial surface)
V.S Monocot Isobilateral Leaf
source: studentsguide.in
1. Xylem 6. Bulliform Cell
2. Phloem 7. Upper Epidermis
3. Sclerenchyma 8. Bundle Sheath
4. Stama 9. Vascular Bundle
5. Sub Stomatal Chamber 10. Mesophyll Tissue
11. Lower Epidermis
Secondary Growth: Increase in girth or thickness or diameter of the axis due to formation of new tissues as a result of joint activity of vascular cambium and extrasteler region respectively. It occurs in the root and stem of gymnosperm and dicots.Secondary growth in dicot stemcompletes in the following steps:
A. Formation of vascular cambium ring:
(i) Intrafascicular cambium: It is primary in origin , present in between primary phloem and primary xylem.
(ii) Interfascicular cambium: It is true secondary meristem. It originates from the parenchyma cells of medullary rays region. It lies in between the vascular bundles .
(iii) Vascular cambium ring: Both intrafascicular and interfascicular cambium joins together and forms cambium ring.
Cells of cambium are of two types:
(a) Fusiform initials: They form tracheids, vessels, fibres and axial parenchyma in secondary xylem and sieve tubes, companion cells , fibres and axial parenchyma in secondary phloem.
(b) Ray initials: These are isodiametric and form ray parenchyma and vascular rays.
(iv) Periclinical division of vascular cambium ring.
(v) Formation of sceondary phloem ( outside the vascular cambium) and secondary xylem produced is 8-10 times grater than secondary phloem.
(vi) Fate of primary phloem and primary xylem being dead and lignified , replaced in the pith region.
Stages of Secondary Growth in a Typical Dicotyledons Stem
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(vii) Formation of secondary structures i.e. Annual rings: sapwood and heartwood, hardwood and softwood etc.
(a) Annual rings: These are formed by the seasonal activity of vascular cambium. Cambium is not active through out the year. During the summer season or spring the cambium is more active and form large sized xylem elements ( vessels) and constitue autumn wood or late wood. Both autumn and spring wood constitue a growth or annual ring. In a year only one growth ring is formed. In successive years numerous growth rings are formed. Counting the annual rings in the main stem it is easy to ascertain the age of the tree. This branch of science is known as Dendrochronology.
Growth rings are distinct or sharply demarcated in the plants of temperate climate e.g. Shimla, Nainital, Mussourie, due to presence of contrasting seasonal variations. Growth rings are not distinct or sharply demarcated in the trees of troical climate ( near equator) e,g, Clcutta, Bombay, madras due to absence of contrasting seasonal variations.
(b) Heart wood and sap wood: The young elements of secondary xylem in the peripheral region constitute sap wood or alburnum. It is physiologically active and light in colour. The water conduction takes place through sap wood.
Sap wood is converted into heart wood or duramen in the central region. It is darker in colour due to deposition of tannins, gums, resins and physiologically inactive ( dead) and provides only mechanical support. During the conversion of sap wood into heart wood the most important change is development of tyloses in the heart wood. Tyloses are balloon like structures develop from xylem parenchyma. These tyloses block the passage of xylem vessels so also called as tracheal plug. The heart wood is commercially used as wood. When the plant is made hollow , it will not die because the water conduction takes place through sap wood. The heart wood is well developed in Morus alba ( Mulberry) The heart wood is absent in Populus and salix palnt. The wood of Tectona grandis is termite resistant. As a tree grows lder thickness of heartwood increases and sap wood remains the same.
Heart wood is much more durable and resistant to microorganisms , insects and pests etc than sap wood. Wood of dicot trees is called porous or hard wood because it consists of vessels( pores). The wood of gymnosperms do not contain vessels ( pores) and is known as soft or non- porous wood. Such wood consists of 90-95% tracheids and 5-10% of ray cells. Sap wood will decay faster if exposed freely to the air.
B. Formation of cork cambium:
Cork cambium or phellogen develops from outer layer of cortex. It produces secondary cortex or phelloderm on innerside and cork or phellem on the outer side. The cells of phellem are dead, suberized and pimpervious to water. Cork cells are airtight and used as bottle stopper or cork. The bottle cork is prepared from the cork of Quercus suber ( Oak tree). Cells of phelloderm are thin walled , living and store food. Phellem , Phellogen and phelloderm are collctively called periderm. Periderm is secondary protective tissue.
Due to pressure of secondary xylem , epidermis ruptures and cortex is largely lost after two or three years of secondary growth. In the cork layer ( bark) the lenticels are present which re meant for gaseous exchange. In cork, lenticels have loosely arranged cells called complementary cells with intercellular spaces. For bottle corks the cork is processed in such a manner so that lenticels come in vertical direction.
Begining of Lenticel Formation under a Stoma Mature Lenticel f –
Epidermis
a - Epidermis
g – Stoma b - Complementary Cells
h - Complementary Cells c. Phellogen
d - Phellem
Bark includes all the dead and living tissues outside the vascular cambium. It may be:
(i)Scaly bark: When develops in strips e.g. Eucalyptus, Psidium.
(ii) Ring bark: When develops in the form of sheet or ring e.g. Betula ( Bhojpatra)
The outermost layer of bark is dead and is called as rhytidome.
In the ancient times the bark of betula was used as paper for writing the manuscripts.
Secondary Growth in Dicot Root:
Vascular bundles in dicot root are radial , exarch and mostly triarch. Vascular cambium is formed secondary from conjuctive parenchyma cells lying just below each phloem strands. The number of cambium strips formed equals the number of phloem strands. The cells of pericycle lying outside the protoxylem also becomes meristematic to form part of strips of cambium. These cambial strips join the first formed cambium strips to form complete but wavy ring of vascular cambium. This cambium ring produces secondary xylem on inner side and secondary phloem on outer side. In roots, the growth rings are not distinct because there is no seasonal variation under the soil. From the outer layers of pericycle arises the phellogen which cuts phellem ( cork ) on the outer side and secondary cortex or phelloderm toward the innerside.
St ages of Secondary Growth in a Typical Dicotyledons Root
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