Chapter 16 Anatomy of Flowering Plants by Teaching Care online coaching classes
The higher plants have highly complex bodies made up of different types of cells. All cells are of same origin but afterwards they gets differentiated into different types of cells. Cells of similar shape and size constitute a group which perform diverse functions. A group of cells performing a particular function is collectively called as tissue. A tissue may be defined as, “a group of similar or dissimilar cells having common origin and performing a specific functions.”
Tissues are mainly divided into three categories :
- Meristematic tissues or Meristems
- Permanent tissue
- Secretory tissue
Meristematic tissues or Meristems.
The word “Meristem” originated from “Meristos” (Greek = continuous division) and the term meristem was introduced by Nageli (1858). A group of cells which are much active and capable of showing continuous divisions and redivisions, is called as meristematic tissue. The various characteristic features of the meristems are discussed below :
- They contain immature and young cells and are capable of repeated
- Intercellular spaces are not present in meristematic
- They contain a homogeneous thin
- They contain large nuclei associated with abundant
- They are metabolically very active but they do not store food
- Only proto-plastids are present instead of plastids, chloroplast
- Dense cytoplasm is present which contains several premature
- Vacuoles are
- Meristematic cells are isodiametric in
- Undifferentiated tissue in which all divides continuously G1 ® S ® G2 ® M.
- Types of meristems : The meristems may be classified on the basis of their mode of origin, position or function :
- According to origin and development : On the basis of origin, meristematic tissues are of three types :
- Promeristem or Primordial meristem : The promeristem originates from embryo and, therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated. A group of initial cells that lay down the foundation of an organ or a plant part, is called promeristem. This group consists of a limited amount of cells, which divide repeatedly to give rise primary meristem. It occupies a small area at the tips of stem and The promeristem gives rise to all other meristems including the primary meristem.
- Primary meristem : A primary meristem originates from promeristem and retains its meristematic It is located in the apices of roots, stems and the leaf primordia. Primary meristem gives rise to the primary permanent tissue.
- Secondary Meristem : They always arise in permanent tissues and have no typical promeristem. Some living permanent cells may regain the meristematic nature. This process in which permanent tissue regains meristematic nature is called dedifferentiation. The secondary meristems are so called because they originate from permanent cells. The phellogen or cork cambium arising from epidermis, cortex or other cells during secondary growth, is an important example of secondary meristem. The secondary meristems produce secondary tissues in the plant body and add new cells for effective protection and
- According to position : On the basis of their position in the plant body meristems are classified into three categories :
- Apical meristem : This meristem is located at the growing apices of main and lateral shoots and These cells are responsible for linear growth of an organ. The initiating cells may be single or in groups. Solitary initial cells are known as apical cells whereas those occurring in groups are called apical initials. Solitary apical cells occur in ferns and other Pteridophytes while apical initials are found in other vascular plants. The apical initials may occur in one or more tiers. Position of apical cells may either be strictly terminal or terminal and subterminal.
- Intercalary meristem : These are the portions of apical meristems
which are separated from the apex during the growth of axis and formation of permanent tissues. It is present mostly at the base of node (e.g., Mentha viridis– Mint), base of internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophyts like Equisetum) or at the base of the leaf (e.g., Pinus). The intercalary meristems ultimately disappear and give rise to permanent tissues.
- Lateral meristem : These meristems occur laterally in the axis, parallel to the sides of stems and roots. This meristem consists of initials which divide mainly in one plane (periclinal) and result increase in the diameter of an organ. The cambium of vascular bundles (Fascicular, interfascicular and extrastelar cambium) and the cork cambium or phellogen belong to this category and are found in dicotyledons and
- According to function : Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types :
Fig : Various meristamatic tissue
- Protoderm : It is the outermost layer of the apical meristem which develops into the epidermis or epidermal tissue system.
- Procambium : It occurs inside the Some of the cells of young growing region which by their elongation and differentiation give rise to primary vascular tissue, constitute the procambium.
- Ground meristem : It constitute the major part of the apical meristem develops ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary
- According to plane of cell division : On the basis of their plane of cell division meristem are classified into three categories :
- Mass meristem : The cells divide anticlinally in all planes, so mass of cells is formed. g., formation of spores, cortex, pith, endosperm.
- Plate meristem : The cells divide anticlinally in two planes, so plate like area e.g., formation of epidermis and lamina of leaves.
- Rib or File meristem : The cells divide anticlinally in one plane, so row or column of cells is
e.g,, formation of lateral root.
(2) Structure and organisation of apical meristem
- Vegetative shoot apex : Shoot apex was first recognized by Wolff (1759) shoot apex is derived from meristem present in plumule of embryo and occurs at the tip of stem and its branches as terminal It also occurs in the inactive state in the axils of leaves as lateral buds. The tip of the shoot apex is dome-shaped and from its flanks at the base of the dome divide to form one or more leaf primordia. This continues throughout the vegetative phase. Many theories have been put forward to explain shoot apex, such as :
- Apical cell theory : This theory was proposed by Nageli (1858). According to this theory, shoot apical meristem consists of single apical This theory is applicable in case of higher algae, bryophytes and in many pteridophytes but not in higher plants (i.e., gymnosperms and angiosperms).
- Histogen theory : It was proposed by Hanstein (1870). According to this theory, the shoot apical meristem consists of three distinct meristematic zones or layers (or histogens).
- Dermatogen : Outermost layer and it forms epidermis and epidermal tissue
- Periblem : It is the middle layer gives rise to cortex and endodermis.
- Plerome : Innermost layer forms pith and stele.
- Tunica corpus theory : This theory was proposed by Schmidt (1924). According to this theory, the shoot apex consists of two distinct
- Tunica : It is mostly single layered and forms The cells of tunica are smaller than corpus. The tunica shows only anticlinal division and it is responsible for
Procambium Ground meristem
Fig : L.S. vegetative shoot apex
- Corpus : It represents the central core with larger cells. Corpus shows divisions in all planes and it is responsible for volume
- Root apex : A group of initial cells, present at the subterminal region of the growing root tip, which is protected by a root cap is called root apical meristem or root It is embryonic in origin and formed from the radicle part of embryo. However, in adventitious roots it is produced from derivatives of root apex. The root apex differs from shoot apex as it is short and more or less uniform due to complete absence of lateral appendages (leaves and branches) and differentiation of nodes and internodes. According to Hanstein (1870) root apex of most of the dicotyledons also consists of three meristematic zones – plerome, periblem and dermatogen (fourth meristem calyptrogen to form root cap only in monocots). Regarding the apical organisation of root following theories have been put forward.
- Korper-Kappe theory : It was proposed by Schuepp (1917). This theory is comparable with the tunica and corpus theory of shoot Korper means body and Kappe means cap.
- Quiescent centre theory : It was proposed by Clowes (1961).
According to him, in addition to actively dividing cells, a zone of inactive cells is present in the central part of the root apex called quiscent centre.
The cells in this region have light cytoplasm, small nuclei, lower concentration of DNA, RNA and protein. These cells also contain fewer number of mitochondria, less endoplasmic reticulum and small dictyosomes.
Types of root apex : It is divided into following four types :
Fig : L.S. root apical meristem
- Ranunculus type : Root apex is made up of only one type of histogen e.g., Plants of family Ranunculaceae, Leguminosae and Amentiferae.
- Casuarina type : Root apex is made up of two types of histogen e.g, Plants of family Casurinaceae, Leguminosae and Proteaceae.
- Common dicot root : Root apex is made up of three layers of
- Common monocot root : Root apex is made up of four layers of
- Reproductive apex : During reproductive phase, the vegetative apices are converted into reproductive Before conversion, the apex stops
producing leaf primordia. The summit of the apex which remained inactive during the vegetative phase, starts dividing. As a result of cell divisions, the apical meristem undergoes change in shape and increase in size. The apex may develop into a flower or an inflorescence. When the apex is to develop into a single flower, the cells at the flanks of the apex produce sepals and petals while the cells in the centre of summit produce stamens and carpels.
Fig : L.S. Reproductive apex (diagrammatic)
Permanent tissues are made up of mature cells which have lost the capacity to divide and have attained permanent shape, size and function due to division and differentiation
in meristematic tissues. The cells of these tissues are either living or dead, thin-walled or thick-walled. Permanent tissues are of three types :
- Simple tissues : Simple tissues are a group of cells which are all alike in origin, form and function. They are further grouped under three categories :
- Parenchyma : Parenchyma is most simple and unspecialized tissue which is concerned mainly with the vegetative activities of the
The main characteristics of parenchyma cells are :
- The cells are thin-walled and
Fig : Parenchyma in T.S.
Intercellular space Vacuoles
- The cells usually are living and possess a distinct
- The cells contain well-developed intercellular spaces amongst
- The cytoplasm is vacuolated and cell wall is made up of
- The shape may be oval, spherical, cylindrical, rectangular and stellate (star shaped) in leaf petioles of banana and canna and some
- This tissue is generally present in almost all the organs of plants, e., roots, stems, leaves, flowers, fruits and seeds.
- If they enclose large air spaces they are called as aerenchyma; if they develop chlorophyll, they are called as chlorenchyma and if they are elongated cells with tapering ends, they are called as prosenchyma.
Functions : They perform the following functions :
- Storage of food e.g., Carrot, Beetroot etc.
- Chlorenchyma helps in
- Aerenchyma helps in floating of the aquatic plants (Hydrophytes) and also help in gaseous exchange during respiration and e.g., Hydrilla.
- In turgid state they give rigidity to the plant
- In emergency they behave like meristematic cells and help in healing of the various plant injuries.
- Sometimes they store secretory substances (ergastic substance) such as tannins, resins and gums and they called as idioblasts.
- Collenchyma : The term collenchyma was coined by Schleiden (1839). It is the tissue of primary body. The main characteristics of are given below :
- The cells of this tissue contain protoplasm and are
- The cell walls are thickened at the corners and are made up of cellulose, hemicellulose and pectin.
- They are never lignified but may posses simple
- They are compactly arranged cells, oval, spherical or polygonal in
- intercellular spaces are present.
- The tissue is plastic, extensible and have capacity to
Fig : (A) Collenchyma L.S. (B) and (C)T.S. of the same
- They provide mechanical strength to younger part where xylum is less
Collenchyma occurs chiefly in the hypodermis of dicotyledonous stems (herbaceous, climbers or plants e.g.
Cucurbeta, Helianthus) and leaves. They are usually absent in monocots and in roots.
- Types of collenchyma : Majumdar (1941) divided collenchyma into three types on the basis of thickening :
- Angular collenchyma : Where the thickening of the cells is confined to the corners of the e.g., Tagetes, Tomato, Datura, Potato, etc.
- Plate or Lamellar collenchyma : When the thickenings are present in the tangential e.g.
hypodermis of sunflower stem.
- Lacunar or Tubular collenchyma : If the thickened cell wall is associated with intercellular spaces of the adjacent e.g. leaf petioles of compositae and malvaceae etc. hypodermis of Cucurbita stem, Salvia, Malva.
- Provide mechanical support to petiole, pedicels, branches of stem, roots and
- If they contain chlorophyll they help in
- It is present at the margins of some leaves and resists tearing effect of the
- Sclerenchyma : It was discovered and coined by Mettenius (1805). The main feature of sclerenchyma are :
- It consist of thick-walled dead
- The cells vary in shape, size and
- They possess hard and extremely thick secondary walls due to uniform deposition of
- In the beginning the cells are living and have protoplasm but due to deposition of impermeable secondary walls they become
Types of sclerenchyma : They are of two types :
- Sclerenchymatous fibres : These are greatly elongated and tapering at both the ends. The fully developed fibre cells are always dead. They are polygonal in transverse section and walls are highly lignified. Intercellular spaces are absent and lumen is highly obliterated. The walls show simple and oblique pits. They provide mechanical strength to the plant. Some of the longest fibre yielding plants are Linum usitatissimum (Flax or Alsi), Corchorus, Cannabis, etc. The fibres are present in hypodermis of monocot stem, in pericycle of many dicots, in secondary wood and vascular bundle sheath in monocot There are three different kinds of fibres :
- Bast fibres : The fibres present in the pericycle (g., Cannabis sativa / Hemp or Bhang), Linum usitatissimum and phloem (e.g., Corchorus capsularis (Jute), Hibiscus cannabinus (Patsan), Calotropis, Nerium, Sunn hemp etc.). These fibre are also known as extraxylary fibres.
Fig : Scalerenchymatous fibres (A) L.S. (B) T.S.
- Wood fibres : Those fibres which are associated with wood or xylem have bordered pits are known as wood fibres. Thick walled wood fibres having simple pits are called libriform fibres whereas thin walled wood-fibres having bordered pits are called fibre-tracheids. A specific type of wood fibre is produced by Quercus rabra and is called gelatinous or mucilagenous fibres.
- Surface fibres : The fibres present over surface of plant organs are called surface fibres. g. Cotton fibres found in the testa of seeds, mesocarp fibres of Coconut (Cocus nucifera).
- Stone cells or Sclereids : They are lignified, extremely thick walled so that the lumen of the cells is almost obliterated and may be spherical, oval, cylindrical, T-shaped and even They are generally found in hard parts of the plant, e.g., endocarp of Walnut and
Coconut. They form part of seed coat in some members of leguminosae. The sclereids provide mechanical support and hardness to the soft parts. Sclereids may be :
- Brachy-sclereids or stone cells : These are small and more or less isodiametric in shape. They occur in the cortex, pith, phloem, and pulp of fruits (g., Pyrus).
- Macrosclereids or rod cells : These are rod- shaped elongated sclereids usually found in the leaves, cortex of stem and outer seed coats.
- Osteosclereids or bone cells : These are bone or barrel-shaped sclereids dilated at their ends. g., leaf of Hakea.
- Astrosclereids or stellate cells : These are star- shaped sclereids with extreme lobes or arms. g., leaf of Nymphaea.
Fig : Stone cells (A, B) from pulp of pear, (C,D) from stem cortex of Hoya, (E, F) from petiole of Camelia, (G) from stem cortex of Trochodendron, (H) from mesophyll cells of fig leaf
- Trichosclereids or internal hairs : These are hair-like sclereids found in the intercellular spaces in the leaves and stem of some
- Complex tissues : A group of more than one type of cells having common origin and working together as a unit, is called complex permanent The important complex tissues in vascular plants are : xylem and phloem. Both these tissues are together called vascular tissue.
- Xylem : The term xylem was introduced by Nageli (1858). Xylem is a conducting tissue which conducts water and mineral nutrients upwards from the root to the leaves.
- Complex tissues : A group of more than one type of cells having common origin and working together as a unit, is called complex permanent The important complex tissues in vascular plants are : xylem and phloem. Both these tissues are together called vascular tissue.
On the basis of origin xylem is of two types
- Primary xylem : It is derived from procambium during primary growth. It consists of protoxylem and
- Secondary xylem : It is formed from vascular cambium during secondary
Xylem is composed of four types of cells
- Tracheids : Term “Tracheids” was given by Sanio (1863). The tracheids are elongated tubelike cells with tapering or rounded or oval ends with hard and lignified walls.
The walls are not much thickened. The cells are without protoplast and are dead on maturity. The tracheids of secondary xylem have fewer sides and are more sharply angular than the tracheids of primary xylem. The cell cavity or lumen of a tracheid is large and without any contents. Tracheids possess bordered pits. Maximum bordered pits are formed in gymnospermous tracheids. They also possess various kinds of thickenings, e.g., annular, spiral,
scalariform, reticulate or pitted tracheids. All the vascular plants have tracheids in their xylem. The main function of tracheids is to conduct water and minerals from the root to the leaf. They also provide strength and mechanical support to the plant.
- Xylem vessels or Tracheae : Vessels are rows of elongated tube-like cells, placed end to end with their end walls Vessels are multicellular with wide lumen. The vessels may be classified into several types according to the thickening developed in their wall. They may be annular, spiral, scalariform, reticulate or pitted. Vessels are absent in pteridophytes and gymnosperms (except Ephedra, Gnetum, Selaginella, Pteridium). In angiosperms (porous wood) vessels are always present (Vessels are absent in family – Winteraceae, Trochodendraceae and Tepacenpaceae of Angiosperm i.e. Lotus, Wintera, Trochodendron). Vessels along with tracheids forms the main tissue of xylem of vascular bundles of the
angiosperms and help in conduction. It also provide mechanical
support to the plant.
Fig : Xylem-A Tracheids, B. Tracheae, C and E. Xylem parenchyma D. Wood fibres (wood sclerenchyma)
On the basis of distribution and size of vessels, porous wood is of two types :
- Diffuse porous wood (Primitive) : Vessels of same size are uniformly distributed throughout the growth or annual ring g., Pyrus, Azadirachta, Eucalyptus, Mangifera sp., Betula. They are characteristics of plants growing in tropical region.
- 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 g. Quercus, Morus, Cassia, Delbergia, Tilea sp.
- Wood (xylem) parenchyma : These are the living parenchymatous cells. As found associated with xylem they are known as wood They serve for the storage of reserve food and also help in conduction of water upwards through tracheids and vessels.
- Wood (xylem) fibres : The long, slender, pointed, dead and sclerenchymatous cells found associated with xylem are termed wood They possess mostly thickened walls and few small pits. These pits are found abundantly in woody dicotyledons. They aid the mechanical strength of xylem and various organs of plant body.
- Phloem (bast) : Term “Phloem” was given by Nageli. Its main function is the transport of organic food materials from leaves to stem and roots in a downward
On the basis of position phloem is of three types :
- External phloem : It is normal type and present outside the xylem g., Mostly angiosperms and gymnosperms.
- Internal or Intraxylary phloem : It originates from procambium and is primary phloem which occurs on innerside of primary xylem. It is primary anamolus structure. g., Members of Apocynaceae, Asclepiadaeae, Convolvulaceae, Solanaceae.
- Induced or Interxylary phloem : It originates from cambium and is secondary phloem which occurs in groups within the secondary It is secondary anamolus structure. e.g., Leptadaenia, Salvadora, Chenopodium, Boerhaavia, Amaranthus.
On the basis of origin phloem is of two types
- Primary phloem : It is formed by procambium during primary growth. It may or may not show differentiation of in protophloem (consists of sieve elements and parenchyma) and metaphloem (develop after protophloem and consists of sieve elements, parenchyma and fiber). During the primary growth the protophloem elements are curshed by the surrounding tissues and This process is known as obliteration consists of sieve elements, parenchyma and fibre.
- Secondary phloem : It is produced during secondary growth by vascular It consists of the following elements :
Sieve element Companion cells Phloem parenchyma
Phloem fibres or bast fibres
(1) Sieve element
- They are long tube-like cells placed end to end, forming a continuous channel in the plant
- Their cell wall is made up of
- Their transverse wall is perforated like a normal sieve and hence they are called as sieve
- Nucleus is not found in these
- Each sieve tube has a lining of cytoplasm near its
- Callus pad may be visible in the winter
- Their main function is to translocate the food material from one part to the
(2) Companion cells
- They are thin-walled cells which are associated with sieve
- They are more or less
- They are connected with the sieve tube through sieve
- They contain nucleus and are therefore, living in
- They are not found in pteridophytes and A
gymnosperms but are always present in angiosperms. Fig : Parts of Phloem (A) L.S. of phloem tissue, (B) T.S. of
phloem tissue, (C) Sieve tubes of Vitis, (D) L.S. of sieve plate
- Phloem parenchyma : The parenchyma associated with the phloem is called phloem The cells are elongated with rounded ends and possess cellulosic cell walls. These cells are living and store food reserves in the form of starch and fats. They are present in pteridophytes and most of dicotyledonous angiosperms. They are absent in monocots.
- Phloem or Bast fibres : The sclerenchymatous fibres associated with the phloem are called as phloem These are also known as bast fibres. The fibres are elongated lignified cells with simple pits. The ends of these cells may be pointed, needle like or blunt. They are non-living cells that provide mechanical support to the organs.
Special or Secretory tissues.
These tissue perform special function in plants, e.g., secretion of resins gum, oil and latex. These tissues are of two types :
- Laticiferous tissues
- Glandular tissues
- Laticiferous tissues : They are made up of thin walled, elongated, branched and multinucleate (coenocytic) structures that contain colourless, milky or yellow coloured juice called These occur irregularly distributed in the mass of parenchymatous cells. latex is contained inside the laticiferous tissue which is of two types :
- Latex cells : A laticiferous cell is a very highly branched cell with long slender processes ramifying in all directions in the ground tissue of the organ. They do not fuse and do not form network. Plants having such tissues are called simple or non-articulated laticifers. g., Calotropis (Asclepiadaceae) Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).
- Latex vessels : They are formed due to fusion of cells and form network like structure in all At maturity, they form a highly ramifying system of channels full of latex inside the organ. Plants having such tissues are called compound or articulated laticifers. e.g., Argemone, Papaver (Papaveraceae), Sonchus (Compositae), Hevea, Manihot (Euphorbiaceae).
- Glandular tissue : This is a highly specialized tissue consisting of glands, discharging diverse functions, including secretory and Glands may be external or internal.
- External glands : They are generally occur on the epidermis of stem and leaves as glandular hair in Plumbago and Boerhaavia, stinging hair secrate poisonous substance in Urtica dioica, nectar secreting glands in flowers or leaves. g., Rutaceae and Euphorbiaceae. Digestive enzyme secreting glands in insectivorous plants e.g., Drosera (Sundew), Nepenthes (Pitcher plant).
- Internal glands : These are present internally and are of several types. g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus, mucilage canals in Cycas. Water secreting glands (hydathodes) in Colocasia (present at the tip of leaves), Tropaeoleum (along margin), etc. The glands which secrete essential oil are called osmophores (osmotrophs).
Acc. to origin and development
Acc. to position
Acc. to function
Acc. to plane of cell division
- Parenchyma : Cells are
- Promeristem :
Gives rise to primary
- Apical meristem : 1. Protoderm : It Responsible for secondary develops into the
- Mass meristem : Forms mass of cell g., formation
living, soft with thin cellulosic wall and intercellular spaces. Present
growth of an organ.
epidermis or epidermal tissue system.
of spores, cortex, pith, endosperm etc.
in all the plant organs e.g., roots,
stems, leaves, fruit, seeds etc.
- Primary meristem : Gives rise to primary permanent
- Secondary meristem :
- Intercalary meristem
: Present at the base of 2. Procambium : node (e.g., Mint) or Gives rise to primary internode (e.g., stems of vascular tissue,
many monocots, i.e. constitute procambium. Wheat, Grasses etc) or at
- Plate meristem :
Increase plate like area
e.g. formation of epidermis and lamina of leaves.
- Collenchyma : Compactly arranged living cells with thick wall of pectin and hemicellulose and no intercellular
Produce secondary tissues
the base of leaf e.g., Pinus.
- Ground meristem :
- Rib or file meristem :
e.g., phellogen or cork 3. Lateral meristem :
cambium, interfascicular Increases diameter of
Develops ground tissue
e.g., hypodermis, cortex,
Form row or column of cells e.g., formation of
Angular Plate or
cambium, cambium etc.
endodermis, pericycle, pith, medullary rays.
e.g., Tagetus, Tomato, Datura, Potato etc.
e.g., hypodermis of Sunflower stem.
e.g., hypodermis of Cucurbita stem.
- Sclerenchyma : Thick walled dead cells due to uniform deposition of lignin.
Sclerenchymatous fibres : Elongated and tapering at both the ends e.g., Cannabis Sativa, Linum, Corchorus, Cocus etc.
Stone cells or tracheids : May be spherical, oval or cylendrical e.g., endocarp of Walnut and Coconut, occur in pulp of some fruits, part of seed coat.
Complex tissue Special or secretory tissue
Xylem Conducting element which conducts water and mineral nutrients upwards from the root to the leaves.
- Tracheids : Elongated tube like cells with tapering or rounded or oval ends, with hard and lignified e.g., present in xylem of all vascular plant.
- Xylem vessels or trachae : Elongated tube like cells, placed end to end. g., always present in angiosperms (except lotus, Winifera, Trochodendron) and absent in pteridophytes and Gymnosperms (except Ephedra, Gnetom, Selaginella Pteridium).
- Wood parenchyma : Living parenchymatous cells associated with
- Wood or xylem fibres : Long, slender, pointed, dead and sclerenchymatous. g., found in woody dicotyledons.
elements which transport the organic food material downward from leaves to stems and roots.
Sieve elements : Nucleus is not found. In angiosperm it occurs as sieve plate while in pteridophytes and
gymnospermum as sieve cells (have nuclei).
Companion cells : Contain nucleus. Not found in pteridophytes and gymnosperms but always present in angiosperms.
Phloem parenchyma : Parenchymatous living cells with cellulosic cell wall and nucleus. e.g., present in pteridophytes and some of dicotyledonous angiosperms but absent in monocotyledons.
Phloem or Bast fibres : Sclerenchymatous fibres associated with phloem abundantly found in sec. phloem e.g., Jute, Hamp.
Glandular tissue : Highly
specialized tissue consisting of glands including secretory or exretory.
- External glands : Occurs in epidermis of stems and leaves as glandular hair in Plumbago and Boerhaavia, stinging hair secrete poisonous substance in Utrica dioica, nector secreting glands in flowers or leaves of Rutaceae, Euphorbiaceae, digestive secreting glands in sectivorous plants g., Nepenthes (Pitcher plant), Drosera (Sundew).
- Internal glands : Present internally g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus, schizogenous canal and mucilage canals in Cycas, water secreting glands hydathodes in Colocasia (present at the tip of leaves); Tropaeoleum (along margin), etc.
Laticiferous tissue : Thin walled branched, elongated and multinucleate structure that contain latex.
- Latex cells : Very highly branched cell which do not fuse and do not form e.g.,Calotropis, Nerium, Vinca, Euphorbia, Ficus.
- Latex vessels : Formed due to fusion of cells and form network like structure in all directions g., Argemone Papaver, Sonchus, Hevea, Manihot.
The tissue system.
The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “Tissue System”. According to Sachs (1975) there are three major tissue systems in plants as follows :
- Epidermal tissue system (2) Ground or fundamental tissue system (3) Vascular tissue system
- Epidermal tissue system : The tissues of this system originate from the outermost layer of apical It forms the outermost covering of various plant organs which remains in direct contact with the environment.
- Epidermis : Epidermis is composed of single layer These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some cases epidermis may be multilayered e.g. Ficus, Nerium, Peperomia, Begonia etc.
The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces. Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types :
- In aerial roots, the multiple epidermal cells are modified to velamen, which absorb water from the atmosphere (g., Orchids).
- Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor It is hygroscopic in nature. e.g., Ammophila. They are thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves.
- Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short The short cells may be cork cells or silica cells.
- Cuticle and Wax : In aerial parts, epidermis is covered by cuticle. The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all it helps in reducing the loss of water by evaporation. Usually the cuticle is covered with wax which may be deposited in the form of granules, rods, crusts or viscous semiliquid masses. Other substances deposited on the cuticle surface may be oil, resin, silicon and salts (cystoliths are crystals of calcium carbonate, e.g., Ficus. Druse and Raphides, e.g., Pistia) are crystals of calcium oxalate. Thick cuticle are found in leaves of dry habitats plants.
- Stomata : Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard Stomata are absent in roots. In xerophytes the stomata are sunken in grooves due to which rate of transpiration is greatly reduced (e.g. Nerium). Usually there is a large air cavity below each aperture, it is called substomatal cavity. In some species the guard cells are surrounded by subsidiary cells or accessory cells which differ morphologically from the other epidermal cells. In monocots e.g., Doob, Maize guard cells are dumb bell shape. Stomata are scattered in dicots leaves but they are arranged in rows in monocots.
Depending upon distribution of stomata, the leaves are :
- Apple-mulberry type : g. Oxalis, Mulberry, Apple.
- Potato type : g. Bifacial (dorsiventral leaves of pea, bean, tomato).
- Oat type : g. Suberect (isobillateral) leaves of most grasses and cereals (monocotyledens).
- Nymphea type : g. Floating leaves of Nelumbo, Nymphia, water lily.
- Potamogeton type : g. Submerged plants like Hydrilla, Vallisneria, Potamogeton.
- Trichomes : These are epidermal outgrowths present temporarily or permanently on almost all plant They may be unicellular or multicellular and vary in size and shape in different species. They may be of different types : stellate hair, glandular hair, short glandular hair, floccose hair, urticating hair and stinging hair.
The trichomes serve for checking excess loss of water and for protection.
- Root hairs : They are enlargements of special epiblema cells called trichoblasts and occurs in a particular zone of young root called root hair A root hair cell has vacuolated protoplast with nucleus present towards the apical part of hair. They are specialised to absorb water from soil crevices. They also hold soil particles.
B D F
Fig : Appendages of epidermis of leaves
- Stellate hair of Alyssum, B. Glandular hair of Pelorgonium C. Short glandular hair of lavandula, D. Floccose hair of Malva, E. Glandular hair of Solanum,
- Urticating hair of Verbascum, G. Stinging hair of Cestus
- Ground or Fundamental tissue system : Ground tissue system includes all the tissues of plant body except epidermal tissue system and vascular tissues. It forms the bulk of body. This tissue system mainly originates from ground The ground tissues constitute the following parts :
- Cortex : It lies between epidermis and the pericycle. The cortex is distinct in dicotyledons but not in monocotyledons where there is no clear demarcation between cortex and It is further differentiated into :
- Hypodermis : It is collenchymatous in dicot stem and sclerenchymatous in monocot It provides strength.
- General cortex : It consists of parenchymatous Its main function is storage of food.
- Endodermis (Starch sheath) : It is mostly single layered and is made up of parenchymatous barrel shaped compactly arranged The inner and radial or transverse wall of endodermal cells have casparian strips of suberin. In roots thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem pathches. These thin walled endodermal cells are called passage cells or transfusion cells. A fully developed endodermis is found in all types of roots. Endodermis with characteristic casparian bands is absent in woody dicot stem, monocot stem and leaves of angiosperms. The young stems of angiosperms show a layer with abundant starch deposition. This layer occurs in the position where endodermis would have been situated which is called as starch sheath. In Selaginella trabeculate endodermis is found due to formation of air spaces between two endodermal cells.
Endodermis behave as water tight dam to check the loss of water and air dam to check the entry of air in xylem elements. Endodermis is internal protective tissue.
- Pericycle : It is a single layered or multilayered cylinder of thin-walled or thick-walled cells present between the endodermis and vascular tissues. In some cases, the pericycle is made up of many layers of sclerenchymatous cells (Cucurbita stem) or in the form of alternating bands of thin-walled and thick-walled cells (Sunflower stem). In case of roots, the pericycle is made up of thin-walled parenchymatous cells which later on gives
rise to lateral roots. In dicot roots the cork cambium originates in the pericycle which results in the formation of periderm. Pericycle also gives rise to a part of vascular cambium in dicot roots.
- Pith or Medulla : It occupies the central part in dicot stem, and monocot root. It is mostly made up 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 known as primary medullary rays or pith rays. They help in lateral
- Vascular tissue system : The central cylinder of the shoot or root surrounded by cortex is called The varying number of vascular bundles formed inside the stele constitute vascular tissue system. Xylem, phloem and cambium are the major parts of the vascular bundle. Vascular bundle may be of following types :
- Radial : The xylem and phloem strands alternate with each other separated by parenchymatous cells. such kinds of vascular bundles are called radial and found mainly in roots.
- Conjoint : A vascular bundle having both xylem and phloem together, is called conjoint. Normally the xylem and phloem occur in the same They occur in stems. Such vascular bundles are of two types :
- Collateral : A vascular bundle in which the phloem lies towards outerside and xylem towards inner side, is called collateral, g., Sunflower.
Collateral bundle having a cambium between xylem and phloem is said to be of the open type, e.g., Dicot stem.
Collateral bundle lacking a cambium between xylem and phloem is said to be of the closed type, e.g., Monocot stem.
- Bicollateral : A vascular bundle having the phloem strands on both outer and inner side of xylem, is called bicollatera e.g., Cucurbita.
- Concentric : A vascular bundle in which one tissue is completely surrounded by the other, is called concentric. The concentric bundles are of two types :
- Amphivasal (Leptocentric) : The phloem lies in the centre and remains completely surrounded by e.g., Dracaena, Yucca.
Fig : Different types of vascular bundles
- Amphicribal (Hadrocentric) : The xylem lies in the centre and remains completely surrounded by e.g., Ferns.
Stelar theory was proposed by Van Tieghem and Douliot (1886). According to this concept primary body of root and stem are basically alike anatomically i.e. each consists of a central stele surrounded by cortex. Stele
includes the vascular tissues and the ground tissue like pericycle and pith, when present. Different types of steles were recognised, a brief review of which are given in table :
|Types of stele||Diagrammatic representation|
|(1) Protostele : This term was given by Jeffrey. It is the simplest and most primitive type of stele in which central core of xylem surround by phloem.|
|(i) Haplostele : It consists of a smooth core of xylem which is surrounded by a ring of phloem. e.g., Rhynia, Selaginella, Lyco podium, etc.||Endodermis Pericycle|
|(ii) Actinostele : Protostele having star shaped xylem core with many radiating arms called actinostele. e.g. Psilotum, Lycopodium etc. It may be of two types :|| |
Phloem Endodermis Xylem
|(a) Plectostele : A protostele in which xylem core broken into a number of parallel plates is known as plectostele. e.g., Lycopodium clavatum.||Phloem Xylem|
|(b) Mixed protostele : A protostele in which xylem broken into small group or patches is known as mixed protostele. e.g., Lycopodium cernuum, Hymenophyllum demissum, etc.|| |
Endodermis Parenchyma Pericycle
|(2) Siphonostele : A protostele with central pith is called siphonostele or medullated stele. It is considered to be derived phylogenetically from protostele and thus represents an advance form. It is of two types :|
|(i) Ectophloic siphonostele : When phloem occurs on the outer side of xylem. e.g., Osmunda.||Xylem Phloem|
|(ii) Amphiphloic siphonostele : When phloem is present on both external and internal sides of the xylem. e.g., Marsilea, Adiantum.|| |
Outer Outer phloem endodermis
|Modification of siphonostele|
|(i) Solenostele : A siphonostele with non- overlapping leaf gaps is known as solenostele. It may be ectophloic or amphiphloic.||Phloem Leaf gap|
|(ii) Dictyostele : A siphonostele with overlapping leaf gaps is known as dictyostele. It has many scattered vascular strands called as meristeles. e.g., Dryopteris, Pteris Ophioglossum.||Inner Xylem Outer phloem pericycle|
Leaf gap Leaf trace
endodermis Inner phloem endodermis
|(iii) Polycyclic stele : When vascular tissue is present in the form of two or more concentric cylinders. e.g., Pteridium, aquilinum, Marattia. It may be polycyclic solenostele or polycyclic dictyostele.||Meristeles Leaf gap Solenostelic|
Leaf trace Siphonostelic cylinder
|(iv) Polysteles : Somtimes more than one steles are present in the axis of some pteridophytes. e.g., 2 steles in Selaginella kraussiana, 16 steles in S. laevigata.|| |
Internal structure of root, stem and leaf.
(1) Functions of different organs and tissues of a plant tissue system
|(i) Functions||(i) Absorb water and minerals.|
(ii) Anchor plant.
(iii) Store materials.
|(i) Transport water and nutrients.|
(ii) Support leaves.
(iii) Help store materials.
|Carry on photosynthesis.|
|(a) Epidermis||Root hairs absorb water and minerals.||Protect inner tissues.||Stomata carry on gas exchange.|
|(b) Cortex||Store products of photosynthesis and water.||Carry on photosynthesis if green.|
|(c) Endodermis||Regulates passage of minerals into vascular cylinder.||Regulates passage of minerals also into vascular tissue, if present.||Regulate passage of minerals into vascular tissue if present.|
|(d) Vascular||Transport water and nutrients.||Transport water and nutrients.||Transport water and nutrients.|
|(e) Pith||Store products of photosynthesis and water.||Store products of photosynthesis.|
(i) Spongy layer
(ii) Palisade layer
Carry on gaseous exchange and photosynthesis.
- Difference between internal structure of root and stem
|(i) Epidermis or Epiblema||Epiblema or piliferous layer without cuticle.||Epidermis usually with cuticle.|
|(iii) Chlorenchyma in cortex||Absent.||Usually present in young stems but absent in old stem.|
|(iv) Endodermis||Very distinct.||Poorly developed or absent.|
|(v) Vascular bundle||Radial.||Conjoint collateral or bicollateral or concentric.|
Origin of Lateral roots : Lateral roots arise endogenously i.e., form the cells inside the endodermis. They arise from pericycle cells.
(3) Difference between dicot and monocot leaf
|Character||Dicot leaf||Monocot leaf|
|(i) Type of leaf||Dorsiventral (bifacial).||Isobilateral.|
|(ii) Stomata||Usually more on lower epidermis.||Equal on lower and upper epidermis (amphistomatic).|
|(iii) Mesophyll||Made up of two types of tissues|
(a) Palisade parenchyma.
|Only spongy parenchyma is present which has very small intercellular spaces.|
|(b) Spongy parenchyma with large intercellular spaces.|
|(iv) Bundle sheath||Made up of parenchyma. Just above and below the vascular bundle some parenchymatous cells or collenchymatous cells are present (upto epidermis).||Made of parenchyma but just above and below the vascular bundles are found sclerenchymatous cells (upto epidermis).|
|(v) Bulliform or motor cells||Absent.||Present on upper epidermis.|
Dicot leaf Monocot leaf
Fig : Comparison of T.S. of a dicot and monocot leaf
Kranz type anatomy occurs in both monocot and dicot leaves of some tropical and arid areas. Karanz anatomy is characteristic feature of C4 plants. The mesophyll is undifferentiated and occurs in concentric layers around vascular bundles. Cells of bundles sheath posses large chloroplast.
(4) Difference between dicot and monocot stem
|Characters||Monocotyledonous Stem||Dicotyledonous Stem|
|(i) Epidermis||Present, cells comparatively smaller and without hair.||Present, cells larger and with hair|
|(ii) Hypodermis||Sclerenchymatous (non-green)||Collenchymatous (green)|
|(iii) Cortex||Absent, but ground tissue is present from hypodermis to the centre of stem||Made up of several layers of parenchymatous tissue.|
|(iv) Endodermis||Absent||One layered, starchy sheath which is usually not well differentiated.|
|(v) Pericycle||Absent||Made up of 1 or more layers of parenchymatous and sclerenchymatous cells.|
|(vi) Medullary rays||Absent||Found in between vascular bundles|
|(vii) Pith (Medulla)||Absent||Abundant, made up of parenchymatous cells situated in the centre of stem.|
|(viii) Vascular bundles||Scattered|
Conjoint, Collateral and closed
|Vascular bundles in a ring|
Conjoint, collateral and open
|Larger towards centre||All of same size|
|Bundle sheath present||Bundle sheath absent|
|Phloem parenchyma absent||Phloem parenchyma present|
|Xylem vessels either Y or V shaped||Xylem vessels more radial|
Fig : Comparision of the T.S. of monocot and dicot stem
(5) Difference between dicot and monocot root
|Character||Dicot Root||Monocot Root|
|(i) Pericycle||Gives rise to secondary roots and lateral meristem||Gives rise to lateral roots only|
|(ii) Vascular bundles||Diarch to hexarch||Hexarch to polyarch|
|(iii) Cambium||Develops at the time of secondary growth||Absent|
|(iv) Pith||Absent or poorly developed||Abundant and fully developed|
|(v) Secondary growth||Takes place||Does not take place|
|Narrow cortex. Endodermis is less thickened and casparian strips are more prominent.||Cortex wide. Casparian strips are visible only in young root. Later on endodermal cells become highly thickened.|
Monocot root Dicot root
Fig : Cmparision of the T.S. of monocot and dicot root
The increase in thickness or girth due to the activity of the cambium and the cork cambium is known as
- Secondary growth in stem : On the basis of the activities of cambium and cork-cambium, secondary growth in stem can be discussed under the following heads :
(i) Activity of cambium (ii) Activity of cork-cambium
- Activity of cambium : The vascular cambium in between xylem and phloem is called intrafascicular or fascicular cambium which is primary in origin. At the time of secondary growth the parenchymatous cells of medullary rays between the vascular bundles become meristematic and form strip of cambium called as interfascicular cambium which is secondary in origin. Both inter and intrafascicular cambium joins together and form cambium ring which is partly primary and partly secondary in origin. By anticlinal divisions the circumference of the cambium increase. By periclinal division cambium produced the secondary xylem and phloem tissues on innerside and outerside. The amount of sec. xylem produced is 8-10 times greater than sec. The cambium has two types of cells :
- The fusiform initials which are elongated and form fibres, sieve cells, sieve tubes, tracheids.
- Ray initials which produce parenchyma cells of the rays in wood and phloem. Ray initials are much shorter than fusiform initials. Certain cells of cambium form some narrow bands of living parenchyma cells passing through secondary xylem and secondary phloem and are called secondary medullary rays. These provide radial conduction of food from the phloem, and water and mineral salts from the
- Annual rings : Activity of cambium is not uniform in those plants which grow in the regions where favourable climatic conditions (spring or rainy season) alternate regularly with unfavourable climatic conditions (cold water or dry hot summer). In temperate climates, cambium becomes more active in spring and forms greater number of vessels with wider cavities; while in winter it becomes less active and forms narrower and smaller vessels. The wood formed in the spring is known as spring wood and that formed in the dry summer or cold winter autumn wood or late wood. Both autumn and spring wood constitute a growth or annual ring. In one year only one growth ring is formed. Thus by counting the number of annual rings in the main stem at the base we can determine the age of a tree. This branch of science is known as dendrochronology. Age is determined by an instrument increment borer. Growth rings are distinct or sharply demarcated in the plants of temperate regions where as in tropical climate (near equator) they are not distinct or sharply demarcated in the
- Activity of cork cambium : Cork cambium or phellogen develops from outer layer of cortex. It produces secondary cortex or phelloderm on innerside and cork or phellum on The cells of phellem are dead, suberized and impervious to water. Cells of phelloderm are thin walled, living and store food. Phellem,
phellogen and phelloderm collectively called as periderm. Periderm is secondary protective tissue. Due to pressure of secondary xylem, epidermis raptures and cortex is largely lost after two or three years of secondary growth.
- Bark : All dead tissues lying outside the active cork-cambium are collectively known as bark. This includes ruptured epidermis, hypodermis and When cork-cambium appears in the form of a complete ring, it is known as ring bark, e.g., Betula (Bhojpatra). If the cork cambium occurs as separate strips and the resulting bark appears in the form of scales, such a bark is known as scaly bark. e.g., Eucalyptus, Psidium guava. The outermost layer of bark is dead and called as rhytidome.
- Lenticels : These are aerating pores formed in the cork through which gaseous exchange takes
They are formed as a result of the action of phellogen. A lenticel appears as a scar or protrusion on the surface of the stem and consists of a radial row of thin-walled cells, known as complementary cells or filling tissue. They are found in old dicot stem, main
Promeristem Provascular (cells all alike) tissue
Epidermis Cortex Primary
(fascicular) Primary Xylem
function is guttation.
- Cork : It consists of dead cells with thick walls heavily impregnated with suberin. These cells are compactly arranged in radial rows without intercellular Cork is impervious to water and prevents its loss from the plant surface. It also protects the inner tissues from the attack of fungi and insects. There is no differentiation of bark, sap wood and heart wood of Date palm.
- Heart wood and sap wood : In old trees, secondary wood is differentiated into a centrally situated darker and harder wood called the heart wood or duramen which are physiologically inactive (almost dead)and an outer light-coloured zone called the sap wood or alburnum which are physiologically active. Dark colour of heart wood is due to the deposition of tannins, resins, gums, essential oils, in
A B C
the cell walls and cell cavities. The water conduction takes place through sap wood. During the conversion of sap wood into heartwood the most important change is development of tyloses in the heart wood. Tyloses are ballon 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 plant. As a tree grows older thickness of heartwood increases and sap wood remains same.
Periderm Phloem Sap wood Heart wood
- Secondary growth in dicot roots : Vascular bundles in dicot roots are radial, exarch and mostly Vascular cambium is formed secondarily from conjuctive parenchyma cells lying just below each phloem strand. Thus the number of cambium strips formed equals the number of phloem strands. The cells of pericycle lying outside the protoxylem also become 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 produced 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 inner side.
Formation of cambium
Periderm Medullary ray
Fig : Secondary growth in dicot root
- Grew is the father of anatomy (1682) and coined the term tissue and parenchyma.
- Haberlandt proposed the names of protoderm (for dermatogen), ground meristem (for periblem) and procambium (for plerome)
- Haberlandt (1914) gave the terms lepton for soft walled conducting part of phloem and hadrom for conducting part (tracheary elements) of
- Strasburger discovered albuminous cells instead of companion cells in the phloem of non-flowering
- In sugarcane there is no distinction of tunica and corpus.
- Reproductive apex is elongated in Sagittaria but it can be 400 times broad in Chrysanthemum.
- Seive cells or seive tube elements resemble RBCs in being without nucleus in the mature
- Cavities are of three types :
- Schizogenous : They are formed by enlargement of intercellular spaces or separation of cells g. oil of Sunflower.
- Lysigenous : They are formed by degeneration of cells, g., oil cavity of Citrus and protoxylem lacunae or water cavity in monocot stem vascular bundles.
- Schizolysigenous : They are formed partly by separation and partly by degeneration of e.g., protoxylem cavity.
- Pith cavity often present in monocot stems (e.g., grass) and occassionally in dicot stems (e.g., Ricinus).
- Wood without vessels is called homoxylous, g., Ranales (winteraceae, tetracentraceae, trochodendraceae). Whereas with vessels is called heteroxylous.
- The wood of Tactona grandis is termite
- The bottle cork is prepared from cork of Quercus suber (Oak tree).
- Lightest wood is of Ochroma pyramidate (O.lagopus).
- Heaviest wood is of Guaiacum In India heaviest wood is of Acacia sundra.
- Most durable soft wood is of Cedrus
- Latex for chewing or chickle gum obtained from Achras sapota, Gutta percha (insulating material) from palaquium gutta alkaloid opium from Papaver sominiferum (poppy), papain (enzyme) from carica papayas, Rubber from Hevea brasiliensis, Ficus
- Reaction wood is a wood formed in bending stems. When reaction wood is formed on the lower side, it is called as Compression wood g., conifers. When it is formed on the upper side, it is called as tension wood e.g., Dicots.
- Wound periderm is similar to natural But it is restricted to the place of injury and is used in producing the commercial cork.
- Maceration is a method of separation of various tissues by disintegration of middle
- In some plants primary structure is abnormal such as presence of medullary bundles in pith g., Boerhaavia, Mirabilis, Achyranthes, Bougainvillea or presence of cortical vascular bundles (inverted) e.g., Casuarina and Nyctanthus.
- A protective tissue found in roots of some plants (Rosaceae, Myrtaceae) having alternate layers of endodermal and parenchyma cells are called periderm.
- Knots are the bases, scars/wounds of fallen branches get covered by growth of secondary They form knots in the wood.
- Abscission is a special layer of parenchymatous cells appears at the base. Abscission is premature fall of plant parts from the plant without causing the A protective layer of suberised thick walled cork cells is formed below the abscission layer to prevent infection or dessication (sometimes it is corky layer).
- Metaxylem consist of two larger and rounded vessels situated on the sides with the pitted tracheids in between
- Protoxylem consists of two smaller vessels situated to wards the centre. The vessels of metaxylem are pitted and those of protoxylem are annular and
- Depending upon the relative position of protoxylem; xylem is of four types :
- Exarch : Protoxylem towards the
- Endarch : Protoxylem towards innerside of
- Mesarch : Protoxylem surrounded by
- Centrach : Protoxylem in the centre of metaxylem.
- Endarch xylem is also called centrifugal as xylem matures from inside to Similarly, exarch xylem is known as centripetal
because differentiation of xylem proceeds from outside to inside e.g. roots.
- Root cap is absent in hydrophytes.
- Root hairs are found in zone of maturation.
- In the leaf, vascular bundles are found in the veins.
- An example of monocots showing secondary growth in stems is Yucca or Draceana.
- Safranine stains lignified elements of the
- The longitudinal section of a root have four zones which occur in the following order (from the tip upward) : Root cap, cell division, cell enlargement, cell