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  Introduction.                                                                                                                                                          

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 :

1. Meristematic tissues or Meristems
2. Permanent tissue
3. 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 divisions.
• Intercellular spaces are not present in meristematic tissue.
• They contain a homogeneous thin wall.
• They contain large nuclei associated with abundant cytoplasm.
• They are metabolically very active but they do not store food material.
• Only proto-plastids are present instead of plastids, chloroplast absent.
• Dense cytoplasm is present which contains several premature mitochondria.
• Vacuoles are absent.
• Meristematic cells are isodiametric in shape.
• Undifferentiated tissue in which all divides continuously G₁ ® S ® G₂ ® M.

1. Types of meristems : The meristems may be classified on the basis of their mode of origin, position or function :
   1. According to origin and development : On the basis of origin, meristematic tissues are of three types :

1. 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 root. The promeristem gives rise to all other meristems including the primary meristem.

2. Primary meristem : A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia. Primary meristem gives rise to the primary permanent tissue.
3. 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 repair.
   2. According to position : On the basis of their position in the plant body meristems are classified into three categories :

1. Apical meristem : This meristem is located at the growing apices of main and lateral shoots and roots. 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.
2. 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.

3. 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 gymnosperms.
   3. According to function : Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types :

Apical meristem

Intercalary meristem

Lateral meristem

a          a

A                        B

Fig : Various meristamatic tissue

1. Protoderm : It is the outermost layer of the apical meristem which develops into the epidermis or epidermal tissue system.
2. Procambium : It occurs inside the protoderm. Some of the cells of young growing region which by their elongation and differentiation give rise to primary vascular tissue, constitute the procambium.
3. Ground meristem : It constitute the major part of the apical meristem develops ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary rays.
   4. According to plane of cell division : On the basis of their plane of cell division meristem are classified into three categories :

1. Mass meristem : The cells divide anticlinally in all planes, so mass of cells is formed. e.g., formation of spores, cortex, pith, endosperm.

2. Plate meristem : The cells divide anticlinally in two planes, so plate like area increased. e.g., formation of epidermis and lamina of leaves.
3. Rib or File meristem : The cells divide anticlinally in one plane, so row or column of cells is formed.

e.g,, formation of lateral root.

(2)Structure and organisation of apical meristem

1. 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 bud. 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 :

1. Apical cell theory : This theory was proposed by Nageli (1858). According to this theory, shoot apical meristem consists of single apical cell. This theory is applicable in case of higher algae, bryophytes and in many pteridophytes but not in higher plants (i.e., gymnosperms and angiosperms).
2. 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 system.
   • Periblem : It is the middle layer gives rise to cortex and endodermis.

• Plerome : Innermost layer forms pith and stele.

3. Tunica corpus theory : This theory was proposed by Schmidt (1924). According to this theory, the shoot apex consists of two distinct zones.
   • Tunica : It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus. The tunica shows only anticlinal division and it is responsible for

Young leaf

Axillary bud

Shoot apex

Tunica Corpus

Epidermis (Protoderm)

Procambium Ground meristem

surface growth.

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 growth.
• 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 apex. 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.

1. Korper-Kappe theory : It was proposed by Schuepp (1917). This theory is comparable with the tunica and corpus theory of shoot apex. Korper means body and Kappe means cap.

Procambium Protoderm

2. Quiescent centre theory : It was proposed by Clowes (1961).

Cortex

Pith

Ground

meristem

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 :

Quiescent centre

Root cap

Fig : L.S. root apical meristem

• Ranunculus type : Root apex is made up of only one type of histogen layer. e.g., Plants of family Ranunculaceae, Leguminosae and Amentiferae.
• Casuarina type : Root apex is made up of two types of histogen layers. e.g, Plants of family Casurinaceae, Leguminosae and Proteaceae.
• Common dicot root : Root apex is made up of three layers of histogen.
• Common monocot root : Root apex is made up of four layers of histogen.
  3. Reproductive apex : During reproductive phase, the vegetative apices are converted into reproductive apices. 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.

Stamen

Petal Sepal

Fig : L.S. Reproductive apex (diagrammatic)

 Permanent tissues.                                                                                                                                              

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 :

1. 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 :

1. Parenchyma : Parenchyma is most simple and unspecialized tissue which is concerned mainly with the vegetative activities of the plant.

The main characteristics of parenchyma cells are :

1. The cells are thin-walled and soft.

Fig : Parenchyma in T.S.

Nucleus

Intercellular space Vacuoles

Cytoplasm

2. The cells usually are living and possess a distinct nucleus.
3. The cells contain well-developed intercellular spaces amongst them.
4. The cytoplasm is vacuolated and cell wall is made up of cellulose.
5. The shape may be oval, spherical, cylindrical, rectangular and stellate (star shaped) in leaf petioles of banana and canna and some hydrophytes.
6. This tissue is generally present in almost all the organs of plants, i.e., roots, stems, leaves, flowers, fruits and seeds.
7. 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 materials. e.g., Carrot, Beetroot etc.
• Chlorenchyma helps in photosynthesis.
• Aerenchyma helps in floating of the aquatic plants (Hydrophytes) and also help in gaseous exchange during respiration and photosynthesis. e.g., Hydrilla.
• In turgid state they give rigidity to the plant organ.
• 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.

2. 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 living.
   • 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 pits.
   • They are compactly arranged cells, oval, spherical or polygonal in outline.

• No. intercellular spaces are present.
• The tissue is plastic, extensible and have capacity to expand.

A

Fig : (A) Collenchyma L.S. (B) and (C)T.S. of the same

• They provide mechanical strength to younger part where xylum is less developed.

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.

1. 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 cells. e.g., Tagetes, Tomato, Datura, Potato, etc.
• Plate or Lamellar collenchyma : When the thickenings are present in the tangential walls. e.g.

hypodermis of sunflower stem.

• Lacunar or Tubular collenchyma : If the thickened cell wall is associated with intercellular spaces of the adjacent cells. e.g. leaf petioles of compositae and malvaceae etc. hypodermis of Cucurbita stem, Salvia, Malva.

(b)Functions

• Provide mechanical support to petiole, pedicels, branches of stem, roots and fruits.
• If they contain chlorophyll they help in photosynthesis.
• It is present at the margins of some leaves and resists tearing effect of the wind.

3. Sclerenchyma : It was discovered and coined by Mettenius (1805). The main feature of sclerenchyma are :
   • It consist of thick-walled dead cells.
   • The cells vary in shape, size and origin.
   • They possess hard and extremely thick secondary walls due to uniform deposition of lignin.
   • In the beginning the cells are living and have protoplasm but due to deposition of impermeable secondary walls they become dead.

Types of sclerenchyma : They are of two types :

1. 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 stems. There are three different kinds of fibres :

• Bast fibres : The fibres present in the pericycle (e.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.

B

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. e.g. Cotton fibres found in the testa of seeds, mesocarp fibres of Coconut (Cocus nucifera).

2. 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 stellate. 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 (e.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. e.g., leaf of Hakea.

• Astrosclereids or stellate cells : These are star- shaped sclereids with extreme lobes or arms. e.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 hydrophytes.
  2. Complex tissues : A group of more than one type of cells having common origin and working together as a unit, is called complex permanent tissue. The important complex tissues in vascular plants are : xylem and phloem. Both these tissues are together called vascular tissue.
     1. 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.

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 metaxylem.
• Secondary xylem : It is formed from vascular cambium during secondary growth.

Xylem is composed of four types of cells

1. 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.

2. Xylem vessels or Tracheae : Vessels are rows of elongated tube-like cells, placed end to end with their end walls dissolved. 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.

B

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 e.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 e.g. Quercus, Morus, Cassia, Delbergia, Tilea sp.

3. Wood (xylem) parenchyma : These are the living parenchymatous cells. As found associated with xylem they are known as wood parenchyma. They serve for the storage of reserve food and also help in conduction of water upwards through tracheids and vessels.
4. Wood (xylem) fibres : The long, slender, pointed, dead and sclerenchymatous cells found associated with xylem are termed wood fibres. 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.
   2. 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 direction.

On the basis of position phloem is of three types :

1. External phloem : It is normal type and present outside the xylem e.g., Mostly angiosperms and gymnosperms.
2. Internal or Intraxylary phloem : It originates from procambium and is primary phloem which occurs on innerside of primary xylem. It is primary anamolus structure. e.g., Members of Apocynaceae, Asclepiadaeae, Convolvulaceae, Solanaceae.

3. Induced or Interxylary phloem : It originates from cambium and is secondary phloem which occurs in groups within the secondary xylem. It is secondary anamolus structure. e.g., Leptadaenia, Salvadora, Chenopodium, Boerhaavia, Amaranthus.

On the basis of origin phloem is of two types

1. 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 disappear. This process is known as obliteration consists of sieve elements, parenchyma and fibre.
2. Secondary phloem : It is produced during secondary growth by vascular cambium. It consists of the following elements :

Sieve element Companion cells Phloem parenchyma

Phloem fibres or bast fibres

(1)Sieve element

1. They are long tube-like cells placed end to end, forming a continuous channel in the plant parts.
2. Their cell wall is made up of cellulose.
3. Their transverse wall is perforated like a normal sieve and hence they are called as sieve tubes.
4. Nucleus is not found in these cells.
5. Each sieve tube has a lining of cytoplasm near its periphery.
6. Callus pad may be visible in the winter season.
7. Their main function is to translocate the food material from one part to the other.

(2)Companion cells

1. They are thin-walled cells which are associated with sieve tubes.
2. They are more or less elongated.
3. They are connected with the sieve tube through sieve pore.
4. They contain nucleus and are therefore, living in nature.
5. 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

3. Phloem parenchyma : The parenchyma associated with the phloem is called phloem parenchyma. 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.
4. Phloem or Bast fibres : The sclerenchymatous fibres associated with the phloem are called as phloem fibres. 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 :

1. Laticiferous tissues
2. Glandular tissues

1. Laticiferous tissues : They are made up of thin walled, elongated, branched and multinucleate (coenocytic) structures that contain colourless, milky or yellow coloured juice called latex. These occur irregularly distributed in the mass of parenchymatous cells. latex is contained inside the laticiferous tissue which is of two types :

1. 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. e.g., Calotropis (Asclepiadaceae) Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).
2. Latex vessels : They are formed due to fusion of cells and form network like structure in all directions. 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).

2. Glandular tissue : This is a highly specialized tissue consisting of glands, discharging diverse functions, including secretory and excretory. Glands may be external or internal.
   1. 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. e.g., Rutaceae and Euphorbiaceae. Digestive enzyme secreting glands in insectivorous plants e.g., Drosera (Sundew), Nepenthes (Pitcher plant).
   2. Internal glands : These are present internally and are of several types. e.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).

Tissues

Meristematic tissue

Permanent tissue

Acc. to origin and development

Acc. to position

Acc. to function

Acc. to plane of cell division

Simple tissue

1. Parenchyma   :   Cells are

1. Promeristem :

Gives rise to primary

1. Apical   meristem   : 1.   Protoderm    :    It Responsible for secondary develops    into           the

1. Mass meristem : Forms mass of cell e.g., formation

living, soft with thin cellulosic wall and intercellular spaces. Present

meristem.

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.

2. Primary meristem : Gives rise to primary permanent tissue.

3. Secondary meristem :

2. 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

2. Plate meristem :

Increase plate like area

e.g. formation of epidermis and lamina of leaves.

2. Collenchyma : Compactly arranged living cells with thick wall of pectin and hemicellulose and no intercellular spaces.

Produce secondary tissues

the base of leaf e.g., Pinus.

3. Ground meristem :

3. 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

Lacunar or

cambium, cambium etc.

an organism.

endodermis, pericycle, pith, medullary rays.

lateral roots.

e.g., Tagetus, Tomato, Datura, Potato etc.

lamellar

e.g., hypodermis of Sunflower stem.

tubular

e.g., hypodermis of Cucurbita stem.

3. 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.

1. Tracheids : Elongated tube like cells with tapering or rounded or oval ends, with hard and lignified walls. e.g., present in xylem of all vascular plant.

2. Xylem vessels or trachae : Elongated tube like cells, placed end to end. e.g., always present in angiosperms (except lotus, Winifera, Trochodendron) and absent in pteridophytes and Gymnosperms (except Ephedra, Gnetom, Selaginella Pteridium).

3. Wood parenchyma : Living parenchymatous cells associated with xylem.

4. Wood or xylem fibres : Long, slender, pointed, dead and sclerenchymatous. e.g., found in woody dicotyledons.

Phloem

Food     conducting

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.

1. 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 e.g., Nepenthes (Pitcher plant), Drosera (Sundew).
2. Internal glands : Present internally e.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.

1. Latex cells : Very highly branched cell which do not fuse and do not form network. e.g.,Calotropis, Nerium, Vinca, Euphorbia, Ficus.

2. Latex vessels : Formed due to fusion of cells and form network like structure in all directions e.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 :

1. Epidermal tissue system (2) Ground or fundamental tissue system (3) Vascular tissue system

1. Epidermal tissue system : The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment.

1. Epidermis : Epidermis is composed of single layer cells. 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 :

1. In aerial roots, the multiple epidermal cells are modified to velamen, which absorb water from the atmosphere (e.g., Orchids).
2. Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. 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.
3. Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells.

2. 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 walls. 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 (cy