Tissues

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
 

Tissues (On the Basis of Stage of Development)

• Meristematic tissues (undifferentiated): Meristem growth in plants is largely restricted to specialised regions of active cell division called meristem.

• A meristem is alocalised region in which actual cell division occurs.

• Permanent tissue (fully differentiated tissue): Cell cycle of permanent tissue is arrested at G₁ stage so cells have no capacity to divide actively, so it is composed of mature cells.

Characteristics of Meristematic Tissues

• Meristematic tissues or meristem is an undifferentiated tissue.

• Cell cycle of meristem is in continuous state of division. It means they have the capacity to divide. So meristematic tissue is composed of immature cells.

• Meristematic cells have ,only primary cell wall which is thin and flexible ( elastic) and made up of cellulose. Secondary cell wall is absent.

• Cells of meristem are small and isodiametric.

• They have dense cytoplasm.

• Normally vacuoles are absent in meristematic cells but if present then small.

• They have prominent and-large nucleus.

• Meristematic cells are metabolically highly active so lack of reserve food in these cells.

• Plastids are absent in meristems. If they are present, then only in the proplastid stage.

• They do not have intercellular spaces. Cells are closely fitted (packed) together so it is a compact tissue

Classification of Meristematic Tissue

(A) Meristematic Tissue Based on Origin and Development

On the basis of origin and development meristems can be divided into following three types:

(i) Promeristem/Embryonic meristem

• This meristem develops in beginning during embryonic stage.

• They develop primary meristem.

(ii) Primary meristem:

• Meristematic cells developed from promeristem are known as primary meristem.

• These cells are always in division phase and form primary permanent tissue.

(iii) Secondary Meristem:

• These ate the meristems developed from primary permanent tissues. They are not present in the embryonic stage of the plant.

• Some of the cells of primary permanent tissues become meristematic and constitute secondary meristem.

• If by the activity of secondary meristems, secondary growth takes place.

Note: Formation of meristem from any permanent tissue is called dedifferentiation. Or

Formation of undifferentiated tissue from differentiated tissue is called dedifferentiation.

(B) Meristematic Tissues Based on Location (Position) in Plant Body

On the basis of position, meristematic tissues are divided into three types:

(a) Apical Meristem: The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems. They are responsible for increase in the length of plant organs.

Example: Root apex. shoot apex. They are responsible for primary growth.

Apical meristem has two regions at embryonic stage:

• First - Promeristem or primordial Meristem which develops in embryonic stage.

• Second - Eumeristem- It is formed from the division of primordial meristem.

According to Haberlandt, eumeristem is divided into three regions (on the basis of function):

(i) Protoderm: It is the outer most layer of eumeristem. By the activity of protoderm epidermal tissue system is formed. It includes Epidermis, Root hair, stem hair etc.

(ii) Procambium: These cells are long and it gives rise to the vascular tissue system. It includes xylem, phloem.

(iii) Ground Meristem: The cells of this region are large, thin walled and isodiametric. Ground tissue system is formed by the activity of these cells. It includes hypodermis, cortex, endodermis, pericycle, pith-rays and pith.

(b) Intercalary Meristem:

• The meristem which occurs between mature tissues.

• This is the separated region of apical meristem. This meristem is found between plant organs.

• By the activity of this meristem length of the plant organs increases:

• They are present in some stem of plants.

• They may be present either at the base of internode e.g. grasses, bamboo and Equisetum etc, or at the base of node e.g. Mint. They are also present at tile base: of leaves e.g. Pinus. By the activity of this meristem, length of leaves increases.

They are short lived and convert into permanent tissue.

(c) Lateral Meristem:

• Lateral meristem occurs in lateral side of plant organ or parallel to the longitudinal axis (Tangential plane) of plant organ.

• Activity of lateral meristem increases the width of plant organ so it is responsible for secondary growth.

• Lateral meristems are both primary and secondary to origin (mostly secondary in origin). There are two examples of primary lateral-meristem

(i) Marginal meristem: It occurs at the margin of leaf. Its activity increases the width of leaf so total growth of leaf is called intercalary marginal growth.

(ii) Intra fascicular cambium or fascicular cambium: This cambium occurs inside the vascular bundle. Except intra fascicular cambium all cambia are secondary in origin.

Lateral meristems are cylindrical.

(C) Classification Based on Plane of Division

(i)  Rib-meristem/File meristem: Meristem in which anticlinal division Occurs in one plane. For example tunica is a type of rib-meristem.

Note: Formation of some cells of cortex and pith takes place by this' meristem,

(ii) Plate meristem: The meristem which divides anticlinally into two planes at right angle to each other. By this division a plate like structure is formed. Formation of leaf blade takes place by the activity of this meristem.

(iii)  Mass-meristem: Meristem which divides in all possible planes resulting it the increase in the volume of plant body (organ). Example: The formation of embryo and endosperm takes place by this kind of meristem.

(D) Classification Based on Rate of Division

Cytohistological zonation theory: According to Foster, meristem is classified into two region on the basis of rate of division:

1.     Summit

2.     Flank

Vegetative shoot apex:

(1)    Summit: The rate of division is slow in this region. This region is located at the 'apex.

(2)    Flank: The rate of division is very fast in this region. This region lies behind the summit and leaf primordia are' formed by this region.

• Time period between initiation of two successive leaf primordia is called Plastochron.

• Shape of vegetative shoot apex is Conical or dome shaped.

• Shape of Reproductive shoot apex is Broad & flat.

Reproductive shoot apex:

• During reproductive phase i.e., at the time of flowering vegetative shoot apex transforms into reproductive shoot apex; This change of shoot apex is induced by florigen & light.

• Summit zone of reproductive shoot apex is more active i.e., rate of cell division is greater and it forms stamens & carpels and flank zone is less active in reproductive shoot apex and it forms sepals and petals.

Composition of Apical Meristem in Different Plants

• Apical meristem is absent in lower Algae and Fungi. All the cells of these plants are divisible, so they do not show apical growth. Thus such type of growth in these plants is' called diffused' growth. Diffused growth also occurs in animals.

• Apical meristem in higher algae (e.g. Fucus, Dictyota, & Sargassum), Bryophytes and some Pteridophytes (eg. Selaginella) is consisting of single cell. This cell is known as apical cell. This apical cell is pyramid in shape and divides into two lateral planes.

• Apical meristem in Ferns, Gymnosperms and Angiosperms consist of many cells.

• Several views have been available in relation to structure and growth habit of apical meristem.

1. Apical Cell theory: This theory proposed by Karl Nageli and Hofmeister and supported by Wolff.

According to Nageli and Hofmeister, the apical meristem is composed of single apical cell. This view is only applicable on Bryophytes and some Pteridophytes and some higher algae (Fucus, Dictyota & Sargassum)

2  Histogen Theory: It was proposed by Hanstein (1870).

According to him, the root and shoot apices are distinguished into three meristematic regions or three layers of histogen cells. These are as follows.

(i) Dermatogen: this is the outermost single layer of cells. These cells forms uniseriate epidermis by anticlinal divisions,

(ii) Periblem: This region is situated just below the dermatogen. It forms cortex (Hypodermis, general cortex and endodermis),

(iii)  Plerome: This is the innermost region. Stele formation takes place by division of these cells. It means formation of pericycle, vascular bundles, pith rays or medullary rays and pith.

• This theory is only true for root apex. It is not applicable for shoot apex of higher plants because in most of the gymnosperms and angiosperms, shoot apex does not have distinct differentiation of three layers.

• Except above described three histogens, a fourth type of histogen is present in monocotyledons. This is known as Calyptrogen. Root cap is produced by Calyptrogen in monocots. Root cap is produced by dermatogen in dicotyledons.

• Exception: There is only one histogen present in Ranunculus. Two histogens occurs in Casuarina.

• Due to presence of root cap position of root apex is sub terminal. so maximum growth in root takes place behind the apex.

3. Tunical corpus theory: This theory was proposed by Schmidt (1924). This theory is applicable on shoot apex; it is based on planes of division. According to this theory two types of layers are found in the shoot apex:

(a) Tunica:

• This is peripheral layer. Epidermis is formed by this layer. In tunica cells, anticlinal division takes place only in one plane.

• Anticlinal division occurs at right angle to longitudinal axis of cell. Surface area of the plant organs increases due to anticlinal divisions.

• When division occurs in single anticlinal plane they do not increase the number of layers.

• Generally, tunica represent only single layer, but sometimes it is multilayered, then the outer most layer forms the epidermis and remaining layers form rest types of the tissue system with the association of corpus.

(b) Corpus:

• The mass of cells present below the tunica is called Corpus.

• The cells of this zone divide in all directions (many planes) due to which volume increases.

• The mass of these cells mainly forms the cortex.

• The cells of corpus are usually larger than the cells of tunica. It forms rest of the tissue system.

4. Mantle core theory: This theory was proposed by Popham and Chan. They compared the mantle to the tunica and core with the corpus. Mantle forms epidermis. According to them corpus or core is distinguished into three zones.

(I) Sub-Apical Meristem: This is present just below the mantle. It's function is re-establishment of mantle if mantle gets damaged.

(II) Central-zone Meristem: This is the inner most zone. This zone is responsible' only for formation of pith.

(III) Peripheral Meristem: This region is only responsible for the formation of cortex, pericycle and vascular tissues.

5. Newman's theory: According .to this theory meristematic tissues of shoot apex are three types

(I) Monoplex: Such type of shoot apex in which meristematic cells are arranged in groups in place of meristematic layers. Such type of shoot apex is found in Ferns.

(II) Simplex: Such type of shoot apex is formed by single layer of meristematic cells. It is found in Gymnosperms. (Except-Gnetales)

(III) Duplex: Such type of shoot apex is formed by two layers of meristematic cells. Peripheral layer is called tunica and inner layer is called corpus. Such types of shoot apex is found in Gnetales and Angiosperms. Tunica-corpus organization occurs in duplex.

6. Korper-Kappe theory:

• It was proposed by Schuepp (1917). According to this theory, the cells of central and peripheral part of the root apex exhibit differences in planes of cell divisions.

• In peripheral region each cell first divides transversely and there after the lower daughter cell divides longitudinally thus forming the shape of T. Such divisions are called the Kappe divisions. In the central region T is inverted (^) as the second division takes place in the upper daughter cell. Such divisions are called the Kerperdivisions, As a result of these T or ^ divisions, the cells in root apex remain arranged in rows.

• By Kappe divisions, the number of rows increases downwardly and by Korper .divisions upwardly.

• This is a rejected theory because there is no relation of arrangement of cells to tissue" formation.

Permanent Tissues

Permanent tissues are composed of cells which have lost the power of division temporarily or permanently. They are formed by division and differentiation of meristematic tissues.

Their cells may be living or dead permanent tissues. These are of three types:

1. Simple tissue (Homogenous tissue)

2. Complex tissues (Heterogenous tissue)

3. Special tissue

(A) Simple tissue: These tissues are made up of similar cells that perform a common function. Simple tissues are of three types:

(a) Parenchyma

(b) Collenchyma

(c) Sclerenchyma
 

Parenchyma

• It is very primitive type of tissue.

• It is first evolved tissue.

• Remaining all different types of tissues are derived from this tissue so it is also called as fundamental tissue

• Parenchyma-name coined by Grew.
   

Characteristic Features

• It is a living tissue.

• Tissue 'first to be differentiated from meristem is parenchyma.

• It is a universal tissue.

• It Body of a bryophyte is mainly composed of parenchyma.

• All the cells of parenchyma are thin walled. Cell wall is made up of pectocellulose (mainly cellulose). So parenchyma is a soft tissue:

• Parenchyma is most common tissue' which is morphologically and physiologically unspecialized. Each cell containing large central vacuole. So the main function of a "parenchyma cell is storage of food.

• Inter cellular spaces are present between cells of this tissue, it is a loose tissue. Intercellular spaces are schizogenous in origin.

• The cells are isodiametric. Flesh of a fruit is mainly composed of parenchyma.

• Shape: The cells of parenchyma are spherical, oval or polyhedral in shape. Each parenchymatous cell contains 14 planes of lateral line, which are maximum possible plane in a cell. These are known as tetrakiidecahedron. This characteristic of cells is due to more flexibility.
   

Modification of Parenchyma

1.  Prosenchyma: The cells of this parenchyma are long with pointed ends. This parenchyma forms the pericyle of roots.

2. Aerenchyma: This parenchyma is made up of rounded cells. These cells surrounds the large air chambers.

Air-chambers are lysigenous in origin. (Oxygen is stored in these chambers which are evolved from photosynthesis which help in respiration). It is found in cortex region, It provide buoyancy to hydrophyte plants,

3. Stellate parenchyma: The cells of this tissue are stellate and branched. Air spaces ate also present but' they are less developed. Main function of this parenchyma is to provide mechanical support.

It is found in the leaf bases of banana. It provide strength to leaf bases. Leaf base of banana performs the function of stem. Rhizome is found in banana.

4. Chlorenchyma: type of parenchyma in which abundant quantity of chloroplasts are found. Two types of chloroplaste are present in dorsiventral leaves.

(a) Palisade tissues: Inter cellular spaces are absent. Their cells are tightly fitted together. They are present 'towards adaxial/ventral/upper side of leaf. Number of chloroplasts are more in palisade tissue as compare to spongy tissue. So upper surface of a leaf appears more green as compared to lower surface.

(b) Spongy tissues: Large intercellular spaces are present. So they facilitates transpiration and gaseous exchange. They are present towards abaxial dorsal lower side of leaf.

5. Mucilage parenchyma: In the mucilage parenchyma large vacuoles and mucilage will be found. e.g. Succulent xerophytic plants. e.g. Aloe. Function storage of water.

Functions of parenchyma:

The main function of this tissue is storage of food. Some cells of parenchyma store waste materials. They are called "idioblast cells". Idioblast cells store and form oils, tannin and crystal of calcium oxalates.

Collenchyma

Term coined by Schleiden.

Main Characteristics

• Collenchyma is a living mechanical tissue. Lignin is absent.

• It is made up of elongated, oval, spherical or polygonal cells.

• Localized deposition of pectocellulose (mainly pectin) & hemi cellulose is the characteristics feature-of collenchyma.]

• Vacuolated cytoplasm is found in the cells of collenchyma.

• Collenchyma originates from ground meristem.

Occurence

• Collenchyma is not a universal tissue. It is found in the stems of herbaceous dicotyledons:

• Collenchyma is absent in parts of woody plant parts, roots and monocotyledons.

• Collenchyma forms the hypodermis of dicotyledon stems. Cells of collenchyma are flexible due to hydrophilic nature of pectocellulose so flexibility occurs in dicotyledonae stems.

• Collenchyma is absent in plants after the secondary growth because plant becomes woody.

• Chloroplast may be found in the cells of collenchyma.

• Lamina margins of leaves also bears collenchyma:

• This protects the cracking of lamina margin due to the action of wind.

Types of Collenchyma

On the basis of place of deposition, it is classified into three types by majumdar:

1. Lamellar/plate collenchyma: The cells of collenchyma arranged in lamellar forms. The cell have thickening on the tangential walls. Due to such type of deposition, cell looks like a lamellar or plates. Ex. Sunflower stem.

2. Angular collenchyma: This type of collenchyma is abundantly found in plants. The cells of this tissue are angular. The deposition of pectocellulose are at the angles of cell. e.g., Stem of Datura, Solanum and tomato.

3. Lacunar collenchyma/tubular collenchyma:

Large Intercellular spaces are present in the cells of this tissue. Deposition of pectocellulose is on the wall of intercellular spaces. Intercellular spaces of collenchyma are thickened. e.g. Cucurbita stem and aerial roots of monstera.

Functions:

• Mechanical as well as biological. It provides tensile strength for extensibility of various growing plant organs.

• Due to the presence of chloroplast, it also participates in the process of photosynthesis,

Sclerenchyma

Name coined by Mettenius.

Main Features

• Sclerenchyma is the main mechanical tissue.

• These cells are long, narrow, thick walled and dead.

• Cell wall is thick and lignified.

• Various types of pits are formed due to the deposition of lignin on hard wall.

Types of Sclerenchyma

On the basis of length of cells and quantity of deposition of wall materials, Sclerenchyma cells are of two types.

(i)  Sclereids: These cells are small, extremely thick walled and their ends are not pointed. Sclereids are isodiametric or irregular in shape. Sclereids cells have more pits and lumen is almost very small. Their pit cavity is branched.

Sclereids are classified by Tschierch, on the basis of their shapes:

(A) Stone cells or Brachy- sdereids or Grit cells:

• These cells are spherical or oval in shape. They are found in endocarp of drupe fruits, so endocarp becomes hard.

• They are present in endocarp of Coconut, Mango, Almond, Walnut etc.

• Besides drupe fruits brachysclereids also present in fleshy (edible) part of pear. Grittiness in pear fruit is due to these sclereids.

(B) Trichosclereids: These are also known as internal hairs. They are spines like, bifurcated cells. These are found in floating leaves.

(C) Astro Sclereids or Stellate sclerenchyma:

• These cells are stellate (star) shaped. They are found in floating leaves.

• Example: Both Astro and Tricho sclereids are present in floating leaves. Fig. Victoria, Nelumbo ( Lotus) and Nymphaea petiole.

(D) Macro-sclereids or Rod cells or Malpighi cells: They are small and rod like cells. They are present in seed coats.

Example:

• They form part of seed coat in legume plants. Due to their presence seed coat becomes hard and dormancy is present in legume seeds.

• In leguminous plants hardest seed coat is found in french bean. So dormancy is found in leguminous seeds.

• In plant kingdom hardest seed coat is found in lotus.

(E) Osteo- Sdereids: These are known as prop­-cells. These are pillar like cells. Both end of pillar like cells spreads to form bone like structure.

Example: These cells are found in leaves of two plants Hakea and Osmanthus.

(ii) Sclerenchymatous fibres:

• These cells are fibrous.

• They are longest cells in the plant body.

• Their both ends are pointed (tapering).

• Due to thick cell wall, lumen is reduced.

• Their cell wall contains simple and bordered pits.

On the basis of structure fibres are classified into two groups:

(a) Libriform fibres: They are highly thickened long fibres. They posses simple pits and narrow lumen.

Libriform fibres are found in phloem, xylem, pericycle and hypodermis (Maximum in phloem).

(b) Fibre Tracheids: They are also highly thickened. Bordered pits are present in these fibres and lumen is broad. They are only found in xylem.

On the basis of position, fibres are divided into three types:

(A) Surface fibres: They are present on the surface of plant. These fibres also called as filling fibres.

(i)  Seed surface fibre:

Example:

• Cotton fibres: Cotton fibres are formed by the outgrowth of seed coat. They are not any type of tissue or cell.

• Cotton fibres are composed of cellulose these fibres are un lignified. So, cotton fibres are not true fibres. Two types of fibres are found in cotton. Long fibres are called 'lint' and small fibres are known as 'fuzz'. Lint fibres are used in cloth industry. Fuzz are filling fibre.

• Cotton fibres are not an example of any type of cell because these fibres are formed by out growth of testa.

• Red silk cotton (Semal fibre)-Obtained from Salmalia malabaricum.

• White silk cotton (Kapok) - Obtained from Ceiba pentendra.

• (Both red and white silk cotton fibres are not true fibres and they are also an example of seed surface fibre.)

(ii) Coir of coconut is also a type of surface fibre. They are derived from the mesocarp. These are true fibres.

(B) Xylary or wood fibres: These are hard fibres. They fibres are not flexibe. They can not be knitted (weaved) easily so they are not useful. These are found in xylem.

Example: Munj fibre (Saccharum munja)

(C) Bast fibres/Extra xylary fibres/Phloem fibre:

• These are also known as commercial fibres. These fibres are flexible and can be knitted (weaved) easily. They have great economic value.

• These fibres are obtained from the phloem and pericycle of plants.

• The bast fibres of Corchorus capsularis (Jute), Crotalaria juncea (Sunn hemp) and Hibiscus sabdariffa (patua) are obtained from the secondary phloem of stem.

• The bast fibres of hemp (Cannabis sativa) and Linum usiiatissimum (flax) are obtained from the pericycle.

• Fibres which are obtained from pericycle are called perivascular fibres,

• Leaf fibres: Manila hemp (Musa textilis) and agave hemp.

• Agave sislana: These are obtained from Sclerenchymatous bundle sheath.

Sclerenchyma is classified on the basis of variation in form, structure, origin and development.

Complex permanent tissue:

• The complex tissues are made of more than one type of cells and these work as a unit. Complex tissue are heterogenous

• Complex tissues are absent in gametophytes.i

Complex tissues are of two types –

(a)    Xylem

(b)    Phloem

Q.1 - The meristem which develops into a primary vascular tissue is [MP PMT 1999]       

(a)        Protonema      

(b)        Promeristem   

(c)        Ground meristem       

(d)        Procambium

Q.2 - Which of the follwing is a secondary meristem [CET Pune 1998]

(a)        Phelloderm     

(b)        Primary cambium       

(c)        Cork cambium

(d)        Promeristem

Q.3 - The cambium is an example of  [CPMT 1998; EAMCET 1995]

(a)        Lateral meristem        

(b)        Intercalary meristem  

(c)        Apical meristem         

(d)        Primary meristem

Q.4 - Fibres are obtained from [JIPMER 2002]

(a)        Xylem, phloem and sclerenchyma     

(b)        Xylem, phloem, sclerenchyma and epidermis

(c)        Xylem, parenchyma, epidermis                      

(d)        Xylem, parenchyma, endodermis

Q.5 - The quiescent centre in root meristem serves as a [AIIMS 2003]

(a)        Site for storage of food which is utilized during maturation

(b)        Reservoir of growth hormones          

(c)        Reserve for replenishment of damaged cells of the meristem

(d)        Region for absorption of water

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