TISSUE SYSTEM and VASCULAR BUNDLES

TISSUE SYSTEM

  

In higher plants several tissues work together in the form of a unit to perform a particular function. These tissues have the same origin. Such tissues form a system which is called tissue system. On the basis of location/position & structure / morphology tissues were categorised by Von Sachs (German scientist) into three types of tissue system. Each system usually consist of an association of tissues which perform specific function.

1. Epidermal/dermal tissue system. This system forms the outer most.covering of the whole plant body and comprises epidermis, stomata and epidermal appendages. e.g Root hairs, trichomes.

• Epidermis: It is outermost layer of the primary plant body and made up of elongated. compactly arranged cells. It is usually single layered and made up of parenchymatous cells Cuticle (Waxy thick layer) is present on the epidermis which prevents the loss of water. Cuticle is absent in roots Epidermal cells are with a small amount of cytoplasm lining the cell wall and a large vacuole.

• Trichomes:- The cells of the epidermis of dicot stem produce hairs called trichomes. The trichomes in the shoot system are usually multicellular. They may be branched or unbranched and soft or stiff. They may even be secretory called glandular hairs. The trichomes help in preventing water loss due to transpiration.

Note : In shoot system trichomes are usually multicellular.

Function: The trichomes help in reduction of water loss.

• Root hairs :- The root hairs are formed due to the elongation of the epidermal cells. These have a vacuolated protoplasm. The thin wall is made up of cellulose and pectic materials. Root hairs are always unicellular.

Function: Root hairs play an important role in absorbing water from the soil.

2. Ground tissue system: - It is the largest tissue system. All tissues except epidermis and vascular bundles constitute the ground tissue. It includes hypodermis, general cortex, endodermis, pericycle and medullary rays (pith rays), pith. In leaf G.T.S. consists of mesophyll. GTS is also called fundamental tissue system. G.TS. is made of simple tissues such as parenchyma, collenchyma & sclerenchyma. The GTS forms the main bulk of the plant.

3. Vascular/conducting tissue system:- 

The V.T.S. consists of complex tissues. xylem and phloem, It is also called specific tissue system.

Note : Primary structure of plant organ or primary plant body is mainly composed of parenchyma.

Types of development of primary xylem :

I. Centrifugal - In this type of development, the protoxylem is formed towards the centre (pith) and metaxylem is formed away from the centre, it means towards the periphery In this condition xylem is known as endarch ex. Stem of angiosperms & gymnosperms.

II. Centripetal - In this type of development protoxylem is formed towards the periphery near the pericycle and metaxylem is formed towards the centre (pith). In this condition xylem is called exarch ex. Roots.

III. Centrifugal and Centripetal :- Elements of metaxylem are formed on both sides of the elements of protoxylem. In this condition xylem is known as mesarch. ex Fern rhizome (underground stem).

VASCULAR BUNDLES

Xylem and phloem are collectively termed as vascular bundles. Which may or may not have cambium.

On the basis of arrangement of elements means location of xylem and phloem vascular bundles are divided into three categories.

Types of vascular bundles 

1. Radial vascular bundles When the xylem and phloem are present separately on different radio in alternate manner, then vascular bundles are called radial vascular bundles

In these vascular bundles xylem is exarch. The order of development of xylem in these vascular bundles is centripetal. Example : Most of the roots (Dicot, monocot, gymnosperm, fern root) 

Exception :- In Radish, carrot, turnip and sugar beet (Beet root) roots, conjoint, collateral vascular bundles are present.

2. Conjoint vascular bundles In the localar bundeswomand loan ton same radius of combine into a bundle. These are of two types :-

Collateral (Open) Bicollateral Conjoint collateral in this type of vascular bundle xylem and phloem re present on the same radius and phloem is present towards the periphery

These are two types :

(i) Open - the cambium is present between the xylem and phloem, then it is said to be open vascular bundle. Ex. stem of dicots/dicotyledons and gymnosperms

(ii) Closed- When cambium is absent between the xylem and phloem conjoint vascular buut then it ched closed vascular bundle.

Ex. Monocotyledonous stem and leaves of angiosperms.

In this type of vascular bundle. xylem is endarch and cool development of sum is centrifugal

(2) Conjoint, bicollateral and open vascular bundle -In this type of V.B two patches of phloem, one of each side of xylem and found. There are two strips of cambium (outer and innert), one oneach side of xylem are found. Only outer cambium is functinal.

Order of development of xylem is centrifugal means endarch condition is found.

ex.  stem of family cucurbitaceae and some plants of family solanaceae.

3. Concentric vascular bundles In these type of vascular bundies either xylem surrounds vascular bundles are always closed. They are of two  :

(a) Amphicribral or Hadrocentric In this type of vascular bundle xylem completely, Surrounded by phloem. It means xylem procent the centre of vascular bundle. Such types of vascular bundles are found in ferns rhizome (Underground stem). The order of development of xylem in ferns time is of both centripetal and centrifugal and we mesarch.

(b) Amphivasal or Leptocentric

• In this type of vascular bundle phlegm is completely surrounded by xylem. It means phloem is present centre of the vascular bundle.

• In this type of vascular bundle, xylem is endarch. stem of Dracaena, Yucca etc

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Secondary Growth

SECONDARY GROWTH

Secondary Growth:

• By the activity of laterel meristem (vascular cambium and cork cambium). Increase in the circumference girth/ thickness of the plant organs is called secondary growth.

• Normal secondary growth takes place in roots and stem of dicotyledons and gymnosperms.

• The tissues involved in secondary growth are two lateral Meristems: vascular cambium.and.cork.cambium.

• Secondary growth is not found in the leaves and monocots.

• Due to lack of cambium in monocotyledons, secondary growth is absent. But exceptionally secondary growth takes place in some monocotyledons. Such as-Palm, Date Palm, Coconut Palm, Yucca, Dracaena, Kingia, Sansevieria, Smilax, Agave etc These plants show abnormal secondary growth.

SECONDARY GROWTH IN DICOT STEM 

A. SECONDARY GROWTH IN STELAR REGION BY VASCULAR CAMBIUM

Secondary growth in stelar region begins earlier than the extrastelar region.

Formation of ring of vascular cambium- A Cambium which is present inside the vascular bundle (Means between primary xylem  meristem and primary phloem) is called intrafascicular cambium This is a type of primary lateral Meristems.

• First of all,cells of medullary  rays adjoining intrafasicular cambium become meristamatic to form interfascicular cambium which is a secondary lateral meristem.

intarafasicular and interfasicular cambium are collectively known  as vascular cambium or intra stelar Vascular cambium.

Vascular cambium is  formed in the form of a complete ring which made up of single layer of cells. 

In dicot stem, the vascular cambium is partly primary and partly secondary in origin.

Two types of cells are found in the ring of this vascular cambium.

1. Fusiform initials

2.Ray Initials

1.Fusiform initials are long with pointed ends, whereas ray initials are spherical/rounded or oval in shape. Fusiform initials are more in number in vascular cambium.

Activity of vascular cambium:

(a) Activity of fusiform initials:

Continuous perclinal divisions (parallel to longitudinal axis) takes place in fusiform then few cells are formed towards  the periphery and these cells are differenciated into secondary phloem or bast and the cells which are formed towards the  centre (towards pith) are differenciated into secondary xylem or wood.

The cambium is generally more active on the innerside than on the outer.

 Normally more secondary xylem is formed  as compared to the secondary phloem due to unequal distribution of hormones. (Secondary xylem is formed 8-10 times more as compared to the sea phloem).

Different stages of secondary growth in Dicotyledonae stem

By the pressure of secendary xylem, all the primary tissues such as primary xylem, pith are pushed towards  the centre.

The primary xylem however remain more or less intact in around the center. The primary phloem and earlier secondary phloem (old secondary phloem) get gradually crushed due to the continued formation and accumulation of secondary xylem.

Activity of Ray Initials :- Due to periclinal division ray initials cuts off (form) parenchymatous cells. These are called vascular rays (Xylem rays & phloem ray) or secondary medullary rays which passes through the secondary xylem and secondary phloem in the radial direction.They conduct water and food in radial direction. The order of development of vascular rays are both centripetal and centrifugal.

a) Formation of Annual Rings

• Annual rings are formed due to unequal activity of vascular cambium.

• The activity of cambium does not remain same. It is chargeable in the whole year

• Activity of vascular cambium is under the control of many physiological and environmental factors .
•  In temperate regions,the climatic conditions are not uniform through the year.

• In the spring season, the vascular cambium is very active and produces a large number of secondary xylem elements having vessels with wider cavities/lumens. The wood formed during this season is called spring wood or early wood.

• The spring wood is lighter in colour and has a lower density whereas the autumn (or winter) wood is darker and hes a higher density.

Note:-The autumn  and  spring wood are formed in the form of  concentric rings called growth rings.

• The two kinds of wood that appear as alternate concentric rings, constitute an annual ring.

• A ring of autumn wood and a ring of spring wood are collectively known as annual ring. The number of annual rings, formed in a tree give the idea of the age of the tree. The study of determination of age of a tree/plant by counting annuals rings is called Dendrochronology.

• A piece of wood start the stem up to in that region from the  base of stem with the help of Increment borer instrument. The annual rings are counted from that piece and again inserted (fitted) into the same stem as the same place.

• More distinct/clear annual rings are formed in that regions where climatic variations are sharp 

• More distinct annual rings are formed in temperate plants Because in temperate  regions, the climate conditions are not uniform  throughout the year.

• Distinct annual rings are not found in tropical plants. Distinct/clear annual rings are not formed in India except Himalayan regions (Shimla, Nainital ete).

• Least distinct annual rings are formed in seashore regions/coastal regions because the climate remains the same throughout the year.

• More clear annual rings are formed in deciduous plants as compared to evergreen plants .(In temperate region).

•  In deserts annual rings are less distinct.

• In annual rings bands of secondary xylem and xylem rays (Ray parenchyma) are present.

HEART WOOD & SAP WOOD:

In old trees the greater part of secondary xylem is dark brown.

The organic compounds like tannins, resins, gums oils and aromatic substances etc. are filled in lumen of tracheids and vessels of secondary xylem. Due to this, central region of secondary xylem becomes dark brown. It is called heart wood or duramen. These substances make it hard, durable and resistant to the attack of micro-organisms and insects. Heart wood comprises dead elements with highly lignified walls. Heart wood. provides mechanical strength to stem

• The peripheral region of secondary xylem which is light in colour, is called sap wood or alburnum.
• The function of sap wood is conduction of water and minerals.

• Heart wood does not conduct water because :

• Cavities of tracheids and vessels are progressively filled with waste materials.

The bladder/balloon like ingrowth of parenchyma cells enter in the lumen of vessels (mainly) & tracheids through the pits. Such bladder like ingrowths are called tyloses or tracheal plugs. Tyloses block the lumen of tracheary elements (vessels & tracheids).

Heart wood provides stiffness to the stem. The waste materials of heart wood are antiseptic in nature. Heart wood is resistant to the attacks of termites and insects and in rainy season it does not imbibe water. Thus it is the best quality of wood.

• Study of wood is known as Xylotomy. The wood is actually a secondary xylem.

• Position of youngest secondary phloem is just outside the vascular cambium.

• Position of oldest secondary phloem is just inside the primary phloem.

• Position of youngest layer of secondary xylem is just inside the vascular cambium.

• Position of oldest layer of secondary xylem is just outside the primary xylem.

• As the time passes amount of heart wood increases more as compared to sap wood.

Classification of wood:-

 A. On the basis of ammount of parenchyma wood is classified into two groups- 

1. Manoxylic wood- such type of wood contains more amount of living parenchyma. It is loose wood eg. Cycas.

2. Pycnoxylic wood- such wood contains less amount of living parenchyma.

Example- Pinus (conifers).

B. Classification based on vessels-

On the basis of presence and absence of vessels, wood is classified into two categories:-

1. Non- porous wood/Homoxylous wood- vessels are absent in this type of wood

 Example:- mostly gymnosperms.

2. Porous wood/ Heteroxylous wood- vessels are present in this type of wood .eg. mostly dicots (angiosperms) on the basis of arrangement of vessels porous wood is divided into two groups.

a. Ring porous wood- vessels are arranged in the rings in this type of wood.

Example:- in temperate region plants ex. Dalbergia.

b. Diffused porous wood- Asystematical distribution of vessels is found in this type of wood.

 Example:- in tropical region plants ex. Azadirachta ( neem).

Most durable wood - Tectona grandis (Teak= sagwan).

SECONDARY GROWTH IN EXTRA STELAR REGION BY CORK CAMBIUM

• Secondary  growth takes place in extra stelar region due to the activity of cork cambium.  Cork cambium is also known as phellogen or extrastelar cambium. the cells of the cork cambium are narrow, thin walled and nearly rectangular. Cork cambium develops usually in cortical region by hypodermis.

• As the stem continues to increase in girth due to activity of vascular cambium the outer cortical and epidermal layers get broken and need to be replaced to provide new protective cell layers. Hence sooner or later another meristamatic tissue called cork cambium or phellogen develops.

• Cork cambium is derived form the hypodermis (outer part of cortex) by dedifferentiation. Cork cambium is single or a couple of layers thick (mainly). It forms secondary tissues in extra stelar region.

• Cork cambium divides periclinally to form some cells towards the outside (towards epidermis) and some cells towards the inside (towards general cortex). Those cells which are formed towards outside become suberized. Due to this, these cells become dead. These dead cells are known as cork or phellem. Those cells which are formed towards the inside are differentiated into parenchyma and may contain chloroplast. These are called secondary cortex or phelloderm.

• Phellem, phellogen and phelloderm are collectively known as periderm.

• The cork is impervious to water due to suberin deposition in the cell wall. 

• Commercial cork is obtained from  Quercus suber (oak). Common bottle  Cork is made from this cork. 

• Due to activity of the cork cambium, pressure builds up on the remaining layers peripheral to phellogen and ultimately these layers dies and slough off.

• Ring of cork cambium remain living and active only for one year. each year, a new cork cambium is formed below the previous cambium. this new cambium derived from the secondary cortex or phelloderm.

• all the tissue which occur outside the innermost cork cambium are collectively termed as rhytidome. rhytidome include cork and tissue which become dead due to presence of cork. 

Lenticels : At certain regions, the phellogen (cork cambium) cuts off forms closely arranged parenchymatous cells on the outer side instead of cork cells. These thin walled, rounded, colourless, parenchymatous cells are called complementary cells. These cells are not suberized. As the complementary cells increase in number, pressure is exerted on the epidermis to which it ruptures, forming a lens-shaped openings called lenticels.

Complementary cells are formed by the activity of phellogen cork cambium.

• Lenticels are found in most of the woody trees. absent  in woody climbers.

• Lenticels are mainly found on woody stems and they are never found on leaves They are also present on some fruits.

•  Lenticels are not found in herbaceous dicots and monocot plants.

 Functions :

Exchange of gases  Lenticels permit the  Exchange of gases  between the outer atmosphere and the internal tissues of the stem. Main function 

Help in transpiration i.e. Lenticular transpiration.

BARK 

There are two views about the bark.

 Old view All the tissues situated outside the cork cambium are called bark. According to old view bark includes mainly dead tissues.

 Modem view Bark is non-technical term  that refers to all tissues exterior to vascular cambium, therefore including secondary phloem.  According to modern view bark includes both living and dead tissues. 

KIND OF BARK 

1.Ring Bark - Sheet bark continuous  Bark of equal thickening is called ring bark, It is formed around the stem in the form of a complete ring. When cork cambium in continuous then ring bark is formed. Example Bhojpatra (Betula utilis) A complete distinct ring bark is formed in this  plant. its bark was used as writing material/as a paper in ancient period, only cork layer was used. Ring bark is also formed in Eucalyptus.

2.Scaly Bark Discontinuous bark of unequal thickening is called scaly bark. This bark is formed around the stem in the form of pieces or fragments or patches.  When cork cambium is discontinuous then scaly bark is formed  eg. Guave (psidium guajava), Neem (Azadirachta indica), Mango (Magnifera indica), and Tamarind=Imli (Tamarindus) etc. Plants.

• Highly distinct scaly bark is fromed in Psidium guajava (Guava).

• Scaly bark is found in most of the woody plants.

• If bark is removed in the form of a ring (Girdling) from the base of main stem then root dies first due to lack of food.

• Girdling is not possible is monocot stem because vascular bundles are scattered.

• If complete bark is removed then plant dies due to excessive water loss.

• Bark that is formed early in the season is called early or soft bark. Towards the end of the season late or hard bark is formed.

• Secondary phloem and periderm are included in bark.

SECONDARY GROWTH IN DICOT ROOT

In the dicot root, the vascular cambium is completely secondary in origin.

• First of all the tissue located just below the phloem bundles means conjunctive tissue becomes meristematic. during the secondary growth in a dicotyledon root and forms separate curved strips of vasular cambium below the phloem bundles. Then after, the cells of pericycle is less  above the protoxylem also become meristematic to form additional strips of cambium. In this way a complete way a complete ring of vascular cambium is formed.

• The portion of vascular cambium which is formed by pericycle is less. The main portion of vascular cambium is formed by connective tissue.

• The shape of vascular cambium is wavy in the beginning, but later on it becomes circular due to the pressure of secondary xylem.

• The portion of vascular cambium formed by conjunctive tissue becomes meristematic first and forms the secondary xylem towards the centre. Ultimately the cambium becomes circular by the pressure of secondary xylem.

The activity of vascular cambium of root is the same as the activity of vascular cambium of stem. Secondary xylem is formed towards the innerside and secondary phloem is formed towards the outer side by vascular cambium. The portion of vascular cambium which is formed by pericycle is responsible for the formation of pith rays or medullary rays. These are made up of parenchyma. These pith rays are known as primary medullary rays (Multiseriate). A few medullary rays or pith rays are also formed from remaining Vascular cambium. These are called secondary medullary rays (uniseriate). Thus two types of medullary rays are found in the secondary structure of roots.

Note : Two types of medullary rays are formed in the dicot roots during secondary growth.The presence of two types of medullary rays is basic characteristic feature of roots. Only secondary medullary rays are formed in dicot stem during the secondary growth. Both of them conduct water and food in radial direction.

Cork cambium is developed from the pericycle in roots. Cork is formed towards the outside and secondary cortex is formed towards the inner side by the cork cambium. Lenticels are also found in roots but less in number as compared to stem. Cortex completely degenerates in roots after the secondary growth of one or two years. This falls down due to the pressure of cork, whereas in stem, it degenerates after the long duration.

1.Secondary growth is essential in roots to provide strength to the growing aerial parts of the plants and fulfill the requirement of water and minerals.

2.Generally clear annual rings are not seen in roots because roots are not effected by the changes of environment.

3.Secondary growth is not found in monocot roots.

In dicot roots, all cambia and pith rays (medullary rays) are secondary in origin.

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CHROMOSOME

GENERAL INTRODUCTION :

At the time of cell division the chromatin material get condensed to form chromosomes, thus chromosome is highly condensed form of the chromatin. Chromosomes are not visible during interphase stage but during different stages of cell division, cells show structured chromosomes in place of the nucleus.

Chromosomes can be best studied at metaphase stage because size of chromosomes is the shortest during metaphase (Shape of chromosome is studied at Anaphase stage)

The number of chromosomes in a gamete is called "Genome" or "A complete set) of chromosomes inherited as a unit from one parent is known as genome.

A single human cell has approximately two metre long thread of DNA distributed among its 46 (23 pairs) chromosomes.

TYPES OF CHROMOSOMES ON THE BASIS OF POSITION OF CENTROMERE

i) Telocentric - When centromere is terminal or Located at the top of chromosome.

ii) Acrocentric- When the centromere is sub-terminal or located near the tip.

iii) Metacentric - When the centromere is located at mid of the chromosome

iv) Sub metacentric - When the centromere located near centre or mid point of chromosome

The ratio of length of the long arm to the short arm of a chromosome is called arm ratio A ratio is maximum in acrocentric chromosome.

Karyotype is external morphology of all Chromosomes of a cell which is specific for each species of living organisms. Karyotype can be studied in metaphase of mitosis. Karyotype includes the number of chromosomes, relative size, position of centromere, length of the arms, secondary constrictions and banding patterns.

Idiogram :-Diagrammatic representation of Karyotype. In idiogram chromosomes are arranged in decreasing order of size. Sex chromosomes are placed in last. Idiogram is specific for every species.

STRUCTURE OF CHROMOSOME

1. Chromatid - At metaphase stage each chromosome is consist of two cylindrical structures - called chromatids Both sister chromatids are Joined together by a common centromere. A chromosome, may have single chromatid (in Anaphase or Telophase) or two chromatids in prophase and metaphase).

•  Each chromatid is consist of a single long thread of DNA associated with histone. Non histone proteins and RNA are also present

2. Centromere:

Each chromosome (at prophase or metaphase) is consist of two chromatids. Both the chromatids of a chromosome are joined or connected by a structure called Centromere. At centromere two protein discs are present which is called Kinetochore.

Kinetochores constitute the actual site of attachement of spindles to chromosomes during cell division.

At the region of centromere the chromosome is comparatively narrower than remaining part of chromosome thus it is termed as Primary constriction.

3. Secondary constriction : Besides primary constrictions, other constriction may also choose which are knoWn as secondary constriction. These constriction are non staining and foundata constant location.

Secondary constriction is also known as NOR Nucleolar organizer region) (3.14.15.21.22 chromosomes in human)

4. Satellite : Part of chromosome remains after the NOR Is known as chromosomes satellite.

5. Telomere : Chromosomes have polarity and polar ends of chromosomes are known as Telomeres.

 Telomere prevents fusion of one chromosomes to other chromosome. Telomere rich in Guanine base.(5 TTAGGG-3).

Enzyme Telomerase synthesize telomere part of chromosome which is a Ribonucleoprotein Telomeres of chromosomes becomes shorter during ageing process.

SPECIAL TYPE OF CHROMOSOMES

Salivary gland chromosome :- This type of chromosome was discovered by E.G. Balbiani, in Chironomia larva.

This chromosome is called Polytene chromosome, because number of chromatids are very high

Swollen areas present at some places in polytene chromosome, which are called as Balbiani rings or puffs. These puffs helps in synthesis of RNA and proteins.

Salivary gland chromosome concerns with metamorphosis and moulting process of insect larva.

 Lamp brush chromosome :- Discovered by Flemming and Ruckert from oocytes of vertebrates (Amphibia) during diplotene stage of cell division. These chromosomes look like lamp - brush, thus called as lamp brush chromosomes.

Axis of lamp brush chromosome is consist of DNA, while matrix is consist of RNA & proteins.

Lamp brush chromosome is concerned with "Vitellogenesis" (Yolk 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. The cells of permanent tissues do not generally divide further.

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

A. Simple tissue (Homogenous tissue)

B. Complex tissues (Heterogenous tissue)

C. Special tissue (Secretory tissue)

Mainly of 2 types i.e. simple and complex tissues :

(A) SIMPLE PERMANENT TISSUE

This tissues is made up of structurally similar type of cells or only one type of cells that perform a common function .Simple Tissues are of three types - L IL. 

1. Parenchyma

2. Collenchyma

3. Sclerenchyma

PARENCHYMA: It is very primitive type of tissue. It is first evolved tissue. Remaining different types of tissues are derived from this tissue, so it is also called fundamental tissue or precursor of other tissues Parenchyma forms the major component within organs.

Parenchyma term was coined by Grew

CHARACTERISTIC FEATURES:

• It is a living tissue
• It is first differentiated tissue.
• It is a universal tissue.
• Pulp of a fruit is mainly composed of parenchyma.
• Body of bryophytes is mainly composed of parenchyma.
• The cells of parenchyma are thin walled. Cell wall is made up of pectocellulose means pectin + cellulose.(mainly cellulose),So parenchyma is a soft tissue.
• Each cell contains large central vacuole. So the main function of a parenchyma cell is storage of food.
• Parenchymatous cells may either be closely packed or have small intercellular spaces
• It is found in cortex. pericycle, medullary raa. pith leaf mESophyll etc It forms major component within organs.

SHAPE:

The cells of parenchyma are generally isodiametric. They may be spherical (rounded), oval elongated or polygonal in shape.

MODIFICATIONS OF PARENCHYMA:

Prosenchyma - The cells of this parenchyma are thick walled, long with pointed ends. This parenchyma forms the pericycle of roots. Function: To provide strength.

Aerenchyma :- This parenchyma is made up of rounded cells. These cells surround the large air chambers.Aerenchyma is usually found in cortex region. It provides buoyancy to hydrophytes (aquatic plants)

Stellate parenchyma : Found in leaf bases of banana.

The cells of this tissue are stellate (star shaped). Main function of this parenchyma is to provide mechanical support/mechanical strength to leaf bases (pseudo stem) of banana.

Chlorenchyma: Such type of parenchyma contains abundant quantity of chloroplasts. It is found in the mesophyll of leaves. Its function is to perform photosynthesis

Mucilage Parenchyma : In the mucilage parenchyma large vacuoles and mucilage are present. eg. Succulent (fleshy) xerophytic plants.eg. Aloe. Opuntia Cactus, Euphorbia. Function - storage of water.

Functions of parenchyma

• The parenchyma performs various functions like storage, photosynthesis. secretion etc.
• 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 oils, tannin and crystals.
• Photosynthesis (by chlorenchyma).

COLLENCHYMA: Term was coined by Schleiden.

Main characteristics :

• Collenchyma is a living mechanical tissue.
• It is made up of more or less elongated cells (In transverse section cells.appear oval, spherical/round or polygonal in shape).
• Localized deposition of pectin (mainly). cellulose & hemicellulose occurs mainly at corners.
• Usually intercellular spaces are absent.
• Generally chloroplasts are found in the cells of collenchyma or cells often contain chloroplasts.
• These cells assimilate food when they contain chloroplasts.

Occurrance :

• Collenchyma is not a universal tissue. It is found in the stems of herbaceous dicotyledons (young dicot stem) below the epidermis either as a homogenous layer (in sunflower stem) or in patches (in cucurbita steam).
•  Collenchyma forms the hypodermis of dicotyledon stems Cells of collenchyma are flexible de to hydrophilic nature of pecto cellulose, so flexibility occurs in dicotyledonous/dicot stems.
• Margins of leaf lamina and petiole of leaves also bear collenchyma, It protects the lamina margins from cracking by the action of wind.
• Collenchyma is absent in mature/woody plant parts (After secondary growth in dicot stem), roots and monocotyledons.

FUNCTIONS :

• Collenchyma performs both functions mechanical as well as biological/vital functions Provides tensile strength against bending & swaying (mechanical function)
• They provide mechanical support to the growing parts of the plant such as young stem and petiole of a leaf.
• Due to the presence of chloroplast, photosynthesis process (assimilation of food) takes place in collenchyma (vital function).

SCLERENCHYMA:

Term was coined by Mettenius

Main features : 

• Sclerenchyma is the main mechanical tissue. It is dead mechanical tissue.
• Cells of sclerenchyma are generally long, narrow, thick walled, lignified without protoplasts and dead (Cells become dead at maturity).
• Sclerenchyma is found in the hypodermis of monocot stem .Function: It provides mechanical support/mechanical strength to plant organs.

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

Types of sclerenchyma :

On the basis of variation in form, structure, origin & development sclerenchyma cells are of two types.

(i) Sclereids

(ii) Sclerenchymatous fibres

SCLEREIDS: These cells are small dead extremely thick walled (highly thickened) and generally their ends are not pointed. Sclereids are of various shaped.(Spherical.oval or cylindrical) Sclereids cells have pits and lumen (cavity) is almost very small/very narrow. Sclereids are commonly found in fruit walls of nuts, pulp of guava, pear & sapota (Stone cells); seed coats of legumes and leaves of tea etc.

SCLERENCHYMATOUS FIBRES :

The fibres are thick walled. elongated and pointed cells.

Fibres are longest cells in plant body. Their both ends are pointed (tapering). Due to thick cell wall, lumen is reduced and generally occurring in groups in various parts of the plant. Their cell wall contains pits.

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

A.Surface fibres :- They are present on the surface of seeds, fruits etc. These fibres are also called filling fibres.

(i) Seed surface fibres -

Example 1: Cotton fibres (Gossypium fibres) - Cotton fibres are out growth of seed coat Testa.

Cotton fibres are composed of cellulose. They are non-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 fibres

(ii) Coir of coconut is also a type of surface fibres. They are derived/obtained from the fibrous mesocarp of coconut (Cocos nucifera). These are true fibres, because they are lignified

B. Xylary fibres/Intraxylary fibres/Wood fibres: These are hard fibres. These fibres are not flexible. These fibres are obtained from xylem (mainly from secondary xylem or wood).Eg.. Munj fibre (Saccharum munja)

C. Phloem fibres/Extra xylary fibres / Bast fibres These are commercial fibres These fibres are flexible and can be Knitted (weave 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 phloem (secondary phloem) of stem.
• The bast fibres of Hemp (Cannabis sativa) and Flax (Linum usitatissimum) are obtained from the pericycle.

• Phloem fibres of jute, flax and hemp are used commercially.
• Fibres are longest plant cells. Longest fibres occur in phloem of Boehmeria nivea (Ramie plant) length-55 cm.
• In plant kingdom hardest, thickest and Largest leaves are found in Victoria regia.

• Longest commercial timbres -Jute fibres

[B] COMPLEX PERMANENT TISSUE

• The complex tissues are made of more than one type of cells or different types of cells and these work together as a unit. Complex tissues are heterogenous.
• Complex tissues are absent in gametophytes.

• During vascularisation in plants differentiation of procambium is followed by the formation of primary phloem and primary xylem simultaneously.
• Complex tissues are also known as vascular tissues or conducting tissues.
• Complex tissues are of two types - 1. Xylem 2. Phloem

(a) XYLEM

• The term Xylem was coined by Nageli.
• The function of xylem is to conduct water & mineral salts upwards from the roots to stem & leaves and to give mechanical strength to the plant parts.
• For efficient conduction of water death of protoplasm is must. Dead tissues are more developed in water scarce conditions.
• In hydrophytes xylem is poorly developed, while in xerophytes xylem is well developed. 
• On the basis of origin, xylem is divided into primary xylem and secondary xylem.
• Primary xylem originates from procambium during vascularisation. Xylem which is formed early in the life of a plant is known as primary xylem. On the basis of primary xylem is divided into two parts. 

        1. protoxylem               2. Metaxylem

• Cells of protoxylem are small as compared to metaxylem. The first formed primary xylem elements are called protoxylem and the later formed primary xylem is called metaxylem.
• Secondary xylem originates from vascular cambium during secondary growth. Xylem which is formed during secondary growth is known as secondary xylem. Secondary xylem is not differentiated into protoxylem and metaxylem.

Xylem is composed of four different kinds of elements. The elements of xylem are

(1) Tracheids, - dead element

(2) Vessels or tracheae, - dead element

(3) Xylem fibres. - dead element

(4) Xylem parenchyma - living element

1. TRACHEIDS :

• Tracheids are primitive conducting elements of xylem.
• Single tracheid is elongated or tube like cell with thick and lignified walls and possess a narrow lumen. The ends of tracheids are tapering or chisel like.
• The tracheids found one above the other and are separated by cross wall / end wall which bears bordered pits.
• Usually bordered pits are present at the end walls of tracheids. The maximum bordered pits are found in the tracheids of Gymnosperm plants.
• Tracheids are dead (without protoplasm) and lignified cells.
• Tracheids are found in pteridophytes, gymnosperms and angiosperms.
• End walls of tracheids are imperforate (not porous) but pitted (Pits are present)
• Tracheids are unicellular.
• The inner layers of cell walls have thickenings which vary in form.
• Types of thickening in tracheids and vessels are annular, spiral, reticulate, pitted and scalariform.
• The deposition of lignin on cell wall is responsible for the formation of different types of thickenis annular (primitive type), spiral, scalariform, reticulate and pitted.
• Annular and spiral type of thickening of lignin is found in protoxylem.
• Reticulate and pitted (mainly) type of thickening of lignin is found in metaxylem.
• Scalariform (ladder like) type of thickening is found in metaxylem tracheids of pteridophytes mata xylem tracheids of Cycas (gymnosperm).
• Maximum deposition of lignin is found in pitted type of thickening and pits are formed in this type of th
• Pits are unlignified areas on lignified walls.

2) VESSELS - TRACHEAE

• Vessel is an advanced conducting element of xylem.

• Vessel is a long cylindrical tube like structure with lignified walls and a wide (large) central lumen cavity.

• Vessel is multicellular, it is made up of many.cells called vessel members or vessel elements.

• Vessel is an example of dead syncyte. Vessel cells are also devoid of protoplasm.

• The end wall is perforated. Thus vessels are more capable for conduction of water than tracheids. Due to presence of perforated end walls, vessels work as a pipe line during conduction of water.

• Vessel members ao interconnected through perforations in their common walls. The perforation may be simple (only one porel) or multiple (several pores). Vessels contain usually simple pits on their lateral walls.

1. Presence of vessels is a characteristic feature of angiosperms Vessels are usually absent in gymnosperms but exceptionally vessels are present in some gymnosperms like Ephedra, Gnetum and Welwitschia (Order Gnetales).

2. Vessels are absent in some angiosperm plants such as Dracaena, Yucca, Dagenaria, Drimys. There are some angiosperm families in which vessel less angiosperms are included e.g. Winteraceae Tetracentraceae and Trochodendraceae.

3. Tracheids and vessels are called tracheary elements of xylem

4. In flowering plants. tracheids and vesselsace.be main water transporting elements.

Syncyte: Structure which is formed by fusion of cells is called syncyte

3. XYLEM FIBRES - WOOD FIBRES :

• They may either be septate or aseptate.

• Xylem fibres provide strength to the tracheids and vessels They have highly thickened wall and obliterated.central lumen.

• That are abundantly found in secondary xylem (wood)

• They are generally not found in gymnosperm wood to gymnosperms are also called soft wood spermatophytes)

4. XYLEM PARENCHYMA

Cell living and thin walled and their cells are madesinof.celulose.

Function: Storage of food material in the form.of starch.or.fat and storage of other substance, like tannins.

Hadrome - Conducting part of xylem is known as hadrome.

Tracheid And vessels are collections own water conducting element or Hadrome', term was proposed by Haberlandt.

PHLOEM 

• The term Phloem was coined by Nagell.
• The main function of the phloem is to conduct/transport food materials usually from the leaves to other parts of the plants.
• On the base of origin, phloum is dassified into two categories primary and secondary phloem.
• Primary phloem originates from procambium during vascularisation and secondary phloem originates from vascular cambium during secondary growth.
• On the basis of development primary phloem is categorised into protophloem and metaphloem.
• The protophloem (first formed primary phloem) has narrow sieve tubes whereas metaphloem (later formed primary phloem) has bigger sieve tubes.
• Phloem remain active for less duration as compared to xylem.
• Phloem consist 4 types of cells / elements.
• Sieve tube elements, companion cells, phloem parenchyma & phloem fibres (In angiosperms).
• Sieve cells albuminous cells, phloem parenchyma and phloem fibres (In Gymnosperms). 

• sieve cell element - In Gymnosperms and pteridophytes 
• sieve tube element In Angiosperms

• Sieve element was discovered by Hartig.
• In Angiosperm plants, sieve tube elements are joined from their ends to form sieve tube. Their end walls are perforated means having sieve pores in a sieve plates   (oblique perforated septal. Translocation of food material takes place through these pores.
• Sieve tube is an example at living syncyte.
• Sieve tube elements are long, tube like structures arranged longitudinally and associated with companion cells.
• Sieve cells and sieve tube elements are living and thin walled.
• A mature sieve tube element possess, a peripheral cytoplasm & a large vacuole but lacks a nucleus.
• Mature sieve tube elements are enucleated living cells (enucleated means without nucleus).
• The function of sieve tubes are controlled by the nucleus companion cells.
• A central large vacuole is present in each sieve cell and sieve tube element and around the central vacuole thin layer of cytoplasm is present.

Notes : 

• Callose deposition takes place on the radius of sieve pores during dropping season/falling season of leaves (autumn), to form a thick layer. This called calls pad /callose pad. It is formed mainly in deciduous plants

• Sieve plate is protected by callose pad, it is protects sieve plate from bacterial infection and drought dryness.
• Callose dissolves during spring season. Callose is a polymer of beta-1, 3-glucan.
• Siee elements contain special type cof Protein P-protein (P=phloem). Most likely function of p-protein is sealing mechanism on wounding along with callose and it is also  related with conduction of food.
•  In Pteridophyte and gymnosperm, sieve cells arranged in zigzag manner. In sieve cells, seve areas are located laterally. Sieve cells are narrow elongated cells.

COMPANION CELLS

• These are thin walled and specialised parenchymatous cells, which are closely associated with sieve tube elements.
• The sieve tube elements and companion cell are connected by pit fields presents in their common longitudinal walls.
• Companion cells laterally associated with each sieve tube element in angiospermic plants.
• Sieve tube element and companion cell originate together. Both of the originate from a single mother cell. So they are called sister cells.
•  Comparison cell a living cell with large elongated nucleus. This nucleus also controls the activity/function  of sieve tube element.
• Companion cells are found only angiosperms.
• The companion cells help in maintaining the pressure gradient in the sieve tubes.

Note: Special type of cells are attached with the sieve cells in gymnosperm and in pteridophytes in place of companion cells. These cells are called albuminous cells/Strasburger cells.

PHLOEM FIBRES/BAST FIBRES

• These are Made up of sclerenchymatous cells.
• These are much elongated, unbranched and have pointed needle like apices. The cell wall of phloem fibres in quite thick.
• These fibres are generally not found in primary phloem but  are found in the secondary phloem.
• At maturity fibres lose their protoplasm and become dead.

PHLOEM PARENCHYMA

• It's cells are living, elongated, tapering, cylindrical which have dense cytoplasm and nucleus.
• The wall is composed of cellulose and has pit fields through which plamodesmatal connection exist between the cells.
• It stores food material and other substances like resins, latex, mucilage etc.
• The main function of phloem parenchyma storage of food material and function of phloem is ray(ray parenchyma) is conduction of food in radial direction.
• Phloem parenchyma absent in most of the monocotyledons.
• The conducting ring of phloem in called Leptome.
• Leptome term was  given by Haberlandt.

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RIBOSOME (ENGINE OF CELL) and OTHER ORGANELLES

Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid RNA) and proteins and are not surrounded by any membrane.

Except mammalian RBC all living cells have ribosomes. (Both prokaryotes & Eukaryotes) Ribosomes are smallest cell organelles.

 Ribosomes are also called as "Organelle within organelle" and "Protein factory of cell"

Types of Ribosomes :

1) Eukaryotic ribosomes :- 80S- Occur in cytoplasm of eukaryotic cells.

2) Prokaryotic ribosomes :- 70 S- Occur in cytoplasm and associated with plasma membrane of prokaryotic cell. Their size is 15 to 20 nanometre.

70 S ribosome also present in mitochondria and chloroplast of eukaryotes.

S=Svedberg unit or Sedimentation rate. It indirectly is a measure of density and size.

Each ribosome composed of two subunits i.e, larger and smaller subunits

80 S - 60 S+ 40s 70 S - 50 S + 30 S

Magnesium ion is essential groups of for the binding the ribosome subunits. Mg form ionic bond with phosphate r- RNA of two subunits. Minimum 0.001 M Mg concentration is required for structural formation of ribosomes.

Chemical Composition of Ribosomes :

70 S 60% r- RNA + 40% proteins 80 S - 60 S - 40 S - 40% r-RNA + 60% proteins rRNA 28S, 5.8 S, 5 S rRNA 18S 50 S - rRNA 23S, 5S 30 S = RNA 16 S

At the time of protein synthesis, several 70 S ribosomes become attached to m-RNA with the help of smaller subunits. This structure is called polyribosome or polysome vs Ribosome Large subunit (50s) contains peptidyl transferase enzyme (23S rRNA) which helps in the formation of peptide bond during protein synthesis. This is an example of Ribozyme. (Noller 1992)

CYTOSKELETON

An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton The cytoskeleton in a cell are Involved in many functions such as mechanical support motility maintenance of the shape of the cell.

MICROTUBULES

Microtubules are composed of protein, Tubulin (Size 25 nm).

In plants microtubules often found associated with cell wall. Probably these transport cell wall material from Golgi body to outside of cell. During cell division these microtubules form spindle fibers.

MICROFILAMENTS

They are composed of contractile protein Actin which concern with muscle contraction, Microtubules and microfilament are part of cytoskeleton-base of cell. [Size 6-7 nm)

INTERMEDIATE FILAMENT

Intermediate filaments has size/diameter in between microfilaments and microtubules. These filaments form basket like structure around the nucleus. [Size 8-12 nm]

CILIA AND FLAGELLA

Cilia (sing. cilia) and flagella (sing: flagellum) are hair-like outgrowths of the cell membrane. Cilia are smal structures which work like oars, causing the movement of either the cell or the surrounding fluid Flagella are comparatively longer and responsible for cell movement. The bacteria (prokaryotic cell also possess flagella but these are structurally different from that of the eukaryotic flagella

The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane.

Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array

(9 doublet + 2 singlet)

Arms of A tubules consist of an enzymatic protein dynein similar to myosin of muscle cells. Dynein have ability of hydrolysis of ATP & Liberates energy for ciliary and flagellar movement.

The central tubules are connected by bridges and is also enclosed by a central sheath, which is one of the tubules of each peripheral doubles by radial spoke. Thus there are nine radial

connected to spokes, The peripheral doublets are also interconnected by lakers. Both the cilium & flagella emerge from centriole like structure called the basal bodies or blepharoplasty.

CENTROSOME and CENTRIOLES

Centrosome is absent in higher plants.

Centrosome containing twa centrioles (diplosome) located just outside the nucleus and lie at right angle 1909 each other. Each centriole is surrounded by amorphous pericentriolar materials.

Centrioles are membraneless cylindrical structure which exhibicari wheel structure in transverse section Centriole mainly consist of 9 evenly spaced peripheral triplet fibrils of tubulin. These triplets are linke with the help of A-C linker.

The central part of the centriole is proteinaceous and called the hub, which Is connected with peripher triplets by radial spokes made of protein (9 + 0 arrangement) Centrioles are self duplicating units.

Function :

In animal cells, centrioles play important role in cell division by arranging spindle fibres between two poles of cell. The location of centrioles during cell division decides the plane of division. The plane of division is always at right angle to the spindle.

Centrioles form the basal body of cilia or flagella.

MICRO-BODIES

These are many, membrane bound minute vesicle contain various enzyme that are present in both plant and animal cells.

1) Peroxisomes and ribosomes :

These are found in both plant and animal cells. Peroxisomes contain catalase enzyme which is concerned with peroxide (HO Metabolism. Catalase degrade the H,O into water and oxygen

In plants, peroxisomes occurs in cells of green tissues and concerned with photorespiration (glycolate pathway).

Peroxisomes are also involved in B-oxidation of fatty acids,

(2) Glyoxysomes :

Glyoxysomes occurs only in plants especially in fatty seeds (castor seed, ground nut seed etc.).

Glyoxysomes are considered as a highly specialised peroxisomes Glyoxylate acid cycle takes place in glyoxysomes. This cycle convert fats into carbohydrates.

Anatomy of Flowering plants

Plant Anatomy:

The branch of botany which deals with study of internal structures and organization of plants or plant organs (plant parts) is known as plant anatomy/study of internal structure of plants is called plant anatomy.

N.Grew (Nehemiah Grew) is known as father of plant anatomy.

K.A. Chaudhary is known as father of Indian plant anatomy.

Note: Book- "The Anatomy of seed plants" was written by Katherine Esau (K.Esau). It was published in 1960. It was referred to as Webster's of plant biology- it is encyclopedia.

TISSUE:

A group of cells having a common origin and usually perform a common function is called tissue.

 The term tissue was coined by Nehemiah Grew.

The tissue were divided into two groups by Karl Nageli.

Meristem: Growth in plants is largely restricted to specialised regions of active cell division called meristems/ A meristem is a localised region in which actual cell division occurs.

Meristem term was given by Nageli. It is derived from a Greek word meristos (means Divided/Divisible).

CLASSIFICATION OF MERISTEMATIC TISSUE MERISTEM/MERISTEM:

MERISTEM BASED ON ORIGIN AND DEVELOPMENT

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

PROMERISTEM/PRIMORDIAL MERISTEM/URMERISTEM

This meristem develops in the beginning during embryonic stage. It forms primary meristem.

eg. Embryonic meristem

PRIMARY MERISTEM

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

• It appears early in the life of a plant and contribute to the formation of the primary plant body.
• Cells are always in division phase and form primary permanent by the process of differentiation. eg. Apical meristem, intercalary meristem, intrafascicular cambium (Fascicular vascular cambium).

SECONDARY MERISTEM

• Secondary meristem develops from primary permanent tissue by the process of dedifferentiation. 
• Secondary meristem appears later than primary meristem. 
• By the activity of secondary meristem, secondary growth takes place. eg. Interfascicular cambium & cork cambium of dicot stem, vascular cambium & cork cambium of dicot root.

[B] MERISTEM BASED ON LOCATION (POSITION) IN PLANT BODY

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

(a) APICAL MERISTEM : It is an example of primary meristem.

The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristers. They are responsible for increase in the length of plant organs. It means they are responsible for primary growth. Examples of apical meristem - Root apex/root apical meristem, shoot apex/shoot apical meristem.

HABERLANDT DIVIDED EUMERISTEM (it is a primary Meristem i.e. apical Meristem) into three region  on the basis of function.

Protoderm : It is the outermost layer of meristem By the activity of protoderm epidermal tissue system is formed. E.T.S. includes epidermis, stomata, stem hair (shoot hair/trichomes) etc.

Procambium : It is made up of elongated cells and it gives rise to the vascular tissue system V.T.S. includes Xylem, phloem (vascular bundles).

Ground Meristem : The cells of this region are thin walled and isodiametric. Ground tissue system is formed by the activity of these cells. G.T.S. includes hypodermis, general cortex. endodermis, pericycle, pith-rays (medullary rays) and pith(medulla). During the formation of the primary plant body, specific regions of the apical meristem produce dermal tissues, ground tissues and vascular tissues.

(b)INTERCALARY MERISTEM :

Is an example of primary meristem.

It is present at the base of internode of monocuts stems e.g.grasses,bamboo,sugarcane etc it is also present at the base of leaves. 

By the activity of this Meristem, length of leaves increases.

Intercalary meristem occurs between mature tissues.

By the activity of this meristem length of the plant organs increases.

They occur in grasses and regenerate parts removed by the grazing herbivores.

Both apical meristems & intercalary meristems are primary meristems because they appear early in the life of a plant and contribute to the formation of primary plant body

(c) LATERAL MERISTEM:

• Lateral meristem occurs on lateral side of plant organs.
• Activity of lateral meristem increases the circumference/ girth/thickness of plant organ.

• All secondary meristems are lateral meristems.

• Lateral meristems are both primary and secondary in origin. (mostly secondary in origin).

Primary lateral meristems:

1.Marginal meristem

2. Intrafascicular cambium

1.Marginal meristem :- It occurs at the margin of young leaf. Its activity causes the width o f not thickness). For total growth of head only primary meristems are responsible.

2. Intrafascicular cambium /fascicular cambium : This cambium occurs inside the vascular bundles of dicot stem  and gymnosperms stems.

Secondary lateral meristems :-Interfascicular cambium and cork cambium (phellogen) of dicot stem and gymnosperm stem. Cork cambium and vascular cambium of dicot roots.

Note : Generally lateral meristems are cylindrical.

Note: The meristem that occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis and appears later than primary meristemis  called the secondary meristem.

COMPOSITION/ORGANISATION OF APICAL MERISTEM IN DIFFERENT PLANTS

• Apical meristem is absent in most of the 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.

• In some algae (eg.- brown algae), bryophytes and some pteridophytes one meristematic cell present  at the apex. Generally the shape of apical cell is pyramid like.
• In ferns, gymnosperms and angiosperms apical meristem consists of many cells.

HISTOGEN THEORY:

• This is most valid theory for root apex organisation.

• This theory was proposed by Hanstein (1870).

According to Hanstein, the apical meristems (root and shoot apices) are distinguished into three histogens (meristematic regions). These are as follows.

1.Dermatogen. This is the outermost histogen & composed of single layer of cells. These cells form uniseriate (single layered) epidermis.

2.Periblem - This region is situated Just below the dermatogen. It forms cortex (hypodermis, general cortex and endodermis).

3.Plerome - This is the innermost histogen. Stele formation takes place by division of these cells. It means formation of pericycle, vascular bundles, pith rays (medullary rays) and pith(medulla).

This theory is true only for root apex. It is not applicable for shoot apex of higher plants because in most of the gymnosperms and angiosperms, shoot apex is not differentiated into three histogens.

Including above described three histogens, a fourth histogen is also present in monocotyledon root apex. This is known as calyptrogen. Root cap is produced by calyptrogen in monocots. Root cap & epiblema/epidermis are produced by dermatogen in dicotyledons.

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

• In hydrophytes root cap is absent eg. Pistia. In place of root cap, root pockets are present.

• Generally, root cap is single but in Pandanus (screw pine), multiple root cap is present.

• Root cap is living, it contains large amount of golgi bodies which secrete mucilage which makes the roots slimy.

• In monocot root, epidermis and calyptrogen are derived from dermatogen.

Note :- Root cap is formed by calyptrogen (If dicot or monocot is not given)

QUIESCENT CENTRE:-

Quiescent centre term was coined by "Clowes". Quiescent centre was discovered by Clowes in maize root.

A group of inactive or less active cells present between the dermatogen and calyptrogen of monocot root is called quiescent centre. The cells of quiescent centre contain less amount of DNA, RNA, light cytoplasm, small nuclei and synthesis of protein is also less.

Function: The quiescent centre in the root meristem serves as a reserve for replenishment damaged cells of the meristem or Inactive cells of quiescent centre become active when previously active initials of calyptrogen get damaged. Quiescent centre is crescentic shape.

TUNICA CORPUS THEORY

This theory was proposed by Schmidt (1924).It is most valid theory for shoot apex organisation of angiosperms. It is based on planes of division. According to this theory two zones are found in the shoot apex:-

(1) TUNICA

• This is peripheral layer. In turica cells, anticlinal division takes place only in one plane Epidermis is formed by tunica.
• Generally, tunica is single layered, but sometimes it is multilayered, than the outer most layer forms the epidermis and remaining layers of tunica form rest types of tissues with the association of corpus.

CORPUS

•  The mass of cells present below the tunica is called corpus. The cells of this zone divides in all directions (all planes) due to which volume increases. The cells of corpus are usually larger than the cells of tunica.
• Function : Formation stele of ground tissue system and vascular tissue system or Formation of cortex and stele.

CYTO-HISTOLOGICAL ZONATION THEORY:- 

According to Foster shoot apical meristem bested into two regions on the basis of rate of division

1. Summit

2 Flanks

VEGETATIVE SHOOT APEX

SUMMIT:

The rate of division is slow in this region. This region is located at the apex

FLANKS:

The rate of division is very fast in this region. This region lies behind/below 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- dome (mainly) or conical shaped.
•  Shape of reproductive shoot apex- Broad & flat
• Shoot apex is terminal in position.
• Growth of leaf primordium - first apical then marginal.
• Function of leaf primordium-Provide protection to shoot apex.

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.

• In reproductive shoot apex, summit zone is more active (rate of cell division fast) and it forms stamens (androecium) & carpels (gynoecium) and flanks zone is less active rate of cell division slow) and it forms sepals (calyx) and petals (corolla).

• During the formation of leaves and elongation of stem, some cells "Left behind" from shoot apical meristem. constitute the axillary bud. Such buds are present in the axils of leaves and are capable of forming a branch or a flower.

• Root apical meristem occupies the tip of a root while shoot apical meristem occupies the distant most region of the stem axis.

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