Meristematic tissue types and classification.

Meristematic tissue types and classification.

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  UNIVERSITY OF ALLAHABAD DEPARTMENTOF BOTANY TOPIC : MERISTEM -ANURAG PANDEY
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   TISSUE :Group of cells which are similar, having a common origin and performing similar function.  A complex of cells of common origin. -Fahn (1928) Tissues are of three types : TISSUE MERISTEMATI TISSUE PERMANENT TISSUE SECRETORY TISSUE
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  MERISTEMATIC TISSUE : The term meristem was coined by Carl Wilhelm von Nägeli (1858).  Term meristem is derived from Greek word “Merismos” that means “divisible”.  Meristematic tissue is group of immature cells that has capacity of division and redivision.  The term of meristem is applied to the region of more or less continuous cell division from which new tissues and organs are produced .  Meristematic tissues are the products of meristem which do not develop into adult cells but remain meristematic .  For example, the root and shoot apices and the cambium are meristems which produce meristematic tissues that divide and differentiate into xylem ,phloem, parenchyma, etc.
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  CHARACTERSTICS OF MERISTEM Ability to grow and divide, small immature cells, Isodiametric, rounded, oval or polygonal, Absence of intercellular spaces.  Walls are thin, elastic and made up of cellulose, dense protoplasm with many small vacuoles and large nuclei .  Pro-plastids are present instead of plastids.  Endoplasmic reticulum is poorly developed.  Mitochondria have simple structure, rate of respiration is very high.  These cell do not store reserve food materials or very low reserve food materials.
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   Functions ofMeristematic Tissue:  Meristems are actively dividing tissues of the plant.  They are responsible for primary (elongation) and secondary (thickness) growth of the plant.  All new organs and their growth occur by the division of meristematic tissue.  Secondary tissues such as wood, cork are also formed due to activity of meristematic tissue.
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  Classification of meristem: Meristematic tissue On the basis of origin Promeristem Primary meristem Secondary meristem On the basis of position Apical meristem Lateral meristem Intercalary meristem On the basis of functions Protoderm Procambium Ground meristem On the basis of plane of division Rib meristem Mass meristem Plate meristem
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  Classification based onorigin/development : 1. Promeristem or Primodial meristem -:  Origin: embryonic origin therefore called Primordial or Embryonic meristem.  It is earliest and youngest meristematic tissue.  It is present at the extreme tip of young and growing root and shoot consisting of only few cells.  It give rises to primary meristem. 2. Primary meristem -:  Origin: From Promeristem  Cell are always active and dividing  Present below promeristem in the shoot and root tip, and also in intercalary position  It gives rise to secondary meristem and primary permanent tissue
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  3.Secondary meristem -: Origin: From primary meristem.  It is developed later in life.  It gives rise to secondary permanent tissue.  E.g. Vascular cambium and cork cambium.
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  Classification based onposition in plant body : Meristem oApical meristem oPresent at growing apices of plant such as root tip and shoot tip. •It divides in different planes and helps to increase the length. •Apical meristem has two distinct zone: •Promeristem zone: contains group of dividing cell (apical initials) •Meristematic zone: contains Lateral meristem 1.Located in between the regions of permanent tissues. 2.It divides periclinally . 3.It helps to increase thickening or girth during secondary growth. 4.Example: vascular cambium (primary meristem) and cork cambium (secondary meristem) Intercalary meristem These are the portions of apical meristems which are separated from the apex during the growth of the axis and remain intercalated between permanent cells. It helps in elongation of plant body. Commonly present in monocots, grass and pines. Two type of intercalary meristem involved in
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  Classification based onfunction -: o Haberlandt (1914) classified meristem on the basis of function. Meristem Protoderm Function: Protection mechanical injury •It gives rise to epidermis layer. •It is the outermost meristematic tissue Procambium Function: Transport of water and nutrition •It gives rise to vascular tissue (xylem and phloem) •It is the innermost meristematic tissue Ground meristem Function: Various functions •It gives rise to cortex, endodermis, pericycle and pith in dicot and hypodermis, ground tissue in monocot.
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  Apical meristem Protoderm Epidermis Cork cambium Procambium Primary xylem Fascicular cambium Primary phloem Ground meristem Pith Cortex Vascular cambium Secondary xylem Secondary phloem
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  Classification based onPlane of Division -: On the basis of plane of division the meristem are grouped under the following three types -  Rib meristem-The cells divide only in one plane (anticlinal)is called Rib meristem. It’s important role is in the development of pith and cortex of young stem.  Mass meristem-Cells divide in three planes or all planes e.g. Early embryo. It’s important role is in the early development of endosperm, sporangia, cortex and pith.  Plate meristem -The cells divide chiefly in two planes giving a plate like increase is called plate meristem. It’s important role is in the development of leaves without increasing thickness.
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  Quiescent Centre orQuiescent Zone: o First time observed by Clowes(1956,58) in root tips of Zea mays. o An inactive center in between root cap and meristematic region. o It is a biconvex structure and made up of thousands of inactive cells. o In this region the rate of cell division is very slow as compared to surrounding cells. o This region carries less amount of DNA, RNA, ER, mitochondria and ribosomes. o This zone is the site of auxin synthesis. Functions:  It acts as reservoir of cells.  It is more resistant to injury and irradiation.  If root growth is stopped or root tip is damaged then this zone restores growth.
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  REFERENCES:  www.biologydiscussion.com  www.onlinebiologynotes.com Esau’s Plant Anatomy third edition.