3-APICAL MERISTEM.pptx

Apical meristem

1. APICAL MERISTEM Professor Hussan Akbar Khan

2. Definition: The meristem at the tip of a stem or root that gives rise primary tissues and is responsible for increase in length rather than girth of the axis.

3. Origin of Apical Meristem: Seed has two parts 1- Plumule 2- Radicle On germination Plumule forms shoot, shoot forms a full shoot system ,shoot tip has shoot apical meristem which is terminal in position. Radicle forms root which form a full system root tip has root apical meristem which is sub-terminal in position.

4. Germinating seed

5. Characteristics of Apical Meristem 1 . They are composed of immature cells which are in a state of division and growth. 2. Usually the intercellular spaces are not found among the cells .

6. 3. The cells may be rounded, oval or polygonal in shape they are always living and thin walled. 4. Each cell of meristematic tissue possesses abundant cytoplasm and one or more nuclei in it.

7. 5. The vacuoles in the cells may be quite small or altogether absent.

8. Types of Apical Meristem: There are two types of apical meristem 1- Shoot apical meristem ( SAM): Present on the apex of shoot . Which gives rise to organs like the leaves and flowers.

9. 2- Root apical meristem (RAM): Present on the apex of root . Which provides the meristematic cells for future root growth.

10. Root Apical Meristem

11. Structure of Shoot Apical Meristem: Shoot apical Meristem is protected by juvenile leaf. It is dome shaped or conical in structure Big in size 1 cm. Outer part is called summit having very less mitotic activity.

12. Below the summit part is called flank which has high metabolic activity so high mitotic division. Periodic cyclic changes occur in flank Flank forms leaf primordia The distance between two leaf primordia is called plastochron .

13. When cells divide they are added towards the base they will give rise the axillary buds. Outside part is called protoderm Inside the protoderm , ground meristem which will form cortex and pith tissues Procambium which will form vascular tissues

14. Because of the axillary buds alternate nodes and internodes are developed, its branches will be exogenous in origin High degree of differentiation is absent in SAM. SAM goes high degree of changes during reproduction.

15. Theories of Meristem Organization and Function: Many anatomists illustrated the root and shoot apical meristems on the basis of number and arrangement of cells , proposed the following theories.

16. 1-Apical Cell Theory: Apical cell theory is proposed by Hofmeister (1852) and supported by Nageli (1859). A single apical cell is the structural and functional unit.

17. Hofmeister Nageli

18. This apical cell governs the growth and development of whole plant body. It is applicable in Algae, Bryophytes and in some Pteridophytes.

19. 2-Histogen Theory   : Histogen theory was proposed by  Hanstein  (1868) and supported by  Strassburgur .   The shoot apex comprises three distinct zones .   1 .    Dermatogen :  It is an outermost layer.   It gives rise to epidermis . 2 .       Periblem :  It is a middle layer. It gives   rise to cortex. 3 .          Plerome :  It is innermost layer. It gives   rise to stele

20. Strassburgur

21. 3- Tunica Corpus Theory   : Tunica corpus theory was proposed by  A. Schmidt  (1924). Two zones of tissues are found in apical meristem . a.       The tunica : It is the peripheral zone of   shoot apex, that forms epidermis . Single layer of cells. It forms dermatogen which give rise to epidermis of stem and leaf. b.        The corpus : It is the inner zone of   shoot apex , that forms cortex and stele of shoot.

22. Schmidt

23. Root Apical Meristem: Root apex is present opposite to the shoot apex. The roots contain root cap at their apices and the apical meristem is present below the root cap. 

24. Structure and Position of Root Apical Meristem: Sub terminal in position Small size less than 1mm At the apex there is root cap Behind the root cap is apical root meristem

25. The different theories proposed to explain root apical meristem organization is given below . 1- Apical Cell Theory :   Apical cell theory is proposed by  Nageli .   The single apical cell or apical initial composes the root meristem. The apical initial is tetrahedral in shape and produces root cap from one side. The remaining three sides produce epidermis, cortex and vascular tissues. It is found in vascular cryptogams.

26. 2- Histogen Theory Histogen theory is proposed by  Hanstein  (1868) and supported by  Strassburgur .  The shoot apex comprises three distinct zones .   1.          Dermatogen/ Calyptrogen :   It is an outermost layer.   It gives rise to epidermis. 2.             Periblem :  It is a middle layer. It gives   rise to cortex. 3.             Plerome :  It is innermost layer. It gives   rise to stele

27. 3- Korper Kappe Theory   : Korper kappe theory was proposed by  Schuepp in 1917 . It is similar to the tunica-corpus theory of the shoot apex. It is based on differences in the planes of cell division. There are two zones in root   apex – Korper and Kappe 1.             Korper zone  forms the body . 2.             Kappe zone  forms the cap.

28. 4- Quiescent Centre Concept: Quiescent cell theory was given by Claws in 1961 in maize. These are the cells that are present in roots is a region of apical meristem that does not multiply or is divide very slowly but these cells are able to restore division where it is required or when the cells around them get damaged. .

29. These centre is located between root cap and differentiating cells of the roots. Quiescent centre is the site of hormone synthesis and also the ultimate source of all meristematic cells of the meristem.