Download Meristematic tissues The term meristem has been derived from

Dr.Muhannad.M.Sahib
Plant histology (lecture3)
College of Biotechnology
Qadysia university
Meristematic tissues
The term meristem has been derived from meristose which means divisible and thus a
meristem can be defined as an immature, not well differentiated cell which has the capacity of
division. True meristem cells (eumeristems) have following important features.
1- They are normally isodiametric, spherical, oval or polygonal in shape.
2- They are compactly arranged and lack intercellular spaces.
3- They have thin, homogenous and cellulosic cell wall. Secondary wall is normally absent.
4- Each cell has dense cytoplasm with a large distinct and prominent nucleus. Vacuoles
and ergastic substances are generally absent.
5- Plastids are indistinct or may be present in the form of proplastids.
6- The most important character of these cells is their capacity to divide. In fact, meristems
are formative regions from where new cells are added to the plant body.
Classification of meristems
Meristems are normally classified on basis of their origin, plane of division, functions and
position in the plant body.
I-
Classification based on origin and development
On the basis of origin and development of initiating cells, three categories are recognized.
1- Promeristem
The term promeristem is used to refer to a group of earliest and youngest meristematic
cells of a growing organ. It is the embryonic meristem. It occupies a small area at the
tip of stem and root.
2- Primary meristem
These are derived from promeristem and found at tips of root, stem and appendages.
The cells divided in all possible planes and form the fundamental part or primary
structure of a plant.
3- Secondary meristem
It is derived from primary permanent tissues which have the capacity of division. These
are always lateral in position and develop either at the time of emergency to heal up the
wounded portion or to effect secondary growth or to form cork.
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Dr.Muhannad.M.Sahib
II-
Plant histology (lecture3)
College of Biotechnology
Qadysia university
Classification on basis of position
On the basis of their position in the plant body, three categories are recognized.
1- Apical meristem
These are found at the apex or growing points of root and shoot and bring about increase
in length.
2- Intercalary meristem
It lies between the regions of permanent tissues and is considered as a part of primary
meristem. It is found at the base of internode in grasses and below the node in mentha.
3- Lateral meristem
These are arranged parallel to the sides of organs. These increase the thickness of the
plant part. Vascular cambium and cork cambium are common examples of such
meristem.
III-
Classification on basis of function
1- Protoderm meristem
2- Procambium meristem
3- Ground meristem
*Several theories have been to explain the mode of growth found in shoot apical meristem
1- Apical cell theory
This theory proposed by Nagli 1878. According this theory, a single apical cell is the structural
and functional unit apical meristem which governs the entire process of apical growth. Such
organization has been found only in lower plants like algae, ferns.
2- Histogen theory
Hanstein postulated this theory revealing that three distinct meristematic zone (layer) are found
in apical region. Each zone consists of a variable number of layers called histogen or tissue
builder.
i-
Dermatogen
The outermost uniseriate layer. Dermatogen cells divide anticlinally and develop into
unilayerd epidermis.
ii-
Periblem
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Dr.Muhannad.M.Sahib
Plant histology (lecture3)
College of Biotechnology
Qadysia university
The middle region composed of isodiametric cell, forms the cortex.
iii-
Plerome
The central or inner mass, forms the stele (vascular tissues, pericycle, pith, rays).
Disadvantage of this theory:
It is not possible to distinguish these three histogen layers in gymnosperms and
angiosperms. Hence this theory was rejected for shoot apical meristem.
3- Tunica-corpus theory
Schmidt 1924 proposed tunica – corpus theory to explain shoot apex organization.
According to this theory, there are two zone in apical meristem.
i-
Tunica: the outer zone consisting of one or more peripheral layers of cells, forming
the outer region by anticlinal divisions.
ii-
Corpus: the central undifferentiated multilayered mass of cells surrounded by tunica
which forms the central part of shoot by irregular divisions.
The tunica cells are smaller and corpus cells are larger. The number of tunica layers may
vary even in same species due to influence of seasonal growth changes.
4- Cytohistological zonation
It was found that in gymnosperms, tunica like layer is not found which led foster 1939 to divide
shoot apex organization of ginkgo biloba on basis of cytohistological zonation, which react
differently to staining. He recognized four inter related zones.
i-
Apical initial zone
The outermost layer of the apical meristem undergoes periclinal and anticlinal divisions and
contributes cells to the peripheral and interior tissues of the shoot.
ii-
Peripheral layer
Encircles the other zones. The peripheral zone typically is the most meristematic of all three
zones and has the densest protoplasts and the smallest cell dimensions. It may be described as
a eumeristem. Leaf primordia and the procambium arise here, as well as the cortical ground
Tissue.
iii-
Central mother cell
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Dr.Muhannad.M.Sahib
iv-
Plant histology (lecture3)
College of Biotechnology
Qadysia university
Rib meristem
Appears directly below the central zone and is centrally located in the apex. It usually becomes
the pith after additional meristematic activity has occurred.
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