The word meristem (Greek: Meristos= divisible) was coined by Nageli (1858). A tissue is a group of cells in origin, structure, and function. The study of tissue is called Histology. A plant is made up of different types of tissues.
There are two principal groups of tissues:
- Meristematic tissues
- Permanent tissues
Meristematic tissues:
The word meristem (Greek: Meristos= divisible) was coined by Nageli (1858). A meristematic tissue is a group of cells in a continuous state of division or retaining their power of division. In plants, the growth is not diffused as in animals.
It is restricted to only certain specific regions. They are called meristematic regions in which meristematic tissues are located. The meristematic tissues divide and redivide and give rise to different organs of the plant body.
Characters of Meristematic Cells
- The cells may be rounded, oval, polygonal, or rectangular.
- They are arranged very closely without any intercellular spaces.
- Their cell wall is thin, elastic, and composed of cellulose.
- They have generally dense cytoplasm with prominent nuclei.
- The vacuoles may be very few and small or sometimes may be absent.
- They do not store reserved food materials.
- The cells may divide in a particular plane or all directions depending upon the position of the meristem in the plant body.
- The cells undergo rapid and repeated divisions and produce new tissues called formative tissue.
Meristems and growth of the plant body:
The plant begins its life from a single diploid zygote. After successive systematic divisions, it gives rise to an embryo. In a young embryo, all the cells can divide.
During the maturation of the embryo, cell differentiation begins. Thus we find different sets of initials that give rise to root and shoot systems of seedlings. The young seedling is an independent plant. It is composed of embryonic tissues and adult tissues.
The embryonic tissues persist in the plant throughout its life. They are confined to apices of the plant body and hence are called apical meristems (Primary meristems).
The meristematic cells divide continuously and add new cells to the seedling. These cells differentiate into different mature tissues of the primary plant body.
After a certain period of elongation, in 8ymnosperms and dicots, the plant body increases in diameter by the activity of the newly formed vascular cambium & cork cambium.
They are called secondary (lateral in position) meristems. The secondary meristems divide and give rise to secondary permanent tissues. After the occurrence of secondary growth, the plant body is called the secondary plant body.
Classification of Meristem
Meristem has been classified into several types based on position, origin, function, and division.
A) Based on origin and stage of development:
Based on their origin and development, the meristems are classified as follows:
(a) Promeristem
b) Primary meristem and
(c) Secondary meristem
Promeristem:
A group of young and undifferentiated, actively dividing apical initials derived from the embryo’s tissue is called Promeristem. It is also called embryonic meristem or primordial meristem. It consists of a few apical initials.
It is present at the extreme tip of a young stem. The cells are isodiametric. They have thin primary walls with early stages of pits. They have dense and active protoplasm.
The pro meristem lays down the foundation of different organs of the primary plant body in the early stage of growth and development. These cells divide and rise to a meristematic zone below it, called primary meristem.
Primary Meristem:
The meristems derived from the embryonic meristems are known as primary meristems. The primary meristems are concerned with forming the primary permanent tissues of the primary plant body.
In the early growth stage, different organs of the plant body are initiated by pro meristem. Further growth and differentiation of various tissues and organizations take place through the activity of primary meristems.
Shoot and root apical meristems, intercalary meristems, and leaf primordia are examples of primary meristems. The cambium strips of vascular bundles are also primary in origin.
Secondary Meristem:
Secondary meristems are not present in the primary plant body. As the name indicates, the meristems which are developed, after a certain period of vegetative growth of the plant body, from the permanent tissues at the time of secondary growth are called secondary meristems.
In gymnosperms and dicots, the secondary growth takes place through the activity of lateral meristems (Secondary meristems) such as vascular cambium (in the stellar region) and cork cambium (in the extra stellar region).
In vascular cambium, intra fascicular cambium is primary in origin where as inter fascicular cambium is secondary in origin. Interfascicular cambium develops from medullary ray cells.
The cork cambium originates from the epidermis, cortex (ex: – Stem), or the pericycle (Eg. root). Secondary meristems add secondary permanent tissues to the primary plant body and give protection and repair wounded regions.
B) Based on the position in the plant body:
Depending upon the position in the plant body, the meristems are classified into three distinct types.
Classification of Meristem
Apical Meristems:
The meristems present universally at the apices of the stem, branches, and roots are called apical meristems. It is responsible for the increase in the length of the plant; it is called primary growth. They take part in the formation and elongation of plant organs.
In higher plants, an apical meristem consists of a group of cells, whereas in cryptogams single cell forms an apical meristem. The apical meristem is distinguished into two zones, upper pro meristem and lower primary meristem.
Intercalary Meristems:
If the meristematic tissues are intercalated between permanent tissues, they are called intercalary meristems. Little portions of apical meristems are believed to get detached and remain embedded between permanent tissues.
They are either present at the base of internodes, at the base of nodes,s or at the base of leaf and leaf sheath (Eg: Equisetum, Pinus & grasses). The inter-nodal regions of the stem increase by the activity of intercalary meristems.
They are short-lived and are converted into permanent tissues. The stem part becomes very weak due to the presence of intercalary meristem.
Lateral Meristems:
As the name indicates, these are present along the sides of the plant body. The vascular cambium and cork-cambium are examples of lateral meristems.
They are called secondary meristems as they differentiate from primary permanent tissues during secondary growth.
The meristematic cells divide in the periclinal plane only. They produce secondary permanent tissues. These meristems are responsible for the increase of girth of the plant body. Example: vascular cambium and cork cambium.
C) Based on function:
Based on function, the apical meristem is distinguished into three zones. These zones have distinct functions and give rise to various kinds of primary tissues of the plant body. The zones are named protoderm, ground meristem, and procambium.
Protoderm:
It is the outermost single-layered structure. The cells always divide in the anticlinal plane. This layer produces an outer protective covering of the plant body known as the epidermal tissue system.
Ground Meristem:
It lies beneath the protoderm. It consists of several layers of meristematic cells. These cells divide into all planes and give rise to the ground tissue system (cortex, endodermis, pericycle, medullary rays, and pith) or all tissues except the epidermis and vascular strands.
Procambium:
Thin-walled elongated procambial strands appear in groups in ground meristem near the central region at an early stage of differentiation. These strands give rise to the vascular tissue system. Example: xylem and phloem.
D) Based on the plane of cell division:
Several meristematic cells and their division plane vary from group to group in the plant kingdom. The cell division plane also differs depending on the position within the plant body. According to the cell division plane, three kinds of meristems are recognized.
Rib meristem or File meristem:
In this type of meristem, the cell division is in one plane. For example, the development of the cortex and pith. The cells divide and produce a row of cells.
This type of meristem is found mainly in filamentous Algae. It also plays a vital role in forming young roots, cortex, and pith of young stems in higher plants.
Plate Meristem:
In this type of meristem, the cells divide into two planes. The surface area of an organ is increased. This type of meristem forms flat structures, such as leaf formation, without increasing thickness. For Example, growth is shown by flat blades of angiosperm leaves.
The Plate meristem and Rib meristem are growth forms mainly occurring in the ground meristem.
Mass Meristem:
The cells divide into all planes in mass meristem to form a tissue bulk. This type of meristem takes part in the formation of the cortex, pith, endosperm, and sporangia.
Key Points
- The word meristem was coined by Nageli (1858). Meristematic tissues are found in specific regions of plants, where they divide and redivide to give rise to different organs.
- Meristem is a group of cells in a continuous state of division or retaining their power of division.
- Meristematic tissues are located in specific regions of plants and give rise to different organs of the plant body.
- Meristematic cells are closely arranged without intercellular spaces and have thin, elastic cell walls.
- Meristems are classified based on origin and stage of development, position in the plant body, function, and cell division plane.
- Apical meristems are present at the apices of the stem, branches, and roots and are responsible for primary growth.
- Intercalary meristems are intercalated between permanent tissues and contribute to stem elongation.
- Lateral meristems, such as vascular cambium and cork cambium, are present along the sides of the plant body and contribute to secondary growth.
- Meristems can be classified based on their function into protoderm, ground meristem, and procambium.
- Meristems can also be classified based on the cell division plane into rib meristem, plate meristem, and mass meristem.
FAQs
What are meristems and the types of meristems?
Meristems can be categorized based on their specific locations within the plant. Apical meristems are found at the tips of roots and shoots, while lateral meristems reside in the vascular and cork cambia. Additionally, intercalary meristems are situated at internodes, the regions between where leaves attach, and at the bases of leaves, particularly in particular monocotyledons like grasses.
What are the 4 meristems?
Within a plant, there exist four distinct types of meristems: the apical meristem, as well as three variations of lateral meristems—vascular cambium, cork cambium, and intercalary meristem.
What is the role of meristem in plant growth?
Meristem is responsible for continuously producing new cells, promoting primary growth in roots and shoots and secondary growth in stems and roots. It enables plants to increase size, repair damaged tissues, and adapt to environmental changes.
Can meristematic cells be genetically modified?
Yes, meristematic cells can be genetically modified, allowing for genetically modified crops with desired traits. This has revolutionized plant breeding and agricultural practices.
How do meristematic zones contribute to plant development?
Meristematic zones ensure a constant supply of new cells for growth, cell elongation, and specialization into distinct tissue types.
References
- Kara Rogers, Mitosis, Britannica, 2020
- M.J. Farabee, Plant Hormones, Nutrition, and Transport, 2007
- Melissa Petruzzello, Meristem, Britannica, 2020
- Oregon State University, Mechanisms for Growth, 2022
- Pratima Bajpai, Biermann’s Handbook of Pulp and Paper (Third Edition), 2018
- Reinhard Renneberg, Biotechnology for Beginners (Second Edition), 2017