When cells having similar origins, structures, and functions aggregate, they form tissues. A tissue is a group of cells having structural and functional similarities. The tissues in the plant body can be broadly characterized into three different types: Meristems or Meristematic tissue, Permanent tissue, and Secretory tissue. In this article, we are going to talk about Meristematic Tissue; what meristematic tissues are, where are they found, their functions, their characteristics, and the different types found in a plant body.
What are Meristematic tissues?
A meristem or meristematic tissue consists of a group of self-perpetuating cells that retain their capability of division, i.e., these cells are capable of continuous division and contribute to the growth of a plant body. The term meristem comes from the Greek word meristos, meaning ‘divisible’. The term meristematic tissue was coined by Carl Wilhelm von Nägeli. These tissues are present in different regions of the plant body and contribute to its primary and secondary growth.
The presence of meristem distinguishes plants from animals in a significant way.
All components of a juvenile animal's body increase as they mature until they reach a genetically specified size for each species. The growth ceases once the animal body reaches its genetically determined size. However, in a plant, the growth occurs throughout the plant’s life span and is restricted to certain specialized areas, called meristems.
Where are they found?
The meristems are found in the plant where active cell division occurs. In a primary plant body, the meristems are found at the root apices and shoot apices and contribute to the elongation of the plant. The root apical meristem is located at the root's tip, while the shoot apical meristem is located at the farthest region of the stem axis. The cells of the apical meristem are organized in four zones: the central zone, the peripheral zone, the medullary meristem, and the medullary tissue. A small collection of slowly dividing cells in the apical meristem is frequently referred to as the central zone. Cells of this zone have functions similar to stem cells in an animal body and are crucial for meristem maintenance. The central zone is surrounded by a ‘peripheral zone’. The cells of this zone have a division rate higher than that of the central zone.
The meristem that is found in vascular and cork cambia, called lateral or secondary meristem, is responsible for the lateral growth of a plant body.
location of meristematic tissue in plants
Characteristics of Meristematic Tissues
The cells of meristematic tissue possess a few qualities that distinguish them from other cells of a plant body. Some of these characteristics of meristems are:
They are living, thin-walled cells.
The cells have a variable shape; they may either be round, oval, or polygonal.
These are immature cells that remain in a continuous state of growth and division.
The intercellular spaces are usually absent in these cells.
The vacuoles in these cells are excessively small or absent.
They have a very dense protoplasm.
These cells possess a very high metabolic rate.
These cells do not store food.
Functions of Meristematic Tissues
The meristematic tissues are actively dividing, self-renewing cells that are responsible for the formation of the primary plant body.
They are responsible for the primary and secondary growth of a plant body, i.e., increase in length and increase in width of a plant, respectively.
Meristematic tissue division is responsible for the formation of all new organs and their expansion.
Meristematic tissue activity also results in the formation of secondary tissues such as wood and cork.
Classification of Meristematic tissues
A wide variety of classification systems have been used to classify meristematic tissues. However, no system is exclusive or rigid. Some of the important ones are:
A. Based on the stage of development
The meristematic tissues are classified as Promeristem or Primordial meristem based on their stage of development. A promeristem is a region of new development in a plant's body where the formation of new organs or organ components begins. The characteristic features of cells of Promeristem are-
They have isodiametric, thin-walled, vacuolate cells.
These cells have active cytoplasm and early stages of pits.
They have a prominent nucleus and unremarkable intercellular spaces.
B. Based on the origin of Initiating cells
This classification system classifies meristematic tissues as primary meristems and secondary meristems. Primary and secondary meristems are two types of meristematic tissues present in the growth zones of plants. These consist of actively dividing cells and are responsible for the primary and secondary growth of a plant body, respectively.
Primary Meristem: The primary meristem comprises the embryonic meristem or primordial meristem and makes up the primary part of a plant's body. It arises early and is present below the promeristem. It is responsible for the longitudinal growth of the body of a plant, i.e., the increase in the length of a plant. The apical and intercalary meristems comprise the primary meristem.
The primary meristems are usually found in the root apices, shoot apices, leaves, and similar appendages.
Secondary Meristem: It appears later in the development process and contributes to the secondary growth or lateral growth of a plant body, i.e., an increase in the girth of a plant. Hence, it is also called lateral meristem. These always arise from the permanent tissues of a plant's body. Primary permanent tissue cells can sometimes regain their ability to divide and become meristematic. The secondary meristem is made up of these tissues. These tissues help to replace early-produced tissues or to protect and restore the injured area.
The two types of secondary meristems found in plants are vascular cambium and cork cambium.
C. Based on the position in a plant body
The meristems are divided into three types based on their location in the plant body: apical meristem, intercalary meristem, and lateral meristem.
Apical meristem: The meristem that is present at the apex of the root and shoot is called the apical meristem. They are seldom found at the apices of leaves. These are responsible for increasing the length of a plant's body.
Intercalary meristem: Intercalary meristems are found lying in between masses of permanent tissues either at the internodes or at the base of the leaves. These are simply portions of apical meristems that have been left behind as the apical meristem progresses in its growth. These are commonly found in monocotyledonous plants.
Lateral meristem: Lateral meristem is arranged parallel to the sides of an organ. They either increase the volume of existing tissue or give rise to new tissue. These are responsible for an increase in thickness or lateral growth of a plant body. The cambium and cork cambium are examples of such types of tissues in a plant body.
meristematic tissue parts
Meristems and Permanent Tissues
What happens when meristems mature? The cells of meristematic tissue remain in a continuous state of division. Some of these cells retain the properties of their parent cells, while others become structurally and functionally specialized by a process called ‘differentiation’ to become permanent. These cells are called permanent cells, and they form permanent tissue.
Permanent tissues have the following characteristics:
They may either be living or dead
Their walls can either be thin or thick.
These tissues consist of specialized, nondividing cells.
The cells are large, having a definite shape and size.
Intercellular spaces are present.
They have large vacuoles.
They have a relatively slow metabolic rate.
Plants use permanent tissue for a variety of functions, including support, protection, photosynthesis, and water, mineral, and nutrient transport.
Conclusion
Meristematic tissues are juvenile tissues that are crucial for the growth and development of a plant body. These tissues possess the ability of continuous division and are responsible for the formation of new plant cells. The cells of such tissue can either be undifferentiated or incompletely differentiated.