The entire living organism is made up of a cell. The cell is the functional and structural unit of life. Living organism is either made up of a single cell called unicellular organism or of many cells called a multi-cellular organism. When the cells having a similar origin, structure and function unite and act as a single unit, it is called tissue. Tissue simply defined as the group of cells having similar structure, origin and function. The organized group of tissue helps to form different organs. Each cell has a definite function. The function of different cells is different from each other.
Types of plant tissues
A tissue is defined as the “a group of cells having a similar origin, structure, and function.” The different types of plant tissue are grouped under the following three heading:
Meristematic tissue (Meristems)
Mature or permanent tissue
Those tissues which are composed of immature or not well develop cells are called Meristematic tissue. These types of tissue are responsible for the cell division. So, this tissue is a group of cells that are in a continuous state of division or retain their power of division. Some of the characteristics of Meristematic tissue are as follows:
They are found in round, oval, polygonal or rectangular shapes.
They don’t have intercellular space between the cells.
The cells in this tissue are not able to store reserve food materials.
The well of this type of tissue is thin and made up of cellulose.
Their nuclei are large and conspicuous.
The cells are capable of division.
The cells have few or no vacuoles.
Classification of Meristematic tissue
Meristematic tissue is also classified into types on the basis of their origin, position and function. They are described below:
1. On the basis of their origin:
They are divided into three types according to the origin are divided into three types. They are as follows-
Promeristem (primordial Meristems): The meristems which are origin from the embryo are called Promeristem. They are also called primordial meristem or embryonic meristem. This type of the meristem is found where the part of the plant is started to grow or initiated. It can be observed in at the apices of shoot and root. Primary meristem is developed from it.
Primary meristem: The meristem which is originated from Promeristem is called primary meristem. The cells are very active towards the cell division and help to grow primary permanent tissues. It lies below the apices of shoot and root at the Promeristem wich gives raise to secondary meristem.
Secondary meristem: It is absent at the beginning of the development of an organ but a raise later after the development of the plant. The secondary permanent tissue is developed from them. Different examples of secondary meristem are vascular cambium of a root, cork cambium and inter-fascicular cambium.
2. On the basis of position:
They are divided into following three types on the basis of their position-
Apical meristem: These are the type of meristem which are located at the growing points or the apices of the root, shoot and branches. These sorts of meristem help to add or increase the length of the plant or its different parts. They mostly grow in two zones which are as follows:-
Promeristem zone- This zone consist of apical initials which are a group of dividing cells.
Meristematic zone- This is again sub-divided into three types. They are-
Protoderm- The outer layer from where the epidermal tissue system is developed is known as Protoderm.
Procambium- This layer lies below the protoderm from where primary vascular tissue is developed.
Ground meristem- This is the meristem which is responsible for the development of cortex and pith.
Instead of using the term apical meristem of the shoot and apical shoot of the root, it is easy to use the term shoot apex and root apex.
Intercalary meristem: It is the portion of apical meristem which gets separated during the growth and development of axis from the apex. In many monocots, it is found at the base of a blade or sheath. It lives for a short period of time and is intermodal. So, combine with the tissue nearby surrounding it. These meristems also add the length to the plant as well as the different organs of plants.
Lateral meristem: These meristems are found at the lateral side of the stem or along the stem. So, they are called lateral meristem. It is made up of single layered rectangular cells which always divide in only one plane and develops new vascular tissue on either side. It produces secondary tissue; the vascular cambium and cork which improve the thickness of the plants. Cambium of the angiosperms and gymnosperms are the suitable example of the lateral meristem. For the good illustration of the lateral meristem, primary cambium which lies between the vascular tissue xylem and phloem in dicot stem is best one.
3. On the basis of function:
Meristem is divided into following three types on the basis of their functions-
Protoderm: The outermost meristematic cells are called protoderm. They help in the formation of the epidermis of the developing organs.
Procambium: The innermost meristematic cells are called Procambium. They are responsible for the development of the vascular tissue like xylem, phloem and cambium by elongation and differentiation.
Ground meristem: It helps in the formation of the hypodermis, cortex, endodermis, pericycle, pith and medullary ray.
Shoot apex (Shoot apical meristem)
In 1924, Schmidt proposed Tunica-Corpus Theory. According to this theory, leaf primordia protect the done shaped shoot apex which can distinguish into outer mantles such as tunica and inner cellular mass is known as the corpus.
Tunica- It is an outer zone which helps in the development of the epidermis or protoderm. Generally cells are small and cell division takes places at the right angle to the surface of the apex and as a result, the surface gets grow.
Corpus- The lower multilayered zone which is responsible for the growth and development of the procambium and ground meristem id known as the corpus. The cells are large and cell division takes place in different planes. Vascular tissue and ground tissue develop from the Procambium and ground meristem respectively.
The shoot apex is divided into three regions according to Histogen theory purposed by Hanstein In 1870. They are:-
Dermatogen- The outermost layer of the cell which helps to form the epidermis of the stem is known as dermatogens.
Periblem- The region which lies beneath the dermatogens and helps in the formation of the cortex and endodermis.
Plerome- It is the centrally located meristematic region of stem apex. It is found just below the periblem which helps to form pericycle, vascular bundles, medullary rays and pith.
In 1914, Haberlandt proposed the nomenclature of protoderm for dermatogens, ground meristem for periblem and Procambium for pleurome.
Root apex (Root apical meristem)
Generally, root apex is found at the tip of the main root as well as other branched roots. Root cap covers it and protects it from the injury. It is divided into other five regions. They are:-
Protoderm- In dicot plants, it helps to form epidermis or epiblema and root cap.
Calyptrogen- This region helps to form the root cap in the monocot plants.
Procambium- This region is responsible for the formation of the vascular tissue.
Ground meristem- This region is responsible for the formation of the pith, cortex, epidermis and pericycle.
Quiescent centre- This the region where cell division is very slow in rate. It is found in the central part of the root cap.
According to the Histogen Theory shoot apex and root apex has three histogens:
Dermatogen- It is responsible for the formation of epiblema and root cap in dicot plants.
Periblem- It is responsible for the formation of cortex and endodermis.
Plerome- It helps to form pith, vascular tissues, pericycle, etc.
1. A tissue is defined as the “a group of cells having a similar origin, structure, and function.”
2. The meristems which are origin from the embryo are called Promeristem.
3. The meristem which is originated from Promeristem is called primary meristem.
4. These meristems are found at the lateral side of the stem or along the stem.
5. In dicot plants, protoderm helps to form epidermis or epiblema and root cap.
6. Dermatogen is responsible for the formation of epiblema and root cap in dicot plants.