What is meant by tissue and tissue systems?
-A group of cells, having the same origin and similar or dissimilar shapes which do the same function is known as tissue.
-Groups or Masses of the cells that are alike in origin, structure, and function from the tissue. The plant body consists of vegetative tissue and reproductive tissue.
-Morphologically, a tissue is a group or mass of cells or vessels, alike in form origin and function.
“All the tissues of a plant which perform the same general function, regardless of position or continuity in the body, may be considered to form together, a tissue system.”
-A plant tissue system can be defined as a functional unit, which connects all organs of a plant.
The tissue may be grouped into three principal groups-
1. Meristems or meristematic tissue
2. Permanent tissue,
3. Secretory tissue.
What is meristem? Discuss each type of meristem with respect to their
structure, function, location and plane of division.
A meristematic tissue consists of a group of cells that remain in a continuous state of division or they retain their power of division.
A meristem is a tissue in most plants containing undifferentiated cells (meristematic cells), found in zones of the plant where growth can take place.
Characteristics of meristem or meristematic tissue:
The characteristic features of meristematic tissue arc as follows-
1. They are composed of immature cells that are in a state of division and
2. Usually, the intercellular spaces are not found among these cells.
3. The cells may be rounded, oval or polygonal in shape; they are always living
4. Each cell of meristematic tissue possesses abundant cytoplasm and one or
more nuclei in it.
5. The vacuoles in the cells may be quite small or altogether absent.
6. These cells have cellulose made thin cell walls.
Meristems Based on Stage of Development:
Promeristem or Primordial:
Meristem-Promeristem is the region of new growth in a plant body where the foundation of new organs or parts of organs is initiated. The cells of this region are isodiametric, thin-walled, vacuolated, with active cytoplasm and early stages of pits. Prominent nuclei and inconspicuous intercellular spaces may be seen. As soon as the cells of this region begin to change in size, shape, and character of wall and cytoplasm, setting off the beginning of tissue differentiation, they are no longer a part of typical meristem; they have passed beyond that earliest stage.
Classification of meristematic tissue:
Meristematic tissue can be divided into different types on the basis of origin, structure, Position, and function, etc. Such as :
1. On the basis of the position
2. On the basis of origin
3. On the basis of cell division
4. On the basis of function.
1.On the basis of origin:
On the basis of origin, the meristematic tissue may be divided into three types.
This type of tissue is limited to the apex of root and stem. From here the next meristem arises. Growth occurs from this area,
ii). Primary meristem:
The meristematic tissue that arises from the embryonic stage is called the primary meristem. The division of this tissue results in the tangential growth of the plant. Primary permanent tissue arises from the primary meristem. The primary meristems are those that build up the primary part of the plant and consist in part of Promeristem. In primary meristems, Promeristem is always the earliest stage. The possession of Promeristem continuously from an early embryonic origin is characteristic of primary meristems. The main primary stems are the apices of roots, stems, leaves and similar appendages.
iii) Secondary meristem:
The meristematic tissue that originates in the next time from any permanent tissue.
Example: Cork cambium.
The secondary meristem appears later at a stage of development of an organ of a plant body. Secondary meristems always arise in permanent tissues and they are always found lying lateral along the side of the stem and root.
2. On the basis of position:
The meristematic tissue may be of three types on the basis of position.
i). Apical meristem:
This type of tissue is present at the apex of the root, stem or branches. These organs increase in length by the division of apical meristematic.
The apical meristem lies at the apex of the stern and the root of vascular plants. Very often they are also found at the apices of the leaves. Due to the activity of these meristerns, the organs increase in length. The initiation of growth takes place by one or more calls situated at the tip of the organ.
ii). Intercalary meristem:
The meristematic tissue remarking in between two permanent tissues is known as intercalary meristem. The intercalary meristems are merely portions of apical meristems that have become separated from the apex during development by layers of more mature or permanent
tissues and left behind as the apical meristem moves on meristems arc intermodal in their position.
iii) Lateral meristem:
This type of tissue is present vertically along the lateral line of root or stem. This type of tissue is also situated in between two permanent tissues. They arise from growth. The intercalary permanent tissue. Thus, they are called secondary meristematic tissue. E.g. Interfascicular cambium.
The lateral meristems are composed Of such initials that divide mainly in one plane (periclinal) and increase the diameter of an organ. They add to the bulk of existing tissues or give rise to new tissues. These tissues are responsible for growth in the thickness of the plant body. The cambium and the cork cambium are examples of this type.
3. On the basis of cell division:
Meristematic tissue may be divided into three types on the basis of cell division.
Such as- i) Mass ii) Plate iii) Rib
i). Mass meristem:
The cell division of this meristematic tissue takes place on all planes. By this type of division, the organ increases in volume. Such as Endosperm tissue, pith, cortex, etc.
ii). Plate meristem:
The cells of the meristematic tissue which divide only into two plants to form a single-layered plate-like structure are known as plate meristem. By this division, the organ increases in volume. Such as- Leaf.
iii). Rib meristem:
The cells of the meristematic tissue divide only on a single plane and the cells arrange in a single layer tangentially forming a rib-like structure of the chest, This tissue is known as rib meristem. This type of division results in the formation of a single cell layer. Such as-Growing root and pith.
4. On the basis of function:
Meristematic tissue may be divided into three types on the basis of functions. Such as- i). Protoderm, ii) Procambium iii) Ground tissue
The outermost layer of the meristematic tissue is called protoderm. The main function of the protodermis to form the epidermis or epiblema of root, stem, and branches. The protoderm is the outermost tissue which develops into epidermis
The meristematic tissue that gives rise to cambium, xylem and phloem is known as procambium. Its main function is to form vascular tissue. The procambium develops into primary vascular tissue.
iii). Ground meristem:
The meristematic tissue that gives rise to the cortex, pith and the medullary ray of the plant body is known as ground meristem.
The ground or fundamental meristem develops into ground tissue and pith; the cells of this region are large, thin-walled, living and isodiametric. In later stages, they become differentiated into hypodermis, cortex, endodermis pericycle, pith rays, and pith,
Functions of meristematic tissue:
1. Plants grow perpendicularly through the division of the special meristem. In this way, small plants grow high and long.
2. Plants grow in girth by the division of the lateral meristematic tissue. In this way, thin plants become thick.
3. Permanent tissue is formed from meristematic tissue.