Tissue – Groups or masses of cells that are alike in origin, structure, and with some common function forms tissue.
Tissue system – The tissues of a plant that perform the same general function, regardless of position or continuity in the body may be considered to form together, a tissue system.
Meristem – The cell which has a capacity of division.
Meristematic tissue – It is a group of cells that are in a continuous state of division or retain their power of division.
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Meristem (Meristematic Tissue)
- The cells of meristematic tissue are immature and are in an active state of division.
- The region where the formation of new cells takes place is known as meristem.
- In Greek, meristems meaning divisible.
Characteristic of meristem
- The cells are usually closely packed with no intercellular spaces.
- The shape of the meristematic cells may be oval, polygonal, rounded or rectangular in shape.
- Each cell has a dense cytoplasm with a large, distinct and distinct nucleus.
- Vacuoles and orgastic substances are generally absent.
- The most important characteristic of these cells is their capacity to divide.
- The plane of division in the cells varies. It can be periclinal (similar to a face) or anticlinal (perpendicular to the surface).
On the basis of position in the plant body, the meristems are classified as –
- Apical meristems
- Intercalary meristems
- Lateral meristems
Apical meristems –
Apical meristems are one of the types of Meristematic Tissue as the very name suggests are located at the apices of shoot, root and other organs.
They are also known as growing points of the plants.
The activity of these meristems brings about an increase in the length of these organs.
This results in the primary growth in plants.
Intercalary meristems –
These represent the detached portion of the apical meristems separated due to the growth of an organ or it is a portion of apical meristem located between the permanent tissues.
They are always internodal.
In the early stages of development of the plant, the whole or part of the internode is meristematic.
Subsequently, a portion of it matures, as a result, patches of meristems are found embedded in permanent tissues.
They occur at the base of internodes and leaves.
They also contribute to the primary growth of the plant.
Also Read: Vascular Tissue System
Lateral meristems –
Lateral meristem occurs on the lateral sides of roots and stems.
They are composed of initials that divide periclinal or radially and form secondary permanent tissues.
They increase the diameter or thickness of an organ.
The vascular cambium (primary meristem) and cork cambium (secondary meristem) are meristems of this type.
It results in secondary growth in plants.
Types of Tissues
Permanent tissues are those where growth has stopped completely or temporarily.
Permanent tissues are further classified into the following two types –
Simple tissue –
It is made up of one type of cells forming a uniform, homogenous system of cells.e.g
Parenchyma, Collenchyma and Sclerenchyma
Complex tissue –
Complex tissues are made up of more than one type of cells that work together as a unit. e.g. Xylem and Phloem.
Characters of Parenchyma
- The cells are nearly isodiametric.
- The cell walls are thin. These are made of cellulose, hemicellulose and pectic substances.
- Parenchymatous cells are living and therefore, contain cytoplasmic organelles and a nucleus.
- These generally act as storage tissue and hence reserve foods are present.
- Parenchyma forms major parts of the cortex and pith of roots and stems.
Characters of Collenchyma
- The cells shape varies from isodiametric to elongated.
- The cell walls are unevenly thickened. These are made of cellulose, pectic and other wall substances but no lignin.
- If the thickening occurs in the corners, these are called angular collenchyma. In some cells, stiffness arises from tangential walls. This type is called lamellar collenchyma.
- collenchymatous cells retain active protoplast at maturity and are capable of further growth and division.
- Collenchyma performs both mechanical and vital functions. The cells are relatively soft and pliable and hence form a major mechanical tissue of herbs.
Characters of Sclerenchyma
- It consists of thick-walled and lignified dead cells.
- Typically, sclerenchyma cells are divided into fibers and Sclereids.
- Fibers are considered to be long cells while sclereids are short cells.
- It is a long and thick cell, many times longer than broad.
- The end of the fibers tapers into sharp points.
- The cell walls are mainly made up of lignin. Cellulose, pectins, and hemicellulose are also present.
- The thick walls show many slit-like pits. Even bordered pits are may be present.
- The lumen or cell cavity is very narrow. There is no protoplasm, the cell being dead.
- Fibers are mostly unicellular but can also be multi-cellular.
- These are short isodiametric cells but sometimes may be elongated also.
- The cell walls are thick and lignified but the cells have no imaging edges.
- The cells are dead and have a very narrow lumen.
- It is a characteristic of the higher cryptogams ie pteridophytes and all groups of the spermatophytes (Angiosperms and Gymnosperms).
- The vascular tissue system consists of a number of vascular bundles which are found to be distributed in the stele.
- The stele is a central cylindrical portion of the stem and the root, commonly surrounded by the endodermis and consists of vascular bundles, pericycle, pith and medullary rays.
- Each vascular bundle consists of xylem and phloem tissues with or without cambium.
- The function of this system is to conduct water and nutrients from roots to leaves through the xylem and translocation of prepared carbohydrates from leaves to other storage organs and growing regions of the plant body through the phloem.
- Structure of xylem It is a complex tissue.
- Most of its components are lignified and dead. Only xylem parenchyma is living with the Xylem fibres component.
- Xylem is a complex tissue, mainly concerned with the conduction of water and mineral salts.
Xylem consists of the following four elements,
- Xylem fibers
- Xylem parenchyma
- The phloem is the principal conducting tissue of the elaborated food substances in the vascular plants.
- Phloem is a complex tissue, mainly concerned with the translocation of food material.
Phloem consists of the following four elements,
- Sieve tubes
- Companion cells
- Phloem fibres
- Phloem parenchyma