Plant Tissue
Tissue can be defined as organized layers or masses of structurally similar cells and performing an organized function in the plant. These tissue combined with each other form the various organs and organ systems of the plant.
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| Plant Tissues |
Plant tissue can be classified mainly in two groups-
Meristematic tissues
Permanent tissue
Meristematic tissues
The tissues made up of cells with indefinite dividing capacity are called meristems or meristematic tissues. These tissues involve mainly in the growth of the plant, it is the growing tissue of the plant.
On the basis of position in plant body meristematic tissue can be classified in the following types:
(i) Apical meristems: They are present at the tips of stem, root and their branches. This is the region of rapidly dividing cells in plants. These tissues lead to growth in length. The division of these cells results in primary (vertical) growth, both at the root and shoot.
(ii) Intercalary meristems: They are derived from the apical meristems. These help in elongation of the organs and are commonly found at the bases of leaves and above (e.g., grasses) or below the nodes (e.g., mint).
(iii) Lateral meristems: They occur on the sides and take part in increasing girth of the plant (secondary growth). The common examples are fascicular vascular cambium (primary meristem), interfascicular cambium and cork cambium or phellogen.
On the basis of origin, meristems may be primary (origin form embryonic cells) or secondary (originated by dedifferentiation of permanent tissues).
Apical, intercalary and fascicular meristems are primary meristems as they originate from the embryonic meristem or promeristem. They retain the meristematic nature throughout plant life.
Vascular cambium, interfascicular cambium, and cork cambium are secondary meristems since they originate as new meristems from the permanent tissues (dedifferentiation) which have already undergone differentiation.
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| Dicotyledonous stem |
Permanent tissues
Permanent tissues are those tissues that have lost the power of cell division. They are fully differentiated tissue. Permanent tissues are derivatives of meristematic tissue. These are the mature tissue and their cells are large and have a definite shape and size.
Permanent tissue can be of two types:
1. Simple permanent tissue
2. Complex permanent tissue
1. Simple permanent tissue
A simple permanent tissue is made up of similar permanent cells that carry out the same function or similar set of functions.
Simple permanent tissues are of three types:
(a) Parenchyma: It consists of thin-walled living cells which have intercellular spaces between them, and their cell wall is made of cellulose and calcium pectate. It serves functions of food storage, absorption, lateral conduction, photosynthesis, providing buoyancy, secretion, etc.
(b) Collenchyma: It consists of refractile non-lignified living cells that possess pectocellulose thickening in specific areas of their walls. It provides mechanical strength to young dicot stems, petioles and leaves and flexibility to the organs. It often contains chloroplasts for photosynthesis.
(c) Sclerenchyma: It consists of dead cells with hard and extremely thick secondary walls due to the uniform deposition of lignin. Sclerenchyma is of two types, sclerenchyma fibers, and sclereids. The sclerenchyma fibers occur in all those parts where mechanical strength is required. Sclereids are found in fruit walls of nuts and seed coats of legumes.
2. Complex permanent tissue
Permanent tissues with more than one type of cells working as a unit are called complex permanent tissues.
Complex permanent tissue includes two types of tissues:
(a) Xylem: Xylem performs the function of transport of water of sap inside the plant and also provides mechanical strength. It consists of tracheids, vessels (both for conduction of water and dissolved salts), xylem fibers (mechanical strength); and xylem parenchyma (stores food and conducts water radially).
The first formed xylem is called protoxylem and later formed xylem is called metaxylem. Xylem can be exarch (protoxylem lies towards the outside of metaxylem), endarch (protoxylem inner to metaxylem), mesarch (protoxylem in between metaxylem) and centrarch (protoxylem in center of metaxylem).
(b) Phloem: Phloem transports organics food inside the body of the plant. It consists of four types of cells, viz. sieve tubes (conducting channels formed of several enucleated cells; due to the presence of sieve pits the end walls are commonly called sieve plates); companion cells (thin-walled living cells on the sides of the sieve tubes); phloem parenchyma (parenchymatous cells which store food, resins, latex, etc.); and phloem fibers.
The first formed primary phloem consists of narrow sieve tubes and is referred to as protophloem and the later formed phloem has bigger sieve tubes and is referred to as metaphloem.
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| Dicotyledonous root |
Tissues together form tissue system which are of three types:
1.Epidermal tissue system
2. Ground tissue system
3. Vascular tissue system
1. Epidermal tissue system
Epidermal tissue system consists of epidermis and epidermal outgrowths. The epidermis is the superficial layer covering the entire surface of the primary plant body and is itself covered with a cuticle on aerial plants.
All the epidermal cells are living (parenchymatous). Epidermal outgrowths may include trichomes (present on the stem, prevent water loss), root hairs (present on the root, absorb water and minerals from soil), etc.
Stomata are the structures present on the epidermis of leaves for the regulation of transpiration and gaseous exchange. The specialized green epidermal cells present around the stomata are called guard cells that regulate stomatal opening. Guard cells in dicots are kidney (bean) shaped and in monocots are dumb-bell shaped. The guard cells are surrounded by two or more epidermal cells called subsidiary cells. When subsidiary cells lie above the guard cells, the stomata are celled sunken.
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| Leaf structure |
2. Ground tissue system
It consists of simple tissues and is derived partly from periblem and pertly form plerome. It is of two types-extrastelar (cortex) and interstellar (includes pericycle, pith, and medullary rays). In leaves, it consists of chloroplast containing cells and is called mesophyll.
3. Vascular tissue system
Vascular tissue includes stele i.e., the central column of a plant body that consists of vascular bundles and pith (if present). It lies inner to pericycle. Each vascular bundle comprises of xylem and phloem and cambium (if present).
Types of vascular bundles
According to the cambium, it is of two types:
(a) Open vascular bundle: If the cambium present between xylem and phloem is called the open vascular bundle. Open vascular bundles are the characteristic of dicotyledons (dicots). They show secondary growth.
(b) Closed vascular bundle: If the cambium absent between xylem and phloem is called the closed vascular bundle. They do not show secondary growth.
According to the position of xylem and phloem:
(a) Radial: In radial vascular bundles the xylem and phloem components are arranged separately in different radii. Radial vascular bundles are typically found in the ROOTS of monocots and dicots.
(b) Conjoint: It is a type of vascular bundle where xylem and phloem lie opposite to each other in the same radius. They are linked with the help of a cambium. It is of two types: Collateral (phloem on the outer side and xylem on the inner side), and Bicollateral (xylem is sandwiched between outer and inner phloem.
(c) Concentric: It is a type of vascular bundle in which either xylem surrounds the phloem or phloem surrounds the xylem. It is of two types: Amphicribal or hadrocentric (phloem surrounds xylem), and Amphivasal or leptocentric (xylem surrounds phloem).
