This is the notes for Plant tissues.
Tissue
• Group of similar or dissimilar cells which perform same function is called tissues.
• There are mainly three types of tissues-
a. Meristematic tissues
b. Permanent tissues
c. Secretory tissues
Meristematic tissue
• Such types of tissues that have cells of dividing capacity is called meristematic tissue.
The meristem cells have following important features:
1. They have cells of dividing capacity. They divide actively throughout life.
2. The cells are always living.
3. Intercellular space between the cells are absent.
4. The cells have few or no vacuoles.
5. The cells are always thin walled and cellulosic.
6. They have dense cytoplasm and distinct nucleus.
7. They may be oval, polygonal or rectangular.
8. These are found in root tip, shoot tip, cambium etc.
Classification of meristems
On the basis of origin
a. Promeristem: It is the early embryonic meristem. It is found in tip of shoot and root.
b. Primary meristem: This types of meristem derived from promeristem and found in the tip of root, shoot, branches.
c. Secondary meristem: It is derived from primary permanent tissues, and have dividing capacity. Eg. Interfascicular cambium, intrafascicular cambium, cork cambium.
On the basis of position
a. Apical meristem: It is found at the apex of root and shoot and increases the length of plant.
b. Lateral meristem: It is found parallel to long axis along the length of plant. It increases the thickness of plant. Cork cambium, vascular cambium. c. Intercalary meristem: It is present at the base of internodes and leaf sheaths of monocots. It increases the length of internodes.
c. Intercalary meristem: It is present at the base of internodes and leaf sheaths of monocots. It increases the length of internodes.
Apical cell theory
1. Shoot apex: The shoot apex is a dome shaped structure enclosed in a bud and protected by young leaves. Theories which explain the mode of growth found in shoot meristem are:
a. Tunica- corpus theory: This theory was proposed by Schmidt(1924). According to this theory dome shaped shoot apex is differentiated into outer one or few layered tunica zone and inner multilayered corpus zone. Tunica gives rise to epidermis while corpus gives rise to cortex, vascular bundle and pith.
b. Histogen theory:
This theory was proposed by Hanstein(1868). According to this theory shoot apex is differentiated into three regions i.e., 1.dermatozone, 2.periblem and 3.plerome.
- Dermatogen is outermost epidermis.
- Periblem lies inner to dermatogen and gives rise to pericycle.
- Plerome is central meristematic region and gives rise to pericycle, vascular bundle, medullary ray and pith.
2. Root apex: It is cup shaped in structure and protected by root cap. The central part of root apex is called quiescent centre from which all the other cells are formed. Just behind the quiescent centre, root apex shows three regions.
- Protoderm: Outermost layer gives rise to epidermis.
- Ground meristem: It lies below the protoderm and gives rise to cortex and endodermis.
- Procambium: It gives rise to pericycle, vascular bundle and pith.
Permanent tissues
• Definition: The group of similar or dissimilar cells which have lost the power of cell division but perform same function is called permanent cells. These are of two types i.e.,
1. Simple permanent tissues
2. Complex permanent tissues
Simple permanent tissues
• Definition: A group of similar types of cells which lost the dividing capacity but perform same function is called simple permanent tissues. The simple permanent tissues are of following types:
1. Parenchyma
2. collenchyma
3. Sclerenchyma
Parenchyma
• Definition: Parenchyma is a simple permanent tissue made up of thin walled living cells. The cells are oval, rounded or polygonal shaped with or without intercellular space. Each cells has large central vacuole and peripheral cytoplasm. Cell wall is made up of cellulose. They are found in cortex of stem and root, pith, mesophyll of leaves, pulp of fruits, endosperm of seeds, flowers etc.
On the basis of shape, size and content they may be
• Procenchyma: cells with tapering end. E.g., pericycle
• Aerenchyma: Cells with wide air cavity. Hydrophyte
• Chlorenchyma: Cells with chloroplast. Palisade
• Idioblast: Cells with cell inclusions like tannins, oils, crystals etc. It helps in storage of excretory products.
Functions of parenchyma
• Mainly they store large amount of food material in the form of glucose, starch, oil, fat etc.
• Parenchyma with chlorophyll (chlorenchyma) helps in photosynthesis.
• Parenchyma with large air cavity(aerenchyma) helps in buoyancy and exchange of gases.
• Being thin walled, they help in transport of water and minerals.
• Idioblast helps in storage of excretory products.
• In fleshy leaves and stem, it represents storage tissues.
Collenchyma
• Definition: Collenchyma is simple permanent tissue with thick walled living cells. Deposition of extra cellulose, hemicellulose, and pectin substance takes place at the edges of cells. Each cells possess large central vacuole and peripheral cytoplasm. The cells are generally circular, oval, angular or elongated in transverse section. These are found in Hypodermis of herbaceous dicots, leaves, stem etc.
On the basis of wall thickenings, collenchyma is of three types. i. e.,
• Angular collenchyma: The wall thickenings are present at the angles. E.g, stem of tomato, Dhatura, Cucurbita, Solanum etc.
• Lamellate collenchyma: The wall thickening occurs on tangential walls. Eg., stem of sunflower.
• Lacunate collenchyma: The wall thickening occurs at the region of inter cellular space.eg., stem of Cucurbita, petiole of Salvia.
Functions of collenchyma
• It provides mechanical strength to herbaceous plants, leaves, petioles etc.
• It provides elasticity to plant organs.
• It prevents the tearing of leaves.
• It provides flexibility to organs and allows their bending without tearing.
• Being a living tissue, it stores foods.
• Chloroplast containing collenchyma helps in photosynthesis.
• It is capable of elongation.
Sclerenchyma
• Definition: Sclerenchyma is simple permanent tissues with highly thick walled dead cells. It is mechanical tissue. The cells have little or no protoplasm. The cell wall is made up of cellulose, hemicellulose and lignin substance. Sclerenchyma is of two types;
1. Fibre
2. Sclereids
Fibre
• The fibres are highly elongated, narrow, spindle shaped thick walled dead cells with pointed end walls. It consists of pit but absence of intercellular space. Fibres are the major mechanical tissues. These are found in xylem and phloem.
Sclereids
Sclereids are highly thickened dead sclerenchyma cells with very narrow cavities. They may be oval, cylindrical, rod shaped, stellate etc. they are of following types:
• Brachysclereids: Isodimetric structure -guava
• Macrosclereids: Rod like structure - legume
• Osteosclereids: Bone like structure - legume
• Astrosclereids: Star shaped – petiole of lotus
• Trichosclereids: Hair like -hydrophytes
Functions of sclerenchyma
• It provides mechanical strength to the plant part.
• It resists plant against the wind.
• A number of fibres are used as commercial purposed. Eg. Jute, hemp, agave, banana fibre.
• Sclereids form protective covering in many legume seeds.
• It produces stony endocarp and protects internal parts in almond, coconut etc.
Complex permanent tissues
• Definition: A group of dissimilar permanent cells which performs common function is called complex permanent tissues. These are conducting tissues i.e., xylem and phloem.
Xylem
• Definition: A complex permanent tissue which transport water and minerals from root, and gives mechanical support to plant is called xylem. It consists of four types of elements i.e., tracheids, trachea(vessels), xylem parenchyma, xylem fibre.
Tracheids: The tracheids are elongated tube like, dead cells with hard, thick, lignified walls, wide lumen and narrow end walls. The wall of tracheids have various types of thickening. On the basis of thickening, the tracheids are of following types: Annular, Spiral, Reticulate, Scalariform, Pitted.
Xylem parenchyma Small, thin walled parenchyma cells associate with xylem is called xylem parenchyma. It stores food and helps in lateral conduction of water.
Xylem fibres: Thick walled dead sclerenchyma cells associate with xylem is called xylem fibres. They are mainly mechanical in function.
Functions of xylem
• It helps in conduction of water and minerals from root to leaf of plant.
• Vessels help in quick movement of water in the plant.
• Xylem parenchyma stores food and helps in the lateral conduction of water and sap.
• Xylem fibres are mainly mechanical in function.
Phloem(Bast)
Definition: A complex permanent tissue which conducts prepared food materials from leaves to different parts of plants is called Phloem. It is living non lignified tissue. It consists of four types of elements i.e., sieve tube, companian cells, phloem parenchyma, phloem fibres.
Sieve tub: The sieve tubes are elongated tubular non lignified cells placed end to end. The end walls are generally bulged out, transverse or oblique and have many pores called sieve plate. Sieve tube lacks nucleus.
Phloem parenchyma: Living parenchyma cells associated with phloem is called phloem parenchyma.
Phloem fibre: Sclerenchyma cells associated with phloem is called phloem fibre.
Functions of phloem
• Sieve tubes transport organic food from leaves to different part of plant.
• Phloem parenchyma stores food material being living cells.
• Phloem fibres provides mechanical strength to plant.
Special tissues
• Those tissue which help in secretion or excretion are called special tissues. Thus they are secretory or excretory in function. The common special tissues are:
1. Laticiferous: It consists of thin walled elongated and branched tubes which secrete a crystallo-colloid fluid called latex. It contains sugar, proteins, gum, alkaloid, fat, enzymes, tannins, mucilage, rubber etc. The latex may be watery (banana), milky (Euphorbia), coloured(Poppy). It is of two types i.e.
- Latex cells
- Latex vessels
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