ANATOMY
When we study various plants, we observe structural similarities and variations in the external morphology of plants. If we study the internal structure, one can find several similarities and dissimilarities. Internal structures show modifications as a result of adaptations by the plants to diverse environments. Study of internal structure of plants is called anatomy. Plants have cells as their basic unit, which are organized into tissues and in turn tissues are organized into organs.
THE TISSUES
A tissue can be defined as a group of cells specialized for a particular function or a set of similar functions. A plant is made up of different kinds of tissues.
Classification of plant tissues
1. Meristematic tissues
$\displaystyle \small \bullet$ They contain immature and young cells and are capable of repeated divisions.
$\displaystyle \small \bullet$ Intercellular spaces are absent in meristematic tissue.
$\displaystyle \small \bullet$ They contain large nuclei associated with abundant cytoplasm.
$\displaystyle \small \bullet$ They are metabolically very active.
$\displaystyle \small \bullet$ Meristematic cells are isodiametric in shape and vacuoles are absent.
$\displaystyle \small \bullet$ Types of meristems based on origin and development
(i) Promeristem or Primordial meristem: The promeristem originates from embryo hence called primordial or embryonic meristem.
(ii) Primary meristem: A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia.
(iii) Secondary Meristem: They always develop in permanent tissues and have no typical promeristem.
$\displaystyle \small \bullet$ Types of meristems based on position
(i) Apical meristem: The meristems are present at the shoot and root tips produced from primary tissues. Some meristematic cells left behind from shoot apical meristem, constitute the axillary buds. Such buds can form a branch or a flower.
(ii) Intercalary meristem: These meristem occurs between mature tissues. They are seen in grasses.
(iii) Lateral meristem: These occur parallel to the sides of stems and roots laterally. They result in the increase in girth of the plants.
2. Permanent Tissues
$\displaystyle \small \bullet$ The cells that are differentiated from meristems and are specialized in their structure and function constitute permanent tissues.
$\displaystyle \small \bullet$ These cells lose their capacity to divide.
$\displaystyle \small \bullet$ Permanent tissues having all cells similar in structure and function are called simple permanent tissues and those having different kinds of cells are called complex tissue.
Simple Permanent Tissues
Simple tissues are a group of cells which are all alike in origin, form and function.
They are further grouped into three types
a) Parenchyma:
$\displaystyle \small \bullet$ It is a most simple and unspecified living tissue.
$\displaystyle \small \bullet$ The parenchyma cells are isodiametric.
$\displaystyle \small \bullet$ They may be spherical, oval, round, polygonal or elongated in shape.
$\displaystyle \small \bullet$ Thy have thin walls made up of cellulose.
$\displaystyle \small \bullet$ They have small intercellular spaces.
$\displaystyle \small \bullet$ They perform various functions like photosynthesis, storage, secretion.
$\displaystyle \small \bullet$ Types of parenchyma are
(i) Simple parenchyma: They are the cells with basic characters, they have active cytoplasm
which essential for metabolism, helps in respiration, and they are in function of storage and
secretion.
(ii) Chlorenchyma: The parenchyma cells carry out photosynthesis are known as chlorenchyma,
they are found in mesophyll of leaves. In the chlorenchyma compactly arranged cylindrical
shaped cells are called palisade parenchyma. The loosely arranged cells are called spongy
parenchyma.
(iii) Aerenchyma: In aquatic plants the parenchyma develops more intercellular space. The air is
present in the space it is called aerenchyma which helps in floating or buoyancy and exchange of gases.
(iv) Stallate: star shaped with elongated several ends which encloses air space in banana. The
parenchyma cells stores the minerals such cells are called idioblasts(raphids). The thick walled parenchyma are called prosenchyma
b) Collenchyma
$\displaystyle \small \bullet$ It consists of cells which are much thickened at corner due to cellulose, hemicellulose and pectin.
$\displaystyle \small \bullet$ The cells of this tissue contain protoplasm and are living.
$\displaystyle \small \bullet$ It occurs in layers below the epidermis in dicotyledonous plants.
$\displaystyle \small \bullet$ Collenchyma cells vary in shape and contain chloroplasts.
$\displaystyle \small \bullet$ Intercellular space is absent in collenchyma tissues.
$\displaystyle \small \bullet$ They provide mechanical support to the growing parts of the plants like young stem. Hence they are called living mechanical tissues.
c) Sclerenchyma
$\displaystyle \small \bullet$ It is a simple permanent tissue consists hard, dead cells and they are in mechanical in function.
$\displaystyle \small \bullet$ The cells are thick walled by the deposition of lignin. The cell wall is uneven in thickened and forms pits.
$\displaystyle \small \bullet$ The cells at young stage were living with vacuolated protoplasm and nucleus.
$\displaystyle \small \bullet$ In matured cells by the lignification they expel out nucleus and protoplasm become
dead cells.
$\displaystyle \small \bullet$ The cells are polygonal in shape and compactly arranged without intercellular space.
$\displaystyle \small \bullet$ There are two types of sclerenchyma cells namely sclerenchyma fibres and sclerieds.
(i) Fibres: They are elongated and polygonal shaped with pointed ends. The cells are originated
partly from procambium and partly from ground meristems. The cells are compactly arranged without intercellular space. The cell wall is thick due to deposition of secondary wall material lignin. The central cavity lumen is highly reduced. The sclerenchyma cells provides mechanical support hence they are called dead mechanical tissue. The fibres associated with phloem are the raw material of industries. From stem of jute gunny bags are prepared, fibres around seeds in cotton use in textiles, the coir in coconut have commercial value.
(ii) Sclerieds: They are commonly called sclerotic cells or stony cells. They are short, broad, isodiametric or irregular in shape. They are widely distributed in plants, cells have many layers of lignin deposition. The lumen is narrow and it is filled with massive deposition, at maturity they lose protoplasm and become dead. These are commonly found in the fruit walls of nuts; pulp of fruits like guava, pear and sapota; seed coats of legumes and leaves of tea. Sclerenchyma provides mechanical support to organs. The main function of sclerieds is mechanical support.
Complex permanent tissue
The complex permanent tissues are composed of different types of cells, but they are together in specific position in the plant body. They consist both living and dead cells of thick cell wall. The two complex tissues are xylem and phloem, they together forms vascular tissues or conducting tissues.
a) Xylem (wood) –water conducting tissue
The term xylem was coined by Nageli. Along with water conduction xylem provides mechanical support to the plant. The word xylem is also referred as wood. This is composed of both living and dead elements. They are thick walled due to deposition of lignin. There are 4 types of xylem elements namely: tracheids, tracheas or vessels, xylem fibres and xylem parenchyma.
(i) Tracheids: They are elongated and polygonal shaped cells, cells are with pointed with chisel shaped ends. The cells are overlapping one another at their ends. They are dead thick walled cells, protoplasm is absent and it forms lumen. Cell wall is thick due to lignification and it is impermeable to water. The main function is water conduction and they are also give mechanical support due lignified walls. Tracheids are primitive elements found in most pteridophytes, gymnosperms and in dicot plants of angiosperms.
(ii) Tracheas or vessels: They are cylindrical shaped, shorter tube like cells. Their end walls are absent. Tracheas are placed one above another forms continuous tube which runs parallel. The cavity or lumen is wide, cells are thick walled and dead. The lignin deposition is similar to tracheids, they have annular, spiral, reticulate, pitted and scalariform deposition. They are good conduction elements than the tracheids. Absence of cross wall between the tracheas, they are advanced elements found in monocots of angiosperms.
(iii) Xylem fibres: They are sclerenchyma fibre which are associated with xylem provides mechanical support. They are thick walled dead cells.
(iv) Xylem parenchyma: Xylem parenchyma cells are living and thin-walled and their cell walls are made up of cellulose. They are the parenchyma cells associated with xylem. They are the only living elements of xylem with living protoplasm and nucleus which helps in storage. Primary xylem is of two types – protoxylem and metaxylem. The first formed primary xylem elements are called protoxylem and the later formed primary xylem is called metaxylem. In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ. This type of primary xylem is called endarch. In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre. Such arrangement of primary xylem is called exarch.
b) Phloem (bast) - food conducting tissue
The word phloem was coined by Nageli (1858). It is a complex permanent tissue, it consist both living and dead cell of either thick walled or thin walled. It is a food conducting tissue which has 4 types of elements namely –sieve elements, companion cells, and phloem parenchyma and phloem fibres
(i) Sieve elements: They are the phloem elements composed of two units namely sieve tubes and sieve plates. The sieve tubes are elongated cylindrical shaped, they are living cells with protoplasm but nucleus is absent hence called as enucleated cells. The cell wall of sieve tubes thick wall due to cellulose and pectin but lignin is absent. The cytoplasm of one cell connected with the next cell through perforated sieve plates, it is called cytoplasmic strands or transcellular strands. Ends of sieve tube are closed by perforated sieve plate. Through the sieve plates cytoplasm connects. Cytoplasmic strands of phloem contains P-protein (phloem proteins). During winter callus pad is formed at sieve plate. The main function of sieve elements is the food conduction.
(ii) Companion cells: They are narrow slender specialized cells associated with sieve tubes. They are metabolically active cells. Companion cells are living containing vacuolated protoplasm and nucleus. The cytoplasm of companion cells and sieve tubes are connected through the plasmodesmata. The companion cells and sieve tubes are produced by longitudinal division of single meristematic cell hence they are called as sister cells. In pteridophytes and gymnosperms the companion cells are absent but presence of another type of cells called albuminous cells, they perform same function.
(iii) Phloem parenchyma: They are living thin walled cells of parenchyma, associated with phloem. It is also called bast parenchyma. The main function of phloem parenchyma is conduction of food in radial direction and food storage. The vascular part of phloem sieve cells is called Leptom. The cells have active protoplasm and nucleus, they are absent in monocots.
(iv) Phloem fibres or bast fibers: They are the only dead elements of phloem. They are sclerenchyma cells associated with phloem. They are thick walled dead cells and cell wall is lignified. Phloem fibre provides mechanical support and they are commercially important. The cell wall of phloem fibres is quite thick. At maturity, these fibres lose their protoplasm and become dead. Phloem fibres of jute, flax and hemp are used commercially. 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.
When we study various plants, we observe structural similarities and variations in the external morphology of plants. If we study the internal structure, one can find several similarities and dissimilarities. Internal structures show modifications as a result of adaptations by the plants to diverse environments. Study of internal structure of plants is called anatomy. Plants have cells as their basic unit, which are organized into tissues and in turn tissues are organized into organs.
THE TISSUES
A tissue can be defined as a group of cells specialized for a particular function or a set of similar functions. A plant is made up of different kinds of tissues.
Classification of plant tissues
1. Meristematic tissues
$\displaystyle \small \bullet$ They contain immature and young cells and are capable of repeated divisions.
$\displaystyle \small \bullet$ Intercellular spaces are absent in meristematic tissue.
$\displaystyle \small \bullet$ They contain large nuclei associated with abundant cytoplasm.
$\displaystyle \small \bullet$ They are metabolically very active.
$\displaystyle \small \bullet$ Meristematic cells are isodiametric in shape and vacuoles are absent.
$\displaystyle \small \bullet$ Types of meristems based on origin and development
(i) Promeristem or Primordial meristem: The promeristem originates from embryo hence called primordial or embryonic meristem.
(ii) Primary meristem: A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia.
(iii) Secondary Meristem: They always develop in permanent tissues and have no typical promeristem.
$\displaystyle \small \bullet$ Types of meristems based on position
(i) Apical meristem: The meristems are present at the shoot and root tips produced from primary tissues. Some meristematic cells left behind from shoot apical meristem, constitute the axillary buds. Such buds can form a branch or a flower.
(ii) Intercalary meristem: These meristem occurs between mature tissues. They are seen in grasses.
(iii) Lateral meristem: These occur parallel to the sides of stems and roots laterally. They result in the increase in girth of the plants.
$\displaystyle \small \bullet$ The cells that are differentiated from meristems and are specialized in their structure and function constitute permanent tissues.
$\displaystyle \small \bullet$ These cells lose their capacity to divide.
$\displaystyle \small \bullet$ Permanent tissues having all cells similar in structure and function are called simple permanent tissues and those having different kinds of cells are called complex tissue.
Simple Permanent Tissues
Simple tissues are a group of cells which are all alike in origin, form and function.
They are further grouped into three types
a) Parenchyma:
$\displaystyle \small \bullet$ It is a most simple and unspecified living tissue.
$\displaystyle \small \bullet$ The parenchyma cells are isodiametric.
$\displaystyle \small \bullet$ They may be spherical, oval, round, polygonal or elongated in shape.
$\displaystyle \small \bullet$ Thy have thin walls made up of cellulose.
$\displaystyle \small \bullet$ They have small intercellular spaces.
$\displaystyle \small \bullet$ They perform various functions like photosynthesis, storage, secretion.
$\displaystyle \small \bullet$ Types of parenchyma are
(i) Simple parenchyma: They are the cells with basic characters, they have active cytoplasm
which essential for metabolism, helps in respiration, and they are in function of storage and
secretion.
(ii) Chlorenchyma: The parenchyma cells carry out photosynthesis are known as chlorenchyma,
they are found in mesophyll of leaves. In the chlorenchyma compactly arranged cylindrical
shaped cells are called palisade parenchyma. The loosely arranged cells are called spongy
parenchyma.
(iii) Aerenchyma: In aquatic plants the parenchyma develops more intercellular space. The air is
present in the space it is called aerenchyma which helps in floating or buoyancy and exchange of gases.
(iv) Stallate: star shaped with elongated several ends which encloses air space in banana. The
parenchyma cells stores the minerals such cells are called idioblasts(raphids). The thick walled parenchyma are called prosenchyma
$\displaystyle \small \bullet$ It consists of cells which are much thickened at corner due to cellulose, hemicellulose and pectin.
$\displaystyle \small \bullet$ The cells of this tissue contain protoplasm and are living.
$\displaystyle \small \bullet$ It occurs in layers below the epidermis in dicotyledonous plants.
$\displaystyle \small \bullet$ Collenchyma cells vary in shape and contain chloroplasts.
$\displaystyle \small \bullet$ Intercellular space is absent in collenchyma tissues.
$\displaystyle \small \bullet$ They provide mechanical support to the growing parts of the plants like young stem. Hence they are called living mechanical tissues.
$\displaystyle \small \bullet$ It is a simple permanent tissue consists hard, dead cells and they are in mechanical in function.
$\displaystyle \small \bullet$ The cells are thick walled by the deposition of lignin. The cell wall is uneven in thickened and forms pits.
$\displaystyle \small \bullet$ The cells at young stage were living with vacuolated protoplasm and nucleus.
$\displaystyle \small \bullet$ In matured cells by the lignification they expel out nucleus and protoplasm become
dead cells.
$\displaystyle \small \bullet$ The cells are polygonal in shape and compactly arranged without intercellular space.
$\displaystyle \small \bullet$ There are two types of sclerenchyma cells namely sclerenchyma fibres and sclerieds.
(i) Fibres: They are elongated and polygonal shaped with pointed ends. The cells are originated
partly from procambium and partly from ground meristems. The cells are compactly arranged without intercellular space. The cell wall is thick due to deposition of secondary wall material lignin. The central cavity lumen is highly reduced. The sclerenchyma cells provides mechanical support hence they are called dead mechanical tissue. The fibres associated with phloem are the raw material of industries. From stem of jute gunny bags are prepared, fibres around seeds in cotton use in textiles, the coir in coconut have commercial value.
(ii) Sclerieds: They are commonly called sclerotic cells or stony cells. They are short, broad, isodiametric or irregular in shape. They are widely distributed in plants, cells have many layers of lignin deposition. The lumen is narrow and it is filled with massive deposition, at maturity they lose protoplasm and become dead. These are commonly found in the fruit walls of nuts; pulp of fruits like guava, pear and sapota; seed coats of legumes and leaves of tea. Sclerenchyma provides mechanical support to organs. The main function of sclerieds is mechanical support.
| Parenchyma | Collenchyma | Sclerenchyma |
|---|---|---|
| Cells are oval and isodiametric | Cells are elongated and oval | Cells are polygonal and tubular |
| Living cells | Living cells | Dead cells |
| Presence of intercellular space | Intercellular space is occupied by pectin | Absence of intercellular space |
| Cell wall is thin | Cell wall thick by pectinization | Cell wall thick by lignification |
| Found in cortex and pith | Found in hypodermis and around vascular bundle | Found in hypodermis and around vascular bundle |
| Storage and photosynthetic function | Living mechanical tissue and photosynthesis | Purely dead mechanical support. |
Complex permanent tissue
The complex permanent tissues are composed of different types of cells, but they are together in specific position in the plant body. They consist both living and dead cells of thick cell wall. The two complex tissues are xylem and phloem, they together forms vascular tissues or conducting tissues.
a) Xylem (wood) –water conducting tissue
The term xylem was coined by Nageli. Along with water conduction xylem provides mechanical support to the plant. The word xylem is also referred as wood. This is composed of both living and dead elements. They are thick walled due to deposition of lignin. There are 4 types of xylem elements namely: tracheids, tracheas or vessels, xylem fibres and xylem parenchyma.
(i) Tracheids: They are elongated and polygonal shaped cells, cells are with pointed with chisel shaped ends. The cells are overlapping one another at their ends. They are dead thick walled cells, protoplasm is absent and it forms lumen. Cell wall is thick due to lignification and it is impermeable to water. The main function is water conduction and they are also give mechanical support due lignified walls. Tracheids are primitive elements found in most pteridophytes, gymnosperms and in dicot plants of angiosperms.
(ii) Tracheas or vessels: They are cylindrical shaped, shorter tube like cells. Their end walls are absent. Tracheas are placed one above another forms continuous tube which runs parallel. The cavity or lumen is wide, cells are thick walled and dead. The lignin deposition is similar to tracheids, they have annular, spiral, reticulate, pitted and scalariform deposition. They are good conduction elements than the tracheids. Absence of cross wall between the tracheas, they are advanced elements found in monocots of angiosperms.
(iii) Xylem fibres: They are sclerenchyma fibre which are associated with xylem provides mechanical support. They are thick walled dead cells.
(iv) Xylem parenchyma: Xylem parenchyma cells are living and thin-walled and their cell walls are made up of cellulose. They are the parenchyma cells associated with xylem. They are the only living elements of xylem with living protoplasm and nucleus which helps in storage. Primary xylem is of two types – protoxylem and metaxylem. The first formed primary xylem elements are called protoxylem and the later formed primary xylem is called metaxylem. In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ. This type of primary xylem is called endarch. In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre. Such arrangement of primary xylem is called exarch.
| Tracheids | Vessels (tracheas) |
|---|---|
| Cells are elongated and slender | Cells are short and broad |
| Cell ends are tapering and chisel shaped | Cells with transverse ends |
| Lumen is narrow and not well adapted for water conduction | Lumen is wide and well adapted for water conduction |
| They are imperforated cells | It has perforation ends |
| Occurs in pteridophytes,gymnosperms and in dicots of angiosperms | Occur in monocots of angiosperms |
b) Phloem (bast) - food conducting tissue
The word phloem was coined by Nageli (1858). It is a complex permanent tissue, it consist both living and dead cell of either thick walled or thin walled. It is a food conducting tissue which has 4 types of elements namely –sieve elements, companion cells, and phloem parenchyma and phloem fibres
(i) Sieve elements: They are the phloem elements composed of two units namely sieve tubes and sieve plates. The sieve tubes are elongated cylindrical shaped, they are living cells with protoplasm but nucleus is absent hence called as enucleated cells. The cell wall of sieve tubes thick wall due to cellulose and pectin but lignin is absent. The cytoplasm of one cell connected with the next cell through perforated sieve plates, it is called cytoplasmic strands or transcellular strands. Ends of sieve tube are closed by perforated sieve plate. Through the sieve plates cytoplasm connects. Cytoplasmic strands of phloem contains P-protein (phloem proteins). During winter callus pad is formed at sieve plate. The main function of sieve elements is the food conduction.
(ii) Companion cells: They are narrow slender specialized cells associated with sieve tubes. They are metabolically active cells. Companion cells are living containing vacuolated protoplasm and nucleus. The cytoplasm of companion cells and sieve tubes are connected through the plasmodesmata. The companion cells and sieve tubes are produced by longitudinal division of single meristematic cell hence they are called as sister cells. In pteridophytes and gymnosperms the companion cells are absent but presence of another type of cells called albuminous cells, they perform same function.
(iii) Phloem parenchyma: They are living thin walled cells of parenchyma, associated with phloem. It is also called bast parenchyma. The main function of phloem parenchyma is conduction of food in radial direction and food storage. The vascular part of phloem sieve cells is called Leptom. The cells have active protoplasm and nucleus, they are absent in monocots.
(iv) Phloem fibres or bast fibers: They are the only dead elements of phloem. They are sclerenchyma cells associated with phloem. They are thick walled dead cells and cell wall is lignified. Phloem fibre provides mechanical support and they are commercially important. The cell wall of phloem fibres is quite thick. At maturity, these fibres lose their protoplasm and become dead. Phloem fibres of jute, flax and hemp are used commercially. 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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