Animal Tissues and their Types: Epithelial Tissues

Welcome to MBC Biology! In this post, I'm presenting you concise description Epithelium.

Simple Columnar Epithelium
Source: Berkshire Community College  Bioscience Image Library, CC0, via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/3/39/Epithelial_Tissues_Simple_Columnar_Epithelium_%2827854452128%29.jpg

 

The tissue is a group of cells that have similar structures, origin, and functions. The study of tissues is called histology. The study of the development and differentiation of tissues is known as histogenesis.

Various tissues combine to form an organ and different organs functioning together constitute an organ system. Various systems constitute a body. Those tissues are of four fundamental types (epithelial, connective, muscular, and nervous tissue) based on their structure and function.

A.    Epithelial tissues

The tissue which forms the outer and the inner coverings of various body parts (skin), external parts, internal organs and body spaces inside are called epithelial tissues or epithelium. The cells are closely packed; there is no intercellular space or matrix exists between the cells. The cells are held together by a cementing substance. Cells lie on a basement membrane, which is not supplied with blood vessels. Epithelium contains nerve endings too.

Based on the shape of the cells and the layers, epithelium is simple, stratified or compound, and modified epithelium.

1.      Simple epithelial tissues

Simple epithelial tissue is composed of single layer cells resting on the basement membrane. They are simple squamous, simple cuboidal, simple columnar, and pseudo-stratified epithelium.

a.       Simple squamous epithelium

Simple squamous epithelium is made of thin, flat, and hexagonal cells with a large central rounded nucleus. The cells are closely packed like the tiles on the mosaic floor. So, this tissue is also called pavement epithelium.

Simple squamous epithelium is found in a covering around coelom, lining of the buccal cavity, blood vessels, kidney, alveoli of lungs etc.

It helps in protection, absorption, filtration, and exchange of gases.

b.      Simple cuboidal epithelium

Simple cuboidal epithelium is composed of cuboidal cells having a centrally located small rounded nucleus in each. Cells lie on a basement membrane.

It is found in a lining on kidney tubules, sweat glands, salivary glands, gut, testis, and ovary.

It is involved in secretion, absorption, and excretion.

It is of two types – ciliated cuboidal and brush bordered cuboidal epithelium.

Ciliated cuboidal: Ciliated simple cuboidal epithelium contains cells with cilia on the free surface. Each cell is associated with secretory goblet cells. They are located on the ducts of the nephron. Cilia help in conducting the mucus and other substances.

Brush bordered cuboidal: Cells in this epithelium have microvilli at the free ends. They are located at the proximal convoluted tubules of the nephron.

c.       Simple columnar epithelium

Simple columnar epithelium consists of tall and narrow cells that are placed side by side forming a layer like a column. Each cell has a large nucleus situated at the basal end. These cells lie on a basement membrane.

It is found in the lining of the goblet cells of the stomach, gastric glands, intestinal glands, gall bladder, ureter, and uterine wall.

It is of two types – simple ciliated columnar and brush bordered columnar epithelium.

Ciliated columnar: Cells in ciliated columnar are with numerous cilia on the free surface. They are associated with secretory goblet cells.  They are located in the oviducts, respiratory passages (bronchioles), etc.

Brush bordered columnar: This simple columnar epithelium has cells with microvilli at the free ends of the cells. They are found in the intestinal mucosa.

d.      Pseudo-stratified epithelium

Pseudo-stratified epithelium consists of the columnar cells. As the cells do not reach free surface and their nuclei appear to be at different levels, they provide a false multilayered tissue. These cells rest up on the basement membrane.

It is found in the lining of trachea, large bronchi, and urinary bladder.

It protects the dust particles entering our respiratory tract.

Figure 1-4 Simple epithelium: 1A. & 1B. simple squamous epithelium; 2A. simple cuboidal cells; 2B. ciliated cuboidal cells; 2C. brush-bordered cuboidal cells; 3A. columnar epithelium; 3B. ciliated columnar cells; 3C. brush-bordered columnar cells; 4. pseudostratified epithelium  

2.      Stratified or compound epithelial tissue

Stratified or compound epithelial tissue is made of several layers of the epithelial cells. It is multi-layered as it contains an upper layer and a lower layer of cells. Cells in a lower layer multiply and give rise to the cells of the upper layers. The cells of a lower layer are called germinative cells.

Stratified or compound epithelial tissue are stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium, and transitional epithelium.

a.       Stratified squamous epithelium

The tissue in which the upper layer of cells consists of large, flat and polygonal or squamous cells, but the cells of germinative layer are either cuboidal or columnar is stratified squamous epithelium. The first formed cells are cuboidal shaped; they are pushed towards the upper surface outwards and become flattened squamous. Stratified squamous epithelium is of two types – keratinized stratified and non-keratinized squamous epithelium.

keratinized stratified epithelium: The uppermost layer consists of dead cells and is hardened due to the deposition of keratin, a protein. The deposition of keratin makes the cell layer water proof. It is located on hair, claws, and nails.

Non-keratinized squamous epithelium: The uppermost layer consists of living cells without keratin. The layer is wet due to the absence of keratin. It is found on wet surfaces like buccal cavity, pharynx, oesophagus, and vagina.

b.      Stratified cuboidal epithelium

It is a stratified epithelium in which the outermost layer consists of cuboidal cells. However, the lower layer has germinative cells either columnar or squamous.

It is found on the lining of the ducts of sweat glands, salivary glands, pancreatic gland, and female urethra, etc.

c.       Stratified columnar epithelium

The outermost layer of this tissue consists of tall columnar cells. However, the germinative cells are cuboidal shaped.

It is found on the lining of the ducts of mammary glands, lining of vasa-differentia, trachea, and bronchi.

d.      Transitional epithelium

Transitional epithelium is composed of three or four layers of cells. Cells in the uppermost layer are dome-shaped; the middle layer cells are club-shaped; and the basal layer cells are cuboidal or rounded cells. It has the capacity to stretch and relax.

It is found in the lining of the urinary bladder, ureters, and uterus, etc.

Figure 5-8 Stratified or compound epithelium: 5A. Stratified squamous epithelium; 5B.Nonkeratinized cells; 5C.Keratinized cells; 6. Stratified cuboidal epithelium; 7. Stratified columnar epithelium; 8. Transitional epithelium.   

3.      Modified epithelium ####

Some epithelial tissues are modified for the specialized functions. These are ciliated epithelium, sensory epithelium, Germinal epithelium, and glandular epithelium.

a.       Ciliated epithelium

Ciliated epithelium contains modified columnar or cuboidal cells. Those cells have cilia at their free surfaces. It forms the lining of the neck of uriniferous tubules, sperm ducts, trachea, and bronchi, etc.

b.      Sensory epithelium

Some of the columnar cells are modified with sensory fibres at their free surfaces. They form the lining of the tongue and the nasal cavity.

c.       Germinal epithelium

The modified cuboidal cells found in the lining of testes and ovary make germinal epithelium. They can divide and develop as gametes (spermatozoa and ova) by meiosis. The germinal epithelium forms the lining of the gonads (seminiferous tubule of testis and lining of ovary).

d.      Glandular epithelium

The modified columnar or cuboidal cells specialized for manufacture and secretion of certain chemical substances make glandular epithelium. The glandular epithelia form glands.

Figure 9. Modified epithelium: 9 A. Ciliated epithelium; 9 B. Sensory epithelium

Glands

The glands are grouped on the basis of number of cells present; the kind of secretion and the duct present; the shape and complexity; the mode of secretion, and the nature of secretion.

Glands based on the number of cells present

There are unicellular and multicellular glands based on the number of cells present in them.

1. Unicellular gland: A single cell scattered in the columnar cells is unicellular gland. Examples are goblet cells or mucus secreting cells.
2. Multicellular gland: It is made of many cuboidal cells that form many tubular invaginations. Examples are sweat glands and gastric glands.

Glands based on the kind of secretion and the duct present

These are exocrine and endocrine glands.

1. Exocrine glands: The glands which pour their secretions through the ducts are called exocrine glands. They secrete enzymes. Glands can be unicellular or multicellular (simple or compound). Examples are salivary, tear, gastric, and intestinal glands.
2. Endocrine glands: The glands that do not possess ducts but pour their secretions directly into the blood vessels are called endocrine glands. They are also called ductless glands. They secrete hormones. Examples are pituitary, thyroid, and adrenal glands, etc.

Glands based on the shape and complexity

Glands based on the shape and complexity or exocrine glands are of two types – simple glands and compound glands.

Simple glands

These glands have a single unbranched duct. The secretory part can be of tubular form (called tubules) or sacs (called alveolar). These can be coiled or uncoiled; branched or unbranched.

1. Simple tubular glands are found in intestinal crypts in the intestine.
2. Simple coiled tubular glands are found in simple sweat glands in the skin of mammals.
3. Simple branched tubular glands are found in the linings of gastric glands and Brunner’s glands of intestine.
4. Simple alveolar glands are found in the mucous secreting glands in the skin of frog.
5. Simple branched alveolar glands are found in the sebaceous or oil glands in the skin of mammals.

Compound glands

These glands have a number of ducts forming a branching pattern. The secretory part can be in the form of tubes (tubules), sacs (alveoli), or both.

1. Compound tubular glands are in the salivary glands.
2. Compound alveolar glands are found in the mammary glands, pancreatic glands etc.
3. Compound tubular-alveolar glands are found in the parts of salivary and mammary glands.

Glands based on the mode of secretion

Depending on the mode of secretion, the exocrine glands are of three types – merocrine glands, apocrine glands, and holocrine glands.

1. Merocrine glands: The secretions are discharged on the cell surface by diffusion or exocytosis without causing any damage or loss in the secretory cells. The cells remain intact e.g. goblet cells, salivary glands, intestinal glands, and sweat glands.
2. Apocrine glands: The secretions are discharged on cell surface causing loss or damage to some parts of secretory cells e.g. mammary glands, eyelid, and ear, etc.
3. Holocrine glands: The secretions are discharged on the cell surface by the rupture of the plasma membrane of secretory cells completely e.g. sebaceous glands in the skin of mammals.

Glands based on the nature of secretion

These are mucous glands, serous glands, and mixed glands.

1. Mucous glands: These glands secrete the mucus. The cells are called mucous cells or mucocytes. The mucus is a proteinous viscous and slimy substance. The goblet cells in the intestine are examples of mucous glands.
2. Serous glands: These glands secrete a clear watery fluid. These cells are called serocytes. Serous cells are found in the parotid salivary glands, intestinal glands, and sweat glands.
3. Mixed glands: Some glands are made of both the mucocytes and serocytes. These glands produce both kinds of secretions. Examples are gastric secretion and pancreatic secretion.

Figure 10 A. unicellular exocrine glands; 10 B. multicellular exocrine glands

Figure 11 A. Exocrine gland; 11 B. Endocrine gland.

Figure 12 Simple and compound glands: 12 A. simple tubular; 12 B. simple coiled tubular; 12 C. simple branched tubular; 12 D. simple alveolar; 12 E. simple branched alveolar glands. 12 F. compound tubular; 12 G. compound alveolar; 12 H. compound tubular-alveolar glands.

Figure 13 Glands based on the mode of secretion

Secondary Growth in Dicot Stem

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Here, I am posting a concise description of the secondary growth in dicot stem.

Schematic diagram of primary and secondary growth in dicot stem 
Image source: https://commons.m.wikimedia.org/wiki/File:Crescita_primaria_e_secondaria_del_fusto.png#file

The process of formation of extra layers or secondary tissues due to activity of vascular cambium and cork cambium for increasing girth or thickness or diameter of plant is called secondary growth. It takes place in stellar regions due to the activity of vascular cambium (see Figure 17) and in extra-stellar regions due to the activity of cork cambium (see Figure 21).

Secondary growth in stellar regions due to the activity of vascular cambium

Secondary growth due to activity of vascular cambium takes place by the formation of cambium ring, formation of secondary tissues, formation of annual rings or growth rings, and formation of heartwood and sapwood.

Formation of cambium ring

The dicot stem has a strip of cambium in between xylem and phloem in vascular bundles. This cambium is known as intrafascicular cambium. During secondary growth, some cells of medullary rays that are present at the level of intrafascicular cambium show meristematic activity to form the cambium between two adjacent vascular bundles, called interfascicular cambium. Intrafascicular cambium and interfascicular cambium join to form a cambium ring.

Formation of secondary tissues

Cambium ring acts as a meristem to add the secondary tissues both towards inner side and outer side. Cambium ring has two types of cells – elongated spindle-shaped fusiform initials and short isodiametric ray initials (see Figure 18). Fusiform initials divide to form secondary phloem towards outer side and secondary xylem towards inner side. The primary phloem and primary xylem are pushed towards periphery and centre by secondary phloem and secondary xylem, respectively. Ray initials divide to form vascular rays or secondary medullary rays on both sides. The activity of fusiform initials is more than the activity of ray initials. So, more secondary vascular tissues are formed than secondary medullary rays. In such condition, secondary medullary rays are compressed by secondary vascular tissues and form vascular rays. Vascular rays are uniseriate (one cell in breadth) or multiseriate (two or more cells in breadth). Vascular rays present in between the secondary xylem are called wood or xylem rays while the part of rays present in between secondary phloem are called phloem rays (see Figure 19).

Formation of annual ring or growth ring

The activity of cambium ring in temperate region is not uniform in spring or rainy season (favourable condition) and in cold winter or dry hot summer (unfavourable condition). Cambium ring is more active during favourable condition and form wide vessels containing more secondary xylem, called spring wood (early wood). But during unfavourable condition, a few small vessels containing secondary xylem, called autumn wood (late wood), are formed. These two kinds of wood appear together as a concentric ring in a year, called annual ring or growth ring. Such rings are formed year after year. Such type of annual ring is called ring porous wood.

The vessels in secondary xylem of the trees of tropical region are almost similar in both the spring wood and autumn wood due to the uniform activity of cambium ring in spring and autumn season. Such type of annual ring is called diffuse porous wood.

Formation of heartwood and sapwood

In old trees, secondary xylem is differentiated into centrally located dark and non-functional wood due to deposition of phytochemicals (oils, resins, gums, tannins, etc.) in cell wall and cell cavities, called heartwood or duramen and the outer light-coloured wood without phytochemicals in cell wall and cell cavities, called sapwood or laburnum (See Figure 20). Heartwood is stronger and more durable than sapwood.

Secondary growth in extra-stellar regions due to activity of cork cambium

The outermost layer of cortical cells (rarely deeper cortical cells/ pericycle cells) goes dedifferentiation and form a meristematic layer of cambium, called cork cambium (phellogen). Phellogen cells divide on both the outer side and inner side to form secondary tissues. The secondary tissue produced on the inner side of phellogen is called secondary cortex or phelloderm (See Figure 21). Phellogen produce cork cells or phellem on the outer side. It consists of dead and compactly arranged rectangular cells that possess suberized cell wall. The activity of cork cambium is more towards outer side than inner side. So, more phellem towards outer side than phelloderm towards inner side are formed. Cork cells prevent the loss of water by evaporation. They also protect the interior cells against the entry of harmful micro-organisms, mechanical injury, and unfavourable conditions.

At places of former stomata of the epidermis, phellogen produces aerating pores in the bark of plant instead of cork cells, called lenticels, for the exchange of gases and transpiration. They occur in woody trees but not in climbers. Lenticels are filled up by loosely arranged thin walled rounded, suberized or unsuberised cells, called complementary cells, which are formed by the division of sub-stomatal parenchyma cells. Complementary cells help in exchange of gases.

Figure 17 Secondary growth in dicot stem: a. & b. promeristem; c. young stem; d. formation of cambium ring; e. secondary growth in the first year; f. secondary growth in third year.

Figure 18 L. S. of Cambium ring showing fusiform and ray initials

Figure 19 T. S. of a part of two years old dicot stem showing secondary growth

Figure 20 T. S. of trunk showing sapwood and heartwood

Figure 21 Secondary growth due to activity of cork cambium

Internal structure of Isobilateral Leaf (Monocot Leaf)

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Isobilateral leaves of Paddy (Oryza sativa)
Image source: MB Chand

Here, I am posting a concise description of the anatomy of isobilateral leaf or monocot leaf.

Isobilateral leaves are found in monocotyledonous plants, such as maize. These leaves are normally erect and identical. Both the surfaces of such leaves are equally exposed to light. They are also called unifacial leaves. The internal structure of isobilateral leaf has epidermis, mesophyll tissue, and vascular bundles (see Figure 17).

1. Epidermis

Epidermis is the single layer of parenchyma found in both sides of leaf. This layer contains nearly equal number of stomata on both the sides, called amphistomatic. Each stoma opens into the substomatal cavity. In maize, cells in the upper stomata are modified into swollen bulliform or motor cells. Each stoma is guarded by a pair of dumb-bell shaped guard cells.

2. Mesophyll tissue

Mesophyll tissue is between the upper and lower epidermis. It contains large rounded parenchymatous cells with large cavities. It has reduced intercellular spaces. It is differentiated into palisade parenchyma and spongy parenchyma. Cells have abundant chloroplast.

3. Vascular bundles

Vascular bundles are parallelly arranged within the mesophyll tissues. They have different sizes. Each vascular bundle is conjoint, collateral, and closed type. Also, it is surrounded by a bundle sheath. Small vascular bundles are with less developed phloem and xylem whereas large bundles are with distinct phloem and xylem. The phloem is towards the lower epidermis and xylem is towards upper epidermis.

Phloem consists of sieve tubes and companion cells to transport food materials.

Xylem consists of two pitted oval metaxylem with tracheids in between them. The xylem parenchyma is less abundant. The protoxylem remains as a lysigenous cavity. Xylem conducts water and minerals.

Figure 17 T. S. of isobilateral leaf

Internal Structure of Dorsiventral Leaf (Dicot Leaf)

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Dorsiventral leaves of Syzygium cordifolia 

Image source: File:Dorsiventral leaves of Syzygium cordifolia IMG 1985.jpg - Wikimedia Commons

In this post, I am presenting a concise description of the anatomy of dorsiventral leaf .

The dorsiventral leaf is found in dicotyledonous plants. It has two surfaces – the upper or adaxial surface and the lower or abaxial surface. Its internal structure has epidermis, mesophyll tissues, and vascular bundles (See Figure 16).

1. Epidermis

Dorsiventral leaf shows distinct upper and lower epidermis.

Upper epidermis: The upper epidermis consists of a single layer of thin-walled and compactly arranged parenchymatous cells without intercellular spaces. Cells lack chloroplasts. Stomata are either absent or are fewer than in the lower epidermis. It is externally covered by cuticle.

Lower epidermis: Lower epidermis is made of single layer of parenchymatous cells. It is externally covered by cuticle. More stomata are present than on upper epidermis. Each stoma is surrounded by two kidney-shaped guard cells.

Epidermis helps in protection of the inner tissues from microbes, exchange of gases, and transpiration.

2. Mesophyll tissue

Mesophyll tissue lies between upper and lower epidermis. It is made of two types chlorophyll bearing parenchyma – palisade parenchyma and spongy parenchyma.

Palisade parenchyma: Palisade parenchyma lies just below the upper epidermis. The cells are radially elongated with numerous chloroplasts. These cells remain in 2 to 3 layers.

Spongy parenchyma: Spongy parenchyma is located between palisade parenchyma and lower epidermis. The cells are irregular and loosely arranged. They have fewer chloroplasts than in palisade parenchyma. They have large intercellular spaces, called air chambers. Some air chambers open into substomatal cavities. They perform photosynthesis.

3. Vascular bundles 

Vascular bundles are present in the mid rib and major veins. Each vascular bundle is made of xylem and phloem. The xylem is towards the upper epidermis (adaxial side) and the phloem towards lower epidermis (abaxial side). Each vein has a single vascular bundle. The vascular bundle is conjoint, collateral, and closed type. The bundle sheath surrounds each vascular bundle.

Xylem: Xylem consists of tracheary elements. Protoxylem lies towards upper epidermis and the metaxylem towards the lower epidermis.

Phloem: Phloem lies below xylem. It consists of sieve tubes and the companion cells.

Figure 16 T. S. of Dorsiventral leaf
Image source: MBC Biology

 

Anatomy of Monocot Stem

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 A maize stem bearing a newly sprout fruit

In this post, I am presenting a concise description of the internal structure of monocot stem.

A typical monocot stem consists of epidermis, hypodermis, ground tissues, and vascular bundles (see Figure 15).

Epidermis

Epidermis is the outermost layer of monocot stem. It is composed of compactly arranged, elongated, barrel-shaped parenchymatous cells without intercellular spaces. It contains stomata. Each stoma has a pair of specialized dumb-bell shaped cells, called guard cells. It helps in the protection of internal tissues, exchange of gases through stomata, minimize the surface transpiration by cuticle, and prevent the entry of harmful organisms.

Hypodermis

Hypodermis lies below epidermis. It is made up of 2- to 3-layered thick-walled dead lignified sclerenchyma. It helps in mechanical support.

Ground tissues

Ground tissue lies below the hypodermis. It is homogenous mass of tissue that is not differentiated into cortex, endodermis, pericycle, pith, and medullary rays. It is made up of oval or rounded parenchymatous cells with intercellular spaces. It helps in storage of food.

Vascular bundles

Vascular bundles in monocot stem are atactostele i.e. vascular bundles are scattered throughout the ground tissue. They are small and numerous towards the periphery but they are large and few towards the centre. They are oval or rounded.

Each vascular bundle contains both phloem and xylem. Xylem lies towards the centre and phloem lies towards the periphery. Cambium is absent. This type of vascular bundle is called conjoint, collateral, and closed. Each vascular bundle is surrounded by sheath of parenchyma, called bundle sheath.

Phloem

Phloem consists of sieve tubes, companion cells, and a few phloem fibres. Phloem parenchyma is absent. Sieve tubes conduct organic matters.

Xylem consists of vessels, tracheids, xylem parenchyma, and a few xylem fibres. Protoxylem is smaller than metaxylem. Protoxylem is first formed xylem whereas metaxylem is later formed xylem. Protoxylem lies at the centre while metaxylem lies towards periphery. This type of xylem is called endarch xylem. Vessels are arranged to form ‘V’ or ‘Y’. Some of the protoxylem vessels and xylem parenchyma cells are separated during rapid growth of stem and form a cavity, called protoxylem cavity (schizo-lysigenous cavity). Protoxylem cavity stores water. Xylem helps in conduction of sap and provides mechanical support.

Figure 15a T. S. of monocot stem (diagrammatic)

Figure 15b. T. S. of monocot stem (detailed view)

Anatomy of Primary Dicot Stem

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A runner bean (Phaseolus coccineus) with young beans 

This post describes the internal structure of the primary dicot stem.

Primary dicot stem consists of epidermis, hypodermis, cortex, endodermis, pericycle, vascular bundles, medullary rays or pith rays, and pith or medulla (see Figure 14a & 14b.).

Epidermis

Epidermis is the outermost single layer of stem. It is made of compactly arranged, elongated, barrel-shaped parenchymatous cells without intercellular spaces. It is covered externally by cuticle. Stomata and several unbranched multicellular hairs or trichomes are also present. Each stoma has a pair of specialized kidney-shaped cells, guard cells. Epidermis helps in protection of internal tissues, exchange of gases through stomata, reduce the rate of surface transpiration by cuticle, prevents the entry of harmful organisms and heat insulation by epidermal hairs.

Hypodermis

Hypodermis lies below epidermis. It is made of 3- to 4-layered collenchyma. Collenchyma is green due to chloroplasts in it. It helps in mechanical support, cell elasticity, storage of food, and photosynthesis.

Cortex

Cortex lies below hypodermis. It is made up of a few to several layers of thin-walled, angular, oval or rounded parenchymatous cells with intercellular spaces. Its main function is to store food.

Endodermis

Endodermis is a single wavy innermost layer of cortex. It is made of barrel-shaped, compactly arranged, parenchymatous cells without intercellular spaces. Cells in it contain large number of starch grains. Due to presence of starch grains in cells, endodermis is also called starch sheath.

Pericycle

Pericycle lies between the endodermis and vascular bundles. It is generally heterogeneous or rarely homogeneous. Heterogeneous pericycle contains both the parenchyma and sclerenchyma. Sclerenchymatous pericycle is just outside the vascular bundles in the form of semicircular patches, called bundle caps. The parenchymatous pericycle is made either of only parenchyma or only of sclerenchyma. The sclerenchymatous pericycle gives mechanical support while the parenchymatous pericycle stores food.

Vascular bundles

Vascular bundles in dicot stem are present as eustele form i.e. ring of vascular bundles present inner to the pericycle around the central pith. They are wedge-shaped structures. Each vascular bundle consists of phloem at outer side, xylem towards inner side, and a strip of cambium in between phloem and xylem. So, the vascular bundle is conjoint, collateral, and open.

Phloem

Phloem lies towards pericycle. It consists of sieve tubes, companion cells, phloem parenchyma, and some phloem fibres. The sieve tubes are main conducting element of phloem that transport food.

Xylem

Xylem lies towards the pith or inner side of the vascular bundle. It consists of tracheids, vessels, xylem parenchyma, and xylem fibres. Tracheids and vessels form small protoxylem and large metaxylem. Protoxylem is the first formed xylem and lies towards centre whereas metaxylem is the later formed xylem and lies towards periphery. This type of xylem is called endarch xylem. Protoxylem has annular thickenings whereas metaxylem has pitted thickenings. Vessels remain in chains or rows. They are with angular outline. Xylem helps in conduction of sap and mechanical support.

Cambium

Cambium lies between xylem and phloem of same vascular bundle. It is a narrow strip of primary meristematic tissue. It is called intrafascicular cambium or fascicular cambium. In transverse section, cambial cells appear rectangular. Cambium helps in increasing the girth of stem by producing secondary phloem towards outer side and secondary xylem towards inner side.

Medullary rays or pith rays

Medullary rays lie between two adjacent vascular bundles. They are parenchyma with radially elongated cells. There may or may not be intercellular spaces. Medullary rays help in radial conduction of food and water. They also transport gases from pith to cortex and vice-versa.

Pith or Medulla

Pith is present at the centre of stem. It contains oval, rounded or polygonal parenchyma cells with or without intercellular spaces. Pith cells help in storage of food.

Figure 14a T. S. of dicot stem (diagrammatic view)

Figure 14b T. S. of dicot stem (detailed view)

Anatomy of Monocot Root

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 Prop root of Maize (Zea mays)

In this post, I am presenting a concise description of the internal structure of monocot root.

In monocot plants, a young and an old root do not have such distinction due to lack of secondary growth. A typical monocot root consists of epiblema, cortex, endodermis, pericycle, vascular bundles, conjunctive tissues, and pith (see Figure 13).

Epiblema
Epiblema is the outermost single layer of the root that is made up of compactly arranged, thin-walled, radially elongated parenchymatous cells without intercellular spaces. It does not have cuticle and stomata. Some cells elongate and make unicellular tubular outgrowths, called root hairs. As it has root hairs, epiblema is also called piliferous layer. Root hairs help in absorption of water and minerals from the soil.

Cortex

Cortex lies below the epiblema. It consists of many layers of thin-walled, oval, rounded or angular parenchyma with intercellular spaces. In older roots, outer one layer (in Smilax) or many layers (in Zea mays) of cortex contain sclerenchymatous tissues, called exodermis. Exodermis gives mechanical support. Cortex helps in conduction of water from the root hairs to the vascular system and storage of food.

Endodermis

Endodermis is the innermost layer of cortex which is made of barrel-shaped compactly arranged parenchymatous cells without intercellular spaces. Young endodermal cells lying opposite to phloem possess an internal strips or bands of suberin and lignin, called Casparian strip. A few thin-walled cells lying opposite to protoxylem are called passage cells. Casparian strips do not allow the movement of substances from cortex to pericycle. However, passage cells conduct fluid inwards and outwards.

Pericycle

Pericycle lies below endodermis. It is made up of thin-walled parenchymatous cells without any intercellular spaces. Pericycle may be uniseriate (single layered in maize) or multiseriate (multilayered in Smilax). It is responsible for the formation of lateral roots.

Vascular bundles

Vascular bundles are made of xylem and phloem. Xylem and phloem are present in different bundles. They are equal in number and lie alternate to each other. So, these vascular bundles are called radial vascular bundles. Vascular bundles are more than six (polyarchy) in monocot root.

Xylem is made of tracheids, vessels, xylem parenchyma, and xylem fibres. Vessels are rounded or oval. Xylem bundles are exarch i.e. protoxylem (the first formed xylem) lies towards periphery while metaxylem (the later formed xylem) lies towards the centre of a root. Xylem helps in conduction of water and minerals. It also provides mechanical support.

Phloem consists of sieve tubes, companion cells, rarely phloem fibres. Phloem transports food.

Conjunctive tissue

One or many layers of thin-walled parenchymatous or thick-walled sclerenchymatous cells are present in between phloem and xylem, called conjunctive tissues. The parenchymatous conjunctive tissues store food whereas sclerenchymatous conjunctive tissues help in mechanical support.

Pith

Pith is the centre of a root. In monocot root, pith is with large area. It is composed of thin-walled, oval, rounded or angular parenchymatous cells with or without intercellular spaces. It stores food.

Figure 13 a. T.s of monocot root (diagrammatic)

Figure 13b. T. S. of monocot root (detailed view)