Plant Anatomy

The study of internal structure of plants through microscope is called plant anatomy or phytotomy.

N. Grew is known as 'Father of plant anatomy'

Plant is multicellular and is composed of many different cells.

The cells of same kind that have same origin and function constitute a group called as "tissue."

In plants, tissue is highest form of development and tissues constitute tissue system.

In plants, 3 types of tissue system are recognized. They are Dermal System, Ground System and Vascular System.

Dermal system gives protection, ground system stores several chemicals and vascular system conducts water, minerals and food.

Types of Plant Tissue

Depending on nature of cells in tissue, plant tissue are classified into 3 groups: Meristematic, Permanent, Secretory.

1. Meristematic tissue (Meristem)

• A group of immature, non-specialized cells which undergo repeated division is called meristematic tissue.
• Meristem therefore responsible for growth.
• Meristem can give rise to different tissue which can form permanent tissue.

The general features of cells of meristem are mentioned below:

1. The cells are immature and unspecialized.
2. They are at the state of continuous division.
3. They are thin walled.
4. They are compactly arranged without intercellular space.
5. They are densely packed with cytoplasm.

Types of meristem

1. On the basis of Origin

1. Promeristem: This meristem is found in embryo of seeds. Therefore, promeristem also called Embryonic meristem. It is earliest type of meristem and is also name as primordial meristem.
2. Primary meristem: Promeristem divides and differentiates into another types of meristem called primary meristem. It is located at the apices of growing parts. This meristem brings increament in length of plant parts. This types of growth is called primary growth.
3. Secondary meristem: The meristem which is developed from cells of primary meristem is called secondary meristem. It brings increament in girth (diameter) of plant parts. The process of increasing girth of plant parts due to activity of secondary meristem is called secondary growth. Cambium is an example of secondary meristem.

2. On the basis of Location

Promeristem is found in seeds and plant has only primary and secondary meristem. Depending on location, primary and secondary meristem are classified into three types:

1. Apical meristem: It is a type of primary meristem and is found in growing tips of roots, stem, leaves and buds. Apical meristem brings primary growth in plant parts.
2. Intercalary meristem: It is a type of primary meristem that is distributed in nodes. This meristem is mixed with other tissue and gives out branches, buds and flower.
3. Lateral meristem: It is an example of secondary meristem and is located on lateral position of plant parts. It brings increament in girth of the plant. This meristem is generally found in dicot stem.

3. On the basis of Function

1. Protoderm: The meristem which found outermost system in plants.
2. Ground meristem: The meristem which forms ground system, where storage of food and other chemicals take place.
3. Procambium: The meristem which forms vascular system in plants.

Shoot Apex (Shoot Apical Meristem - SAM)

Shoot apex has meristems that repeatedly divides and forms different layers in plants parts. Then there are two theories that explain differentiation of SAM.

1. Tunica - Corpus Theory

   This theory was proposed by Schmidt in 1924. According to him, SAM has two distinct regions: the outer is called Tunica and inner is called Corpus. The tunica is few layered thick and is composed of smaller cells. It is differentiated into dermal system. Inner to tunica is many layered thick Corpus, which is composed of larger cells. The corpus cells later gives rise to ground system and vascular system.

2. Histogen theory

   This theory was proposed by Hanstein and stated that there are 3 layers, which are collectively called Histogen (tissue builder). According to him, SAM has following Histogen:

   • a) Dermatogen: It is outermost layer of SAM and the cells form it are differentiated into Dermal layer.
   • b) Periblem: Inner to Dermatogen is called periblem which is middle layer and forms ground tissue.
   • c) Plerome: It is central region of SAM and forms vascular system.

Functions of Meristem

• Meristem results growth and development.
• Primary meristem causes primary growth which is an increment in length of plant parts.
• Secondary meristem causes increment in girth. This kind of growth is called secondary growth.
• It helps in healing wounds.
• SAM results several plants like leaf, buds, flower, branches etc.

2. Permanent tissue

"A group of living or dead cells which have specific shape and function is called permanent tissue."

• It has lost the power of division.
• It has derived from the cells of meristematic tissue.
• It takes part in absorption of water and minerals, conduction, photosynthesis, mechanical support.

Permanent tissues is two (2) types: Simple permanent and Complex permanent.

a) Simple permanent tissue 

When the cells are of same kind the permanent tissue is called Simple permanent tissue.

• The simple permanent tissue may or may not have intercellular spaces.
• It is common types of permanent tissue and has several roles.

Based on the shape of the cells, Simple permanent tissue is 3 types:

1. Parenchyma
2. Collenchyma
3. Sclerenchyma

a. Parenchyma

• The term parenchyma was coined by N. Grew.
• It is most common types of simple permanent tissue and in distributed in almost all parts of plant.
• The cells of parenchyma are loosely arranged with intercellular spaces.
• The cells are thin walled and living with distinct nucleus.

Types of parenchyma:

1. Aerenchyma: The parenchyma which has wide intercellular space is called Aerenchyma. The intercellular space is called air cavity that provides buoyancy to the plant. It is found in free floating hydrophytes.
2. Chlorenchyma: The parenchyma in which the cells are packed with chlorophyll is called chlorenchyma. It takes part in photosynthesis and is distributed in the leaves and all other green parts.
3. Prosenchyma: The parenchyma which is composed of elongated and compactly arranged cells is called Prosenchyma. It provides mechanical support and rigidity to plants parts.
4. Velamen: The parenchyma which is available in the roots of Epiphyte is called Velamen. It takes part in absorption of moisture and minerals from atmosphere. The cells are living and thin walled.
5. Palisade Parenchyma: The parenchyma in which the cells are elongated and compactly arranged with abundant distribution of chlorophyll is called palisade parenchyma.
6. Spongy parenchyma: The parenchyma in which cells are loosely arranged and are distributed with lesser amount of chlorophyll is called spongy parenchyma.

Functions of parenchyma:

1. It stores several chemicals like water, organic compounds.
2. Chlorenchyma takes part in photosynthesis.
3. Aerenchyma help in exchange gases.
4. Aerenchyma gives buoyancy to plant part.
5. Prosenchyma give mechanical support.

b. Collenchyma

The Simple permanent tissue which is made of similar kind of cells and has deposition of cellulose, hemicellulose and pectin is called collenchyma.

• The term collenchyma was coined by Schleiden.
• It is a group of thick walled, living and compactly arranged cells.
• It provides rigidity with flexibility to plants parts.
• The intercellular spaces among the cells are packed with cellulose, hemicellulose and pectin.

Types of collenchyma: Based on pattern of deposition, collenchyma is following types:

1. Angular: The collenchyma in which cellulose, hemicellulose and pectin lie at the corners where two cells meet is called Angular.
2. Lacunar: The collenchyma in which deposition of cellulose, hemicellulose and pectin occurs in the intercellular spaces is called Lacunar.
3. Tangential (Lamellar): When deposition occurs in between the layers of cells, collenchyma it called tangential collenchyma. The deposition appears to be plate. Therefore it is also called plate collenchyma.

Functions of collenchyma:

1. It provides mechanical strength to plant parts.
2. It provides few degree of flexibility of plants parts.
3. It help in photosynthesis to some degree.
4. It helps in storage of food.

c. Sclerenchyma

It is a group of simple permanent tissue and is made of thick walled dead cells. Each cell has plenty of deposition of cellulose, hemicellulose and pectin. As the cells are thick walled Sclerenchyma provides mechanical strength. It is found in hard part of plants.

Types of Sclerenchyma: Depending on amount of deposition sclerenchyma is of 2 types:

1. Fiber: The Sclerenchyma which is composed of elongated narrow thick walled cells is called fiber. It provides mechanical strength to plant part. The fiber has great commercial importance in particular of rope and textiles. The dead tissue found on the surface of coconut is an example of fiber.
2. Sclerid: Sclerids are dead cells which have extreme deposition of cellulose, hemicellulose and pectin. They have different shape like spherical or oval or cylindrical or dumbbell shaped. They provide mechanical protection.

Types of Sclerid:

1. Brachysclereids: It is also called as stone cells, are spherical, oval or rounded and highly thickened dead cells with tubular pits.
2. Macrosclereids: They are rod like or columnar shaped. They are common in seed coats of many leguminous plants.
3. Osteosclereids: They are barrel shaped cells and look like bone. They are found in the leaves and seed coats of several monocotyledons.
4. Astrosclereids: They are star shaped and are found in the petioles.
5. Trichosclereids: They are hair like branched or unbranched cells found in intercellular spaces and stems of some aquatic plants.

Function of Sclerenchyma:

1. It provides mechanical strength to plants parts.
2. It provides protection to plants parts from extreme environmental conditions.
3. Fiber has commercial value in production of ropes and textile.
4. Sclerids provides stiffness to plant parts.

Complex permanent tissue

When tissue is made of 2 or more type of cells, the tissue is called complex permanent tissue. It includes xylem and phloem which conduct water and food. Therefore, xylem and phloem are also called conducting tissue.

• Xylem and phloem form a bundle called vascular bundle. Therefore, they are also called vascular tissue.

Xylem

The term xylem was introduced by Nageli. It is a chief conducting tissue of vascular plant responsible for solutes; it also provide mechanical strength.

Components of xylem: It is composed of different kinds of elements that is:

1. Tracheids (Death): It is elongated tube like dead cell with of cytoplasm with tapering ends. They have hard, thick & lignified walls. They have empty lumen with mature. The walls usually have one or more rows of bordered pits. These are only conducting tissue in the woods of ferns and gymnosperm. They consist water & minerals from root through the empty lumen without any obstruction by living tissue passes through pit from one tracheid to another.
2. Vessels or Trachea: These are elongated tube consisting of a series of drum shaped cells placed one above the other with their ends walls perforated or dissolve. Each cells appears circular, oval or polygonal with wide lumen; they become dead and lose their cytoplasm due to the deposition of lignified secondary walls. These are found in Angiosperm. The long empty tube provide an ideal system for transportation of water in tall plants & also provide mechanical support.
3. Xylem wood parenchyma (death): It is made up of generally small and thin wall parenchyma cell associated with parenchyma. It stores food, resin and latex and help in lateral conduction of water sap. It is only one living element of xylem.
4. Xylem or wood fibers (living): They are sclerenchyma fibers with much thicker wall and overlapping ends. They have narrow lumen than vessels. They cannot conduction water but being stronger provides mechanical strength to vessels & tracheids. They are abundant woody dicot.

Function:

• Xylem conduct water and minerals salt.
• The components of xylem like tracheid, vessel, sclerenchyma have thick lignified wall so they give mechanical to the plant body.

Types of Xylem: Depending on the stage of development & structure xylem is 2 types:

1. Primary xylem: Primary xylem further differentiate into protoxylem and metaxylem.
   • Protoxylem: The first formed xylem element are called protoxylem.
   • Metaxylem: The later formed primary xylem is called metaxylem.

Depending on the placement or management of protoxylem and metaxylem. The primary xylem is said to be endarch, exarch, mesarch or centarch.

• Endarch: Protoxylem lies toward the centre while the metaxylem lies towards the periphery. These arrangement occurs in leaf and stem.
• Exarch: Protoxylem lies towards the periphery while the metaxylem lies towards the centre. These arrangement occurs in the root.
• Mesarch: In mesarch protoxylem lies in the middle of metaxylem or vice versa. Eg: dorsiventrally leaf.
• Centarch: When the protoxylem lies at the centre of metaxylem. Eg: fern.

2) Secondary Xylem: It is formed during secondary growth. Formed from 2nd meristem after redifferentiation.

Phloem (Leptome, Bast)

Phloem is a living conducting tissue. Phloem cells are basically for the translocation of food materials from leaf to various parts of the plant for storage and growth.

• Phloem is a complex tissue composed of four types of cells.

1. Sieve tube (element): The sieve tubes are long tube like structure formed by end fusion of cells. The transverse wall is perforated by pores & looks like sieve plate. The sieve plate allow the flow of solution from one sieve element to another. The sieve tube don't have nucleus but have thin layer of cytoplasm around the periphery and a large vacuole. Though they lack a nucleus, they are living and are dependent on adjacent companion cells. Sieve tube are found in Angiosperm cells; lose phase are formed in winter which seals the sieve pores but in spring it gets melt.
2. Companion cells: These are specialized living parenchymatous cells associated with the sieve tube. These are elongated narrow cells which helps sieve tube in the transport of food materials. They have dense cytoplasm with the prominent nucleus. Companion cells are connected to sieve tube by simple pits. They are present in Angiosperm. They help in maintaining the pressure gradient in the sieve tube and food conduction.
3. Phloem Parenchyma: These are also thin walled parenchymatous cells. They are elongated tapering cylindrical cells that have dense cytoplasm and nucleus. The cell wall is composed of cellulose and has pits. They mainly stores food materials and are present in dicot and pteridophytes. They also stores food mater. substances like resins, latex and mucilage. They are absent in monocot.
4. Phloem fibers (Bast fibers): They are similar in structure to xylem fibers. They mostly occurs in secondary phloem & are made up of sclerenchymatous cells. They are elongated, unbranched and have pointed needle like apices. They are non-living cells hence provide mechanical support and gives strength and rigidity to the organ.

Types of phloem: It is 2 types:

1. Primary phloem: It is earlier or first formed phloem in the growing plant and is responsible for primary growth of the plant (with the help of primary meristem). It is formed from procambium. It is again two types:
   • Protophloem: The first formed primary phloem consist of narrow sieve tube.
   • Metaphloem: The later formed phloem has bigger sieve tube.
2. Secondary phloem: The phloem which is formed during secondary growth with its help of lateral meristem. It formed form vascular cambium.

3) Secretory tissue

It produced different types of chemical; some are in the form of enzymes and hormones to be utilized by the plant which others like rubber and gum are the secretion which are great of economic value. The plant have two types of secretory tissue.

1. Laticiferous tissue: It consist of thin walled, branched, elongated duct which contains numerous nuclei in a thin layer of cytoplasm. The ducts are irregularly distributed in parenchymatous cells. These are supposed to be organs of food storage or waste product. The duct are 2 types:
   • i) Latex vessels: They are made up of more or less parallel duct formed by fusion of branches. The duct are interconnected. Eg: found in sunflower.
   • ii) Latex cell: They are made up of elongated individual cells. They don't fuse with each other. Eg: found in rubber plant.
2. Glandular tissue: They may be unicellular or multicellular glands which may secrete chemical substances. They may be present externally or internally.
   • External glands:
     • Water secreting hairs
     • Glandular hairs - produces gummy substance as in tobacco
     • Poisonous glands - produces irritants
     • Honey glands - produces nectar as in flower
     • Enzyme secreting gland - Carnivorous plants
   • Internal glands:
     • Oil glands - secrete essential oil in fruits & leaves of orange, lemon etc.
     • Mucilage secreting glands - Eg: Betel
     • Resin secreting gland - sunflower, pinus
     • Digestive gland - Drosera
     • Water secreting gland - Ex: Pistia

The Tissue System

On the basis of morphological character, development & function, Sachs (1875) distinguish 3 division system in plants.

1. Epidermal tissue system (Integumentary)
2. Fundamental tissue system (Ground tissue system)
3. Vascular / Conducting system (Fascicular system)

a) Epidermal Tissue System [Epi = upon, derma = skin]

• The epidermal tissue system forms the outermost covering of the whole plant body i.e. stem, root, leaves which remains in direct contact with the environment.
• It protect the plants from under (unwanted) loss of water or mineral, external injuries.
• The tissue consist of epidermal cells, stomata, trichomes and hairs depending on the part of the plant body they are present.
• They are specialized in many to protect the tissue.

The two types of protective tissue present are:

1) Epidermis

• It forms the outermost layer of all soft part of the plant like young stem, roots, leaves and flower. It is generally one cell thick.
• The cells are elongated compactly arranged & without in intercellular spaces. The cells are parenchymatous in nature and colourless sap.
• They have small amount of cytoplasm and a large vacuole.

a) Cuticle & wax:

The outer cell wall is often covered with in a thin waterproof layer of waxy substance called cuticle is absent in roots. Structurally a cuticle layer consist of cellulose lamellae interrupted with pectin substance and cutin.

b) Stomata:

Stomata are the minute opening of leaves or green part of the plant. Stomata is surrounded by ground cells which encloses stomata pores.

• The concave cells of guard cells have minute space or opening in between them cell structure stomatal opening.
• The guard cells possesses chloroplast and regulate opening and closing of stomata.
• The stomata aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.
• In dicot plant guard cells are kidney shaped where in monocot they are dumbbell shaped.

c) Trichomes:

These are epidermal outgrowth present temporarily or permanently on almost all plant parts. They may be unicellular or multicellular and vary in size and shape in different species. Trichomes are of different types such as hairs unicellular or multicellular hairs, scales and water vesicles or bladders.

Function:

• Epidermis acts as a protective tissue covering the plant body.
• The cuticle checks excessive evaporation of water.
• Epidermis protects the plant from excessive heat or cold and from the attack of parasitic fungi & bacteria.
• It allows exchange of gases and transpiration through stomata.
• The water vesicles or bladders helps in storage of water.

2) Cork:

As dicot plants grows older their stem & roots develops a thick and waterproof outer covering. Their epidermis is replaced by layering cork cells. Cork is the peripheral waterproof tissue (called suberin) present in older woody stem and roots. It is composed of dead cells. The cells are roughly rectangular in shape. The cells are compactly packed & without intracellular spaces. They are arranged in multicellular layer. Cork is usually brownish in colour.

Functions:

• It acts as protective tissue.
• Suberin being waterproof prevent the loss of water.
• Cork insulates the trees from freezing temperature in cold and helps in conservation of water.
• Cork is light, highly compressible and doesn't catch fire easily.
• One of the oldest uses of cork is as stopper for bottles because it is non-reactive and doesn't get damaged by liquids. So it is ideal material for corking conical flask in labs, wine bottle.

b) Ground tissue system

• The ground tissue system consist of simple tissue like parenchyma, collenchyma, sclerenchyma.
• This tissue system is present in whole plant except epidermis and vascular bundle.
• Parenchymatous cells are usually present is cortex, pith and medullary rays.
• In leaf the ground tissue cells contain chloroplast and tissue is called mesophyll.

1. Hypodermis: It is found below the epidermis. The hypodermis of dicotyledon stem may be collenchymatous, sclerenchymatous or chlorenchymatous whereas that of monocotyledons is usually sclerenchymatous. The hypodermis is mainly consider as the zone of protecting and supporting tissue.
2. Cortex: It is the region between epidermis and endodermis. It is the major component of ground tissue system and it is represented by main layers loosely arrange parenchyma cells. It is distinct in dicot but not in monocotyledons.
3. Endodermis: It is the single layer of compactly arrange parenchymatous cells present between cortex and pericycle.
   • The cells of endodermis are elongated without intracellular spaces and appears barrel shaped round or oval.
   • The cells of endodermis are characterised to possessed Casparian strips or bands in their radical or transverse walls.
   • These strips are made up of waxy substance like suberin.
4. Pericycle: It is present between endodermis and vascular tissue. In case of root the pericycle made up of thin wall parenchymatous cells which later become sclerenchymatous.
5. Medullary rays: The primary medullary rays are present in between the adjusted vascular bundle. In the primary structure of the dicot stem they are made up of parenchymatous cell and extend from pith to the periphery. The cells originated from the apical meristem. They serve the function the later transport.
6. Pith: The central portion of root & stem are occupied by pith. It is usually made up of parenchymatous cell. The pith become hollow cavity. The main function of pith is storage of water & food material.

c) Vascular Tissue System

Vascular tissue consist complex tissue - xylem & phloem. The conducting tissue of the plant. The xylem and phloem are usually found in bundle hence called vascular bundle. The main function of bundle is conduction of water and minerals, translocation of organic solute and to give minerals mechanical support to the plant body. This tissue originated from the procambium of apical meristem.

Types of vascular bundle

According to the arrangement of xylem & phloem the vascular bundle are 3 types:

1. Radial
2. Conjoint
3. Concentric

• Radial bundle: Xylem & phloem are present on different radii alternating with each other. The bundles are separated by parenchymatous tissue. Present in monocot & dicot root.
• Conjoint: Xylem & phloem are present on the same radius in one bundle. Found in stem.
  • Collateral: Xylem is placed towards inside & phloem towards outside.
    • Closed: Cambium is absent. Ex: Monocot stem.
    • Open: Cambium is present between xylem and phloem. Ex: Dicot stem.
  • Bicollateral: Cambium + Phloem occurs twice. Xylem forms the middle line. Ex: Cucurbita.
• Concentric: A vascular bundle in which tissue is completely surrounded by the other.
  • Amphivasal (Leptocentric): The xylem lies in the centre & remains completely surrounded by phloem. Eg: Dracaena, Yucca.
  • Amphicribral (Hadrocentric): The phloem lies in the centre & remains completely surrounded by xylem. Eg: Ferns.

Anatomy of Root

Internal structure of monocot roots has following parts:

1. Epiblema or piliferous layer: It is the outermost single layer thin walled cells. It bears numerous unicellular roots hairs in its younger regions and helps to hold the plant to the soil and absorb water and minerals salt.
2. Cortex: It consist of many layers of parenchymatous cells. These are large thin walled and have number of intracellular spaces. Water and minerals can pass through these intracellular spaces as well as the cell wall.
3. Endodermis: It is the innermost layer of the cortex which is one cell thick and forms a complete ring. Endodermis has barrel shaped cells without any intracellular spaces. The cells of endodermis opposite to protoxylem are thin walled and called passage cells which allow water to pass into the xylem vessels. The radical cell wall contains water impermeable waxy layer of suberin as the Casparian strips in the form of bands. Suberin being water proof prevent the flow of water and salt through it and thus creating & maintaining physical force called root pressure.
4. Stele: It forms the inner cylinder all the tissues on the inner side of endodermis such as pericycle, vascular bundle & pith form the stele. Stele is bound by pericycle & encloses vascular bundle.
   • a) Pericycle: It is single layer consisting of thin wall cells. A few cell may become sclerenchymatous.
   • b) Conjunctive tissue: They are loosely arrange parenchyma cell found in between vascular bundle. They stores water.
   • c) Vascular bundle: They are radial, polyarch, exarch. Large number more than six; xylem and phloem are orange in groups alternate to each other.
   • d) Pith: It consist of parenchymatous cells filling of the centre. This cells contain starch grains. Monocot have large pith.

Internal structure of dicot root:

A young dicot root is called primary root. It is generally cylindrical in shape.

1. Epiblema or piliferous layer: It is the outermost layer of root. It is made up of compactly arranged cells. A few cells elongates to form root hairs (absorb water).
2. Cortex: It is a simple multilayer consist of thin wall leaving with numerous intracellular spaces. It fills the space upto endodermis.
3. Endodermis: It is single layer. The cells barrel shaped compactly arranged without any intracellular spaces & poses Casparian thickening in their radical and tangential walls. The cells outside the protoxylem don't have thickening and called passage cell. The endodermis controls the movement of fluid air from outside to interior & vice versa.
4. Pericycle: It is single layer consisting of thin walled cells.
5. Vascular bundles: They are radial, xylem and phloem are arranged in alternate radii. They are surrounded by parenchymatous conjunctive tissue. The xylem is exarch and protoxylem faces toward periphery. The number xylem stands are 4.
6. Pith: It is located centrally and it is very small & parenchymatous.

Anatomy of Dicot Stem

A typical dicot stem shows the following internal structure from periphery to centre:

1. Epidermis: It is the outerlayer of the stem consisting of thin & flatten compactly arrange single layer cells. The outerwall of the cells are highly cutinized or coated with wax. The single layer of cell may bears multicellular hair and few stomata. Thus it protect the tissue from injury as well as diseases cause by the entery of fungal & bacteria. It also prevent loss of water.
2. Cortex: It is the layer just below the epidermis and extend upto endodermis. It consist of:
   • a) Hypodermis: It is the form of few layer just below the epidermis consisting of collenchymatous cells. The intercellular spaces are thickened by cellulose. The cells contains chloroplast and it provides mechanical support and flexibility to the plant.
   • b) General Cortex: This cell it present just below the hypodermis and consist up of few layers of parenchymatous cells. The cells are thin walled, larger round & may contain chloroplast. They have large intracellular spaces & store food and it also consist of resin canal.
   • c) Endodermis: It is present just beneath the general cortex in the form of single layer barrel shaped. The cell are compactly arranged with intercellular spaces & form wavy layer operating cortex from vascular tissue. The cells of endodermis contain starch hence the layer is called starch sheath.
3. Vascular bundle: It is in the form of central cylinder which is also called stele and it differentiate into 4 regions:
   • a) Pericycle: It lies below the endodermis and phloem of vascular bundle in the form of semi lunar patches. It consist of sclerenchymatous cell with lignified cell walls and it lies just about the vascular bundle is know as bundle cap or hard bast.
   • b) Medullary ray: They are rose of radial elongated, parenchymatous cell present in between the vascular bundle. They transport water & food material.
   • c) Pith: It form the centre of the stem and consist of thin walled, rounded parenchymatous cell with large intercellular spaces. This cell may serve to storage food materials.
   • d) Vascular bundle: These are arranged in ring in wedge shape (eustelic condition). These are conjoint & open. Each bundle has phloem toward outside & xylem toward inside with cambium present in between. The bundle have protoxylem toward the centre hence endarch arrangement.

Anatomy of Monocot Stem

A typical monocot stem show the following internal structure from periphery to centre:

1. Epidermal tissue system: The epidermis is outermost single layered protective covering made up of living parenchymatous cell. The outer wall of epidermal cell are highly cutinized. A few stomata may be present but the hair of usually absent.
2. Ground tissue system: In case of monocot the cortex, endodermis, pericycle and pith are absent. The entire mass from hypodermis to centre is called ground tissue system.
   • a) Hypodermis: It is in the form of 2-3 layer of lignified sclerenchymatous cells just below the epidermis. These are provide mechanical strength to the stem. [It is absent in asparagus and weeds]
   • b) Ground tissue: It is in the form of thin wall parenchymatous cell filling the entire upto the centre. A number of vascular bundle are scattered in the ground tissue.
3. Vascular bundle: They are numerous and scattered in parenchymatous cell ground tissue. They are small toward periphery & larger toward centre. Each vascular bundle conjoint, collateral, endarch and closed. It is surrounded by sclerenchymatous ground tissue. The protoxylem & metaxylem arranged in V or Y shaped. Due to the break down of inner protoxylem & parenchyma during earlier stages of growth, there is water containing cavity called lysigenous cavity.

Secondary Growth

Apical meristem undergoes division & differentiation as a result primary tissue is form. This causes the increases in the length of the plant which helps in primary growth of plant. Similarly, secondary meristem undergoes division and differentiation form secondary tissue which increase girth or width of the plant. Normally dicot and gymnosperms shows secondary growth but it is absent in monocot plant. Some monocot shows abnormal secondary growth. Eg: Dracaena, Yucca, Agave, Palm & Smilax. The tissue involved in secondary growth are two lateral meristems. These are:

1. Vascular Cambium
2. Cork Cambium

The secondary growth occurs due to the addition of secondary tissue by the action of vascular cambium & cork cambium.

• Vascular cambium adds secondary tissue (xylem and phloem).
• Cork cambium adds secondary tissue in the cortex region.

1) Vascular Cambium

It including following steps:

1. Formation of vascular cambium: The dicotyledon stem characterizes by the presence of cambium in between primary xylem + phloem in each vascular bundle. It is called fascicular cambium. These cambium is derived from procambium of shoot apical meristem.
2. Formation of Cambium: Secondary growth occurs by the formation of new cambium in the primary medullary rays (parenchymatous cell). The primary medullary rays become meristematic and develops a new cambium called interfascicular cambium. The intrafascicular and interfascicular cambium joins both each other to form complete ring called vascular cambium or cambium ring.
3. Activities of Cambium ring: The vascular cambium produces secondary phloem to the outside & secondary xylem to the inside by mitotic division. The amount of secondary xylem produce is much more than the secondary phloem because cambium is more active to the inner side than outside. The cambium cells gradually to increase the growth in circumference tangentially and sec. xylem to toward the pith.
4. Formation of secondary medullary rays: The secondary xylem occupies major portion of the stem forming a hard compact mass. At some places the cambium forms a narrow band of parenchyma which passes through the sec. xylem and sec. phloem in radial direction. These are secondary medullary rays (living cells). They forms a connection to transport of water & minerals and food radially across the stem and also store food during winter.

Formation of Annual Rings: The vascular cambium remains active throughout the life of the plant. But its activities get affected by the seasonal various. Hence annular rings are more distinct in temperate region then in tropical region. In autumn when the plant do not required active transport of nutrients and ascent of sap, the xylem element remains compact and develop thick wall with the narrow lumen. So the band of xylem produces in the season is called autumn wood. On the other hand the plant need active transport of nutrient in the spring seasons. The cambium grows much actively in this season & produces broad thin walled xylem with wide lumen is called spring wood. Thus the two bands of secondary xylem i.e. autumn and spring wood are used in one year and are called annual ring / growth ring. So we can find out age of tree by counting the number of annual ring. This study is known as dendrochronology.

Difference between Spring wood and Autumn wood:

Heart wood and Sap wood: In the older tree two types of wood can be recognized: heart wood and sap wood.

Tyloses: Bladder or ballon like in growth of xylem parenchyma into vessels or tracheids through pith. It blocks the lumen of tracheary element so heart wood has tyloses that is why they are not involved in conduction.

Formation of Cork Cambium and Phellogen

As the secondary growth continuous for several years the stem continuously grows in thickness due to increasing secondary vascular tissue. This increase may result in cracking and breaking other tissue. To avoid of such of breaking of external tissue the plant develop a few cambium ring called cork cambium (phellogen) in the outer region. The cork cambium is secondary lateral meristem which develops from permanent tissue in the region of epidermis, hypodermis & cortex. It produces death cell or cork towards outer side & living cell of secondary cortex (phelloderm) toward inner side. This layer of cork, cork cambium & secondary cortex constitution the protective covering called periderm. The cork cell are highly lignified and checks the lose of water from the general surface of stem. Sometime layers of cork are interrupted by lenticels. At certain region the cork cambium cuts of closely arranged parenchymatous cell on the outer side. These parenchymatous cell soon ruptures the epidermis forming lens-shaped opening called lenticels. Lenticels allow the exchange of gases between the outer atmosphere and inner tissue of the stem. Lenticels occurs in the most of the woody trees. Presence of woody stem, respiratory roots. It may also found in some fruits & absent in leaves.

Anatomy of Leaf

The basic function of leaf is to carry photosynthesis that is convert solar energy into chemical energy in the food. They also carry out transpiration out the stomata. It is estimated that only about 1% of water absorb is used for photosynthesis and rest 99% escapes from leaves in the form of water vapour.

Types of leaves: The leaves are of 2 types depending on the arrangement of tissue:

1. Dorsiventral leaf (Bifacial): The leaves possesses distinct dorsal & ventral surface are called dorsiventral. The palisade is usually present below upper epidermis. Eg: present mostly in dicot plant.
2. Isobilateral leaf (Equifacial): The leaves in which upper and lower surface are similar are called isobilateral leaf. The mesophyll is not differentiate into palisade and spongy parenchyma. Eg: present mostly in monocot only.

Anatomy of Dorsiventral leaf (Dicot)

A dorsiventral leaf receives more sunlight on the upper surface and this account for the specific distribution of tissue within leaf. Internally they show 3 distinct tissue system.

1. Epidermis: It covers both the upper and lower surface of leaf.
   • a) Upper Epidermis: It covers the upper surface and consist of single row of large and flatten cell. The outer surface is covered by a thick layer of wax called cutin. The cell lacks chloroplast & the stomata are either absent or widely dispersed.
   • b) Lower Epidermis: It covers the lower surface of leaf. It has thin layer of cuticle and has minute opening called stomata. Each stomata is surrounded by two kidney shaped cell called guard cells.
2. Mesophyll: It consist of several layer which contains chloroplast. It is differentiate into 2 layer:
   • (i) Palisade Parenchyma: It consist of one or more layers of tightly packed elongated cells. The cells are arranged in a row & contains many chloroplast.
   • (ii) Spongy parenchyma: It consist of more or less row of irregularly arranged cells. The cells contains less chloroplast. They have large number of air spaces by which diffuse of gases occurs.
3. Vascular bundle: These are arranged all along the central vein called midrib and veins of leaf. In each bundle xylem lies toward the upper epidermis and phloem is found toward to lower epidermis. Conjoint, collateral, endarch, closed types of vascular bundle are present. The bundle sheath is mostly parenchymatous (thick walled). Sometime sclerenchymatous but the bundle sheath extension is parenchymatous without chloroplast.

Anatomy of Isobilateral leaf (Monocot)

Like dicotyledonous leaves, monocot also possess three distinct tissue system.

1. Epidermis: Each leaf has upper and lower epidermis made up of single layer of compactly arranged cells. The outer cell of epidermal cell are coated with thick or thin cuticle. Here there equal number of stomata present on both the surface of epidermis hence it is called Amphistomatic condition.

   Note: Some cell of epidermis bear large bubble like cell in addition to ordinary epidermal cells. These bigger cells are called bulliform or motor cells. These cells help of rolling & unrolling of leaves to regulate loss of water.

2. Mesophyll: The bulk of tissue line between upper epidermis and lower epidermis is called mesophyll. All the cells of the mesophyll contains chloroplast & perform photosynthesis. The cells are separated by intercellular spaces and are interconnected to open into substomatal chamber to maintain continuity with external atmosphere through stomata.
3. Vascular bundle: The monocot leaves are characterized by possessing parallel venation. Vascular bundle are conjoint, collateral, endarch, closed and surrounded bundle sheath. The bundle sheath is connected with upper & lower epidermis by group of compact of parenchymatous and sclerenchymatous called bundle sheath extension. Each vascular bundle contains of xylem facing toward upper epidermis and phloem toward lower epidermis.