PLANT TISSUES & ANATOMY

Plant histology is the branch of botany dealing with the study of tissues. A tissue may be defined

as an aggregation or grouping of cells (similar or dissimilar) specialized to perform a particular

function.

Nehemiah Grew is called “Father of Plant Anatomy”. It was he who coined the term tissue. The

pioneers in plant anatomy were Nehemiah Grew and Marcello Malpighi

General Classification of Plant tissues:

Meristematic Tissue (Meristem):

In Greek meristos means divisible. Meristematic tissue is composed of similar cells that are

capable of dividing and forming new cells. Karl Nageli used the term in 1858.

Characteristics:

1. Meristematic cells are immature

and capable of dividing.

2. Cells are small, undifferentiated

and embryonic.

3. They are actively metabolizing

cells.

4. Cells are arranged compactly

without intercellular spaces.

5. Neighboring cells are interconnected by plasmodesmata.

6. Cells are isodiametric in shape, but may be polygonal, oval, rectangular in shape

7. The cell wall is thin and made up of cellulose.

8. The cytoplasm is dense and nucleus is large and prominent.

9. Vacuoles are absent, if present then inconspicuous.

10. The endoplasmic reticulum, mitochondria etc. are less elaborate.

11. The plastids are in the pro-plastid stage.

12. Ergastic substances are absent.

Classification of Meristems:

I. Based on Origin and development:

a) Pro-meristem or Primordial meristem or Embryonic meristem:

i. It is a group of earliest and youngest meristematic cells occupy a growing organ in

shoot and root apex.

ii. By the process of cell division pro-meristem produces the primary meristem.

Ex: Shoot apex, root apex

b) Primary meristem:

i. It is derived from pro-meristem.

ii. Cells produced by them differentiate into primary permanent tissues

iii. They constitute the primary parts of the plant.

Ex: Intercalary meristem, intra-fascicular cambium.

c) Secondary meristem:

i. It is derived from primary permanent

tissues.

ii. It takes place from during the

secondary growth of the stem and root.

Ex: Inter-fascicular cambium, cork

cambium.

II. Based on Position:

a) Apical Meristem:

i. Occupies the apes of roots and stems

ii. Responsible for increase in length i.e.

primary growth

iii. In lower plants like bryophytes and

pteridophytes the apical meristem is

represented by an apical cell where as

in higher plants apical meristem is

represented by group of cells called

apical initials.

iv. Tissues derived from apical meristem are called primary permanent tissues.

Ex: Shoot and root apices

b) Intercalary meristem:

i. It is derived from apical meristem and inserted between two masses of permanent

tissues.

ii. They become separated from the main body of the meristem by the formation of

permanent tissues in between them.

iii. It is short lived.

iv. Their division is responsible for a rapid elongation of the stem that occurs just before

flowering.

v. Tissues derived from the intercalary meristems are also called primary permanent

tissues.

• Intercalary meristem occurs at the base of the leaf. Ex:Pinus.

• At the base of each internode (above the node): Ex Grasses, Equisetum.

• Occur below the node of mint (Mentha)

c) Lateral meristem:

i. Occupy a lateral or peripheral position i.e. parallel to the sides of the organs.

ii. Tissues derived from them are called secondary permanent tissues.

Ex: Intra-fascicular cambium. inter-fascicular cambium, cork cambium(phellogen).

III. Based on Function:

a) Protoderm:

i. It is the outermostlayer of

apical meristem. It

differentiates to produce

epidermis.

b) Ground meristem:

i. It lies between the

protoderm and pro-

cambium.

ii. Its division

productsdifferentiate into

fundamental or ground

tissue system and that

consists of hypodermis,

general cortex, endodermis, pericycle, medullary rays and pith.

c) Pro-cambium:

i. It occupies the center and consists of parallel strands of cells.

ii. It isresponsible for producing primary vascular tissues (xylem and phloem) and

cambium.

Simple Permanent Tissues:

Permanent tissues are the differential or mature tissues that have completed its growth and are

usually incapable of meristematic activity.

The process by which cells produced by meristematic tissue, undergo various physiological and

morphological changes during maturation in order to become specialized for a particular

function is called differentiation.

or

Differentiation is a progressive change leading to specialization.

Permanent tissues are classified into two types:

1) Simple permanent tissues: Cells in the simple permanent tissues are of same type

structurally and functionally

Ex: Parenchyma, collenchyma, Sclerenchyma

2) Complex permanent tissues: Consists of several types of cells which differ from one

another structurally and for functionally

Ex: Xylem and Phloem

Simple permanent tissues:

1. Parenchyma:

a. Parenchyma is the most basic least specialized tissue and common in all plants. Hence

it is called the fundamental tissue.

b. The cells are isodiametric, oval, rectangular or sometimes polyhedral.

c. Cells are loosely arranged with intercellular spaces.

d. Cells are thin walled with prominent nucleus and peripheral cytoplasm.

e. Cells possess central large vacuole.

f. Cell wall composed of cellulose.

Distribution: It is present in the soft parts of plants, cortex and pith of stems and roots, mesophyll

of leaves, fleshy part of fruits, flowers, and endosperm of seeds. It is also associated with xylem

and phloem as xylem parenchyma and phloem parenchyma respectively.

Types of Parenchyma:

1. Chlorenchyma:

a. They are the parenchyma cells with

chloroplasts.

b. Chlorenchyma usually present in the

mesophyll tissue of leaves.

c. They are also present in the outer cortex

young stem.

d. The green color of young stem and aerial

roots are due to the presence of

chlorenchyma.

e. Two types of chlorenchyma are found; palisade parenchyma- consists of compactly

arranged cylindrical cells, spongy parenchyma- consists of loosely arranged parenchyma

cells.

f. Chlorenchyma cells carry out photosynthesis.

2. Aerenchyma:

a. Made up of network of parenchyma cells enclosing large air cavities.

b. It is common in aquatic plants (Hydrilla, Eichhornia etc.) and a few land plants (Petiole

of Canna, Banana pseudo stem)

c. Air cavities of aerenchyma can store air.

d. In aquatic plants, aerenchyma assists in floating i.e. buoyancy

e. Respiratory gases (CO2 or O2) stored in aerenchyma can be utilized by the plants when

required.

Idioblasts: These are specialized storage parenchyma which helps in the storage of excretory

materials like oils, tannins crystals etc.

Prosenchyma: They are thick walled, elongated parenchyma. They possess interpenetrating

tapering (pointed) ends. Prosenchyma is usually present in the vascular tissues of higher plants.

Functions of Parenchyma:

1. Parenchyma forms the fundamental ground tissue system in plants.

2. Parenchymatous cells can store water and mucilage (Opuntia, Aloe)

3. They can store reserve food materials as starch grains (Potato), protein grains (pulses) or

oil droplets (endosperm of coconut).

4. Chlorenchyma of leaves and young stem assist in photosynthesis.

5. Aerenchyma in aquatic plants provides buoyancy. The air cavity also helps in gaseous

exchange.

6. Epidermis is made up of parenchyma cells and provides protection.

7. Parenchyma associated with and xylem and phloem helps in transport of water and food

substances.

8. Parenchyma cells help in healing of wounds and take part in regeneration.

9. Meristematic activities of parenchyma in stem cutting results in the development of buds

and adventitious roots i.e. they act as packing tissue.

2. Collenchyma tissue: The term collenchyma was coined by M. Schleiden in 1839. In Greek

kolla means glue.

a. Collenchyma is called the living mechanical tissue.

b. They are elongated longitudinally and have

pointed or oblique ends.

c. They appear oval, polygonal or circular in T.S.

d. The cell walls are thickened at the corners or

other regions (tangential wall) in an uneven

manner.

e. The thickening is due to the deposition of

hemicellulose and pectin, with high amount of

water.

f. Collenchyma cells have peripheral cytoplasm and small nucleus surrounding a large

central vacuole.

g. Very rarely collenchyma cells may possess chloroplasts.

h. Collenchyma is absent in roots and completely absent in monocots.

Distribution: They are found in the hypodermal region of stem (Sunflower, Eupatorium), also

occurs in petioles, pedicels, peduncle, sometimes in leaves on either sides of veins ( dicot leaf).

They are rarely present in monocots and roots.

Types of Collenchyma: Three types of collenchyma are present based on pectin deposition.

1. Angular Collenchyma:

a. Here the cells possess thickenings at corners i.e. the angle between the cells.

b. Cells are compactly arranged without intercellular spaces.

c. The thickenings are like pillars providing a great mechanical resistance to the plant body.

d. They are usually found below the epidermis forming a region called hypodermis.

Ex: Stem of Datura, Ficus etc.

2. Lacunar Collenchyma:

a. The cells are either oval or spherical and enclose a small intercellular space.

b. The deposition occurs only along the borders of intercellular spaces.

c. It is found in fruit wall.

Ex: Cucurbita, Malva etc.

3. Lamellar Collenchyma:

a. The thickening is found in the tangential walls only.

b. Cells are compactly arranged.

Ex: Stem of Sunflower, Eupatorium

Functions of Collenchyma:

1. Its primary function is to provide the mechanical support. Hence is described as the

living mechanical tissue.

2. The cells possess some degree of elasticity hence also provide tensile strength to

growing regions.

3. Collenchyma with chloroplasts carries out photosynthesis.

4. The cells undergo dedifferentiation and become meristematic that helps in wound

healing and formation of secondary tissues.

3. Sclerenchyma: The term

sclerenchyma was coined by

Mettenius in 1865. In Greek skleros

means hard.

a. It is a dead mechanical tissue.

b. The cells are thick walled, due to

deposition of secondary wall

material lignin. The deposition of

lignin is called lignification.

c. The cells lose their protoplasts

and die at maturity

d. It encloses a cavity called lumen.

e. At certain places of the cell walls

there are depressions called pits,

where there no lignification.

Types of Sclerenchyma:

There are two types of Sclerenchyma

tissue; Sclerenchyma fibres and sclereids

1. Sclerenchyma fibres:

a. The cells are dead, spindle shaped with lignified walls.

b. Simple pits are present.

c. They possess narrow lumen.

d. Fibres are arranged vertically and compactly without intercellular spaces to form strands

or bundles.

e. In cross section, the cells appear mostly polygonal in outline and highly lignified walls

with simple pits.

Distribution: The fibres are commonly found in the form of strands below the pericycle of dicot

stem. They are also present in xylem, phloem, cortex and pith. The mesocarp of fruits is formed

of fibres (coconut), and seed coat (cotton).

Types of Fibres:

i. Intraxylary fibres/ Wood fibres / Xylary fibres: Sclerenchyma associated with xylem is

called intraxylary fibres.

ii. Extraxylary fibres: Sclerenchyma fibres which occurs in any tissues other than xylem.

Ex: phloem fibres, pericycle fibres, cortical fibres etc.

2. Sclereids:

a. Sclereids are reduced form of sclerenchyma cells with highly thickened, lignified walls

that form small bundles or durable layers of tissue in plants.

b. Sclereids variable in shape and size.

c. Cells may be isodiametric, columnar, hair-like, bone-like, star shaped, or irregular in

shape.

d. Cell wall is very thick, hard and strongly lignified showing concentric layering and

lumen is very much reduced.

e. Pits may be simple or branched canal like cavities.

f. Mature sclereids are dead cells without protoplast.

Types of Sclereids: Depending on the shape the Sclereids are classified as following;

i. Brachy-sclereids/ Stone cells:

a. The cells occur in groups and resemble the heap of stones.

b. They are more or less isodiametric

c. Found in soft parts of plants like cortex, pith, and phloem.

Ex: Pulp of custard, pear, and guava

ii. Macro-sclereids/ Rod cells/ Malphigian cells: Cells are elongated, rod shaped or columnar

forming a palisade layer. Ex: Seed coat of pea, Bengal gram etc., epidermis of onion,

leaves of xerophyte.

iii. Osteo-sclereids/ Prop cells: Bone shaped cells with dilated ends. Ex: Seed coats of

monocots and dicots, leaves of xerophytes.

iv. Astero-sclereids/ Stellate cells: Star shaped cells with many lobes or arms. Ex: Intercellular

spaces of tea, Stem of hydrophytes (Nymphaea).

v. Tricho-sclereids/ Trichoblasts/ Internal hair: Long hair like, sparingly branched (usually

trilobed) cells. Ex: Stems and roots of Monstera, Leaves of Olea, leaves of Nymphaea.

Functions of Sclerenchyma Fibres:

a. Provide strength, support, and rigidity to the plant and plant organs in which they are

found.

b. They can withstand compression and pull; it acts as dead mechanical tissue.

c. It occurs in the form of sheath around vascular tissues in monocots.

d. Long sclerenchyma fibres are of commercial importance- Phloem fibres (Corchorus

capsularis, Cannabis sativa) form raw material in Jute industry. Mesocarp of coconut

used in coir industry.

Functions of Sclereids:

a. Provide firmness and hardness to the plant parts in which they are found.

b. Gives grittiness in pulp of some fruits.

Complex Permanent Tissues

The complex permanent tissues are composed of several different types of cells but remain

together in specific positions in the plant body to perform a specialized function. The two

complex permanent tissues are xylem and phloem. They are together described as vascular or

conducting tissues.

The tissue associated with conduction of water, minerals and food materials in plants are called

vascular tissue. Plants with a well-developed conductive system are thus called as vascular

plants. Vascular plants are also known as tracheophytes (‘trachaea’ = vessels, a component of

xylem, ‘phyta’= plants).

The vascular systems in plants composed of two types of tissues;

1. Xylem: Tissue for the conduction of water and minerals.

2. Phloem: Tissue for the conduction of food materials.

Both the xylem and phloem are complex tissues composed of more than one types of cells.

Xylem and phloem are closely organized in plants. The vascular bundles found in the primary

structures of plants are formed by the association of xylem and phloem.

1. Xylem:

The xylem is one of the conductive tissues in plants. It is a complex tissue composed of many

types of cells. The term xylem was proposed by Nageli (1858) and he derived the word from

a Greek word ‘xylos’ meaning wood. The main function of xylem is to conduct water and

minerals from roots to leaves. The secondary xylem also provides mechanical support due to

the presence of thick lignified cell wall.

The xylem composed of four types of cells. Among these cells, some cells are living and

some are dead. The four elements of xylem are:

i. Tracheids

ii. Xylem Vessels/ Trachea

iii. Xylem Fibres

iv. Xylem Parenchyma.

Of these first three are dead and the latter is living.

i. Tracheids:

a. Tracheids are the fundamental cell type in the xylem. They are elongated tube like cells

with tapering ends and chisel like in appearance.

b. The cells are non-living at maturity

and the mature cells are empty

without protoplast.

c. They have highly lignified

secondary cell wall and the cells

appear angular/oval/ polygonal in

cross section. The average length of

tracheid is 5 – 6 mm.

d. Major portions of the cell wall of

tracheids are perforated with pits.

e. They also possess pit pairs between

two adjacent tracheids at their

common walls.

f. Pits may be simple circular pits or

advanced bordered pits.

Distribution of Tracheids among plants:

• Tracheids are the only xylem

element in Pteridophytes. In

Gymnosperms, major portion of the

secondary xylem composed of

tracheids.

• In Angiosperms, tracheids occur

with other xylem elements. In some

primitive Angiosperms such as Drimys, Trochodendron, Tetracentron, the xylem

composed only of tracheids (vessels absent).

Patterns of Secondary thickening/ Lignification in Tracheids:

The secondary cell wall material (lignin) is laid down on the lateral walls of the tracheids in

specific patterns (lignification). The most common patterns are the following types:

1. Annular Thickening: Secondary wall thickening occurs as rings arranged one above the

other. Annular thickening is considered as the most primitive type of wall thickening.

2. Spiral Thickening (Helical Thickening): Here the secondary wall materials are deposited

in the form of spirals along the inner wall of the tracheids.

3. Scalariform Thickening (Ladder like Thickening): The wall materials are deposited as

transverse bands along the wall. The bands are with few interconnections.

4. Reticulate Thickening (Net-like Thickening): Here the wall thickening pattern is net- like

(reticulate).

5. Pitted Thickening: It is the most advanced type of secondary wall thickening in tracheids.

Here, the secondary wall materials are evenly distributed over the inner portion of the cell

and the cell wall looks more or less uniform in their thickness with few un-thickened

circular areas called pits. Many pits are distributed over the cell wall. Nature and

arrangement the pits vary in different plant groups. The pits may be circular or elongate

bordered type.

Types of Pits:

a. Simple pits: These are advanced form of pits found in angiosperms. In the simple pits, the

pit cavity remains of the same diameter and the pit or closing membrane also remains

simple and uniform in its structure.

b. Bordered pits: They are abundantly found in tracheids of confers and vessels of

angiosperms. They are more complex and variable in structure than simple pits. It has an

overarching secondary wall which encloses the part of the pit cavity called pit border which

opens to the outside by a small round opening called pit aperture.

The space within the pit borders is called the pit chamber or cavity. At the centre of the pit

membrane there is a swollen thickened structure made of middle lamella called torus.

ii. Xylem Vessels

a. Vessels (also called as trachea) are the second category of xylem elements composed of

short and tube like cells with oblique ends.

b. Vessels are arranged as a series in an end to end fashion to the long axis of the organ in

which they occur.

c. Components of the vessel are called vessel segments or vessel element.

d. Each vessel elements are

shorter than tracheids in their

length; however, the

diameter of the vessel lumen

is much larger than that of

tracheids.

e. The cells are non-living and

they are devoid of protoplast

at maturity. Numerous pits

are present in the lateral

walls of the vessels for

communication.

f. Even vessels exhibit

lignification in the same

pattern as that of tracheids.

g. The each end wall of the

vessel possesses plates with

pores called perforation

plates.

Distribution of vessels among plants:

• Xylem Vessels occur mainly in the xylem of Angiosperms. Usually, vessels are absent in

Pteridophytes and Gymnosperms.

• The wood of Gnetum, an advanced Gymnosperm, contains plenty of vessels.

• Usually perforations occur at the end wall, sometimes lateral perforations also occur on

the walls.

iii. Xylem Fibres:

a. Xylem fibres are the third non-living components of xylem and it is also called xylary

fibres. They do not contain protoplast at maturity.

b. Cells are with very thick lignified secondary cell wall.

c. The main function is to provide mechanical support.

iv. Xylem Parenchyma:

a. Xylem parenchyma is the fourth component of xylem. It is the only living component in

the xylem.

b. The cells are with plenty of cytoplasm and prominent nucleus. They have thin cellulosic

cell wall.

c. Lignified secondary cell wall is absent in xylem parenchyma.

d. Very rarely parenchyma cells in the secondary xylem undergo secondary growth.

e. Parenchyma in the xylem can store starch, oil and other ergastic substances.

Tyloses: Tyloses are the outgrowth of xylem parenchyma to the lumen of tracheids or vessels of

the secondary xylem through pit openings.

• Tyloses accumulate resins and other secondary materials in their protoplasm.

• They are responsible for the characteristic odor of wood.

• They also prevent the degradation of wood by termites and mites.

Functions of Xylem:

a. Conduction of water from roots to leaves

b. Conduction of minerals and nutrients from roots to leaves

c. Provide mechanical support

d. Ray parenchyma forms tyloses which store ergastic substances

e. These ergastic substances give the wood a characteristic color and odor

f. Ergastic substances present in the tyloses also protect the wood from termites and

Classification of Xylem:

i. Based on origin xylem is classified into two types:

a. Primary Xylem: It is produced during the primary growth of the plant from the pro

cambium of the apical meristem.

b. Secondary Xylem: It is produced during the secondary growth from the vascular

cambium. The secondary xylem is mostly of pitted vessels.

Primary Xylem is further classified into;

▪ Protoxylem: The first formed xylem is named as protoxylem. Its tracheary elements are

smaller and narrower and usually have annular and spiral thickening.

▪ Metaxylem: the later formed xylem is called metaxylem. Its tracheary elements are

larger and wider. They usually have scalariform and pitted thickening.

ii. Based on the position of the protoxylem classification is as follows;

a. Endarch: If the protoxylem is directed towards the centre and metaxylem towards

periphery due to centrifugal growth the xylem is named as endarch. It occurs in stem

b. Exarch: If the protoxylem is directed towards the periphery and metaxylem towards

centre due to centripetal growth the xylem is named as exarch. It occurs in roots.

2. Phloem:

Phloem is a complex tissue system in plants. It is the food conducting tissue of vascular

plants. Together with xylem, they form the vascular tissue system. The phloem composed of

several types of cells among which some are living cells and some are dead.

The term ‘phloem’ was introduced Nageli (1858) from a Greek word ‘phloios’ meaning

‘bark’. The ‘bark’ is a non-technical term describing all tissue outside the secondary xylem

of the plant. Botanically the bark includes secondary phloem, cortex, primary phloem and

periderm. The current post describes the structure, composition and classification of phloem.

Phloem is a complex tissue, composed of 4 types of cells (phloem elements);

i. Sieve elements

ii. Companion cells

iii. Phloem parenchyma

iv. Phloem fibres

Of these first three are living and the latter is dead.

i. Sieve elements:

a. Sieve elements are the fundamental cell type in the phloem. They are the food conducting

cells in the phloem.

b. They are living cells in the phloem with protoplasm. Nucleus is absent in the sieve

elements

c. Lignified secondary cell wall is absent in sieve elements.

Two types of sieve elements are found in the phloem;

A. Sieve cells:

o Sieve cells are less specialized and primitive type of sieve elements.

o They occur in the phloem of Pteridophytes and Gymnosperms.

o They are absent in Angiosperms.

o They are elongated cells with steep inclined end walls. The sieve areas are located on

the lateral wall of sieve cells.

o Very rarely the end wall also possesses sieve areas.

B. Sieve tubes:

o They are advanced type of sieve elements.

o They occur in the phloem of Angiosperms.

o They are longitudinal series of cells connected one above the other.

o Each cell is called sieve tube member or sieve tube element.

o In sieve tubes, the sieve area is located on the end wall called sieve plate. The sieve

area in sieve tubes is more specialized and advanced than sieve cells.

o The end wall of sieve tubes may be inclined to transverse in their arrangement.

Structure of Sieve tube elements:

a. Sieve tubes are with primary cell wall only. However, the cell wall is much thicker than

simple parenchyma. The cell wall appears as very thick and shining in cross sections.

b. Due to the shining appearance of the cell wall, they are called as nacreous layer.

Nacreous wall consists of cellulose and pectic substances as lamellate structures.

c. They do not contain lignin in their wall.

d. The wall area of sieve tube with pore is called sieve area.

e. Adjacent sieve tubes are interconnected by cytoplasmic strands through the pores. The

sieve pores are comparable with primary pit fields on the parenchymatous cells.

f. The width of sieve pore may reach up to 15μm.

g. Wall portion bear highly specialized sieve area is called sieve plate. The sieve plate is

similar to the perforation plates in xylem vessels.

h. Two types of sieve plates occur in phloem: Simple sieve plate: a sieve plate with single

sieve area Compound sieve plate: a sieve plate with many sieve area

i. Surface of sieve area looks like a depression with numerous dots.

j. The dots are protoplasmic materials which pass through pores called pore contents. The

cell wall areas between pore are called ‘bars’.

k. Each pore is lined with a specialized carbohydrate called callose. During the ageing of

sieve tubes, excessive deposition of callose takes places forming callose plugs.

Cell content of Sieve tubes:

a. The cells of sieve tubes undergo many changes during development and maturation.

Nucleolus of the cell degenerates.

b. Endoplasmic reticulum becomes smooth and regenerates into stacks. By the end of the

maturation, ER completely disorganizes.

c. Dictyosomes and ribosomes in the cells disappear. Only plastids and mitochondria are

retained in the cells as cell organelles.

d. Two types of plastids are seen in the sieve tubes: One type of plastids accumulates starch

as small grains. The other type of plastids accumulates plenty of proteins.

e. Plasmodesmata are present in the sieve tubes for cell to cell communications.

f. The tonoplast (membrane of vacuoles) disappears during the maturation of sieve tubes

g. Sieve elements have proteinaceous substances called P-proteins (phloem specific

proteins).

ii. Companion cells

a. Companion cells are specialized parenchymatous cells associated with the sieve tubes of

Angiosperms.

b. Sieve tube member and companion cell arise from common mother cell. Number of

companion cell per sieve tube member varies (one to many).

c. Companion cells are narrower than sieve tube elements. The cells are thin walled cells

with only primary cellulosic cell wall.

d. They contain a large and prominent nucleus. They connect to sieve tube elements through

plasmodesmata. They contain many plastids and ribosomes.

e. Companion cells are metabolically very active. Physiologically it is associated with sieve

tube members Companion cells provide energy to sieve tube elements for phloem

transport.

f. Companion cells are absent in lower plants (Pteridophytes and Gymnosperms).

g. In Conifers and Ginkgo some phloem parenchyma cells are closely associated with sieve

cells. These cells are called albuminous cells.

h. There is no ontogenic relationship between albuminous cell and sieve tube cells (they are

not originated from a common mother cell).

iii. Phloem parenchyma

a. Parenchyma occurs in both primary and secondary phloem. They are thin walled cells

with protoplasm and nucleus.

b. They have primary pit fields on their cell wall. Some parenchyma cells in the phloem can

store starch as grains.

iv. Phloem Fibres:

a. They are dead cells. Possess lignified secondary cell wall. They have simple or slightly

bordered pits on their wall. In some plants, phloem fibres are very long.

b. Phloem fibres are also called as bast fibres. They are long, soft and silky in appearance,

hence have immense economic importance.

Classification of Phloem:

1. Based on the origin, two types of phloem occur as

a. Primary Phloem: Primary phloem is developed from pro-cambium. It is developed as

part of the primary growth of the plant.

b. Secondary phloem: Formed from vascular cambium during the secondary growth of the

plant. Secondary phloem fibres form the bast fibres in some plants. In Hevea brasiliensis,

the latex is obtained from the secondary phloem.

Two types of primary phloem are observed;

a. Protophloem: Protophloem is the first formed phloem in the plants. Sieve elements of

protophloem are narrow and elongated.

b. Metaphloem: Metaphloem is developed after protophloem. Metaphloem is the only food

conducting tissue in some plants (herbs). Metaphloem elements are with well

differentiated sieve areas.

Function of Phloem:

a. Conduction of food materials.

b. Phloem fibres provide mechanical support.

c. Ray parenchyma in the phloem can store ergastic substances.

Anatomy of Root, Stem and Leaf

1. Anatomy of Dicot Root: Dicot root shows following distinct region in its Transverse

section with following features:

a. Epiblema or Epidermis: It is the outermost uniseriate made up of thin walled, compactly

arranged tubular living parenchymatous cells. Usually epiblema is characterized by

absence of stomata and cuticle. Some or most of the epidermal cells have tubular

extensions on their outer surfaces called root hairs. Due to presence of unicellular root

hairs it also helps in absorption of water and minerals from soil. Epiblema provides

protection.

b. Cortex: It is extensive made up of thin walled, multilayered region of polygonal

parenchymatous cells. They usually have intercellular spaces. The cortical cells have no

chloroplast but may contain leucoplast for storage of starch grains. The cortex is

responsible for transportation of water and salts from the root hairs to the center of the

root.

c. Endodermis: It is the innermost layer of cortex and covers the stele. It consists of

compactly arranged barrel shaped parenchyma without intercellular spaces. Most of the

cells are characterized by the presence of special thickening of suberin and lignin on their

radial and tangential walls called casparian strips. These cells allow radial diffusion of

water and minerals through the endodermis. Endodermis acts as a barrier between cortex

and the vascular cylinder.

d. Pericycle: It is the outermost layer of stele and composed of uniseriate layer of

parenchymatous cells without intercellular spaces.

e. Vascular bundles: They are 2-8 in number, radially arranged and closed (no cambium).

Xylem and phloem bundles are separated from each other by parenchymatous cells called

conjuctive or complementary tissue.

a. Xylem is exarch (i.e. protoxylem towards the periphery and metaxylem towards

the centre) and consists of tracheids, vessels, xylem parenchyma and xylem fibres.

b. The phloem forms oval masses beneath the pericycle, alternating with xylem

bundles. Phloem consists of sieve tubes, companion cells and phloem

parenchyma. Usually phloem fibres are absent or reduced.

f. Pith: It is feebly developed and centrally located. It consists of thin walled, polygonal

parenchyma cells with intercellular spaces. In dicots roots, it may be reduced or absent. It

helps in storage of food materials.

2. Anatomy of Monocot Root: T.S. of Monocot roots shows following distinct regions:

a. Epiblema is the outermost single layer made from compactly arranged parenchymatous

cells without intercellular space. Usually epiblema has no stomata but bears unicellular

epidermal root hairs. The root hairs and thin walled epidermal cells take part in the

absorption of water and minerals from the soil.

b. Cortex is a multi-layered well developed and made from oval parenchymatous cells with

intercellular spaces. The intercellular spaces usually help in gaseous exchanges, storage

of starch, etc.

In monocots and several old roots, few layers of cortex just below epiblema give rise to a

single or multilayered cuticularised sclerenchymatous region called exodermis. Cortex

helps in mechanical support to the roots (like hypodermis to stem).

c. Endodermis is innermost layer of cortex made from barrel shaped parenchyma. It forms

a definite ring around the stele. These cells are characterized by the presence of

casparian stripes. It is U-shaped deposition of suberin and lignin. However some cells

just opposite to the protoxylem remain thin-walled and retain their living protoplasts.

Such cells are called passage cells or transfusion cells.

d. Pericycle is uniseriate and made from thin walled parenchymatous cells. It is outermost

layer of stelar system. Usually it is made from parenchymatous cells but it may become

sclerenchymatous in older roots. Several lateral roots arise from this layer. Hence, lateral

roots are endogenous in origin.

e. Vascular bundle is radial, polyarch (presence of many alternating xylem and phloem

bundles). Xylem and phloem are found at different radii alternating with each other

(radial). The number of xylem and phloem vary from, 8 to 46 (100 in Pandanus). The

xylem is exarch, i.e. the protoxylem lies towards periphery and metaxylem toward center.

Conjunctive tissue is parenchymatous tissues which separates xylem and phloem bundles.

It may become sclerenchymatous in older roots.

f. Pith is large, well developed in monocot root. It occupies the central portion and made

from thin walled parenchymatous tissue with intercellular spaces. It contains abundant

amount of starch grains.

3. Anatomy of Dicot Stem: The microscopic structure of T.S. of dicot stem shows following

distinct regions:

a. Epidermis is the outermost layer of (dicot) stem with multicellular epidermal stem hairs/

trichomes. The cells are living, barrel shaped and compactly arranged without

intercellular spaces and chloroplasts. They may contain stomata for gaseous exchange.

The epidermis is externally covered by thick cuticle. The epidermal multicellular stem

hairs help in protection and heat loss.

b. Cortex is present below the epidermis in several layers. It can be differentiated into:

• Hypodermis consists of 3-5 layers of collenchyma without spaces. It contains

thickenings by the deposition of extra cellulose with pectin and gives mechanical

support. Due to the presence of chloroplast, it also helps in photosynthesis.

• General cortex encloses intercellular spaces with resin ducts, each surrounded by a

layer of small thin walled spherical or oval cells. It is made from loosely arranged

parenchymatous. The cortical cells may contain chloroplast and perform

photosynthesis. Some oil ducts line with epithelial cells. It also helps in gaseous

exchange and storage of food materials.

• Endodermis is uniseriate innermost cortex made from more or less barrel shaped

compactly arranged parenchymatous tissues. It stores starch and often called starch

sheath.

c. Pericycle several cells in thickness, with sclerenchyma and intervening masses of

parenchyma (heterogenous). They have thick wall sclerenchymatous tissue found on

patches above the phloem or bast, hence also termed as hard bast or bundle cap. The hard

bast helps in mechanical support and parenchyma helps in storage of food materials.

d. Medullary rays are parenchymatous, radially elongated or polygonal cells lie in between

vascular bundles. It helps for the radial conduction of water and food materials. It is

extension of pith, hence also called pith rays.

e. Vascular bundles are conjoint, collateral, open and arranged in a broken ring (eustele).

Each bundle is composed of outer phloem and inner xylem on the same radius with a

strip of cambium in between them (open type).

• Phloem lies outside of the vascular bundle and composed of sieve tubes, companion

cells and phloem parenchyma (all are living) and few bast fibers. Companion cells

are associated with sieve tubes.

• Xylem lies toward the pith or center and composed of xylem vessels, tracheids,

xylem fibres (wood fibres) and xylem parenchyma (wood parenchyma). The xylem

is endarch (i.e. protoxylem lie towards the pith, while metaxylem lies towards

periphery). Protoxylem is smaller with annular, reticulate or spiral thickenings,

while metaxylem is wider with pitted vessels. The xylem parenchyma is mostly

found around the protoxylem.

• Cambium is a thin strip of two or three layered cells which are radically arranged. It

lies between xylem and phloem. The cells appear almost rectangular in a transverse

section and have thin wall. Due to its position, it is also called intrafascicular

cambium. Cambium is a type of lateral meristematic tissues which increases the

thickness of the plant through secondary growth.

f. Pith is extensively developed and occupies the central portion of the ground tissue. It is

made from rounded or oval, thin walled parenchymatous cell with large amount of

intercellular spaces. Pith helps in the storage of food materials.

4. Anatomy of Monocot Stem: The T.S. of monocot stem contains following regions:

a. Epidermis is the outermost uniseriate cuticularised layer of parenchyma with stomata. It

is made from arranged barrel shaped cells usually without epidermal stem hairs;

trichomes.

b. Hypodermis is just internal to epidermis, made up of compactly arranged sclerenchyma

without intercellular spaces. It provides mechanical strength to the plant.

c. Ground tissue is parenchymatous and not differentiated into cortex, endodermis,

pericycle and pith as in dicot. These cells are smaller, polygonal, compactly arranged

(toward center) or loosely arranged (toward peripheral). The scatter vascular bundles are

imbedded in these tissues. These cells contain reserve materials.

d. Vascular bundles are conjoint, collateral and closed (no cambium). The vascular

bundles are scattered in ground tissue without any order (atactostele). The larger

vascular bundles lie towards the center (less in number) and smaller towards and

periphery (more in number). Each vascular bundles have oval outline and surrounded by

a sclerenchymatous bundle sheath which encloses xylem and phloem.

• Xylem consists of vessels, tracheids, xylem parenchyma and limited xylem fibres.

Vessel is Y-shaped with larger two round pitted metaxylem vessels forming the arms

and smaller annual or spiral protoxylem vessels, forming the base.

• A large water cavity lysigenous cavity, i.e. formed from disintegration of some

protoxylem is present in the inner side of the protoxylem. It stores water and also

called lacuna or water cavity. The xylems are endarch with outer metaxylem and

inner protoxylem.

• Phloem lies outside the xylem in the vascular bundle. It consists of sieve tubes,

companion cells and phloem fibers, but lacks phloem parenchyma. The outer

phloem is protophloem occurs in the form of crushed mass and functional

metaphloem lie inner portion.

5. Anatomy of Dorsiventral Leaf: (Dicot Leaf): The internal structure of the dorsiventral

leaf shows these regions with following features:

Dorsiventral leaves: In a dicot leaf the mesophyll is usually differentiated into upper

palisade and lower spongy parenchyma. Such leaves are called dorsiventral or bifacial

leaves.

a. Epidermis: It is outermost covering of the leaf that forms the boundary between the

atmosphere and underlying mesophyll. It is present on both ventral or adaxial (upper

epidermis) and dorsal or abaxial (lower epidermis) surface. Both the upper and lower

epidermis are covered by a cuticular layer. It consists of uniseriate, compactly arranged

thin walled parenchymatous tissue without chloroplast (but present in upper epidermis).

Usually upper epidermis is thickly cuticularised. Lower epidermis contains numerous

stomata without chloroplast; while upper epidermis contains high amount of chloroplast

and no stomata, thus the leaf is hypostomatic.

b. Mesophyll: It is the ground tissue between the upper and lower epidermis. In dorsiventral

leaf, it is differentiated into palisade parenchyma and spongy parenchyma.

• Palisade parenchyma lies towards the upper epidermis and consists of one, two or

three layers of elongated cells, densely packed with numerous chloroplasts.

• Spongy parenchyma lies towards the lower epidermis and made from loosely

arranged, irregular, thin walled cells parenchymatous tissues with large intercellular

spaces, air cavities and few chloroplasts, Hence, it helps in gaseous exchange in

transpiration and photosynthesis.

c. Vascular bundles: Each bundle is conjoint, collateral and closed. Vascular bundle is

surrounded by large parenchymatous bundle sheath or border parenchyma. Collenchyma

may also be associated with bundle sheath cells.

Xylem lies toward the upper epidermis and phloem toward lower epidermis. Single mid-

vein vascular bundle is larger and several smaller veinlet vascular bundles are smaller.

Smaller vascular bundles are freely scattered in mesophyll cells of leaf.

The bundle sheath extensions are parenchymatous/ collenchymatous in nature.

6. Anatomy of Isobilateral Leaf: (Monocot Leaf): The internal structure of isobilateral leaf

shows distinct layers;

Isobilateral leaves: In the leaves of monocots there is no differentiation of mesophyll into

palisade and spongy and are referred to as isobilateral or unifacial leaves.

a. Epidermis is single layered, present on both surfaces and cuticularised and stomata are

present on both surfaces (amphistomatic). It is composed of compactly arranged oval or

barrel shaped thin walled parenchymatous cells. A few cells in the upper epidermis

may become larger, less cuticularised; lens shaped and found on groups called motor

cells or bulliform cells. These cells become empty and large and regulate the curling and

uncurling (rolling up) of the leaves during dry conditions.

b. Mesophyll is the ground tissue that is present between the two epidermal layers. It is not

differentiated into palisade and spongy parenchyma and contains chloroplasts. It is

composed of cells that are almost spherical, oval or angular with irregular intercellular

spaces. Mesophyll tissues are not found in the mid-vein region. In mid vein region,

sclerenchymatous cells extend from the vascular bundle to the lower and upper

epidermis. This extension of sclerenchyma is called bundle sheath extension.

c. Vascular bundles: The bundles are conjoint, collateral, closed and each covered by

parenchymatous bundle sheath cells containing starch grains. Sclerenchymatous cells

may be present on both sides of the large bundles. The larger bundles have distinct

phloem towards the lower epidermis and xylem toward upper surface.

• Xylem consists of two pitted oval metaxylem; in between them, tracheids are also

found. Xylem parenchyma are less numerous. Protoxylem is represented by a

lysigenous cavity.

• Phloem has sieve tubes and companion cells. The small bundles are surrounded by

individual sheaths and contain not distinct and less developed phloem and xylem.

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