Ch.06   Anatomy of Flowering Plants

• Study of internal structure of plant is called anatomy.

• In plants cells are the basic unit.

• Cells organized into tissues and tissues organized into organs.

THE TISSUES

A group of cells having common origin and perform one function.

Plant tissues are classified into two types: -

• Meristematic tissue.

• Permanent tissue

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Meristematic Tissues

Growth in plants is restricted to specific regions with active cell division called meristems.

Different types of meristems are: -

Apical meristem:

• Occurs in the shoot and root tips.

• Primary meristem- Increase the length of plant

Intercalary meristem:

• Present in-between mature tissues.

• Primary meristem

• Occurs in grasses and regenerate parts removed by grazing.

Lateral meristem:

• Occurs in the mature regions of roots and shoots.

• Also, known as secondary meristem.

• Responsible for producing secondary tissues.

• Fascicular vascular cambium, interfascicular cambium and cork cambium are example of lateral meristem.

Axillary bud:

• During formation of leaves and elongation of stem, some cells ‘left behind’ from shoot apical meristem, constitute the axillary bud.

• Present in the axils of leaves and are capable for forming a branch or a flower.

Permanent Tissue

• Cells produced from primary or secondary meristem stop dividing and differentiated structurally and functionally, termed as permanent cells.

• A group of permanent cell constitutes the permanent tissues.

• Permanent tissues having similar in structure and function are called simple tissues.

• Permanent tissues having many different types of cells are called complex tissues.

Simple tissues:

• Simple tissues made of only one type of cells.

Parenchyma:

• Forms major component within organs.

• Cells are isodiametric.

• Thin cell wall made of cellulose.

• Cells may be spherical, oval, round, polygonal or elongated shape.

• Cells are closely packed or have small intercellular space.

• Perform various functions such as photosynthesis, storage, secretion.

Collenchyma:

• Found either in homogeneous layer of in patches.

• Cell wall thickened in the corner due to deposition of cellulose, hemicelluloses and pectin.

• Cells are oval, spherical or polygonal in shape

• Often contain chloroplasts.

• No intercellular spaces.

• Provide mechanical support to the growing part of the plant such as young stem and petiole of a leaf.

Sclerenchyma:

• Consists of long, narrow cells

• Cell wall is thick and lignified.

• Cell wall with few or numerous pits.

• Cells are usually dead and without protoplast.

• Provides mechanical support to the organs.

• Sclerenchyma are of two types on the basis of origin, form, structure.

Fibres:

• Thick walled

• Elongated and pointed cells

• Generally, occurs in group in various parts of the plant.

Sclereids:

• Spherical, oval or cylindrical in shape.

• Highly thickened dead cells with very narrow cavities (lumen).

• Commonly found in fruit walls of nuts; pulp of guava, pear and sapota; seed coats of legumes and leaves of tea.

Complex tissues:

• Made of more than one type of cells and these work together as a unit.

• Xylem and phloem constitute the complex tissues in plants.

Xylem

• Functions as conducting tissues for water and minerals from roots to stem and leaves.

• Provides mechanical support to the plant.

• It consists of four different kinds of elements

• Tracheids

• Vessels

• Xylem fibres and

• Xylem parenchyma.

Tracheids

• Elongated or tube like cells.

• Thick and lignified walls and tapering ends.

• Cells are dead and without protoplasm.

• Inner layers of cell walls have thickenings which vary in forms.

Vessels.

• Is a long cylindrical tube-like structure made up of many cells called vessel members.

• Each with lignified walls and a large central cavity.

• Cells are devoid of protoplasm.

• Vessel members are interconnected through perforations in their common walls.

• Presences of vessels are the characteristics of the angiosperm.

Xylem fibres:

• Have highly thickened walls and obliterated central lumen.

• Either septate or aseptate.

Xylem Parenchyma:

• Cells are living and thin-walled.

• Cell walls are made up of cellulose.

• They stored food materials in the form of starch or fat.

• Also, store materials like tannins.

• The radial conduction of water takes place by the ray parenchymatous cells.

• The primary xylem is of two types-

Protoxylem

Metaxylem.

• The first formed xylem elements are called protoxylem. The later formed primary xylem is called metaxylem.

• Endarch: the protoxylem lies towards pith and metaxylem towards the periphery of the organ (in stem)

• Exarch: in root the protoxylem lies towards periphery and metaxylem lies towards the centre.

Phloem:

• Transports food materials usually from leaves to other part of plant.

• It is composed of four elements: - Sieve tube elements, Companion cells, Phloem parenchyma, Phloem fibres.

Sieve tube elements:

• Long tube like structure arranged longitudinally

• Associated with companion cells.

• End walls are perforated to form sieve plates.

• A mature sieve element possesses peripheral cytoplasm and a large vacuole but lacks nucleus.

• The function of sieve elements controlled by nucleus of companion cells.

Companion cells:

Specialized parenchymatous cells.

• Closely associated with sieve tube elements.

• Connected with sieve tube element by pit field.

• Helps in maintaining pressure gradient in the sieve tubes.

Phloem parenchyma:

• Made up of elongated tapering cylindrical cells

• Have dense cytoplasm and nucleus.

• Cell wall made of cellulose and has pits through plasmodesmatal connections exist between the cells.

• Store food materials and other substances like resins and latex and mucilages.

• It is absent in monocotyledons.

Phloem fibres:

• Also, known as bast fibres.

• Made of sclerenchymatous cells.

• Absent in primary phloem but present in secondary phloem.

• Much elongated, unbranched and have pointed, needle like apices.

• Cell wall is quite thick.

• On maturity lose their protoplasm and become dead.

• Phloem fibres of jute, flax and hemp are used commercially.

• The first formed primary phloem consists of narrow sieve tubes and referred as protophloem.

• The later formed phloem has bigger sieve tubes and is referred to as metaphloem.

THE TISSUE SYSTEM:

Based on their structure and location there are three types of tissue systems.

1. Epidermal tissue system.

2. Ground or fundamental tissue system.

3. Vascular or conducting tissue system.

Epidermal Tissue System

Forms the outermost covering of the whole plant body and comprises:

1. Epidermal cells.

2. Stomata

3. Epidermal appendages like trichomes and hairs.

Epidermis

• consists of single layer parenchymatous cells.

• Cells are elongated, compactly arranged, which form continuous layer.

• Epidermis is usually single layered.

• Outside the epidermis covered with waxy thick layer called cuticle.

• Cuticle absent in epidermis of root.

Stomata

• These are the structure present in the epidermis of leaf

• Stomata regulate the process of transpiration and gaseous exchange.

• Each stoma composed of two bean shaped cell called guard cells.

• In grasses the guard cells are dumb-bell shaped.

• Outer wall of guard cell is thin and inner wall is thick.

• Guard cell possesses chloroplast and regulates the opening and closing of stomata.

• Epidermal cells near guard cell called subsidiary cells.

• Stomatal aperture, guard cells and subsidiary cells together called stomatal apparatus.

• The root hairs are unicellular elongations of the epidermal cells and help absorb water and mineral from the soil.

Trichomes:

• On stem the epidermal hairs are called trichomes.

• Trichomes are usually multicellular.

• May be branched or unbranched and soft or stiff.

• Sometimes secretory.

• Trichomes help in preventing water loss due to transpiration.

The Ground Tissue System

• All the tissues except epidermis and vascular bundles constitute the ground tissue.

• It consists of simple tissues such as parenchyma, collenchyma, Sclerenchyma.

• Parenchymatous cells are present in cortex, pericycle, pith and medullary rays.

• In leaves, the ground tissue consists of thin-walled chloroplast containing cells called mesophyll.

The Vascular Tissue System

• Vascular system consists of complex tissues – xylem and phloem.

• Xylem and phloem together constitute the vascular bundle.

• In dicot presence of cambium between xylem and phloem called open vascular bundle.

• Vascular bundle without cambium is said to be closed vascular bundle.

• Radial vascular bundle: xylem and phloem arranged alternate manner on the different radii.

• Conjoint vascular bundle: xylem and phloem are situated at the same radius of vascular bundle.

ANATOMY OF DICOT AND MONOCOT PLANTS

Roots

Dicotyledonous Root:

• The outermost layer is epidermis.

• Presence of unicellular root hairs in epidermis.

• The cortex constitutes many layer thin-walled parenchyma cells with intercellular spaces.

• The innermost layer of cortex is endodermis.

• Endodermis consists of single layered barrel-shaped cells without intercellular spaces.

• Presence of casparian strip in the endodermis.

• Next to endodermis there is few layer parenchymatous cells form pericycle.

• Initiation of lateral root and vascular cambium during secondary growth takes place from the cells of pericycle.

• The parenchymatous cells present in-between xylem and phloem is called conjuctive tissue.

• The number of xylem and phloem bundle is three or four.

• All the tissues on the inner side of endodermis such as pericycle, vascular bundles and pith constitute the stele.

Monocotyledonous Root:

Monocot root have similar tissues as in dicot except:

• It contains more than six xylem bundles called polyarch.

• Pith is large and well developed.

• Do not undergo any secondary growth.

Stems

Dicotyledonous Stem:

• Outermost layer is epidermis.

• Epidermis covered with thin layer of cuticle and has trichomes and few stomata.

• The cells arranged in multiple layers’ in-between epidermis and pericycle constitute the cortex.

Cortex has three sub-zones:

• Hypodermis: a few layers of collenchymatous cells below epidermis.

• Cortical layers: consists of rounded thin walled parenchymatous cells with intercellular spaces.

• Endodermis: it is the innermost layer of cortex. Cells are rich in starch grains and are referred to as starch sheath.

• Pericycle: present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of Sclerenchyma.

• Medullary rays: a few layers of radially placed parenchymatous cells present in between vascular bundles.

• Many vascular bundles arranged in a ring.

• Each vascular bundle is conjoint, open and endarch protoxylem.

• The central portion of stem constitutes the pith.

Monocotyledonous Stem:

It has similar tissues with the dicot stem except in following-

• Sclerenchymatous hypodermis.

• Vascular bundles are scattered in the ground tissue.

• Each vascular bundle is covered by bundle sheath cells.

• Vascular bundles are conjoint and closed.

• Peripheral vascular bundles are smaller than central one.

• Phloem parenchyma is absent.

• Water containing cavities are present within the vascular bundles.

Leaves

Dorsiventral (Dicotyledonous) Leaf:

Vertical section of a Dorsiventral leaf shows three main parts:

1. Epidermis.

2. Mesophyll cells.

3. Vascular systems.

Epidermis

• covers both upper (adaxial) and lower (abaxial) surface of the leaf has a conspicuous cuticle.

• Abaxial surface has more stomata than the adaxial epidermis.

• Tissue between upper and lower epidermis called mesophyll.

Mesophyll cells are two types:

1. Palisade parenchyma

2. Spongy parenchyma

• Adaxially placed palisade parenchyma is made up of elongated cells arranged vertically, parallel to each other.

• Spongy parenchyma: oval or round and loosely arranged cells below the palisade parenchyma.

• Vascular bundles are seen in the midrib and veins.

• The vascular bundles are surrounded by a layer of thick walled bundle sheath cells.

 Isobilateral (Monocotyledonous) leaf:

It is similar with Dorsiventral leaf in many respect except –

• Stomata are equally distributed on upper and lower epidermis.

• Mesophyll cells are not differentiated into palisade and spongy.

• In grasses, certain adaxial epidermal cells along the veins modified themselves into large, empty, colorless cells called bulliform cells.

• Causes rolling of leaves to reduce transpiration during water stress.

SECONDARY GROWTH

• lateral meristem -Vascular cambium or cork cambium; sheet like cylinder of meristem that gives rise to plant secondary growth

• vascular cambium -Ring of meristematic tissue that produces secondary xylem (wood) and phloem

• cork cambium -In plants, a lateral meristem that gives rise to periderm

Vascular Cambium

• Divisions of vascular cambium cells produce secondary xylem on the cylinder’s inner surface, and secondary phloem on its outer surface

• Displaced cells of the vascular cambium divide in a widening circle, so the tissue’s cylindrical form is maintained

Cork Cambium

• Cork cambium forms and gives rise to periderm

• Periderm consists of parenchyma and cork, and the cork cambium that produces them

• Bark consists of all of the living and dead tissues outside the vascular cambium A

• The cork component of bark protects, insulates, and waterproofs a stem or root surface

bark

• Secondary phloem and periderm of woody plants

cork

• Component of bark

• Protects the surfaces of woody stems and roots

• Has densely packed rows of dead cells with walls thickened by a waxy substance (suberin)

Secondary xylem (wood) is classified by its location and function, as in heartwood or sapwood

wood -Accumulated secondary xylem

heartwood -Dense, dark accumulation of nonfunctional xylem at the core of older tree stems and roots

sapwood -Functional secondary xylem between the vascular cambium and heartwood in an older stem or root

• Rings visible in heartwood and sapwood are regions of early and late wood

• Early wood forms during wet springs

• Late wood indicates a dry summer or drought when no large-diameter xylem cells were made for water uptake

• In most temperate zone trees, one ring forms each year

Secondary Growth

– In many plants, secondary growth thickens branches and roots during successive growing seasons

– Extensive secondary growth of eudicots and conifers produces wood

– Tree rings can be used to study past environmental conditions