Plant Growth and Develpoment

8/8/2019 Plant Growth and Develpoment

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PLANT GROWTH AND DEVELPOMENT

What is Growth?

It is a characteristic of living beings in which an irreversible permanent increase in size ofan organ or its parts occur. In smaller living beings, an increase in the size of a cell canalso be termed as growth.

Characteristics of Plant growth

Plant Growth is Indeterminate

Plants retain the capacity of unlimited growth throughout life.

Meristems are present in plants that have the ability to divide and self perpetuate.

Open form of growth New cells are always being added to the plant by

meristem.

Primary Growth Occurs due to root apical meristems and shoot apical

meristems

Secondary growth Occurs due to the appearance of lateral meristems, vascular

cambium, and cork cambium later in the life of certain dicots and gymnosperms

Plant Growth is Measurable

Growth is measured by measuring the parameters that are directly proportional to

increase in protoplasm.

Increase in weight (fresh and dry weight both), length, area, volume, and cell

number are some parameters.

Choice of parameters depends upon type of plant.

Examples:For pollen tube length is the parameterFor water melon cell size is usedFor dorsiventral leaf surface area is used

Phases of Growth

Three Phases of growth meristematic, elongation, and maturation

Meristematic Phase

Cells rich in protoplasm

Cells possess large conspicuous nuclei

Cell wall Primary, thin, and cellulosic with abundant plasmodesmatalconnections

Constantly dividing cells at root and shoot apex are in this phase.

Elongation Phase

Cells enlarge and show increased vacuolation.

New cell wall deposits.


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Cells proximal to the merismatic zone (root and shoot tip) are in this phase.

Maturation Phase

Cells attain their maximum size.

Wall thickening and protoplasmic modifications take place completely.

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Growth Rate

Types of Growth Rate

Growth rate: Increase in growth per unit time

Plants show two types of growthArithmetic and Geometricaccording to the increaseshown by the growth rate

Arithmetic growth

Only one daughter cell continues to divide while others differentiate or mature.

Example root elongating at a constant rate

Mathematically, Lt= L0 + rtWhere: Lt= Length at time, tL0 = Initial lengthr= Growth rate

On plotting length against time, a linear curve is obtained.


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Geometric Growth

Initial growth is slow (lag phase), followed by a rapid increase in growth

(log/exponential phase), followed by a phase where growth slows down(stationary phase)

Example all cells, tissues and organs typically show this type of growth

Mathematically, W1 = W0ert

W1 = Final sizeW0 = Initial sizer= Growth ratet= Time of growthe = Base of natural algorithms

On plotting the size or weight of the organ against time, a sigmoid or S-shaped

curve is obtained.


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Ways to compare growth quantitatively

Differentiation, Dedifferentiation, and Redifferentiation

Differentiation

In this process, cells derived from root apical and shoot apical meristems andcambium differentiate and mature to perform specific functions.

Structural changes occur in plant cell (both cell wall and protoplasm).

For example, cells develop strong, elastic, and lignocellulosic cell wall for longdistance transport of water.

Dedifferentiation

Process in which living differentiated cells regain their capacity to divide

For example: Formation of meristems such as interfascicular cambium and cork

cambium from fully differentiated parenchyma cells

Redifferentiation

Process in which differentiated cells that have lost their ability to divide are

reformed from dedifferentiated cells

Redifferentiated cells have the ability to perform specific functions.

Just like growth, differentiation in plants is also open since cells arising from

same meristem may differentiate to form different structures depending upon itslocation.

Development what does it mean?

Includes all changes that an organism goes through during its life cycle

Plasticity Ability of plants to follow different pathways in response to environment orphases to form different kinds of structuresSome examples are heterophylly in larkspur and Buttercup. In these plants, leaves havedifferent shapes based on the phase of life cycle as well as the habitat.

Development can also be termed as growth + differentiation

Development is controlled by intrinsic as well as extrinsic factors.

Intrinsic Genetic factors and plant growth regulators


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Extrinsic light, temperature, water, oxygen, etc.

Two ways to compare growth quantitatively: By measuring (i) absolute growth rate and(ii) relative growth rate

Absolute growth rate:

Measurement of total growth per unit time

In the given figure, an absolute increase is shown in the areas of the leaves A and

B to form leaves A1 and B1.

Relative growth rate:

Growth of a given system per unit time expressed on a common basis; e.g., per

unit of initial parameter In the given figure, both leaves increase by 10 cm2, but a relatively greater growth

has occurred in leaf A.

Conditions for Growth

Include: water, oxygen, nutrients

Water:

Cell enlargement requires water.

Water provides medium for enzymatic activities.

Oxygen:

Releases metabolic energy needed for growth.

Nutrients:

Source of energy

Required for synthesis of protoplasm

Temperature:


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Plays an important role in growth. Any deviation from the optimum temperature

hampers growth.

Environment signals (light and gravity)

Differentiation and Development in Plants

Differentiation, Dedifferentiation, and Redifferentiation

Differentiation

In this process, cells derived from root apical and shoot apical meristems and

cambium differentiate and mature to perform specific functions.

Structural changes occur in plant cell (both cell wall and protoplasm).

For example, cells develop strong, elastic, and lignocellulosic cell wall for longdistance transport of water.

Dedifferentiation

Process in which living differentiated cells regain their capacity to divide

For example: Formation of meristems such as interfascicular cambium and corkcambium from fully differentiated parenchyma cells

Redifferentiation

Process in which differentiated cells that have lost their ability to divide are

reformed from dedifferentiated cells

Redifferentiated cells have the ability to perform specific functions.

Just like growth, differentiation in plants is also open since cells arising from

same meristem may differentiate to form different structures depending upon itslocation.

Development what does it mean? Includes all changes that an organism goes through during its life cycle

Plasticity Ability of plants to follow different pathways in response to environment orphases to form different kinds of structuresSome examples are heterophylly in larkspur and Buttercup. In these plants, leaves havedifferent shapes based on the phase of life cycle as well as the habitat.

Development can also be termed as growth + differentiation

Development is controlled by intrinsic as well as extrinsic factors.

Intrinsic Genetic factors and plant growth regulators

Extrinsic light, temperature, water, oxygen, etc.


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Division AlgaeClassification within Angiosperms

Artificial system of classification

Given by Linnaeus

Based on vegetative characters and androecium structures

Gave equal importance to vegetative and sexual characteristics

Natural system of classification

Based on morphology, anatomy, embryology, and phytochemistry

Given by George Bentham and Joseph Dalton Hooker

Phylogenetic system of classification - based on evolutionary relationship

Numerical Taxonomy

Based on all observable characteristics

Numbers and codes assigned to all characters

Easily carried out using computers

Cytotaxonomy Based on cytological information such as chromosome number,structure, behaviour

Chemotaxonomy Based on chemical constituents of plant to resolve doubts and

confusions

Division Algae

Includes chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic(freshwater and marine) organisms

Some occur in association with fungi (lichens) and animals (on sloth bear).

Size ranges from microscopic unicellular forms such as Chlamydomonas to colonial


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forms such as Volvoxand to filamentous forms such as Ulothrix and Spirogyra.

Massive plant-like bodies are seen in some marine forms (such as kelps).

Economic Importance

Carbon dioxide fixation on earth is majorly carried out by algae.

Important as primary producers of energy-rich compoundsExample Sargassum,Laminaria, andPorphyra used as food

Some brown and red algae species produce water-holding hydrocolloids.Example Algin (brown algae) and carrageen (red algae)

Agar produced by Gelidium and Gracilaria is used to grow microbes and in preparationof ice creams and jellies.

Chlorella and Spirulina are protein-rich unicellular algae, used as food supplements.They are also known as space food.

Major classes of algae:


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Chlorophyceae

Commonly called green algae

May be unicellular, colonial, or filamentous

Grass green in colour due to abundance of chlorophyll a and b

Chloroplast of most of the Chlorophyceae contains pyrenoids.

Pyrenoids Storage bodies containing proteins in addition to starch

Food storage occurs in the form of oil droplets in some algae.

Cells have rigid cell wall: inner layer made of cellulose, outer layer made of pectose

Phaeophyceae (Brown algae) Primarily marine forms

Show great variation in size and form

Range from simple-branched, filamentous forms (Ectocarpus) to profusely branchedforms such as kelps (may reach a height of 100 m)

Possess chlorophyll a, c, carotenoids, and xanthophylls

Vary in colour from olive green to various shades of brown (depending on amount ofxanthophyll and fucoxanthin)

Food stored as complex carbohydrates such as laminarin or mannitol

Vegetative cells have cellulosic wall covered on the outside by gelatinous coating ofalgin.


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Cell contains a centrally located vacuole and nucleus in addition to plastids.

Union of gametes takes place in water or within oogonium (oogamous species).

Gametes are pyriform (pear-shaped).Example Ectocarpus,Dictyota,Laminaria, Sargassum, andFucus


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Rhodophyceae (Red algae)

Commonly called red algae due to the presence of red pigment, r-phycoerythrin

Mainly marine forms with bulk mass inhabiting warmer areas

Occur in well-lighted regions i.e., close to the surface of water and also in deeper areas

Red thalli of most of these species are multicellular. Some have complex bodyorganization.

Food is stored as Floridian starch similar to amylopectin and glycogen in structure.

Example Polysiphonia, Gelidium, Gracilaria,Porphyra

Division Bryophyta

Known as amphibians of plant kingdom since they live on land, but depend on water forsexual reproduction

Usually occur in cool, damp, and shady areas

Play an important role in plant succession on bare rocks/soils

Plant body more differentiated than algae

Thallus-like plant body is attached to substratum by unicellular or multicellular rhizoids.

Lack true roots, stem and leaves; may possesses root-like, stem-like, and leaf-likestructures


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Sporophyte is dependent on gametophyte for food. Hence, it remains attached to thegametophyte.

Few cells of sporophyte undergo meiosis to produce spores (haploid).

Spores germinate to form gametophyte.Economic Importance

Provide food for herbaceous mammals, birds, and insects

Peat provided by Sphagnum is used as fuel.

Sphagnum is also used as packing material in trans-shipment of living material becauseof their water-holding capacity.

They form dense mats on the soil and hence prevent soil erosion.

Mosses along with lichens form the pioneer community in land and desert succession.

Classes of bryophytes

Liverworts


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Grow in moist, shady habitats

Plant body is thalloid.

Thallus is dorsiventrally appressed to the substrate.

Leafy members have tiny leaf-like appendages on stem-like structures

Mosses


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Gametophyte

Predominant stage

Sporophyte

More elaborate than liverworts

Consists of foot, seta, and capsule


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Capsule contains spores.

Spores formed by meiosis

Elaborate mechanism of spore dispersalExample Funaria,Polytrichum, and Sphagnum

Division Pteridophyta

General Characteristics

The dominant plant body is sporophyte.

First terrestrial plants to possess xylem and phloem

Found in cool, damp, shady places

Have well-differentiated true stem, leaves, and roots

Leaves may be microphylls as in Selaginella or macrophylls as in ferns.

Sporophytes bear sporangia, which develop in association with leaves called sporophylls.

In some pteridophytes, sporophylls form distinct, compact structures called strobili orcones (Selaginella, Equisetum).

Sporangia produce spores by meiosis in spore mother cells.

Spores germinate to form small, multicellular, free-living photosynthetic thalloidgametophyte called prothallus.

Major classes:

Gametophyte

Require cool, damp, shady places to grow

Also require water for sexual reproduction


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Zygote produces well-differentiated, multicellular sporophyte.

Sporophyte

Example of heterospory Selaginella and Salvinia


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Division Gymnospermae

Word gymnosperms,gymnos naked,sperma seeds

Ovules not enclosed by any ovary wall

Seeds formed after fertilization are not covered (i.e., naked).

Include medium-sized trees, shrubs, and tall trees

Contains the worlds largest plant Sequoia - the giant redwood

Plants have tap roots. Roots in some genera show symbiotic associations.

Mycorrhiza shows association of fungi withPinus roots.

Coralloid roots ofCycas show association withN2-fixing Cyanobacteria.


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Leaves are well-adapted to withstand extreme conditions. In conifers, needle-like leaveswith thick cuticle and sunken stomata reduce surface area and water loss.

Spores produced within sporangia, borne on sporophylls, which form strobili or cones

Male and female strobili may be borne on same tree (Pinus) or on different trees (Cycas).

Megaspore mother cell divides mieotically to form four megaspores.

Megaspore mother cell is a differentiated cell of nucellus. Nucellus protected by

envelopes is known as an ovule. Male and female gametophytes do not have independent existence, hence remain within

sporangia.

Steps in fertilization:

Pollen grain released from microsporangium

Carried by air currents

Come in contact with ovules

Discharge of pollen content on mouth of archegonia

Fertilization

Formation of zygote


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Development of naked seed

Division Angiosperms

Large group of plants inhabiting a wide range of habitats

The pollen grains and ovules are developed in structures called flowers. Seeds enclosed by fruits

Range from tiny, almost microscopic, specimens (Wolfia) to tall trees likeEucalyptus

Two main groups are:

Monocotyledons having one cotyledon in their seeds

Dicotyledons having two cotyledons in their seeds

The male sex organ in a flower is a stamen.

Each stamen consists of:

a slender filament an anther at the tip

The anther produces pollen grains by meiosis.

The female sex organ is a pistil or carpel

Each pistil consists of:

an ovary

a style

a stigma

The ovary encloses one or more ovules.

Within the ovule (the highly reduced female gametophyte) embryo sacs are present.

Embryo sac is a seven-celled, eight-nucleated structure. Embryo sac contains

One egg cell

Two synergids

Three antipodal cells

One central cell

The polar nuclei fuse to form a secondary nucleus (diploid).


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Pollen grains, after dispersal from anthers are carried by the wind or other agents to thestigma of the pistil; termed as pollination

Fertilisation in angiosperms is termed as double fertilisation.

The synergids and antipodals degenerate after fertilisation.

The ovules develop to form seeds, and the ovaries develop into fruits.

LIFE CYCLE OF AN ANGIOSPERM

Plant Life Cycles

There is alternation of generations between haploid gametophyte and diploid sporophytein the life cycle of a plant.


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In plants, both haploid and diploid cells can divide by mitosis.

Hence, there are two different plant bodies haploid and diploid.

The haploid plant body produces gametes by mitosis and represents a gametophyte.

Mitotic division is encountered in diploid cells when zygote divides by mitosis to

produce sporophytic plant body after fertilization. This sporophyte produces haploid spores by meiosis.

Spores in turn undergo mitosis to form haploid plant body.

Types of Life Cycles in Plants

Haplontic Life cycle

In this, sporophyte is represented by one-celled zygote.

There is no free living sporophyte.

Zygote undergoes meiotic division to produce spores, which divide mitotically and formgametophyte.

Gametophyte is the dominant phase in this life cycle as it is dominant, free living, andphotosynthetic.

Algae such as Spirogyra and some species ofChlamydomonas have this type of lifecycle.

Diplontic Life Cycle

In this case, diploid sporophyte is the dominant phase as it is free living andphotosynthetic.


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Gametophyte is single to few-celled.

Example All seed-bearing plants, gymnosperms, and angiosperms, some algae-likeFucus

HaplodiplonticLife Cycle

Intermediate condition

Both gametophyte and sporophyte are free-living and multicellular, but have differentdominant phases.

In Bryophytes, haploid gametophyte is dominant, independent, and

photosynthetic. It alternates with short-lived multicellular sporophyte totally orpartially and is dependent on gametophyte for nutrition and anchorage.


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In Pteridophytes, diploid sporophyte is dominant, independent, and

photosynthetic. It alternates with short-lived haploid gametophyte, which isindependent of sporophyte.

Plant Growth and Develpoment

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