Plant Physiology And Morphology: Roots, Stems, Leaves, Flowers, Photosynthesis And Plant Hormones

Plant morphology is the branch of biology that deals with the external form and structure of plants.

It studies plant organs such as:

• Roots
• Stems
• Leaves
• Flowers
• Fruits
• Seeds

Plant physiology studies the internal life processes of plants, such as water movement, mineral nutrition, photosynthesis, respiration and hormonal control.

Part 1: Plant Morphology

Plant Diversity

Plants show diversity in form, size, life span, habit, habitat and nutrition.

Based on habit, plants are classified into:

• Herbs
• Shrubs
• Trees

Herbs

Herbs are small plants with soft, green and non-woody stems.

Examples

• Wheat
• Buttercup
• Henbane
• Canna

Shrubs

Shrubs are medium-sized woody plants.

They usually have many branches arising from the base.

Examples

• Rose
• Jasmine
• Capparis

Trees

Trees are tall plants with a thick woody stem called the trunk.

Examples

• Palm
• Pinus
• Casuarina
• Eucalyptus
• Dalbergia
• Banyan

Root System

The root is usually the underground part of the plant.

It helps in anchorage, absorption of water and minerals, storage and sometimes reproduction.

Types Of Root System

The two main root systems are:

• Adventitious root system
• Tap root system

Adventitious Root System

Roots that arise from any part other than the radicle are called adventitious roots.

They are common in monocots.

Examples

• Grass
• Monstera

Modifications Of Adventitious Roots

Adventitious roots may become modified for storage, support, respiration, absorption, reproduction or floating.

Tuberous Roots

Tuberous roots arise from stem nodes and become swollen without any definite shape.

Example:

• Sweet potato

Fasciculated Roots

These are tuberous roots that arise in clusters from the base of the stem.

Examples:

• Dahlia
• Asparagus

Palmate Tuberous Roots

In Orchis, a pair of succulent tuberous roots is present. One root may perish every year while a new one forms beside it.

Nodulose Roots

These roots become swollen at the tips due to accumulation of food.

Examples:

• Mango ginger
• Arrowroot

Epiphytic Roots

Some roots hang freely in the air and absorb moisture with the help of a special sponge-like tissue called velamen.

Velamen is a modification of the epidermis.

Examples:

• Vanda
• Dendrobium

Reproductive Roots

Some fleshy adventitious roots develop buds that grow into new plants.

These roots help in vegetative propagation.

Examples:

• Sweet potato
• Dahlia

Floating Roots

Floating roots develop from the nodes of floating aquatic plants.

They store air, become spongy and help the plant float.

Example:

• Jussiaea or Ludwigia

Tap Root System

In the tap root system, the primary root develops from the radicle and gives lateral branches.

The tap root is the true root.

It produces many lateral roots endogenously.

The tap root system is commonly found in dicot plants.

Modifications Of Tap Roots

Tap roots may become swollen to store reserve food material.

In such plants, the primary root stores food, while secondary roots remain thin and absorptive.

Examples are commonly seen in storage roots such as carrot, radish and turnip.

Stem

The stem is the aerial part of the plant that bears leaves, branches, flowers and fruits.

It also helps in conduction, support, storage and vegetative propagation.

Functions Of Stem

• Supports leaves, flowers and fruits
• Conducts water and minerals
• Conducts food
• Stores food in some plants
• Helps in vegetative propagation
• May perform photosynthesis when green

Modified Stems

Stems may become modified for storage, support, protection or vegetative propagation.

Potato

Potato is a modified stem.

Its “eyes” are buds, which show that potato is stem in origin.

Onion

Onion is a bulb and is a modified stem.

Its fleshy scales store food.

Leaf

The leaf is a green, flattened structure that usually arises from the node of the stem.

It is the main photosynthetic organ of the plant.

Functions Of Leaf

• Photosynthesis
• Transpiration
• Gas exchange
• Storage
• Protection

Photosynthesis In Leaves

The principal function of the leaf is to manufacture organic food.

Green cells of the leaf contain chlorophyll and act as the main seat of photosynthesis.

Transpiration In Leaves

Transpiration is the loss of water from the leaf surface.

It creates tension in the water column and helps in ascent of sap.

Storage In Leaves

Leaves may store food in some plants.

Example:

• Onion leaf bases

Protection By Leaves

Leaves may get modified into spines to protect plants from grazing animals.

Examples:

• Opuntia
• Barberry

Leaf Modifications

Leaves may be modified for special functions.

Phyllode

In phyllode, leaflets disappear and the petiole becomes flattened and green.

It performs photosynthesis like a leaf blade.

Example:

• Acacia

Leaf Bladder

In some plants, leaves are modified into bladders for catching insects.

Example:

• Bladderwort or Utricularia

Pitcher Leaf

In pitcher plants, the leaf becomes modified into a pitcher-like structure.

The pitcher may collect rainwater and organic debris.

Example:

• Pitcher plant

Succulent Leaves

Succulent leaves store water and nutrients.

They help plants survive in dry conditions.

Example:

• Bryophyllum

Inflorescence

Inflorescence is the arrangement and distribution of flowers on the floral axis.

It is mainly of two types:

• Racemose
• Cymose

Racemose Inflorescence

Racemose inflorescence is indeterminate.

The main axis grows indefinitely and does not end in a flower.

Flowers are produced in acropetal succession, where older flowers are at the base and younger flowers are near the tip.

Cymose Inflorescence

Cymose inflorescence is determinate.

The main stem ends in a flower, which limits further growth.

Flowers are arranged in basipetal order, where older flowers are at the top or centre and younger flowers are towards the base or periphery.

Flower

A flower is a highly modified shoot meant for sexual reproduction in plants.

Types Of Flowers

Bisexual Flower

A flower that has both male and female reproductive organs is called a bisexual flower.

Examples:

• Hibiscus
• Rose
• Sunflower

Unisexual Flower

A flower that has either male or female reproductive organs is called a unisexual flower.

Examples:

• Coconut
• Papaya
• Cucumber

Neuter Flower

A flower without male and female reproductive organs is called a neuter flower.

Example:

• Ray florets of sunflower

Structure Of A Bisexual Flower

A flower is a specialized shoot in which leaves are modified into floral structures.

The flower is usually supported by a stalk called the pedicel.

Some flowers may be without a stalk. Such flowers are called sessile.

The enlarged tip of the stalk is called the receptacle or thalamus.

Four Floral Whorls

The floral parts are arranged on the thalamus in four whorls.

Calyx

Calyx is the first whorl.

It consists of green sepals.

Corolla

Corolla is the second whorl.

It consists of brightly coloured petals.

Androecium

Androecium is the male reproductive whorl.

It consists of stamens.

Each stamen has:

• Filament
• Bilobed anther

The anther produces pollen grains.

Male gametes are formed within pollen grains.

Gynoecium

Gynoecium is the female reproductive whorl.

It consists of the pistil.

Each carpel has three parts:

• Stigma
• Style
• Ovary

The stigma receives pollen grains.

The style connects the stigma with the ovary.

The ovary contains ovules.

Complete And Incomplete Flowers

A flower that has all four floral whorls is called a complete flower.

A flower in which one or more whorls are missing is called an incomplete flower.

Fruit

A fruit is the matured ovary of a flower, with or without seeds.

After fertilization:

• Ovary forms fruit
• Ovules form seeds

True Fruit

A true fruit is formed only from the ovary after fertilization.

It is also called eucarp.

Example:

• Grape

Parthenocarpic Fruit

A fruit formed without fertilization and without seeds is called a parthenocarpic fruit.

Examples:

• Banana
• Seedless grapes

Part 2: Plant Physiology

Plant physiology explains the working processes inside plants.

Important topics include:

• Water relations
• Transpiration
• Mineral nutrition
• Photosynthesis
• Respiration
• Plant hormones

Water Relations In Plants

Plants absorb water mainly through roots.

Water then moves upward through xylem against gravity.

This upward movement of water is called ascent of sap.

Osmosis

Osmosis is the movement of water through a selectively permeable membrane from a dilute solution to a concentrated solution.

Imbibition

Imbibition is absorption of water by solid particles without forming a solution.

Example:

• Dry wood absorbs water
• Seeds swell when kept in water

Plasmolysis

Plasmolysis is the shrinkage of plant protoplasm due to exosmosis.

Example:

• A plant may die if excess fertilizer is applied without enough water because water moves out of cells.

Transpiration

Transpiration is the loss of water from aerial parts of the plant, mainly leaves.

Curtis called transpiration a necessary evil in 1926.

It is called necessary because it helps in water movement and ascent of sap.

It is called evil because it causes loss of water.

Types Of Transpiration

• Cuticular transpiration
• Lenticular transpiration
• Stomatal transpiration

Stomatal Transpiration

Stomatal transpiration occurs through stomata.

Stomata are pores guarded by guard cells.

Guard cells contain chloroplasts and are surrounded by subsidiary cells.

Shape Of Guard Cells

Plant Type	Guard Cell Shape
Dicots	Kidney-shaped
Monocots	Dumbbell-shaped

Opening And Closing Of Stomata

Stomata open when guard cells take up K⁺ ions from surrounding cells.

This causes osmotic entry of water into guard cells.

Stomata close when K⁺ ions move out and guard cells lose turgidity.

Factors Affecting Transpiration

• Transpiration increases with increase in temperature.
• Transpiration decreases in high humidity.
• Stomata usually open during day and close at night.

Measuring Transpiration

A potometer is used to measure transpiration.

Antitranspirants

Antitranspirants reduce the rate of transpiration.

Examples:

• Abscisic Acid
• Phenyl Mercuric Acetate

Guttation

Guttation is the loss of water in liquid droplets from small pores called hydathodes.

Examples:

• Rose
• Strawberry
• Tomato

Mineral Nutrition

Plants obtain minerals from the soil through the root system.

De Saussure in 1804 first demonstrated that plants obtain minerals from soil.

Analysis of plant ash shows about 92 mineral elements in different plants.

Out of these, 16 elements are classically considered essential for plants.

Modern classification includes 17 essential elements.

Types Of Essential Elements

Macronutrients

Macronutrients are required in large amounts.

Examples:

• Carbon
• Hydrogen
• Oxygen
• Nitrogen
• Sulphur
• Phosphorus
• Potassium
• Calcium
• Magnesium

Micronutrients

Micronutrients are required in very small amounts.

Examples:

• Iron
• Copper
• Zinc
• Manganese
• Molybdenum
• Chlorine

Trace Elements

Trace elements are used to detect metabolic pathways in plants.

Examples:

• C¹⁴
• N¹⁵
• P³²

Photosynthesis

Photosynthesis is the process by which green plants prepare food using carbon dioxide and water in the presence of sunlight and chlorophyll.

Chloroplast is the site of photosynthesis.

Most photosynthesis occurs in green leaves.

Importance Of Photosynthesis

• Prepares food for plants
• Releases oxygen
• Converts light energy into chemical energy
• Supports food chains

About 90% of total photosynthesis in the world is done by algae in oceans and freshwater.

Photosynthesis Equation

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂

This reaction occurs in the presence of sunlight and chlorophyll.

Pigments In Photosynthesis

Green plants carry out photosynthesis because they contain:

• Chlorophyll-a
• Chlorophyll-b
• Accessory pigments

Leaves appear green because chlorophyll absorbs red and blue light and reflects green light.

Magnesium is an important constituent of the chlorophyll molecule.

Factors Affecting Photosynthesis

• Carbon dioxide
• Light
• Water
• Temperature

Reactions Of Photosynthesis

Photosynthesis includes two main reactions:

• Light reaction
• Dark reaction

Light Reaction Or Hill Reaction

Light reaction occurs in the grana of chloroplast.

It involves:

• Photolysis of water
• Production of ATP
• Production of NADPH₂
• Photophosphorylation

The rate of photosynthesis is measured in quantum yield.

The quantum requirement in photosynthesis is 8.

Photosystems

There are two photosystems:

• PS-I
• PS-II

Reaction centres:

• P700 is the reaction centre of PS-I
• P680 is the reaction centre of PS-II

Dark Reaction Or Blackman’s Reaction

Dark reaction occurs in the stroma or matrix of chloroplast.

It is also known as:

• Calvin cycle
• C₃ cycle

In this cycle, one mole of hexose sugar is formed using:

• 18 ATP
• 12 NADPH₂

Carbon dioxide fixation occurs in the presence of RuBisCO, the most abundant enzyme.

C₄ Cycle

In some plants, the C₄ cycle occurs instead of the C₃ cycle.

The first stable product of carbon dioxide fixation is a four-carbon compound called oxaloacetic acid.

This is the first product of the Hatch and Slack pathway.

C₄ plants show Kranz anatomy.

Examples Of C₄ Plants

• Sugarcane
• Maize
• Cyperus
• Amaranthus

CAM Plants

In CAM plants, carbon dioxide fixation occurs at night.

CAM stands for Crassulacean Acid Metabolism.

Examples

• Opuntia
• Bryophyllum
• Agave

Bacterial Photosynthesis

Some purple and green sulphur bacteria can prepare food in the presence of light and absence of oxygen.

This is called bacterial photosynthesis.

Type	Example
Green sulphur bacteria	Chlorobium
Purple sulphur bacteria	Chromatium
Non-sulphur bacteria	Rhodospirillum

Chemosynthesis

Chemosynthesis is the formation of organic food using chemical energy by some aerobic bacteria.

Examples:

• Nitrifying bacteria – Nitrosomonas, Nitrococcus, Nitrobacter
• Sulphur bacteria – Beggiatoa, Thiothrix, Thiobacillus
• Iron bacteria – Ferrobacillus, Leptothrix, Cladothrix
• Hydrogen bacteria – Bacillus pantotrophus
• Carbon bacteria – Bacillus oligocarbophilus

Respiration In Plants

Respiration is the biochemical process in which organic food materials, mainly glucose, are oxidised inside living cells to release energy.

This energy is stored temporarily in the form of ATP.

Respiration occurs in all living plant cells continuously, during both day and night.

Respiration Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Respiration is a catabolic and exothermic process.

Types Of Respiration

Aerobic Respiration

Aerobic respiration occurs in the presence of oxygen.

Glucose is completely oxidised into carbon dioxide and water.

It releases a large amount of energy, about 36-38 ATP molecules.

It mainly occurs in mitochondria.

Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen.

Glucose is partially broken down and less energy is released.

In yeast and some microorganisms:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy

In animal muscles during oxygen shortage:

C₆H₁₂O₆ → 2C₃H₆O₃ + Energy

Respiratory Quotient

Respiratory Quotient, or RQ, is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration.

Formula:

RQ = Volume of CO₂ evolved / Volume of O₂ consumed

Plant Hormones

Plant hormones are chemical substances that regulate plant growth and development.

Important plant hormones are:

• Auxins
• Gibberellins
• Cytokinins
• Ethylene
• Abscisic Acid

Auxins

Auxins are growth-promoting hormones.

They help in cell elongation and are involved in phototropic movement.

Phototropism, or bending of the plant towards light, is controlled by auxin.

IAA is an important natural auxin.

Gibberellins

Gibberellins promote stem elongation and growth.

They are also involved in bolting.

Antigibberellins suppress the effect of gibberellins.

Examples of antigibberellins:

• Phosphon-D
• Maleic hydrazide
• Chlorocholine Chloride

Cytokinins

Cytokinins promote cytokinesis, or cell division.

Kinetin was first isolated from a degraded sample of DNA.

Zeatin was the first natural cytokinin isolated from maize endosperm.

Cytokinins are synthesized in roots and stems.

Functions Of Cytokinins

• Promote cell division
• Promote cell enlargement
• Prevent senescence
• Help in enzyme synthesis
• Promote flowering in short-day plants

The auxin-cytokinin ratio helps in root-shoot initiation.

Ethylene

Ethylene is the only gaseous plant hormone.

It promotes transverse growth.

It is known as the ripening hormone.

Functions Of Ethylene

• Hastens fruit ripening
• Promotes ageing of plant organs
• Acts mainly as a natural growth inhibitor

Ethylene is used for artificial ripening of green fruits.

Abscisic Acid

Abscisic Acid, or ABA, acts as a growth inhibitor.

It counteracts the effect of other plant hormones.

Functions Of ABA

• Causes dormancy in buds and seeds
• Promotes ageing in leaves
• Inhibits mitosis
• Promotes abscission of leaves, flowers and fruits
• Suppresses growth of dormant lateral buds

Other plant hormones include:

• Florigens
• Caline
• Traumatin

Key Takeaways

• Plant morphology studies the external structure of plant organs.
• Roots, stems, leaves, flowers and fruits may show modifications for special functions.
• Leaves are the main organs of photosynthesis and transpiration.
• Flowers are modified shoots meant for sexual reproduction.
• Fruits are mature ovaries.
• Transpiration helps in ascent of sap but also causes water loss.
• Plants require macro and micronutrients for normal growth.
• Photosynthesis converts light energy into chemical energy.
• Respiration releases energy from food in the form of ATP.
• Plant hormones regulate growth, development, dormancy, ripening and ageing.

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