Detailed Notes- 2-Chapter 5 Morphology of Flowering Plants

5.5 THE FLOWER

1. Definition and Purpose:

   • The flower is the reproductive unit in angiosperms, meant for sexual reproduction.

2. Structure of a Typical Flower:

   • A typical flower consists of four different whorls arranged successively on the swollen end of the stalk or pedicel, called the thalamus or receptacle.
   • These whorls are: calyx, corolla, androecium, and gynoecium.
   • Calyx and corolla are accessory organs, while androecium and gynoecium are reproductive organs.
   • In some flowers, like lilies, the calyx and corolla are not distinct and are collectively termed as the perianth.

3. Sexual Characteristics:

   • A flower with both androecium and gynoecium is bisexual.
   • A flower having either only stamens or only carpels is unisexual.

4. Symmetry:

   • Flowers may exhibit actinomorphic (radial symmetry) or zygomorphic (bilateral symmetry).
   • Actinomorphic flowers can be divided into two equal radial halves in any radial plane passing through the center.
   • Zygomorphic flowers can only be divided into two similar halves in one particular vertical plane.
   • Asymmetric (irregular) flowers cannot be divided into two similar halves by any vertical plane passing through the center.

5. Number of Floral Appendages:

   • Flowers may have trimerous (in multiples of 3), tetramerous (in multiples of 4), or pentamerous (in multiples of 5) floral appendages.

6. Presence of Bracts:

   • Flowers with reduced leaves (bracts) found at the base of the pedicel are called bracteate, while those without bracts are ebracteate.

7. Position of Floral Parts:

   • Flowers can be described as hypogynous, perigynous, or epigynous based on the position of calyx, corolla, and androecium in respect to the ovary on the thalamus.
   • In hypogynous flowers, the gynoecium occupies the highest position with the other parts situated below it, and the ovary is superior.
   • In perigynous flowers, the gynoecium is in the center, and other parts are located on the rim of the thalamus at the same level, making the ovary half-inferior.
   • In epigynous flowers, the thalamus margin grows upward enclosing the ovary completely, and the ovary is inferior.

5.5.1 Parts of a Flower

1. Calyx:

   • The calyx is the outermost whorl of the flower, and its individual parts are called sepals.
   • Sepals are typically green, leaf-like structures that protect the flower in the bud stage.
   • The calyx may be either gamosepalous (sepals united) or polysepalous (sepals free).

2. Corolla:

   • The corolla is the second whorl of the flower, composed of petals.
   • Petals are usually brightly colored to attract insects for pollination.
   • Similar to the calyx, the corolla may be gamopetalous (petals united) or polypetalous (petals free).
   • The shape and color of the corolla vary greatly among plants, with options like tubular, bell-shaped, funnel-shaped, or wheel-shaped.

3. Aestivation:

   • Aestivation refers to the mode of arrangement of sepals or petals in the floral bud with respect to other members of the same whorl.
   • Common types of aestivation include valvate, twisted, imbricate, and vexillary.
   • Valvate aestivation occurs when sepals or petals just touch each other at the margin without overlapping.
   • Twisted aestivation is when one margin of the appendage overlaps the next one, as seen in China rose, lady’s finger, and cotton.
   • Imbricate aestivation happens when the margins of sepals or petals overlap each other but not in any particular direction.
   • Vexillary (or papilionaceous) aestivation is found in pea and bean flowers, where the largest petal overlaps the two lateral petals, which then overlap the two smallest anterior petals.

5.5.1.3 Androecium

1. Composition:

   • The androecium is composed of stamens, which represent the male reproductive organ of the flower.

2. Structure of a Stamen:

   • Each stamen consists of a stalk or filament and an anther.
   • The anther is usually bilobed, with each lobe containing two chambers called pollen sacs. Pollen grains are produced within these pollen sacs.

3. Sterile Stamens:

   • A stamen that does not produce pollen is called a staminode.

4. Attachment and Union of Stamens:

   • Stamens may be attached to other floral parts such as petals or among themselves.
   • When stamens are attached to the petals, they are called epipetalous (as in brinjal), or epiphyllous when attached to the perianth (as in lily flowers).
   • Stamens in a flower may remain free (polyandrous) or may be united to varying degrees.
   • They may be united into one bundle (monoadelphous), two bundles (diadelphous), or more than two bundles (polyadelphous).
   • There may also be variations in the length of filaments within a flower, as observed in Salvia and mustard.

5.5.1.4  Gynoecium

1. Composition:

   • The gynoecium is the female reproductive part of the flower and is composed of one or more carpels.

2. Structure of a Carpel:

   • A carpel consists of three main parts: stigma, style, and ovary.
   • The ovary is the enlarged basal part of the carpel, containing one or more ovules attached to a flattened, cushion-like placenta.
   • The style is an elongated tube connecting the ovary to the stigma.
   • The stigma is usually located at the tip of the style and serves as the receptive surface for pollen grains.

3. Types of Carpels:

   • Carpels may be present as single units (as in lotus and rose) or may be fused (syncarpous) as in mustard and tomato.

4. Fruit Development:

   • After fertilization, the ovules develop into seeds, and the ovary matures into a fruit.

5. Placentation:

   • Placentation refers to the arrangement of ovules within the ovary.
   • Different types of placentation include marginal, axile, parietal, basal, central, and free central.
   • Marginal placentation forms a ridge along the ventral suture of the ovary, with ovules borne on this ridge in two rows (as in pea).
   • Axile placentation occurs when the placenta is axial, and ovules are attached to it in a multilocular ovary (as in china rose, tomato, and lemon).
   • In parietal placentation, ovules develop on the inner wall or peripheral part of the ovary.
   • Basal placentation involves the development of the placenta at the base of the ovary, with a single ovule attached to it (as in sunflower and marigold).

5.6 THE FRUIT

1. Definition and Formation:

   • The fruit is a characteristic feature of flowering plants, representing the mature or ripened ovary developed after fertilization.
   • If a fruit is formed without fertilization of the ovary, it is called a parthenocarpic fruit.

2. Composition:

   • Generally, the fruit consists of a wall called the pericarp and seeds.
   • The pericarp may be dry or fleshy.

3. Structure of Pericarp:

   • When the pericarp is thick and fleshy, it is differentiated into three layers:
     • Outer epicarp
     • Middle mesocarp
     • Inner endocarp

4. Types of Fruits:

   • In some fruits like mango and coconut, the fruit is classified as a drupe.
   • Drupe fruits develop from monocarpellary superior ovaries and are usually one-seeded.
   • In mango, the pericarp is well differentiated into an outer thin epicarp, a middle fleshy edible mesocarp, and an inner stony hard endocarp.
   • In coconut, which is also a drupe, the mesocarp is fibrous.

5.7 THE SEED

1. Seed Formation:

   • After fertilization, the ovules develop into seeds.

2. Seed Composition:

   • A seed is composed of two main parts: the seed coat and the embryo.

3. Embryo Structure:

• The embryo consists of several parts:

• Radicle: The embryonic root of the plant.

• Embryonal axis: The central axis of the embryo from which the radicle and other parts develop.

• Cotyledons: Structures that serve as the first leaves of the seedling. The number of cotyledons varies among different plants.

  • Monocotyledonous plants (such as wheat and maize) have seeds with a single cotyledon.

  • Dicotyledonous plants (such as gram and pea) have seeds with two cotyledons.

5.7.1 Structure of a Dicotyledonous Seed 

1. Seed Coat:

• The outermost covering of the seed.

• Comprises two layers:

  • Outer testa

  • Inner tegmen

1. Hilum:

   • A scar on the seed coat where the developing seeds were attached to the fruit.

2. Micropyle:

   • A small pore above the hilum.

3. Embryo:

   • Located within the seed coat.
   • Consists of:
     • Embryonal axis: The central axis of the embryo.
     • Cotyledons: Usually two, often fleshy and full of reserve food materials.
     • Radicle: The embryonic root.
     • Plumule: The embryonic shoot.

4. Endosperm:

   • In some seeds like castor, formed as a result of double fertilization, it is a food-storing tissue.
   • In seeds like bean, gram, and pea, the endosperm is not present in mature seeds, and such seeds are called non-endospermous.

5.7.2 Structure of Monocotyledonous Seed

1. Endosperm:

   • Generally, monocotyledonous seeds are endospermic, meaning they contain endosperm tissue that stores food.
   • In some monocots like orchids, the seeds are non-endospermic.

2. Seed Coat:

   • In cereals like maize, the seed coat is membranous and typically fused with the fruit wall.

3. Endosperm Structure:

   • The endosperm is bulky and serves as the primary food store for the developing embryo.
   • It is separated from the embryo by a proteinous layer called the aleurone layer.

4. Embryo:

   • The embryo is small and situated in a groove at one end of the endosperm.
   • It consists of:
     • One large and shield-shaped cotyledon known as the scutellum.
     • A short axis with a plumule (embryonic shoot) and a radicle (embryonic root).

5. Sheaths:

   • The plumule and radicle are enclosed in protective sheaths:
     • The plumule is enclosed in a sheath called the coleoptile.
     • The radicle is enclosed in a sheath called the coleorhiza.

5.8 SEMI-TECHNICAL DESCRIPTION OF A TYPICAL FLOWERING PLANT

Habit: The mustard plant (Family: Brassicaceae) is an annual or biennial herbaceous plant, growing upright with a main stem and branching at the upper portions.

Vegetative Characters:

• Roots: The plant possesses a taproot system with lateral roots branching off.
• Stem: The stem is erect, cylindrical, and bears nodes and internodes. It is green when young, becoming woody and brown with age.
• Leaves: The leaves are simple, alternate, and pinnately lobed. They have a petiole and lamina, with toothed margins.

Floral Characters:

• Inflorescence: The plant bears racemose inflorescences, with flowers arranged acropetally along the main axis.
• Flowers: The mustard plant produces bisexual flowers, actinomorphic in nature. Each flower consists of five sepals (K), five petals (C), six stamens (A), and a superior ovary (G).

Floral Diagram:

• The floral diagram provides a visual representation of the flower’s structure, arrangement, and relationship between its parts.
• In the diagram for the mustard plant, the mother axis is represented by a dot at the top. The calyx (K), corolla (C), androecium (A), and gynoecium (G) are drawn in successive whorls, with the gynoecium at the center.

Floral Formula:

• The floral formula summarizes the floral structure using symbols, indicating the number of parts in each whorl and their relationships.
• In the floral formula for the mustard plant, symbols represent various floral parts (Br for bracteate, K for calyx, C for corolla, A for androecium, G for gynoecium). Fusion and adhesion between parts are indicated by brackets and lines respectively.

5.9 SOLANACEAE

Vegetative Characters:

• Habit: Mostly herbs, shrubs, and occasionally small trees.
• Stem: Typically herbaceous, sometimes woody; aerial, erect, cylindrical, branched, and may be solid or hollow. Can be hairy or smooth. Underground stem seen in plants like the potato (Solanum tuberosum).
• Leaves: Alternate arrangement, usually simple but may rarely be pinnately compound; stipules are absent. Venation is reticulate.

Floral Characters:

• Inflorescence: Solitary, axillary, or arranged cymosely as seen in Solanum species.
• Flower: Bisexual and actinomorphic.
• Calyx: Composed of five sepals that are fused, persistent, and show valvate aestivation (overlapping but not fused edges).
• Corolla: Consists of five united petals, also exhibiting valvate aestivation.
• Androecium: Typically with five stamens, positioned opposite the petals (epipetalous).
• Gynoecium: Consists of a bicarpellary ovary that is superior (above the attachment point of other floral parts), bilocular (divided into two chambers), and syncarpous (fused carpels). The placenta inside the ovary is swollen, bearing numerous axile ovules.

Fruits and Seeds:

• Fruits: Commonly berries or capsules.
• Seeds: Numerous and endospermous (contain endosperm tissue).

Floral Formula: 

Economic Importance:

• Solanaceae family plants hold significant economic importance:
  • Source of food: Including tomatoes, brinjals (eggplants), and potatoes.
  • Spice production: Chili peppers.
  • Medicinal use: Examples include belladonna and ashwagandha.
  • Fumigatory purposes: Tobacco.
  • Ornamental plants: Such as petunias.