The Flower

The Flower

Parts of a Flower (4 Whorls)

A typical bisexual flower, the reproductive structure of angiosperms, is composed of four main whorls or sets of organs, arranged concentrically on the receptacle (the enlarged tip of the flower stalk, or pedicel, to which the floral parts are attached). These whorls are:

1. Calyx (Sepals):

   • Structure: This is the outermost whorl of the flower, typically consisting of leaf-like structures called sepals. Sepals are usually green, but in some flowers, they can be colored like petals (petaloid sepals). All sepals collectively form the calyx. They can be free (polysepalous) or fused (gamosepalous).
   • Function: The primary function of the calyx is to protect the inner, more delicate parts of the flower, especially when it is in the bud stage. They enclose and safeguard the developing petals, stamens, and pistil from mechanical injury, desiccation, and sometimes from herbivores.

2. Corolla (Petals):

   • Structure: Located just inside the calyx, this whorl is made up of petals. Petals are often brightly colored, large, and sometimes fragrant, varying greatly in shape, size, and texture among different species. All petals collectively form the corolla. Like sepals, petals can be free (polypetalous) or fused (gamopetalous), forming a tube or bell shape.
   • Function: The vibrant colors, attractive shapes, and often pleasant fragrances of petals, along with the presence of nectar glands (nectaries) at their base, serve to attract various pollinators (like insects, birds, and bats). They act as visual and olfactory signals, guiding pollinators towards the reproductive organs.

3. Androecium (Stamens):

   • Structure: This is the male reproductive whorl, located inside the corolla. It consists of one or more stamens. Each stamen is typically composed of two main parts:
     • Anther: A bilobed (two-lobed) structure that contains pollen sacs (microsporangia). These sacs are where pollen grains (which contain the male gametes or microspores) are produced through meiosis.
     • Filament: A slender stalk that supports the anther, positioning it appropriately for pollen dispersal.
   • Function: The primary function of the androecium is the production and release of pollen grains, which are essential for fertilization.

4. Gynoecium (Pistil/Carpel):

   • Structure: This is the innermost and female reproductive whorl, located at the very center of the flower. It consists of one or more carpels. A single carpel or a group of fused carpels forms a pistil. Each carpel (or pistil) typically has three distinct parts:
     • Stigma: The receptive tip of the pistil, often sticky or feathery, designed to capture and hold pollen grains.
     • Style: A slender, stalk-like structure that connects the stigma to the ovary. It provides a pathway for the pollen tube to grow down to the ovules.
     • Ovary: The swollen basal part of the pistil, located at the base of the style. It contains one or more ovules. Each ovule contains an egg cell (female gamete) and, after fertilization, develops into a seed.
   • Function: The gynoecium's main function is to receive pollen, facilitate fertilization, and protect the developing ovules, which will mature into seeds within the fruit.

Pollination

Pollination is a crucial step in the sexual reproduction of flowering plants (angiosperms) and conifers (gymnosperms). It is defined as the transfer of pollen grains from the anther (male reproductive part) to the stigma (female receptive part) of a flower, or in gymnosperms, to the ovule.

There are two main types of pollination:

• Self-Pollination (Autogamy): This occurs when pollen grains are transferred from the anther to the stigma of the same flower (autogamy) or to another flower on the same plant (geitonogamy). Self-pollination ensures seed production even in the absence of pollinators and maintains genetic purity, but it limits genetic diversity.
• Cross-Pollination (Allogamy): This involves the transfer of pollen grains from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species. Cross-pollination promotes genetic recombination and leads to greater genetic diversity, which can enhance a species' adaptability to changing environments. It often requires external agents for pollen transfer.

Agents of Cross-Pollination (Pollinating Agents)

Cross-pollination relies on various external agents, which can be abiotic (non-living) or biotic (living):

1. Wind (Anemophily):

   • Characteristics of Wind-Pollinated Flowers: These flowers are typically small, inconspicuous, and lack bright colors, scent, and nectar, as they don't need to attract animals. They produce enormous quantities of very light, dry, and smooth pollen grains that can be easily carried by wind currents. The anthers are often large and exposed to release pollen effectively, and the stigmas are large, feathery, or branched to efficiently trap airborne pollen.
   • Examples: Grasses (e.g., Maize, Wheat, Rice), Pine, Oak, Ragweed.

2. Water (Hydrophily):

   • Characteristics of Water-Pollinated Flowers: This is a less common method, primarily found in aquatic plants. The flowers are usually small and inconspicuous. Pollen grains are often light and unwettable, sometimes released on the water surface (e.g., Vallisneria) or dispersed underwater (e.g., Hydrilla, seagrasses).
   • Examples: Vallisneria, Hydrilla, Zostera (seagrass).

3. Insects (Entomophily):

   • Characteristics of Insect-Pollinated Flowers: These flowers are highly adapted to attract insects. They are often large, brightly colored, and may have specific patterns (nectar guides) visible under UV light to guide insects. They produce sweet fragrances and nectar (a sugary liquid) as a reward for pollinators. Pollen grains are typically larger, sticky, or spiny, allowing them to adhere easily to the insect's body. Stigmas are often sticky or have specific shapes to ensure pollen attachment.
   • Examples: Rose, Sunflower, Hibiscus, Pea, Orchids, Apple, Citrus.

4. Birds (Ornithophily):

   • Characteristics of Bird-Pollinated Flowers: These flowers are often large, brightly colored (especially red or orange), and lack strong scents (as birds have a poor sense of smell). They produce abundant, dilute nectar to meet the high energy demands of birds. The flowers are typically tubular or funnel-shaped, allowing birds to insert their beaks. Pollen is often sticky.
   • Examples: Hummingbird-pollinated flowers (e.g., Fuchsia, Hibiscus), Bird of Paradise.

5. Bats (Chiropterophily):

   • Characteristics of Bat-Pollinated Flowers: These flowers typically open at night, are large, dull-colored (white or cream), and emit strong, musky, or fruity odors to attract nocturnal bats. They produce large amounts of nectar and pollen.
   • Examples: Baobab, Saguaro cactus, some Agave species.

Fertilization

Fertilization in flowering plants is the crucial process where the male gamete (from the pollen grain) fuses with the female gamete (egg cell) within the ovule to form a zygote. This event initiates the development of the seed and fruit.

Process of Fertilization (Double Fertilization in Angiosperms):

1. Pollen Germination: After successful pollination, a compatible pollen grain lands on the receptive stigma. The stigma secretes a sugary fluid that stimulates the pollen grain to germinate. A small tube, called the pollen tube, begins to grow out from the pollen grain.
2. Pollen Tube Growth: The pollen tube elongates and grows downwards through the style, navigating through the stigma and style tissues. The growth is guided by chemical signals from the ovule.
3. Entry into Ovule: The pollen tube eventually reaches the ovary and enters an ovule, typically through a small opening called the micropyle.
4. Discharge of Male Gametes: As the pollen tube enters the ovule, it discharges two male gametes (sperm nuclei) into the embryo sac (the female gametophyte within the ovule).
5. Double Fertilization: This is a unique characteristic of angiosperms:
   • First Fertilization (Syngamy): One male gamete fuses with the egg cell to form a diploid (2n) zygote. The zygote will develop into the embryo, which is the rudimentary plant within the seed.
   • Second Fertilization (Triple Fusion): The other male gamete fuses with the central cell (which contains two polar nuclei) to form a triploid (3n) primary endosperm nucleus. This nucleus will develop into the endosperm, a nutritive tissue that provides food for the developing embryo.

After double fertilization, the ovule develops into a seed, and the ovary matures into a fruit.

Formation of Fruit

Following successful fertilization, a series of profound developmental changes occur within the flower, leading to the formation of the fruit and seeds. This transformation is a remarkable example of plant development and ensures the protection and dispersal of the next generation.

• Ovary: The most significant transformation occurs in the ovary. After fertilization, the ovary wall begins to thicken and mature, developing into the fruit wall, also known as the pericarp. The entire ovary, including its contents, develops into the fruit. The fruit's primary role is to protect the developing seeds and aid in their dispersal.
• Ovules: Each ovule inside the ovary, having been fertilized, develops into a seed. The integuments (outer layers) of the ovule harden to form the protective seed coat, and the fertilized egg (zygote) develops into the embryo, while the primary endosperm nucleus develops into the endosperm (nutritive tissue).
• Other Floral Parts: Most other floral parts typically wither, dry up, and fall off after fertilization, as their function in attracting pollinators and facilitating fertilization is complete. This includes the:
  • Sepals: Usually wither and fall off, though in some fruits (e.g., tomato, brinjal, strawberry), they may persist and remain attached to the fruit.
  • Petals: Almost always wither and fall off.
  • Stamens: Wither and fall off.
  • Style and Stigma: Wither and fall off, though sometimes the persistent style can be seen (e.g., on a corn cob, the silks are persistent styles).

In essence, the fruit is a mature or ripened ovary, developed after fertilization, containing seeds.

Parts of Fruits

Fruits are mature ovaries that develop after fertilization, enclosing and protecting the seeds. They play a crucial role in seed dispersal. Fruits can be broadly classified based on the nature of their pericarp (fruit wall) at maturity:

• Dry Fruits: In these fruits, the pericarp becomes dry, hard, and often woody or papery at maturity. They are further categorized based on whether they dehisce (split open) to release seeds or remain indehiscent (do not split open).
  • Dehiscent Dry Fruits: Split open at maturity to release seeds (e.g., Pea pod, Bean, Cotton, Poppy).
  • Indehiscent Dry Fruits: Do not split open at maturity; the seed remains enclosed within the fruit (e.g., Groundnut, Rice, Wheat, Maize, Sunflower).
• Fleshy Fruits: In these fruits, the pericarp remains soft, juicy, and often edible at maturity. They are typically adapted for dispersal by animals.
  • Examples: Mango, Apple, Tomato, Orange, Grape, Berry, Peach, Cherry.

Parts of the Pericarp of Fleshy Fruits

The pericarp, or fruit wall, of fleshy fruits is typically differentiated into three distinct layers, each with a specific role:

1. Epicarp (Exocarp):
   • Structure: This is the outermost layer of the fruit, forming the skin or peel. It can be thin (e.g., grape), thick (e.g., orange), or hairy (e.g., peach).
   • Function: Its primary function is to provide protection to the inner parts of the fruit and the developing seeds from mechanical injury, desiccation, and pathogens. It may also contain pigments that give the fruit its characteristic color, attracting dispersers.
2. Mesocarp:
   • Structure: This is the middle layer of the pericarp, and it is often the fleshy, juicy, and edible part of the fruit. Its thickness and texture vary greatly among different fruits (e.g., thin in cherry, thick and fibrous in mango, pulpy in tomato).
   • Function: The mesocarp is primarily responsible for storing food (sugars, starches, vitamins) and water, making the fruit palatable and nutritious. This palatability encourages animals to consume the fruit, thereby aiding in seed dispersal.
3. Endocarp:
   • Structure: This is the innermost layer of the pericarp, which directly encloses the seed(s). Its texture can vary significantly. It can be thin and membranous (e.g., in citrus fruits like orange), hard and stony (e.g., in drupes like mango, peach, plum, where it forms the pit or stone), or cartilaginous.
   • Function: The endocarp provides an additional layer of protection for the delicate seed(s) inside. In stony fruits, it forms a hard protective shell around the seed.

Seed

A seed is a fundamental unit of reproduction in flowering plants (angiosperms) and conifers (gymnosperms). It is a mature, fertilized ovule that contains a dormant embryo (a miniature plant), a supply of stored food (to nourish the embryo during germination), and is enclosed within a protective seed coat. Seeds are crucial for the survival and dispersal of plant species.

Parts of a Seed

Despite variations in size and shape, most seeds share common structural components:

• Seed Coat: This is the tough, outer protective layer of the seed. It develops from the integuments of the ovule and serves to protect the embryo and stored food from mechanical injury, desiccation (drying out), and attack by pathogens or insects. It can be thin and papery or thick and hard.
• Embryo: This is the miniature, undeveloped plant contained within the seed. It is the result of the fusion of the male and female gametes during fertilization. The embryo consists of several key parts:
  • Embryonic Axis: The central axis from which the root and shoot systems develop.
  • Radicle: This is the embryonic root. It is the first part of the embryo to emerge during germination and develops into the primary root system of the seedling, anchoring the plant and absorbing water and nutrients.
  • Plumule: This is the embryonic shoot. It consists of a tiny shoot apex and young leaves, and it develops into the shoot system (stem and leaves) of the seedling, responsible for photosynthesis.
  • Cotyledon(s) (Seed Leaves): These are embryonic leaves that are part of the embryo. Their primary function is to store food reserves (e.g., starch, proteins, fats) for the developing embryo until it can photosynthesize independently. In some seeds, they may also help in absorbing and transferring food from the endosperm to the embryo.
• Endosperm (Optional): In many seeds, particularly monocots, the endosperm is a separate nutritive tissue that develops from the primary endosperm nucleus after double fertilization. It stores food (mainly starch) and provides nourishment to the developing embryo. In some dicots (e.g., pea, bean), the endosperm is absorbed by the cotyledons during seed development, and the cotyledons become the primary storage organs.

Types of Seeds

Seeds are broadly classified based on the number of cotyledons present in the embryo:

• Monocotyledonous (Monocot) Seed: These seeds contain a single cotyledon. The cotyledon in monocots is often shield-shaped (scutellum) and primarily functions to transfer nutrients from the endosperm to the embryo during germination. The endosperm is usually prominent and serves as the main food storage tissue.
  • Examples: Maize, Rice, Wheat, Barley, Onion, Grasses.
• Dicotyledonous (Dicot) Seed: These seeds contain two cotyledons. The cotyledons are typically fleshy and store the food reserves for the embryo. In dicots, the endosperm may be present (e.g., castor bean) or absent (e.g., pea, bean, gram), having been absorbed by the cotyledons during development.
  • Examples: Bean, Pea, Gram, Sunflower, Mango, Mustard.

Germination

Germination is the process by which a seed sprouts and grows into a new plant.

Conditions Required for Germination

1. Moisture (Water): Seeds need water to swell up, soften the seed coat, and activate enzymes that break down stored food.
2. Warmth (Suitable Temperature): Each seed has an optimal temperature range for germination. Enzymes involved in germination function best at these temperatures.
3. Air (Oxygen): Oxygen is required for the respiration of the embryo to release energy for growth.

Seed Germination of Different Seeds

• Bean Seed (Dicot - Epigeal Germination): The cotyledons are pushed above the soil surface by the elongation of the hypocotyl (the part of the stem below the cotyledons).
• Maize Seed (Monocot - Hypogeal Germination): The cotyledon remains below the soil surface, and the plumule emerges above ground.