Seed – Structure, Types, Development, Germination, Dispersal

What is Seed?

• A seed is a fundamental unit of reproduction in flowering plants, serving as the mature form of the fertilized ovule. Encased within this small structure lies an embryonic plant, accompanied by stored nutrients and a protective coat, all of which are crucial for its eventual germination.
• This tiny embryo, suspended in a dormant state, results from the fertilization process within a flower. As the reproductive mechanism of phanerogams, or flowering plants, seeds carry the genetic blueprint necessary for the continuation of their species. The dormant state of a seed is not merely inactive; it is a survival strategy. This dormancy, often triggered by desiccation and the presence of abscisic acid, ensures that the seed remains viable until it encounters the right conditions for germination.
• Germination marks the transformation of the seed into a growing plant. During this phase, the stored nutrients within the seed fuel the initial growth until the plant can photosynthesize on its own. Seeds are adapted to survive adverse environmental conditions, a trait known as perennation, allowing them to remain viable across seasons or even years.
• The dispersal of seeds is a critical stage in the life cycle of flowering plants. Seeds can be spread over vast distances by various agents such as wind, water, or animals, facilitating the propagation of the species.
• Beyond their role in reproduction, seeds have immense value as a source of food and other products. Cereals like wheat and rice are staple foods derived from seeds, while others, such as sunflower and soybean, are processed for oil production. Seeds are also the source of many essential oils, stimulants like coffee, and spices like pepper, highlighting their significance in both agriculture and industry.

Size and shape of Seed

The size and shape of seeds exhibit considerable diversity, reflecting their adaptation to various ecological niches and functional requirements. This variability plays a crucial role in dispersal, survival, and germination strategies.

1. Size:
   • Seed size ranges widely across different plant species. For instance, the smallest seeds are those of orchids, which lack reserve nutrients and rely on symbiotic relationships with mycorrhizal fungi for nutrition. Conversely, the largest seeds are found in the double coconut palm, showcasing the extreme end of seed size variability.
   • Generally, seed size is influenced by genetic factors. For example, wheat seeds are relatively uniform in size due to their genetic makeup. In some species, such as peas, seed size can vary within the same plant. Seeds located in the central part of the pod are often larger, as they experience less competition for nutrients compared to those at the pod’s periphery.
2. Shape:
   • Seed shape also varies significantly. Seeds can be round, elongated, flat, or wrinkled. The shape can affect how seeds are dispersed and their subsequent germination. For instance, the flat seeds of the tropical mora may face restrictions during germination due to their shape, which can impact their ability to penetrate the soil effectively.
   • In contrast, the seeds of Panicum turgidum, which are convex, can germinate from their rounded side rather than the flat side. This demonstrates how specific seed shapes can adapt to different environmental conditions and enhance survival.
3. Surface Characteristics:
   • Seeds also differ in surface texture, ranging from smooth to hairy or wrinkled. These surface characteristics can influence seed dispersal mechanisms and protection against environmental factors. Hairy seeds, for example, may be adapted to cling to animals for dispersal, while smooth seeds might be better suited for wind dispersal.

Type of seed

Seeds can be classified based on various criteria, including the number of cotyledons and the presence or absence of endosperm. Understanding these classifications helps in comprehending seed development and plant propagation.

Classification of seed Based on Cotyledons

1. Monocotyledonous Seeds:
   • Definition: These seeds contain a single cotyledon, which is the first leaf or leaf-like structure to appear during seedling development.
   • Examples: Common examples include rice, maize, and orchids. Monocot seeds generally exhibit parallel-veined leaves and floral parts in multiples of three.
2. Dicotyledonous Seeds:
   • Definition: These seeds possess two cotyledons. The cotyledons in dicots often serve as nutrient stores for the developing embryo.
   • Examples: Examples of dicot seeds are peas, beans, mangoes, and mustard. Dicots typically show net-veined leaves and floral parts in multiples of four or five.

Classification of seed Based on Endosperm Presence

1. Albuminous Seeds:
   • Definition: Albuminous seeds retain a significant amount of endosperm, which provides nourishment to the embryo during the early stages of development. The cotyledons in these seeds are usually thin and membranous.
   • Examples:
     • Dicots: Castor and cotton are examples where the endosperm is prominent.
     • Monocots: Maize, wheat, and rice exhibit albuminous characteristics with a substantial endosperm supporting the embryo.
2. Exalbuminous Seeds:
   • Definition: Exalbuminous seeds lack a significant endosperm at maturity. Instead, they store nutrients within the thick, fleshy cotyledons. During early seed development, the embryo utilizes the endospermic tissue, leading to a non-endospermic condition in the mature seed.
   • Examples:
     • Dicots: Peas, beans, and grams are examples where the cotyledons serve as the primary nutrient store.
     • Monocots: Orchids are an example of exalbuminous seeds where the endosperm is absent, and the cotyledons are thick and fleshy.

Structure of seed

The structure of a seed is intricately designed to ensure the survival and successful germination of the plant. Seeds generally consist of three primary components: cotyledons, the embryo, and the seed coat. However, the specific structure varies between monocotyledonous (monocot) and dicotyledonous (dicot) seeds. Below is a detailed examination of the structural differences between these two types of seeds:

Structure of Monocotyledonous Seed

1. Seed Coat:
   • The seed coat in monocots is typically thin and membranous. It remains attached to the fruit wall, providing a protective layer around the seed.
2. Endosperm:
   • The endosperm is a bulky tissue that serves as the primary source of nutrients for the developing embryo. In monocots, the endosperm is usually present, storing energy in the form of starch, proteins, and fats. Some exceptions, like orchids, may lack a prominent endosperm.
3. Aleurone Layer:
   • This protein-rich layer lies between the endosperm and the embryo. The aleurone layer is crucial as it secretes enzymes that break down stored food, making nutrients available to the embryo during germination.
4. Embryo:
   • The embryo is a small, fleshy structure located within a groove in the endosperm. It is diploid and consists of several key parts:
     • Scutellum: A large, shield-like cotyledon that facilitates nutrient absorption from the endosperm to nourish the growing embryo.
     • Embryonal Axis: The axis contains the radicle and plumule. The radicle develops into the root, growing downward, while the plumule gives rise to the shoot, growing upward.
     • Coleoptile and Coleorhiza: These are protective sheaths that cover the plumule and radicle, respectively, safeguarding the delicate tissues during germination.

Structure of Dicotyledonous Seed

1. Seed Coat:
   • The seed coat in dicots is derived from the ovule’s integuments and consists of multiple layers:
     • Testa: The outer, thick layer that provides a robust protective barrier for the seed.
     • Tegmen: The inner, thinner layer that offers additional protection to the inner seed structures.
     • Hilum: A small scar on the seed coat where the seed was attached to the fruit. It resembles a navel and serves as a point of connection.
     • Micropyle: A tiny pore near the hilum that allows the entry of water, oxygen, and pollen tube during fertilization and germination.
     • Raphe: A ridge-like structure that runs along the seed’s groove and remains fused with the testa.
2. Embryo:
   • The embryo in dicots consists of several essential components:
     • Cotyledons: Two fleshy structures that store nutrients and provide sustenance to the developing embryo. Dicots typically have two cotyledons, hence their name.
     • Embryonal Axis: The central axis of the embryo, containing the plumule and radicle. The plumule develops into the shoot, while the radicle forms the root. The axis is further divided into two regions:
       • Hypocotyl: The section of the embryonal axis located below the cotyledons, leading to the radicle.
       • Epicotyl: The portion above the cotyledons that will develop into the plumule.

Development of Seed

Seed development is a complex process that unfolds through several distinct stages: cell division and differentiation, accumulation of food reserves, maturation, and water loss. This process begins with fertilization and proceeds through various specialized mechanisms depending on the type of plant.

Stages of Seed Development

1. Cell Division and Differentiation:
   • Fertilization: The development of a seed starts with the fertilization of the ovule, which follows pollination. In angiosperms, the ovule is surrounded by two layers: the integument and the nucellus. The nucellus undergoes meiosis and subsequent mitotic divisions to produce an eight-nucleate embryo sac.
   • Pollination and Fertilization: The pollen tube penetrates the embryo sac and releases two male gametes. One male gamete fuses with the egg cell to form a diploid zygote, which will develop into the embryo. The second male gamete fuses with two polar nuclei to form a triploid endosperm, which serves as the nutrient source for the developing embryo.
2. Accumulation of Food Reserves:
   • Endosperm Formation: The endosperm accumulates food reserves essential for the embryo’s growth. In albuminous seeds, such as those of maize and wheat, the endosperm remains prominent and constitutes a significant portion of the seed. In contrast, in some seeds like those of Tectona, the embryo absorbs most of the nutrition, leading to a minimal or absent endosperm by maturity. In such cases, the cotyledons take over the role of nutrient storage.
3. Maturation:
   • Seed Coat Formation: The integuments of the ovule develop into the seed coat, which has two distinct layers: the outer testa and the inner tegman. This protective coating shields the seed during its development and aids in dispersal.
   • Perisperm: In some seeds, remnants of the nucellar tissue, known as perisperm, may persist and function as an additional food reserve. However, in many cases, the perisperm disappears during seed development.
4. Water Loss:
   • Drying Out: As seeds mature, they undergo a significant reduction in water content, which is crucial for seed dormancy and preservation. This process involves the loss of moisture, which ensures that the seed remains viable until environmental conditions are favorable for germination.

Seed Development in Gymnosperms

1. Ovule Structure:
   • In gymnosperms, the ovule is enclosed by a single integument that is fused with an ovuliferous scale. At fertilization, the nucellus separates from the integument only at the micropyle region.
2. Gametophyte Development:
   • Similar to angiosperms, the nucellus undergoes meiosis and mitosis to form a multicellular, haploid female gametophyte. This gametophyte develops into archegonia, each containing two large egg cells.
3. Fertilization and Seed Formation:
   • One male gamete fertilizes an egg cell to form a zygote, which develops into the embryo. Another male gamete may either be aborted or fertilize additional archegonia but does not participate in forming endosperm.
4. Mature Seed Structure:
   • The mature gymnosperm seed comprises a seed coat derived from the integument, a diploid perisperm (if present), and a haploid gametophyte tissue that nourishes the embryo. This gametophyte tissue serves a similar function to the endosperm in angiosperms. The embryo in gymnosperms includes structures such as the plumule, radicle, and cotyledons, which may vary in number up to 18 in species like Pinus.

Germination of Seed

Seed germination is a crucial biological process wherein a seed develops into a new plant. This process begins with the activation of the embryo’s essential characteristics and occurs under suitable environmental conditions, including optimal atmosphere and soil.

Stages of Seed Germination

Seed germination is a complex physiological process that unfolds in a series of well-defined stages. This process transforms a dormant seed into a fully functional seedling capable of independent growth. Below is a detailed examination of each stage in the germination process:

1. Imbibition:
   • Description: The initial stage of germination begins with imbibition, during which seeds absorb water rapidly. This hydration causes the seed coat to swell and soften.
   • Function: Imbibition activates the seed’s internal physiological processes. This phase marks the start of enzyme activation, which is crucial for initiating the metabolic activities necessary for germination.
   • Outcome: The seed enters a lag phase characterized by increased respiration, protein synthesis, and the mobilization of stored nutrients.
2. Radicle Emergence and Root Development:
   • Description: As the seed coat ruptures, the radicle, or primary root, emerges. This root structure begins to penetrate the soil and establish a connection with the underground water sources.
   • Function: The radicle’s development facilitates the absorption of water and nutrients from the soil. This is essential for the subsequent growth of the seedling.
   • Outcome: Following the radicle’s emergence, the plumule, or shoot, begins to grow upward. This shoot will eventually develop into the stem and leaves of the mature plant.
3. Seedling Development:
   • Description: In the final stage of germination, the cells within the seed become metabolically active. These cells undergo elongation and division, leading to the formation of the seedling.
   • Function: The growth and division of cells are critical for the development of the plant’s structural components, such as stems and leaves. This stage completes the transition from a dormant seed to an actively growing plant.
   • Outcome: The seedling emerges as a fully formed plant structure capable of continuing growth and development in its environment.

Conditions Required for Germination

1. Water:
   • Function: Water is essential for initiating and sustaining the germination process.
   • Details: Seeds often require significant amounts of water relative to their dry weight to begin germination. Water facilitates several key processes:
     • Hydration: It hydrates the seed and initiates metabolic activities.
     • Oxygen Dissolution: It provides dissolved oxygen necessary for respiration.
     • Seed Coat Softening: It softens the seed coat, which aids in the breaking of the seed coat.
     • Nutrient Conversion: Water converts insoluble food reserves into soluble forms that can be transported to the developing embryo.
2. Oxygen:
   • Function: Oxygen is crucial for the energy production required during germination.
   • Details: During germination, seeds rely on aerobic respiration to produce the energy needed for growth. Oxygen must be available in the soil’s pore spaces; seeds buried too deeply may experience oxygen deficiency, impeding germination.
3. Temperature:
   • Function: Temperature influences the rate of biochemical reactions and overall germination speed.
   • Details: The optimum temperature range for most seeds is between 25-30°C. However, different seeds have varied temperature requirements, with some requiring lower or higher temperatures ranging from 5 to 40°C to activate germination processes.
4. Light or Darkness:
   • Function: Light or the absence of light can serve as an environmental cue for germination.
   • Details: Certain seeds require exposure to light to trigger germination, while others may need darkness. This environmental trigger can influence the germination process, ensuring that seeds germinate under optimal conditions for their growth.

Factors Affecting Seed Germination

Seed germination is influenced by a range of factors, which can be broadly categorized into external and internal factors. Each of these factors plays a critical role in determining whether a seed will successfully germinate and develop into a plant.

External Factors Affecting Seed Germination

1. Water:
   • Function: Water is essential for initiating the germination process.
   • Details: Both insufficient and excessive water can impede germination. Adequate hydration is necessary for:
     • Seed Swelling: Water absorption causes the seed to swell and soften the seed coat.
     • Nutrient Activation: Water helps dissolve and mobilize stored nutrients for the growing embryo.
     • Metabolic Processes: Water supports metabolic activities required for germination.
2. Temperature:
   • Function: Temperature influences the rate of biochemical reactions and metabolic activities.
   • Details: Optimal temperatures are required for germination. Deviations from this range can:
     • Slow Metabolism: Lower temperatures can slow down metabolic processes and promote fungal growth.
     • Inhibit Germination: Extremely high temperatures can inhibit germination by denaturing enzymes and disrupting metabolic functions.
3. Oxygen:
   • Function: Oxygen is crucial for aerobic respiration, which provides the energy needed for seed growth.
   • Details: Insufficient oxygen can affect germination as seeds respire vigorously to release energy. Poor oxygen availability can:
     • Impede Respiration: Restrict energy production necessary for germination.
     • Affect Growth: Delay or prevent the emergence of the radicle and other structures.
4. Light or Darkness:
   • Function: Light or the absence of it can act as a trigger for germination.
   • Details: Some seeds require light to germinate, while others need darkness. This factor influences:
     • Germination Timing: Ensures that seeds germinate under conditions that favor successful growth.

Internal Factors Affecting Seed Germination

• Seed Dormancy:
  • Function: Dormancy prevents seeds from germinating even when external conditions are favorable.
  • Details: Several factors contribute to seed dormancy:
    • Seed Coat: A hard, impermeable seed coat can restrict water uptake and gas exchange.
    • Immature Embryo: Seeds with undeveloped or immature embryos cannot germinate.
    • Growth Regulators: Certain seeds contain plant hormones or inhibitors that prevent germination until conditions are optimal.
    • Time Requirement: Some seeds require extended periods to overcome dormancy and initiate germination.

Category	Factor	Function	Details
External Factors	Water	Initiates germination and supports growth	– Adequate hydration is essential for seed swelling. – Helps dissolve and mobilize nutrients. – Supports metabolic activities.
	Temperature	Influences biochemical reactions and metabolism	– Optimal temperatures are necessary. – Low temperatures slow metabolism and promote fungal growth. – High temperatures can inhibit germination.
	Oxygen	Supports aerobic respiration	– Essential for energy production. – Insufficient oxygen impedes respiration and growth. – Poor oxygen availability delays radicle emergence.
	Light/Darkness	Acts as a germination trigger	– Some seeds require light; others need darkness. – Affects timing and conditions for germination.
Internal Factors	Seed Dormancy	Prevents germination under favorable conditions	– Seed Coat: Restricts water uptake and gas exchange. – Immature Embryo: Prevents germination. – Growth Regulators: Inhibit germination. – Time Requirement: Some seeds need extended periods to overcome dormancy.

Types of Germination

1. Epigeal Germination:
   • Process: In epigeal germination, the radicle anchors the seedling while the hypocotyl elongates and pushes above the soil surface. This exposure allows the cotyledons to emerge, and the plumule develops into the primary shoot and leaves.
   • Examples: Common in many dicots, such as beans and sunflowers.
2. Hypogeal Germination:
   • Process: In this type, the cotyledons remain underground while the plumule elongates and grows above the soil. This method allows the cotyledons to remain as a source of nourishment for the developing plant.
   • Examples: Typical in monocots, such as rice and maize.
3. Vivipary Germination:
   • Process: In vivipary, seeds begin to germinate while still attached to the parent plant. The radicle elongates, detaches from the fruit, and falls into the soil, where it continues to develop into a new plant.
   • Examples: Found in mangrove species like Rhizophora and Sonneratia.

Seed Dispersal

Seed dispersal is a fundamental ecological process that ensures the distribution of seeds away from their parent plant. This process not only facilitates plant reproduction but also aids in the colonization of new areas, helping to maintain plant biodiversity. Seed dispersal can occur through various mechanisms, each tailored to the seed’s characteristics and the environment.

Modes of Seed Dispersal

1. Barochory (Gravity Dispersal):
   • Mechanism: Seeds are dispersed by the force of gravity. Heavier seeds fall away from the parent plant and may roll or be buried in the soil. This method often results in localized dispersion.
   • Examples: Apples, mangoes, and coconuts, where the fruit or seed falls directly to the ground due to its weight.
2. Anemochory (Wind Dispersal):
   • Mechanism: Lightweight seeds are carried by the wind. Structures such as wings or tufts help facilitate this process, allowing seeds to travel considerable distances.
   • Examples: Dandelions, cotton seeds, and maple seeds, which have adaptations like parachutes or wings that aid in wind transportation.
3. Hydrochory (Water Dispersal):
   • Mechanism: Seeds with adaptations that allow them to float are dispersed by water. The distance traveled depends on the water’s current and the seed’s buoyancy.
   • Examples: Mangrove seeds and water lilies, which have flat, hollow structures that enable them to drift on water surfaces.
4. Zoochory (Animal Dispersal):
   • Mechanism: Animals play a crucial role in seed dispersal, either by consuming fruits and excreting the seeds elsewhere (endozoochory) or by carrying seeds attached to their fur or feathers (epizoochory).
     • Endozoochory: Seeds are ingested by animals and later excreted, often in nutrient-rich droppings that provide a suitable environment for germination.
       • Examples: Blueberries, tomatoes, and cherries.
     • Epizoochory: Seeds with hooks or spines attach to the external surfaces of animals and are transported to new locations.
       • Examples: Clover leaves and burdock plants.
5. Ballochory (Explosive Dispersal):
   • Mechanism: Seeds are ejected forcefully from the fruit due to internal pressure or tension. This explosion propels the seeds away from the parent plant.
   • Examples: The dynamite tree, okra, and lupins, where the fruit bursts open to scatter the seeds.
6. Anthropochory (Human Dispersal):
   • Mechanism: Humans contribute to seed dispersal, often unintentionally, through activities such as agriculture, gardening, and transport. This mode can lead to wide-ranging seed distribution.
   • Examples: Annual meadow grass, wild chamomile, and stinging nettle, which are spread by human actions, including cultivation and seed handling.

Functions of Seeds

1. Embryo Protection and Nourishment:
   • Function: Seeds house the embryo, which is the initial stage of a new plant. The seed provides a protective environment that safeguards the embryo from physical damage and desiccation.
   • Details: Inside the seed, the embryo is surrounded by tissues that supply essential nutrients. These reserves support the embryo’s growth and development until it can begin photosynthesis.
2. Nutrient Storage:
   • Function: Seeds store nutrients that are crucial for the early stages of seedling development.
   • Details: Depending on the type of seed, nutrients may be stored in the cotyledons (in dicots) or the endosperm (in monocots). These stored nutrients provide the energy needed for germination and the initial growth of the seedling.
3. Dispersal:
   • Function: Seeds facilitate the dispersal of plant species to new locations, which helps in colonizing new areas and reducing competition among seedlings.
   • Details: Seeds can be dispersed by various agents, including wind, water, animals, and mechanical means. This dispersal mechanism ensures that seeds can spread to diverse environments, increasing their chances of survival.
4. Dormancy:
   • Function: Seeds can enter a dormant state to survive unfavorable environmental conditions.
   • Details: During dormancy, seed metabolic activities are reduced, allowing seeds to withstand extreme conditions such as drought or cold. This state ensures that seeds do not germinate until environmental conditions are favorable for seedling survival.
5. Species Continuity:
   • Function: Seeds are vital for maintaining plant species over time.
   • Details: By producing seeds, plants ensure the continuation of their species. Seeds carry genetic information that allows for the propagation of plant traits and adaptation to changing environments.

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