1.7 Seed structure and function

Seeds contain everything a new plant needs to start growing: an embryo, food reserves, and a protective coat. Understanding seed structure and function helps you make sense of plant reproduction, survival strategies, and why seeds behave so differently across species.

Seed composition

A seed is the reproductive unit of a flowering plant. Despite huge variation across species, nearly all seeds share three core components: an embryo, stored food (either in endosperm or cotyledons), and a seed coat. Each component has a distinct job in getting a new plant established.

Embryo structure

The embryo is a miniature plant packed inside the seed. It has four main parts:

• Radicle — the embryonic root. This is the first structure to emerge during germination, anchoring the seedling and absorbing water.
• Plumule — the embryonic shoot. It develops into the stem and first leaves.
• Cotyledons — the seed leaves. These store food reserves like starch, proteins, and lipids, feeding the seedling until it can photosynthesize on its own. Monocots have one cotyledon; dicots have two.
• Hypocotyl — the region connecting the radicle to the cotyledons. During germination, it elongates to push the plumule above the soil surface.

Endosperm roles

The endosperm is nutritive tissue surrounding the embryo. Not all seeds use it the same way:

• In endospermic seeds (like cereals such as wheat and rice), the endosperm persists as the main storage tissue, packed with starch and proteins.
• In non-endospermic seeds (like legumes such as beans and peas), the endosperm is absorbed during development, and food reserves are transferred to the cotyledons instead.

The endosperm also helps regulate dormancy and germination by controlling water uptake and enzyme activity.

Seed coat layers

The seed coat develops from the integuments of the ovule and serves as the seed's armor. It has two primary layers:

• Testa — the outer layer, often hard and thick. It protects against mechanical damage, drying out, and pathogens.
• Tegmen — the inner layer, usually thin and membranous. It helps regulate water and gas exchange.

Some seeds have additional structures that aid dispersal: an aril (a fleshy covering that attracts animals) or mucilage (a sticky substance that helps seeds adhere to soil or animals).

Seed dormancy

Dormancy prevents a seed from germinating until conditions are favorable for survival. Without dormancy, a seed might sprout during a brief warm spell in winter and then die. Both internal factors (genetics, hormones) and external factors (temperature, light) control dormancy.

Dormancy types

• Physiological dormancy — the most common type. Chemical inhibitors like abscisic acid (ABA) block germination until specific environmental cues (certain temperatures or light conditions) are met.
• Physical dormancy — a hard, waterproof seed coat prevents water from reaching the embryo. Common in legumes. The coat must be scarified (scratched, cracked, or weathered) before germination can begin.
• Morphological dormancy — the embryo is underdeveloped when the seed is dispersed and needs additional time to grow before it can germinate.
• Combinational dormancy — involves more than one type at once, such as both physical and physiological dormancy.

Environmental factors

• Temperature — some seeds require cold stratification (weeks of cold exposure) to break dormancy, mimicking winter conditions.
• Light — depending on the species, light can either promote or inhibit germination. The wavelength of light matters too (red light often promotes germination; far-red light inhibits it).
• Water — essential for imbibition, the first step of germination.
• Soil conditions — pH, salinity, and nutrient levels can all influence whether dormancy breaks.

Hormonal regulation

Two hormones play opposing roles in dormancy:

• Abscisic acid (ABA) induces and maintains dormancy. It inhibits germination and promotes the buildup of storage proteins.
• Gibberellins (GAs) counteract ABA. They trigger the mobilization of stored food and stimulate production of enzymes that break down reserves.

The ratio of ABA to GA, along with the seed's sensitivity to each hormone, determines when germination occurs. Other hormones like ethylene and brassinosteroids play supporting roles.

Seed germination

Germination is the process by which the embryo inside a seed develops into a seedling. It involves water uptake, metabolic reactivation, and the physical emergence of the new plant.

Germination stages

1. Imbibition — the dry seed absorbs water and swells, reactivating metabolic processes.
2. Radicle emergence — the seed coat ruptures and the radicle (embryonic root) pushes out, anchoring the seedling and beginning water and nutrient absorption.
3. Shoot elongation — in epigeal germination, the hypocotyl elongates and pulls the cotyledons above the soil (think beans). In hypogeal germination, the epicotyl elongates while the cotyledons stay underground (think peas).
4. Seedling establishment — cotyledons expand (if above ground), the first true leaves develop, and the plant transitions to photosynthesis, becoming self-sufficient.

Water uptake

Water uptake during imbibition follows three distinct phases:

1. Rapid initial uptake — the dry seed quickly absorbs water.
2. Plateau phase — water uptake slows as the seed prepares metabolically.
3. Final uptake phase — water absorption increases again, coinciding with radicle emergence.

The rate of uptake depends on seed coat permeability, seed composition, and environmental moisture. Too much water can cause seed rot or oxygen deprivation.

Metabolic activation

As water enters the seed, dormant metabolic pathways restart:

• Respiration ramps up, generating ATP (energy) for growth.
• Enzymes are activated: amylases break down starch into sugars, proteases break down proteins into amino acids, and lipases break down fats.
• These simpler compounds fuel cell division and the construction of new tissues in the growing seedling.

Seed dispersal

Dispersal moves seeds away from the parent plant. This reduces competition between parent and offspring, allows colonization of new habitats, and improves the species' chances of long-term survival.

Dispersal mechanisms

• Wind (anemochory) — seeds with wings (maple), feathery plumes (dandelion), or extremely small size (orchids) catch air currents.
• Animal (zoochory) — seeds eaten and excreted by animals (endozoochory) or hitchhiking on fur and feathers via hooks or sticky surfaces (epizoochory).
• Water (hydrochory) — seeds that float using air pockets, corky tissues, or waxy coatings. Common in plants near rivers, lakes, or oceans.
• Gravity (barochory) — the simplest method. Seeds fall from the parent plant and roll short distances.
• Explosive/ballistic dispersal — fruits dry out or are disturbed and forcefully eject seeds. Examples include violets and touch-me-nots.

Adaptations for dispersal

Each dispersal method has driven the evolution of specific seed features:

• Hooks, barbs, and sticky surfaces for clinging to animals (burdock is a classic example).
• Fleshy arils or elaiosomes (nutrient-rich appendages) that attract animals, especially ants and birds.
• Low mass and high surface area for wind-dispersed seeds, including structures like the pappus (the fluffy part of a dandelion seed).
• Air pockets and waxy coatings for buoyancy in water-dispersed seeds, like coconuts.

Ecological significance

• Dispersal lets plants escape unfavorable local conditions and reduces sibling competition.
• The spatial pattern of seed dispersal shapes the structure of plant communities and influences interactions with herbivores, pollinators, and pathogens.
• Dispersal is critical for ecosystem recovery after disturbances like fires, floods, or land clearing.
• The coevolution of plants and their dispersal agents (like fruit-eating birds) has created complex ecological relationships where both partners depend on each other.

Economic importance

Seeds are foundational to human civilization. They provide food, animal feed, fiber, and raw materials for industry. Their economic significance extends well beyond the farm.

Agricultural crops

• Most major crops are grown from seed: cereals (wheat, rice, maize), legumes (soybeans, peas), and oilseeds (sunflower, canola).
• Seed quality directly affects crop yield, uniformity, and resilience to stress.
• Technologies like hybrid seed production, genetic engineering, and seed treatments (coatings with fungicides or nutrients) have significantly boosted agricultural productivity.
• Breeding programs continually improve nutritional value, disease resistance, and adaptability to different climates.

Seed industry

The global seed industry covers the production, processing, distribution, and marketing of seeds. It's valued at tens of billions of dollars and includes both multinational corporations and local producers. The industry is regulated by intellectual property laws, seed certification standards, and phytosanitary rules that ensure seed quality and prevent the spread of plant diseases.

Seed banks

Seed banks store seeds under controlled conditions to preserve genetic diversity. Their primary goals are conserving rare or endangered species and maintaining backup supplies in case of crop failures or natural disasters.

Notable seed banks include:

• Svalbard Global Seed Vault (Norway) — a backup facility built into a mountain in the Arctic, holding duplicates of seeds from gene banks worldwide.
• Millennium Seed Bank (UK) — aims to conserve seeds from the world's wild plant species.
• National Center for Genetic Resources Preservation (USA) — preserves genetic material for agricultural research.

Seed development

Seed development begins after fertilization, when the ovule transforms into a mature seed. This process involves coordinated genetic, hormonal, and environmental signals.

Embryogenesis

Embryogenesis is the formation of the embryo from the zygote (fertilized egg cell). The process follows a general sequence:

1. The zygote undergoes repeated cell divisions to form a globular embryo.
2. The globular embryo differentiates into distinct regions that become the radicle, plumule, and cotyledons.
3. The pattern of development varies by species, particularly in cotyledon number and the embryo's size relative to the endosperm.

This process is tightly regulated by networks of genes, transcription factors, and signaling pathways that control cell division and tissue patterning.

Maturation process

Maturation is the final stage of seed development. During this phase:

• The embryo and endosperm accumulate storage reserves: starch, proteins (like albumins and globulins), and lipids.
• The seed coat hardens, providing physical protection.
• Moisture content gradually drops, shifting the seed into a quiescent (inactive) state that allows long-term survival.
• The seed acquires desiccation tolerance, the ability to survive extreme drying.

Desiccation tolerance

Desiccation tolerance allows seeds to survive with as little as 5-10% moisture content. This adaptation is what makes long-term seed storage possible.

The seed achieves this through protective compounds:

• LEA (late embryogenesis abundant) proteins stabilize cellular membranes and proteins during drying.
• Heat shock proteins prevent protein misfolding.
• Antioxidants neutralize reactive oxygen species that would otherwise damage cells.

Not all seeds are equally tolerant. Orthodox seeds (cereals, legumes) handle desiccation well and can be stored for years at low temperature and moisture. Recalcitrant seeds (avocado, mango) are sensitive to drying and have very limited storage life.

Seed viability

Seed viability is the ability of a seed to germinate and produce a healthy seedling under favorable conditions. Maintaining viability matters for agriculture, conservation, and ecosystem restoration.

Longevity factors

How long a seed stays viable depends on both intrinsic and extrinsic factors:

• Intrinsic: species genetics, initial seed quality, dormancy mechanisms, seed coat hardness, moisture content, and antioxidant levels.
• Extrinsic: storage temperature, relative humidity, and oxygen concentration.

Seeds with hard, impermeable coats, low moisture, and high antioxidant content tend to last longest.

Storage conditions

For most orthodox seeds, optimal long-term storage conditions are:

• Temperature: 0-5°C (refrigerator range)
• Relative humidity: 15-25%
• Oxygen: low concentration

Seeds are typically kept in sealed, moisture-proof containers. Seed banks and research institutions use cold storage facilities (refrigerators or freezers) for long-term preservation.

Germination testing

Germination testing determines what percentage of a seed lot will produce normal seedlings. The standard procedure:

1. Place a sample of seeds on a moist substrate (filter paper or sand).
2. Incubate under controlled temperature, light, and moisture for a set period.
3. Count the number of normal seedlings produced.
4. Calculate germination percentage = (normal seedlings / total seeds tested) × 100.

For faster results, alternative tests exist. The tetrazolium test uses a chemical stain to distinguish living from dead tissue without waiting for actual germination. The electrical conductivity test measures how much solute leaks from seeds soaked in water (more leakage = lower viability).

Seed morphology

Seed morphology covers the external and internal physical features of seeds: size, shape, color, and surface texture. These characteristics reflect evolutionary adaptations and are useful for species identification.

Size and shape

Seed size spans an enormous range. Orchid seeds can be less than 1 mm, while coconut seeds reach up to 30 cm. Size often correlates with dispersal strategy: smaller seeds travel more easily by wind or water, while larger seeds are more commonly dispersed by animals.

Seed shape varies from spherical to ovoid, elliptical, or irregular. Shape affects how a seed interacts with soil (penetration, orientation during germination) and its vulnerability to predators.

Surface features

The seed surface can be smooth, rough, wrinkled, or covered in structures like hairs, spines, or ridges. These features often relate to dispersal: hooks and barbs catch on animal fur, while smooth surfaces suit wind or water transport.

Two surface landmarks are useful for identification:

• Hilum — the scar where the seed was attached to the fruit.
• Raphe — a ridge or seam visible on some seeds, marking the path of the vascular bundle that supplied nutrients during development.

Internal structures

Internally, seeds vary in the arrangement and proportions of embryo, endosperm, and seed coat. The embryo may be straight, curved, or coiled. In endospermic seeds, the embryo is relatively small compared to the surrounding endosperm. In non-endospermic seeds, the embryo (especially the cotyledons) fills most of the seed.

The chemical composition of the endosperm also varies: it can be starchy (cereals), oily (castor bean), or protein-rich (legumes). These differences reflect the nutritional strategy each species uses to fuel early seedling growth.

Seed classification

Seeds can be classified by morphology, embryo structure, storage reserves, or germination behavior. These classification systems help organize the huge diversity of seed types and are practical tools for agriculture, taxonomy, and ecological research.