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Seeds are the powerhouses of plant reproduction, containing everything a new plant needs to start growing. They're composed of an embryo, food reserves, and a protective coat, each playing a crucial role in the plant's survival and spread.

Seed structure and function are vital to plant life cycles. From dormancy mechanisms that ensure germination at the right time to dispersal adaptations that help plants colonize new areas, seeds are marvels of evolutionary engineering.

Seed composition

  • Seeds are the reproductive units of flowering plants that contain the embryo, stored food reserves, and protective seed coat
  • The composition of seeds varies among species but generally includes an embryo, endosperm, and seed coat
  • Understanding seed composition is crucial for studying plant reproduction, dispersal, and survival strategies

Embryo structure

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Top images from around the web for Embryo structure
  • The embryo is a miniature plant consisting of a radicle (embryonic root), plumule (embryonic shoot), and one or two cotyledons (seed leaves)
  • Cotyledons store food reserves (such as starch, proteins, and lipids) and provide nutrients to the developing seedling during germination
  • The number of cotyledons differs between monocots (one cotyledon) and dicots (two cotyledons)
  • The hypocotyl connects the radicle and the cotyledons and elongates during germination to push the plumule above the soil surface

Endosperm roles

  • The endosperm is a nutritive tissue that surrounds the embryo and provides sustenance during seed development and germination
  • In endospermic seeds (such as cereals), the endosperm persists and is the primary storage tissue for starch and proteins
  • Non-endospermic seeds (such as legumes) have a reduced or absent endosperm, with food reserves stored in the cotyledons
  • The endosperm also plays a role in seed dormancy and germination by regulating the water uptake and enzymatic activities

Seed coat layers

  • The seed coat, derived from the integuments of the ovule, protects the embryo and endosperm from mechanical damage, desiccation, and pathogens
  • The outermost layer, the testa, is often hard and thick, providing structural support and defense against external factors
  • The inner layer, the tegmen, is usually thin and membranous, regulating water and gas exchange
  • Some seeds have additional layers, such as the aril (a fleshy covering) or the mucilage (a sticky substance), which aid in dispersal and germination

Seed dormancy

  • Seed dormancy is an adaptive mechanism that prevents germination under unfavorable conditions and ensures the survival of the species
  • Dormancy is influenced by both intrinsic (genetic and hormonal) and extrinsic (environmental) factors
  • Understanding seed dormancy is essential for managing agricultural crops, controlling weeds, and conserving biodiversity

Dormancy types

  • Physiological dormancy is the most common type, caused by the presence of inhibitors (such as abscisic acid) or the requirement for specific environmental cues (such as light or temperature)
  • Physical dormancy is imposed by the hard, impermeable seed coat that prevents water uptake and gas exchange (common in legumes)
  • Morphological dormancy occurs when the embryo is underdeveloped at the time of seed dispersal and requires additional growth before germination
  • Combinational dormancy involves a combination of the above types, such as physiological and physical dormancy

Environmental factors

  • Temperature plays a crucial role in regulating seed dormancy, with some seeds requiring cold stratification (exposure to low temperatures) to break dormancy
  • Light can stimulate or inhibit germination, depending on the species and the quality (wavelength) of light
  • Water availability is essential for seed imbibition and the initiation of germination processes
  • Soil composition, such as pH, salinity, and nutrient levels, can also influence dormancy and germination

Hormonal regulation

  • Abscisic acid (ABA) is the primary hormone that induces and maintains seed dormancy by inhibiting germination and promoting the synthesis of storage proteins
  • Gibberellins (GAs) counteract the effects of ABA and promote germination by mobilizing stored reserves and inducing the production of hydrolytic enzymes
  • Other hormones, such as ethylene and brassinosteroids, also play a role in regulating dormancy and germination
  • The balance between ABA and GAs, as well as the sensitivity of the seed to these hormones, determines the dormancy status and the timing of germination

Seed germination

  • Seed germination is the process by which an embryo develops into a seedling, marking the beginning of a plant's growth and development
  • Germination is a complex process that involves the activation of metabolic pathways, the mobilization of stored reserves, and the growth of the embryonic tissues
  • Understanding seed germination is crucial for agriculture, horticulture, and ecological restoration projects

Germination stages

  • The first stage of germination is imbibition, where the dry seed absorbs water and swells, leading to the activation of metabolic processes
  • The second stage involves the rupture of the seed coat and the emergence of the radicle (embryonic root), which anchors the seedling and absorbs water and nutrients
  • The third stage is the elongation of the hypocotyl (in epigeal germination) or the epicotyl (in hypogeal germination), which pushes the cotyledons and the plumule above the soil surface
  • The final stage is the establishment of the seedling, where the cotyledons expand, the first true leaves develop, and the plant becomes autotrophic

Water uptake

  • Water uptake is the critical first step in seed germination, as it activates enzymes, solubilizes stored reserves, and provides a medium for metabolic reactions
  • The rate and extent of water uptake depend on the permeability of the seed coat, the composition of the seed, and the environmental conditions
  • Imbibition occurs in three phases: rapid initial uptake, a plateau phase with limited water uptake, and a final phase of increased water uptake associated with radicle emergence
  • Excessive water uptake can be detrimental, as it may lead to seed rot or oxygen deprivation

Metabolic activation

  • As the seed imbibes water, metabolic pathways are activated, including respiration, protein synthesis, and enzyme production
  • Stored reserves, such as starch, proteins, and lipids, are mobilized and broken down into simpler compounds (such as sugars and amino acids) to support seedling growth
  • The activation of enzymes, such as amylases, proteases, and lipases, is crucial for the mobilization of stored reserves
  • The increase in respiratory activity provides energy (ATP) for the germination process and the synthesis of new cellular components

Seed dispersal

  • Seed dispersal is the movement of seeds away from the parent plant, enabling the colonization of new habitats and reducing competition for resources
  • Dispersal mechanisms have evolved in response to various environmental factors, such as climate, topography, and biotic interactions
  • Understanding seed dispersal is essential for studying plant population dynamics, community structure, and ecosystem functioning

Dispersal mechanisms

  • Wind dispersal is common in seeds with adaptations such as wings (maple), plumes (dandelion), or small size (orchids)
  • Animal dispersal occurs when seeds are consumed and excreted by animals (endozoochory) or attached to their fur or feathers (epizoochory)
  • Water dispersal is prevalent in aquatic plants or plants growing near water bodies, with seeds that can float or survive prolonged immersion
  • Gravity dispersal is the simplest form, where seeds fall and roll away from the parent plant (barochory)
  • Explosive or ballistic dispersal involves the forceful ejection of seeds from the fruit, often triggered by mechanical disturbance or drying (violets, touch-me-nots)

Adaptations for dispersal

  • Seeds may have specialized structures, such as hooks, barbs, or sticky surfaces, that facilitate attachment to animal fur or feathers
  • Some seeds have fleshy or nutritious structures (such as arils or elaiosomes) that attract animals and promote dispersal
  • Seeds dispersed by wind often have low mass, high surface area, or specialized structures (such as pappus or wings) that increase buoyancy
  • Seeds dispersed by water may have air pockets, corky tissues, or waxy coatings that enable them to float and survive in aquatic environments

Ecological significance

  • Seed dispersal allows plants to colonize new habitats, escape unfavorable conditions, and reduce competition with the parent plant or siblings
  • The spatial distribution of seeds influences the structure and composition of plant communities and the interactions with other organisms (such as herbivores, pollinators, and pathogens)
  • Seed dispersal plays a crucial role in the regeneration of disturbed or degraded ecosystems, such as after fires, floods, or human activities
  • The coevolution of plants and their dispersal agents (such as frugivorous animals) has led to the development of complex ecological networks and mutual dependencies

Economic importance

  • Seeds are the foundation of agriculture, providing food, feed, fiber, and industrial raw materials for human society
  • The economic value of seeds extends beyond their role in crop production, encompassing various industries and applications
  • Understanding the economic importance of seeds is crucial for developing sustainable agricultural practices, ensuring food security, and conserving plant genetic resources

Agricultural crops

  • Seeds are the primary propagation material for most agricultural crops, including cereals (wheat, rice, maize), legumes (soybeans, peas), and oilseeds (sunflower, canola)
  • The quality and performance of crop seeds directly influence the yield, uniformity, and resilience of the resulting plants
  • Advances in seed technology, such as hybrid seeds, genetic engineering, and seed treatments, have revolutionized agriculture and increased crop productivity
  • The development of improved crop varieties through breeding and selection has enhanced the nutritional value, disease resistance, and adaptability of seeds to various environmental conditions

Seed industry

  • The seed industry encompasses the production, processing, distribution, and marketing of seeds for agricultural and horticultural purposes
  • Seed companies invest in research and development to create new plant varieties, improve seed quality, and develop innovative seed technologies
  • The global seed market is valued at billions of dollars, with major players including multinational corporations and local seed producers
  • The seed industry is subject to various regulations and policies, such as intellectual property rights, seed certification, and phytosanitary measures, to ensure the quality and safety of seeds

Seed banks

  • Seed banks are facilities that store and preserve the genetic diversity of plants by maintaining collections of seeds under controlled conditions
  • The primary goal of seed banks is to conserve plant genetic resources, particularly rare, endangered, or economically important species
  • Seed banks play a crucial role in safeguarding the world's food supply, providing a backup source of seeds in case of crop failures, natural disasters, or genetic erosion
  • Major seed banks include the Svalbard Global Seed Vault (Norway), the Millennium Seed Bank (UK), and the National Center for Genetic Resources Preservation (USA)

Seed development

  • Seed development is the process by which an ovule transforms into a mature seed following fertilization
  • The development of seeds involves a complex interplay of genetic, hormonal, and environmental factors
  • Understanding seed development is essential for improving crop productivity, enhancing seed quality, and studying plant reproduction and evolution

Embryogenesis

  • Embryogenesis is the formation and development of the embryo from the zygote, the fertilized egg cell
  • The zygote undergoes a series of mitotic divisions (cleavage) to form a globular embryo, which then differentiates into the embryonic tissues (radicle, plumule, and cotyledons)
  • The pattern of embryo development varies among species, with differences in the number and arrangement of cotyledons, the presence or absence of endosperm, and the size of the embryo relative to the seed
  • Embryogenesis is regulated by a network of genes, transcription factors, and signaling pathways that control cell division, differentiation, and patterning

Maturation process

  • Seed maturation is the final stage of seed development, characterized by the accumulation of storage reserves, the acquisition of desiccation tolerance, and the preparation for dormancy
  • During maturation, the embryo and endosperm cells synthesize and deposit storage compounds, such as starch, proteins (like albumins and globulins), and lipids, which will support seedling growth upon germination
  • The seed coat develops and hardens, providing protection against mechanical damage, desiccation, and pathogens
  • Maturation is accompanied by a gradual reduction in seed moisture content, which is essential for the transition to a quiescent state and the ability to withstand long-term storage

Desiccation tolerance

  • Desiccation tolerance is the ability of seeds to survive extreme water loss (down to 5-10% moisture content) without losing viability
  • The acquisition of desiccation tolerance is a crucial adaptation that enables seeds to remain viable for extended periods and to withstand adverse environmental conditions
  • Desiccation tolerance involves the synthesis of protective compounds, such as late embryogenesis abundant (LEA) proteins, heat shock proteins, and antioxidants, which stabilize cellular structures and prevent damage from reactive oxygen species
  • Orthodox seeds (such as cereals and legumes) are desiccation-tolerant and can be stored for long periods under low moisture and temperature conditions, while recalcitrant seeds (such as avocado and mango) are sensitive to desiccation and have a limited storage life

Seed viability

  • Seed viability refers to the ability of a seed to germinate and produce a normal, healthy seedling under favorable conditions
  • Maintaining seed viability is crucial for the long-term conservation of plant genetic resources, the success of agricultural production, and the restoration of natural ecosystems
  • Understanding the factors that influence seed viability and the methods for assessing and promoting it is essential for seed science and technology

Longevity factors

  • Seed longevity is the period over which a seed remains viable and capable of germination
  • Longevity is influenced by intrinsic factors, such as the genetic makeup of the species, the initial quality of the seed, and the presence of dormancy mechanisms
  • Extrinsic factors, such as storage temperature, relative humidity, and oxygen concentration, also play a crucial role in determining seed longevity
  • Seeds with hard, impermeable seed coats, low moisture content, and high levels of antioxidants generally have longer lifespans than those lacking these characteristics

Storage conditions

  • Proper storage conditions are essential for maintaining seed viability and prolonging longevity
  • The optimal storage conditions for most orthodox seeds are low temperature (0-5°C), low relative humidity (15-25%), and low oxygen concentration
  • Sealed containers, such as moisture-proof bags or airtight jars, are used to control the storage environment and prevent fluctuations in temperature and humidity
  • Cold storage facilities, such as refrigerators or freezers, are commonly used for long-term seed preservation, particularly in seed banks and research institutions

Germination testing

  • Germination testing is a method for assessing the viability and performance of a seed lot by determining the percentage of seeds that produce normal seedlings under controlled conditions
  • The standard germination test involves placing a sample of seeds on a moist substrate (such as paper or sand) and incubating them under optimal temperature, light, and moisture conditions for a specified period
  • The germination percentage is calculated based on the number of normal seedlings produced relative to the total number of seeds tested
  • Other viability tests, such as the tetrazolium test or the electrical conductivity test, can provide rapid estimates of seed viability without the need for actual germination

Seed morphology

  • Seed morphology refers to the external and internal structure of seeds, including their size, shape, color, and surface features
  • The morphological characteristics of seeds are highly diverse and reflect the evolutionary adaptations of different plant species to their environments
  • Understanding seed morphology is essential for taxonomic identification, ecological studies, and the development of seed processing and planting technologies

Size and shape

  • Seed size varies widely among species, ranging from the tiny seeds of orchids (less than 1 mm) to the large seeds of coconuts (up to 30 cm)
  • The size of a seed is often related to its dispersal mechanism, with smaller seeds being more easily dispersed by wind or water, while larger seeds are more likely to be dispersed by animals
  • Seed shape can be spherical, ovoid, elliptical, or irregular, depending on the species and the constraints imposed by the fruit or seed coat
  • The shape of a seed can influence its ability to penetrate the soil, its orientation during germination, and its susceptibility to predation

Surface features

  • The surface of a seed can be smooth, rough, wrinkled, or ornamented with various structures, such as hairs, spines, or ridges
  • Surface features often play a role in seed dispersal, with hooks or barbs facilitating attachment to animal fur, while smooth surfaces may promote wind or water dispersal
  • The texture and permeability of the seed surface can also influence water uptake during imbibition and the susceptibility to pathogen attack
  • Some seeds have specialized structures, such as the raphe (a ridge or seam) or the hilum (a scar marking the point of attachment to the fruit), which can be used for identification purposes

Internal structures

  • The internal structure of a seed consists of the embryo, endosperm (if present), and seed coat layers
  • The arrangement and relative proportions of these structures vary among species and can be used for taxonomic classification
  • The embryo may be straight, curved, or coiled, and its size relative to the endosperm can range from small (in endospermic seeds) to large (in non-endospermic seeds)
  • The presence or absence of endosperm, as well as its chemical composition (starchy, oily, or proteinaceous), can provide insights into the nutritional reserves available for the developing seedling

Seed classification

  • Seeds can be classified based on various criteria, such as their morphology, embryo structure, storage reserves, or germination behavior
  • Seed classification systems are used to organize the diversity of seed types and to facilitate their study, management, and utilization
  • Understanding the different seed classifications is

Term 1 of 34

Abscisic acid
See definition

Abscisic acid (ABA) is a plant hormone that plays a critical role in regulating various physiological processes, particularly in response to stress conditions such as drought and salinity. It helps plants conserve water, promotes seed dormancy, and facilitates the aging process, making it essential for survival and adaptation.

Key Terms to Review (34)

Term 1 of 34

Abscisic acid
See definition

Abscisic acid (ABA) is a plant hormone that plays a critical role in regulating various physiological processes, particularly in response to stress conditions such as drought and salinity. It helps plants conserve water, promotes seed dormancy, and facilitates the aging process, making it essential for survival and adaptation.

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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

Term 1 of 34

Abscisic acid
See definition

Abscisic acid (ABA) is a plant hormone that plays a critical role in regulating various physiological processes, particularly in response to stress conditions such as drought and salinity. It helps plants conserve water, promotes seed dormancy, and facilitates the aging process, making it essential for survival and adaptation.



© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Glossary
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