Seed Function and Germination

Functions of Seeds Seeds are far more than just tiny packages of genetic information—they are specialized structures that solve several critical problems for plant survival and reproduction. Protecting and nourishing the embryo. Seeds enclose a living embryo along with stored food reserves (endosperm or cotyledons), providing the seedling with immediate nutrition. This gives young plants a significant head start compared to spores, which are dispersed without food reserves. The seedling can begin photosynthesis and root development before its stored nutrients are exhausted. Enabling dispersal. Seeds move offspring away from the parent plant to locations where environmental conditions may be more favorable for germination and growth. This reduces competition between parent and offspring and allows plants to colonize new areas. Different seed structures—from winged seeds to fleshy fruits—reflect specialized dispersal mechanisms. Timing germination strategically. Seed dormancy synchronizes germination with optimal environmental conditions. Rather than germinating immediately when dispersed, dormant seeds wait for the right season. This also spreads germination over time, reducing the catastrophic loss that would occur if all seeds from a plant germinated at once and conditions suddenly became unfavorable. Generating genetic variation. Because seeds result from sexual reproduction, they create genetic recombination and phenotypic variability. This variation provides the raw material for natural selection and improves population adaptation to changing environments. Understanding Seed Dormancy Dormancy is a state of reduced metabolic activity in which a seed will not germinate even when water and appropriate temperature are present. This is fundamentally different from a seed simply being dead or unable to grow—dormant seeds are alive but have physiological, physical, or structural barriers preventing germination. The critical point: dormancy is adaptive. It prevents germination at the wrong time of year. A seed that germinated in late fall when temperatures drop would likely fail; dormancy mechanisms evolved to wait until spring conditions arrive. Exogenous Dormancy (Barriers Outside the Embryo) Exogenous dormancy involves barriers in the seed coat or surrounding structures, not in the embryo itself. Physical dormancy occurs when a hard, impermeable seed coat prevents water from entering the seed, even if moisture is available. The seed cannot absorb water needed to activate germination. The solution is a "water gap"—a small opening or specialized tissue that must be disrupted by environmental cues. For example, some seeds only become permeable after exposure to specific temperatures or after being scratched during soil movement. Chemical dormancy involves germination-inhibiting chemicals (often inhibitor compounds located in the seed coat) that block germination. These must be leached away by water or chemically deactivated before the embryo can overcome the seed coat and grow. Think of this as the seed coat containing a chemical "brake" on germination. Endogenous Dormancy (Barriers Within the Embryo) Endogenous dormancy originates from the embryo itself or from internal physiological factors. There are three main types, and they differ in important ways. Morphological dormancy occurs when the embryo is underdeveloped or undifferentiated—essentially incomplete. The embryo must grow to a species-specific minimum size before it can germinate. This doesn't require special environmental triggers; the embryo simply needs time to develop. After that development period, the seed will germinate whenever conditions allow (water and suitable temperature). Physiological dormancy is caused by internal hormonal factors, particularly abscisic acid (ABA), a hormone that prevents the embryo from overcoming the seed coat resistance and growing outward. This is perhaps the most common type of dormancy. The key difference from morphological dormancy: the embryo is fully developed, but hormonal inhibition prevents it from germinating. Specific environmental signals—usually particular combinations of temperature and moisture—break this hormonal inhibition. For example, many seeds germinate after a cold period (winter), when cold temperatures lower ABA levels or make the embryo more responsive to growth-promoting hormones. Morphophysiological dormancy combines both problems: the embryo is underdeveloped and subject to physiological inhibition. Breaking this dormancy requires both allowing the embryo to grow to adequate size and applying dormancy-breaking treatments (like cold exposure). This is more complex than the other types because both barriers must be addressed. The tricky distinction: With morphological dormancy, time alone is enough; with physiological dormancy, you need the right environmental signal; with morphophysiological dormancy, you need both time and the right signal. Combinational Dormancy (Multiple Barriers) Some seeds have both a water-impermeable seed coat (physical dormancy) and physiological inhibition of the embryo. Breaking this dormancy is flexible—either barrier may be overcome first depending on the species. The seed might first become water-permeable through temperature exposure, allowing water entry that then softens the physiological inhibition. Alternatively, physiological dormancy might be broken first by cold exposure, making the seed more capable of overcoming the physical barrier. Secondary Dormancy (Dormancy Reactivated After Dispersal) Secondary dormancy develops after seed dispersal when unfavorable environmental conditions trigger dormancy mechanisms in seeds that were previously non-dormant. For example, a seed dispersed in summer might initially lack dormancy and be ready to germinate. But if soil temperatures become excessively hot, secondary dormancy reactivates, delaying germination until fall or the following spring. This is different from the primary dormancy mechanisms discussed above—it's a responsive mechanism that activates in response to unsuitable conditions. Environmental Dormancy (Conditional Germination) A few terms in seed biology describe germination requirements that aren't true dormancy—the seed isn't prevented from growing by internal barriers, but rather responds to specific environmental cues. Photodormancy describes seeds requiring either a specific period of darkness or light to trigger germination. Some seeds must experience a light signal (red light especially) to recognize that they're not buried too deeply. Others need darkness. This is a light-sensing response, not a physiological block to germination. Thermodormancy refers to sensitivity to temperature extremes. While the term includes "dormancy," it's really a temperature response rather than true dormancy. Many seeds are inhibited by excessively warm temperatures and will only germinate when soil temperature drops to a suitable range. Others require a cold period to activate germination. These are conditional responses to environmental signals. Temperature and Germination Timing Temperature plays multiple roles in seed germination, sometimes enabling germination and sometimes preventing it. Many seeds exhibit thermodormancy and will germinate only when soil temperature reaches a warm range in early to midsummer. This ensures germination occurs when conditions favor seedling growth. However, other seeds show the opposite pattern—they require cool soils and are inhibited by excessively warm soil. Celery seeds, for example, germinate poorly when soil is too warm. Notably, these thermodormancy requirements often diminish as seeds age or dry out. A seed that required cold exposure to germinate may eventually lose this requirement after months of storage, allowing it to germinate at warmer temperatures. <extrainfo> Special Cases Vivipary in mangroves: Certain mangrove species produce viviparous seeds that break dormancy and germinate while still attached to the parent plant, rather than waiting until after dispersal. This is an unusual adaptation to their specific coastal environment. Effects of selective breeding: Many cultivated garden plant seeds have lost dormancy through generations of selective breeding. Gardeners favor seeds that germinate readily, so dormancy has been selected against in domestic varieties. </extrainfo> Methods to Induce Germination Understanding how to break dormancy is practically important for agriculture and horticulture, and it also reinforces understanding of what dormancy mechanisms are. Scarification involves physically or chemically disrupting the seed coat. Mechanical scarification uses sandpaper, pins, or hammers to create tiny breaks in hard coats, allowing water to enter and breaking physical dormancy. Chemical scarification involves soaking seeds in hot water or mild acids to soften the coat chemically—essentially using external chemistry to do what temperature naturally does. Stratification (also called moist-chilling) addresses physiological dormancy by mimicking winter conditions. Seeds are moistened and then exposed to cool temperatures (typically around 1–10°C) for weeks or months. This treatment lowers ABA levels and prepares the seed for germination. A practical approach is sowing seeds in late summer in outdoor seedbeds, allowing them to overwinter naturally—the winter cold provides the stratification treatment. Leaching removes chemical inhibitors by soaking seeds in running water for 12–24 hours. This is particularly effective for seeds with chemical dormancy, as water gradually dissolves and carries away germination-inhibiting compounds. Smoke and fire signals: Some fire-adapted species from naturally burned ecosystems respond to smoke or liquid smoke. These compounds either crack hard seed coats or provide chemical signals mimicking fire exposure, breaking dormancy and triggering germination after fire clears competing vegetation. Natural dispersal methods: In nature, passage through an animal's digestive tract weakens the seed coat while simultaneously adding fertilizer from feces—a dual benefit to the offspring.