Structure, Types, Germination, Dispersals, Uses microbiologystudy

A plant seed is a mature, fertilized ovule that is the reproductive unit of seed plants. It has an embryo, reserve nutrients, and a protective seed coat to ensure plant species survival and dispersal.

Seeds are dormant until they receive favorable conditions for germination, allowing plants to reproduce and establish new habitats.

Structure of Plant Seed

Seeds generally consist of 3 parts: Cotyledons or embryonic leaves, the embryo from which growth commences, and the outer protective covering called the seed coat. However, the structure of seeds and their parts differ in dicot and monocot.

Structure of monocotyledonous seed

1. Seed coat: The seed coat is membranous and attached to the fruit wall.
2. Endosperm: Endosperm is a bulky storage organ that nourishes the growing embryo. Monocots are generally endospermic with few exceptions, like an orchid.
3. Aleuron layer: It is the proteinous layer in between the outer membrane of endosperm and embryo that secretes the enzyme for the degradation of reserve food.
4. Embryo: The embryo is a small fleshy body present in the groove at the end of the endosperm. The embryo is diploid. It consists of the following parts:
   • Scutellum: This is the single large, shield-shaped cotyledon that nourishes the growing embryo
   • Embryonal axis: They are radicle and plumule formed at two ends of the seed. The radicle grows downward and gives rise to the root of a new seedling, while the plumule grows upward and gives rise to a shoot or stem.
   • Coleoptile and coleorhiza: Radicle and plumule are enclosed in a sheath called coleoptile and coleorhiza, respectively.

Structure of dicotyledonous seed

Seed coat

It is developed from the integument of the ovule and has the following parts.

• Testa: It is the outer thick layer that performs a protective function.
• Tegman: It is an inner thin membranous layer that protects the inner layer of the seed.
• Hilum: It is the point on the seed coat from which the seed is attached to the endocarp of the fruit. It is equivalent to the naval of humans.
• Micropyle: It is the small round pore above the hilum for the entry of pollen tube, water, and oxygen into the embryo.
• Raphe: It is the ridge portion in the groove that remains fused with testa.

Embryo

• Cotyledon: They are the fleshy structure present in the embryonal axis and provide nutrition for developing embryos. Dicot has two cotyledons.
• Embryonal axis: It is the miniature plant present in between the two cotyledons and consists of plumule and radicle, which later developed into shoot and root, respectively. The portion between cotyledon and radical is called hypocotyl, whereas the point between cotyledon and plumule is called epicotyl.

Plant Seed size and Shape

• Seed size, color, and shape may vary from smaller to large, wrinkled, hairy to smooth. The shape and size of the seed are important for dispersal, survival, and germination.
• The world’s smallest seed known are found in orchids that are devoid of reserve nutrient and lives in symbiosis with mycorhizal fungi, carnivorous plant, and parasitic plants. The seed of the double coconut palm is the largest seed known. 
• In many plants, seeds are uniform in size, and seed size depends upon the genetic factors, e.g., wheat.
• In peas, the seed form on the central part of the pod is relatively larger because of the competition for the nutrient between ovules on the placenta.
• Seeds of Camelina sativa mimic flax seed size and nature that are grown in a particular region.
• The flatness of the seed restricts the germination of tropical mora, but the exception is seen in Panicum turgidum seed which can germinate from the convex side rather than flat.

Types of seeds of plants

Plant seeds are categorized according to different characteristics such as the number of cotyledons, existence of endosperm, way of dispersal, and reproduction pattern. The following are the detailed classifications:

According to the number of cotyledons

The cotyledons are the leaves of the seeds that store or absorb food for the developing embryo.

• Monocotyledonous Seeds (Monocots)- These seeds consist of a single cotyledon (seed leaf). The endosperm is typically present and supplies nutrients to the embryo. The seed coat is commonly united with the wall of the fruit. Examples: Maize (Zea mays), Wheat (Triticum), Rice (Oryza sativa), Palm, Onion etc.
• Dicotyledonous Seeds (Dicots)- They possess two cotyledons (seed leaves). The cotyledons contain food that assists in the germination of the seeds. The embryo contains a well-developed radicle and plumule. Examples: Pea (Pisum sativum), Bean (Phaseolus vulgaris), Mustard, Mango, Groundnut etc.

Depending on the presence of endosperm

Endosperm is the nutrient-storing tissue of the developing embryo. It is present in some seeds and used up during seed development by others.

• Endospermic (Albuminous) Seeds– These seeds have some endosperm stored in them, serving as food during germination. Occurring primarily in monocots but also in certain dicots. Example: Rice, Maize, Wheat, Coconut, Castor, Barley.
• Non-Endospermic (Exalbuminous) Seeds- These seeds don’t have endosperm and the nutrients are stored by the cotyledons in seed development. These type of seeds occurs primarily in dicots. Example: Pea, Bean, Mustard, Sunflower, Groundnut.

Based on dispersal mechanisms

Adaptations in plants have been made to scatter their seeds by various external forces.

• Wind-Dispersed Seeds (Anemochory)- These are light seeds, at times having wings or hairy structures. Examples: Maple (Acer), Dandelion (Taraxacum), Cotton (Gossypium), Drumstick (Moringa), and Milkweed.
• Water-Dispersed Seeds (Hydrochory)– These seeds are light, floating, and waterproof, enabling them to float and disperse long distances. Examples: Coconut (Cocos nucifera), Lotus (Nelumbo), Water Lily (Nymphaea), Mangrove (Rhizophora).
• Animal-Dispersed Seeds (Zoochory)- These seeds possess fleshy, edible fruit or sticky bodies (hooks, barbs) for adhering to animal fur. Example: Edible Seeds: Mango (Mangifera indica), Cherry, Guava, Tomato; Hooked/Sticky Seeds: Burdock (Arctium), Xanthium (Cocklebur), Goosegrass
• Explosion or Self-Dispersed Seeds (Autochory)- Some seeds burst apart and expel their seeds by force to neighboring regions. Examples: Pea, Balsam (Impatiens), Poppy (Papaver), Witch Hazel (Hamamelis).

Depending upon the mode of reproduction

• True Seeds– These seeds are formed from fertilized ovules and hold the genetic material of both the parent plants. Examples: All flowering plants such as rice, wheat, beans, and mango.
• Apomictic Seeds– These seeds form in the absence of fertilization and produce genetically identical offspring to the parent plant. It occurs in various fruit trees and specific grass species. Example: Citrus (Lemon, Orange), Dandelion, Blackberries.

Plant Seed Germination

Seed germination is the phenomenon by which a resting seed starts to grow and develop into a new plant under favorable environmental conditions. Germination is important in plant reproduction and ecosystem maintenance. Germination is initiated when a seed takes in water, activates its metabolic processes, and grows.

Conditions required for germination

Several external and internal factors control seed germination:

Water (Moisture) – Seeds imbibe water, softening the seed coat and triggering enzymes.

Oxygen – Required for cellular respiration to produce energy for growth.

Temperature – Regulates enzymatic activity, with most seeds needing approximately 15°C to 30°C.

Light/Darkness – Light (e.g., Lettuce) or darkness (e.g., Onion) is required for germination by some seeds.

Hormonal Regulation – Growth is promoted by Gibberellins (GA), but germination is inhibited by Abscisic Acid (ABA).

Stages of Seed Germination

Imbibition – The seed takes in water, expanding and ending dormancy.

Enzyme activation – Hydrolytic enzymes release stored starch, proteins, and fats into a form usable by the cell.

Respiration initiation – Cellular respiration rises, generating ATP for cell growth and division.

Radicle emergence – The first emergence is the root (radicle), which anchors the seedling into the ground.

Plumule growth – The shoot (plumule) grows, extending toward the surface.

Seedling development – The seed coat is dropped, and the plant starts independent growth using photosynthesis.

Plant Seed Development

• Seed development generally consists of 3 stages: cell division and differentiation, accumulation of food reserve, maturation, and water loss.
• Seed development generally starts from fertilization which is preceded by pollination.
• In angiosperm, the ovule consists of two-layer the integument and nucellus. The nucellus cell undergoes meiosis and mitotic division to form eight nucleated embryo sacs. Pollen tube enters unto embryo sac and releases two gametes. One male gamete unites with the egg cell to form a diploid zygote that later develops into an embryo, while the other gamete unites with two polar nuclei to form a triploid endosperm that stores food for the growing embryo.
• The integuments of ovule change to seed coat with two distinct covering; outer testa and inner tegman.
• In some cases, there may be remnants of nucellar tissues called perisperm that stores food, but in most cases, it disappears.
• Endosperm nourishes the embryo by storing food material. In albuminous seed, endosperm remains conspicuous and a greater part of the seed than the embryo, while in other seeds like Tectona, the embryo uses all nutrition in development, and endosperm may disappear by the time of maturity. In such cases, the cotyledon is the organ for storage.
• Embryo development may vary that we may distinguish all the parts of embryo-like cotyledon, plumule, radicle, etc. In some angiosperms, more than one embryo may be developed in a single seed called polyembryony.
• In gymnosperm, there is a single integument that is fused with an ovuliferous scale consisting of paired ovules. At fertilization, the nucellus is separated from the integument only in the micropyle region. Meiosis and mitosis in nucellus take place the same as in angiosperm and form female gametophyte (multicellular, haploid) that grows further, replacing nucellus and differentiated into archegonia, each consisting of two large egg cells.
• One of the two gametes from the pollen tube is fertilized with the egg cell to form a zygote that later develops onto the embryo, while another one is either aborted (in Pinus) or may fertilize with other archegonia but never unite with polar nuclei to form endosperm.
• The mature seed consists of a seed coat developed from the integument, diploid perisperm developed from nucellus e.g., Pinus pinea, but in most cases, it disappears during development, and the haploid gametophyte tissue which nourishes the embryo, which is equivalent to the endosperm of angiosperm and the last one is embryo consists of plumule, radicle, cotyledon same as an angiosperm. Cotyledon number may vary in gymnosperm up to 18 (in pinus).

Types of Seed Germination

Epigeal Germination – The type of germination in which cotyledons emerge above ground as the hypocotyl develops is called epigeal germination (e.g., Bean, Castor).

Hypogeal Germination – In this type of germination cotyledons are beneath the ground while the epicotyl develops above (e.g., Pea, Maize).

Vivipary – The type of germination in which seeds germinate on the parent plant itself is called vivipary (e.g., Mangrove).

Functions of plant seeds- Seeds are essential for plant survival, reproduction, and dispersal:

Reproduction – Seeds facilitate the perpetuation of plant species by producing new individuals.

Storage of Nutrients – Cotyledons and endosperm supply nutrients for early growth.

Protection of Embryo – The seed coat (testa) protects the embryo from mechanical injury, desiccation, and disease.

Genetic Variation – Seeds form from fertilization, ensuring biodiversity through genetic recombination.

Dispersal Mechanism – Seeds are dispersed by wind, water, animals, or mechanical action, avoiding overcrowding.

Economic Importance – Seeds are a primary food source (e.g., wheat, rice, corn) and are vital for agriculture and industry.

Means of dispersal of seeds

Seed dispersal enables plants to disperse to new locations, promoting genetic diversity and survival. The primary dispersal mechanisms are:

Wind Dispersal (Anemochory)- Seeds are light, winged, or feathery, which enables them to be dispersed by the wind.

Examples: Dandelion, Cotton, Maple, Drumstick etc.

Water Dispersal (Hydrochory)- Seeds are floating and contained in waterproof coverings, which enables them to float to new places.

Examples: Lotus, Water Lily, Mangrove plants, etc.

Animal Dispersal (Zoochory) – it is of two types-

Endozoochory: Fleshier, food-rich fruits are consumed, and seeds are voided at some other site. E.g., Mango, Cherry, Guava, etc.

Epizoochory: Hook- or barbed seeds get fastened to the fur or feathers of animals. E.g., Xanthium, Burdock, etc.

Self-Dispersal (Autochory)

Some plants bear explosive devices that propel seeds out forcefully from the parent plant.

Example: Balsam (Touch-Me-Not), Pea, Poppy etc.

Functions of Plant Seed

1. Most flowering plants are reproduced by means of seed.
2. Seeds contain embryos, and nourishment of the embryo helps them to develop into the new plant.
3. Seeds disperse by various agents and establish in the new place. 
4. Seed undergoes a dormant state during an unfavorable environment.
5. Seed helps to keep species in existence.

Adaptations of Plant Seeds

Seeds have developed many adaptations to improve their survival and dispersal effectiveness.

Seed Dormancy – Some seeds are adapted to stay inactive for long durations until the conditions become favorable to protect the embryo from unfavorable conditions. E.g. Lotus seeds.

Hard Seed Coat – Some seeds contain a hard seed coat that shields the embryo from severe environmental conditions, animal digestion, and mechanical damage. E.g. Date Palm, Coconut.

Lightweight Seeds – Some seeds are adapted as lightweight, designed for wind dispersal, which ensures extensive colonization E.g. Dandelion, Cotton, etc.

Fleshy Fruits – Fleshy fruit containing seeds attract animals that eat the fruit and disperse seeds through digestion (e.g., Apple, Mango).

Floating Seeds – Some seeds are designed to allow water dispersal, assisting plants to spread throughout aquatic and coastal areas (e.g., Coconut, Lotus).

Explosive Mechanism – Certain seeds explode forcefully to distribute offspring over a large distance (e.g., Balsam, Pea).

Symbiotic Germination – Certain seeds, such as Orchids, need special fungi for germination, establishing mutualistic relationships.

Common issues regarding plant seeds

Seeds are essential in plant reproduction, but several environmental and biological conditions can adversely affect their viability, germination, and growth.

Low germination rate- Certain seeds do not germinate because they have been stored improperly, lack water, or are exposed to unsuitable temperatures. Dormancy due to hard seed coats or unripe embryos may also hinder germination.

Fungal and bacterial infections– Pathogens like Fusarium, Pythium, and Rhizoctonia can cause seed rot, disease damping-off, and mold. Damp conditions and inadequate air circulation raise the risk of infection.

Pest damage– Weevils and beetles deposit eggs within seeds, which when larvae emerge, cause damage. Rodents and birds also eat seeds before they have an opportunity to germinate.

Poor soil conditions- Low nutrient soil content, inappropriate pH, or drainage can hinder seedling growth. Soil salinity and heavy metals are harmful to young plants.

Weed competition- Competitive weeds may shade germinating seeds from light, water, and nutrients, resulting in stunted or weak seedlings.

Impacts of climate change- Unstable weather patterns, extreme heat, drought, or out-of-season frosts may inhibit seed germination. Certain seeds need cold stratification, but increasing global temperatures can interfere with this process.

Mechanical damage- Seeds may be injured during handling, transportation, or harvesting, lowering their viability. Prevention of these problems includes correct seed storage, disease control practices, ideal planting conditions, and seed protection from pests and environmental stresses.

Interesting and fun facts about plant seeds

Oldest viable seed – A 32,000-year-old Silene stenophylla seed was successfully germinated after being buried in permafrost.

Largest Seed – ‘Coco de Mer’ (Lodoicea maldivica) bears the world’s largest seed, weighing up to 30 kg.

Smallest Seed – Some orchids produce dust-like seeds that are less than a grain of sand in size.

Explosive Seeds – Touch-Me-Not (Impatiens) and Sandbox Tree (Hura crepitans) seeds are ejected forcibly.

Floating Seeds – Coconut seeds float in seawater for months before germinating on far-off beaches.

Airborne Seeds – Dandelion seeds can travel hundreds of kilometers on wind currents.

Carnivorous Plants produce seeds too – The Venus Flytrap and Pitcher Plant also produce seeds like any other flowering plant.

Conclusion

Seed plants are vital for the continuation of plant life and species, contributing significantly to ecosystem biodiversity. Beginning with their complicated germination strategy to a specialized mechanism for seed dispersal, the seeds continue life in diverse environmental settings. With numerous ecological agents and biological disturbances, viability as well as the germination capacity of the seeds can be reduced. Withstanding these hindrances is beneficial while conserving the seeds as well as promoting agricultural, forest reproduction, and vegetation restoration activities.

References

1. Xue, X., Du, S., Jiao, F., Xi, M., Wang, A., Xu, H., Jiao, Q., Zhang, X., Jiang, H., Chen, J., & Wang, M. (2021). The regulatory network behind maize seed germination: Effects of temperature, water, phytohormones, and nutrients. The Crop Journal, 9(4), 718–724. https://doi.org/10.1016/j.cj.2020.11.005
2. DID YOU KNOW THESE FUN FACTS ABOUT SEEDS? – Payne’s Nurseries. (2025, March 3). Payne’s Nurseries. https://www.paynes.com/know/
3. Black, R. (2012, February 20). Ancient plants back to life after 30,000 frozen years. BBC News. https://www.bbc.com/news/science-environment-17100574
4. Double coconut: The largest seed in the world. (n.d.). Kew. https://www.kew.org/read-and-watch/double-coconut-largest-seed-in-the-world
5. Admin. (2023, July 14). Seed germination – Process, Necessity, and its Major Factors. BYJUS. https://byjus.com/biology/seed-germination/
6. Factors that Impact Seed Germination. (n.d.). ECHOcommunity. https://www.echocommunity.org/en/resources/ce935a6b-32ed-4d0b-baf5-5411b3a7d600
7. The Seed: Definition, types, seeds, structure, videos, solved examples. (2020, May 12). Toppr-guides. https://www.toppr.com/guides/biology/morphology-of-flowering-plants/the-seed/
8. Admin. (2020, July 29). Parts of a seed- explore the seed coat, endosperm, embryo. BYJUS. https://byjus.com/biology/parts-of-a-seed/
9. Barclay, G. F. (2015). Anatomy and morphology of seed plants. Encyclopedia of Life Sciences. https://doi.org/10.1002/9780470015902.a0002068.pub2
10. Trisha. (2015, August 27). Seeds and their Morphological Features (With Diagram). Biology Discussion. https://www.biologydiscussion.com/plants/seeds-and-their-morphological-features-with-diagram/6334
11. NEETprep. (n.d.). NCERT Ebook for Morphology of Flowering Plants – Morphology of Flowering Plants – Chapter 5 – NCERT Biology – XI. https://www.neetprep.com/ncert/1618-Morphology-Flowering-Plants-Morphology-Flowering-Plants–NCERT-Chapter-PDF