Seeds contain everything a plant needs to get started growing. Despite their small size, seeds are complex and packed with different parts and nutrients that allow them to grow into mature plants. In this article, we will explore the anatomy and composition of seeds to understand what’s inside and how it helps them function.
The Parts of a Seed
While seed structures vary between plant species, most seeds contain three main parts:
Seed Coat
The seed coat, also known as the testa, is the hard, outer layer of a seed. It protects the embryo and endosperm from damage and prevents the seed from drying out. The seed coat also regulates the intake of water to initiate germination when conditions are right. It is usually made up of one or two layers of cells that are thick and impermeable to water. In some seeds, the seed coat is thin, soft and permeable. The seed coat gets its hardness from substances like lignin, suberin, mucilage, and tannins present within its cells.
Embryo
The embryo is the young, undeveloped plant inside the seed. It contains three key parts:
- Plumule – the part that will grow into the shoot system including stems, leaves, and flowers
- Radicle – the part that will grow into the root system
- Cotyledons – seed leaves that are the embryo’s source of nutrients until it can produce its own leaves
The embryo has everything needed to grow into a mature plant. It is a living but dormant organism until germination when it breaks out of the seed coat and begins to sprout.
Endosperm
The endosperm is a food storage tissue that provides nutrition to support the embryo during germination and early growth. It contains carbohydrates, proteins, and oils. The endosperm makes up the major bulk and weight of the seed. Seeds high in endosperm like corn contain abundant food reserves to power early seedling growth.
Other Seed Parts
While all seeds contain the coat, embryo, and endosperm, there are additional optional parts found in some seeds:
Fruit Walls
In some seeds, remnants of the ovary wall or fruit tissue may be fused tightly to the seed coat. For example, the juicy pulp of a tomato seed is botanically part of the fruit wall.
Hilum
The hilum is a scar on the seed coat marking where it detached from the funiculus, which connected it to the parent plant. The hilum is visible as a tiny hole or indentation on the surface of many seeds.
Micropyle
The micropyle is a tiny opening in the seed coat near the hilum. It allows water to enter and initiate germination. The pollen tube penetrates the micropyle during double fertilization to deliver sperm nuclei to the embryo sac.
Storage Materials
Seeds contain stored food reserves to nourish the embryo during germination. The storage materials provide a concentrated energy source to power early seedling growth until the plant can produce its own food via photosynthesis.
Carbohydrates
Starch is the most common carbohydrate stored in seeds. Grains and legumes contain abundant starch in their endosperm and cotyledons that break down into sugars to feed the embryo. Starch appears as dark specks and streaks inside some seeds when cut in half.
Proteins
Proteins in the form of enzymes and amino acids are stored in seeds to support the embryo’s growth. Non-endospermic seeds that lack food reserves are typically high in protein. Soybeans, peanuts, and other legumes contain lots of protein.
Oils
Oily seeds like sunflower and cotton seeds store triglycerides and fatty acids to provide concentrated energy for the embryo. Corn and soybean seeds also contain high percentages of oil in their endosperm and cotyledons.
Micronutrients
In addition to macro-nutrients like carbohydrates, proteins and fats, seeds also contain an array of essential micronutrients:
Vitamins
Thiamine, niacin, folate, and other B-complex vitamins are stored in seeds. Oilseeds also contain fat-soluble vitamins A, D, E, and K. Vitamins support enzyme functions in metabolism and growth.
Minerals
Important minerals like calcium, magnesium, phosphorus, potassium, iron and zinc are found in most seeds. Minerals support cellular functions and growth of seedlings.
Antioxidants
Some seeds are high in antioxidant compounds like vitamin E, selenium, and lycopene which protect seed tissues from damage. Dark pigments in seed coats also contain antioxidants.
Other Compounds
Seeds contain additional compounds that support dormancy, germination, and protection:
Phytic Acid
Found in bran and hulls of seeds, phytic acid is the storage form of phosphorus. It has antioxidant properties but can bind to minerals and inhibit their absorption.
Enzyme Inhibitors
Protease and amylase inhibitors found in seeds likely provide a defense mechanism before germination by inhibiting digestion. Activity of these enzymes increases during germination.
Phenolic Compounds
Phenols and tannins in seed coats act as chemical defenses against microbes, insects, and animals. They also regulate germination and dormancy.
Hormones
Plant hormones like abscisic acid and gibberellins help control dormancy and germination and stimulate growth after seeds sprout.
Seed Nutrient Composition
Here is the typical nutrient composition of seeds from various plant families:
| Seed Type | Moisture % | Protein % | Fat % | Carbs % |
|---|---|---|---|---|
| Cereal Grains | 10-13 | 7-15 | 1-5 | 73-83 |
| Legumes | 8-14 | 20-40 | 1-7 | 63-74 |
| Oilseeds | 5-12 | 20-30 | 30-50 | 15-30 |
| Nuts | 4-8 | 12-25 | 45-75 | 10-24 |
| Vegetables | 50-80 | 20-35 | 10-24 | 5-15 |
As shown, the macronutrient composition of seeds varies widely. Cereal grains are predominantly carbohydrate. Legumes are high in protein. Oilseeds and nuts contain lots of fat. Vegetable seeds have high moisture and protein.
Seed Dormancy and Germination
Seeds are biologically programmed to lay dormant until conditions are optimal for germination and growth. Seed dormancy mechanisms include:
Physical Dormancy
An impermeable seed coat prevents water from entering until scarification weakens the coat. Common in legumes, nuts, and fruits with hard pits.
Physiological Dormancy
Phytohormones within the embryo must be activated before germination can occur. Common in wild plants and weedy species.
Morphological Dormancy
An undeveloped embryo must mature before the seed can germinate. Found in some angiosperms.
Combinational Dormancy
A combination of physical and physiological dormancy within one seed creates double dormancy.
Seeds break dormancy and begin germination when conditions of water, oxygen, temperature, and sometimes light or chemical triggers are right. During germination, enzymes activate within the embryo and seed food reserves break down to nourish growth of the seedling.
Ecological Role of Seeds
Seeds play a crucial role in ecology and evolution of plants. Their unique features allow plants to disperse progeny across environments and survive through time. Key ecological functions of seeds include:
Dispersal
Dispersal away from the parent plant increases chances of survival. Seeds spread via wind, water, animals, force ejection, gravity, and adhesion. Some seeds can travel thousands of miles.
Dormancy & Longevity
Seed dormancy allows plants to wait for optimal conditions for growth. Some seeds can survive for centuries before germinating when conditions allow. The oldest viable seed sprouted after 32,000 years.
Variation
The mixing of genes during sexual reproduction creates genetic variation in seed populations. Variation allows plants to evolve adaptations to environmental changes.
Protection
The seed coat and fruit tissues protect the embryo from stresses like heat, cold, drought, and predators. Biochemical defenses also deter pathogens and pests.
Propagation
Seeds allow plants to multiply and propagate effectively. A single plant can produce thousands of genetically unique seeds over a lifetime.
Importance of Seeds
In addition to their biological role, seeds have immense economic and nutritional importance for humans:
Staple Crops
Starchy cereal grains like maize, wheat, and rice are seeds that provide staple food calories for humans across the world.
Oil Supplies
Oilseed crops like soybean, sunflower, rapeseed, cottonseed, palm provide the majority of the world’s food oil supplies.
Animal Feed
Seeds of corn, cereals, oilseeds, and legumes are widely used as feed grain for livestock animals that provide meat, milk, and eggs.
Industrial Uses
In addition to food and feed uses, seeds provide raw materials for biofuels, cosmetics, soaps, lubricants, and other non-food products.
Nutrition
Many seeds like beans, nuts, and grains are nutritionally dense foods with vitamins, minerals, fiber, protein, healthy fats and antioxidants.
Conclusion
Despite their small size, seeds are complex living structures with distinct parts and vital nutrients that enable new plants to sprout and grow. The seed coat, embryo, endosperm, and food reserves create a protective package optimized for dispersal, dormancy, and germination. Agricultural cultivation of seeds has allowed human civilizations to proliferate through reliable grain supplies. Next time you see a tiny seed, remember that it is a highly advanced system for creating and propagating plant life.