Seeds: Structure, Germination, and Seedling Growth
A seed is a small package containing a plant embryo, a food supply and a protective coat, and germination is the process by which the embryo resumes growth and becomes a seedling. In brief: a seed germinates when it has water, oxygen and a suitable temperature. For a bean seed, the root usually emerges first, followed by the shoot, and the stored food in the seed leaves feeds the seedling until it can make its own food.
Seeds allow plants to survive harsh seasons and spread to new places. This guide explains the structure of a seed, what is needed for germination, how a seedling grows, how to design a germination experiment and which mistakes are frequent in exams and in the garden.
What is a seed?
A seed forms from a fertilized ovule inside the flower of a seed plant. It contains a young plant, the embryo, in a resting state. When conditions are suitable, the embryo grows into a new plant.
Seeds are a reproductive and dispersal unit. They can stay dormant for weeks, years or, in a few species, centuries, which lets the plant wait for good conditions. Flowering plants (angiosperms) enclose their seeds in fruits, while conifers and other gymnosperms carry them on the scales of cones.
What is the structure of a seed?
Seeds differ in size from a speck of dust to a coconut, but most share three main parts.
- Seed coat (testa): A tough outer layer that protects the embryo from drying out, damage and some pests. A small hole called the micropyle lets water enter.
- Embryo: The young plant. It has an embryonic root called the radicle, an embryonic shoot called the plumule and one or two seed leaves called cotyledons.
- Food store: Stored starch, oils or proteins. In beans it is held in the cotyledons. In maize and wheat it is held in a tissue called the endosperm.
Monocot and dicot seeds
Flowering plants are often divided into monocots and dicots according to the number of cotyledons. A bean is a dicot with two large cotyledons that hold the food. A maize grain is a monocot with one cotyledon, and its endosperm holds most of the food.
How does seed germination work?
Germination begins when the dry seed absorbs water, a process called imbibition. The seed swells, the seed coat splits and enzymes become active. Enzymes break down stored starch into sugars, and the sugars supply the energy for growth.
The radicle is usually the first part to emerge. It grows downward and anchors the seedling. Next, the shoot grows upward. In some plants the cotyledons are carried above ground and turn green, as in sunflower seedlings. In others they stay below ground, as in peas.
What are the requirements for germination?
Most seeds need three conditions for germination. Light is not needed by most seeds, although some species respond to it.
Water
Water softens the seed coat, activates enzymes and allows chemical reactions to start. A dry seed has very low metabolism.
Oxygen
The growing embryo needs energy from aerobic respiration, which needs oxygen. A seed buried in waterlogged soil or deep under thick mud may not germinate for this reason.
Suitable temperature
Enzymes work best within a range of temperatures. Many temperate seeds germinate best between about 15 and 25 degrees Celsius. The ideal temperature differs by species. Some seeds also need a cold period, or a seed dormancy-breaking treatment such as scratching of the coat, before they can germinate.
How does a seedling grow?
After emerging, the seedling uses food from the seed to grow its first roots and leaves. This is why seeds are often thicker and richer in stored food than the young plant looks.
Once the first true leaves open and turn green, the seedling begins to photosynthesize. From that time, the seed food reserves are no longer the only source of energy, and the cotyledons shrivel or fall off in many species.
Further growth takes place at regions of dividing cells called meristems, at the tips of roots and shoots. Young seedlings respond to light and gravity: the shoot grows toward light and the root grows downward.
Worked example: a germination experiment
A student wants to find out whether oxygen is needed for germination. Cress or mustard seeds work well because they germinate within days.
- Prepare three test tubes, each with 10 seeds on damp cotton wool.
- Tube A is the control: it is loosely closed and kept at room temperature, so it has water, oxygen and suitable warmth.
- Tube B has seeds covered with a layer of boiled, cooled water topped with a thin layer of oil. Boiling removes dissolved oxygen, and the oil stops new oxygen entering.
- Tube C has dry cotton wool with no water.
- Keep all tubes in the same place with the same light for five days and count the germinated seeds.
A typical result is that most seeds in tube A germinate, few or none in tube B and none in tube C. This suggests that both water and oxygen are needed. The experiment is fair because the seeds, the light and the temperature are the same in all tubes. Only the factor being tested is different. Repeating the test with more seeds makes the results more reliable.
Germination requirements at a glance
| Condition | Role in germination | What happens if it is missing |
|---|---|---|
| Water | Activates enzymes, softens the coat | Seed stays dry and dormant |
| Oxygen | Allows aerobic respiration | Little or no growth |
| Warmth | Allows enzymes to work efficiently | Very slow or no germination in cold, damage in extreme heat |
What is known about seeds historically?
Humans have collected and saved seeds for thousands of years, and the beginning of agriculture depended on this practice. Gregor Mendel used pea seeds in the 1850s and 1860s for his experiments in inheritance, observing traits such as seed shape and color.
Modern seed banks, such as the Svalbard Global Seed Vault in Norway, store seeds at low temperatures as insurance against loss of crop varieties. Reports of the longest-surviving seeds vary, and claims of extremely old seeds that sprouted usually involve special conditions.
Why does seed biology matter in practice?
Farmers and gardeners test the germination rate of seeds before sowing. A packet with a 90 percent germination rate gives about 90 seedlings from 100 seeds under good conditions. Knowing the right depth, temperature and moisture improves the result.
Seed storage, seed treatments and the breeding of new varieties all depend on understanding dormancy and germination. Food producers also use germination: barley is germinated and dried to make malt for brewing.
Which mistakes are common?
Mistake 1: Saying that seeds need light to germinate
Most seeds germinate in darkness underground. A few species do need light, but the general requirements are water, oxygen and warmth.
Mistake 2: Confusing germination with growth in general
Germination ends when the seedling has emerged and starts growing. The later increase in size is growth of the young plant.
Mistake 3: Believing that soil is needed for germination
Seeds can germinate on damp paper or cotton wool. Soil becomes important later because it supplies minerals and support.
Mistake 4: Giving too much water
Soaked seeds can run out of oxygen and rot. Damp, not flooded, conditions give the best results.