What is germination?
It all starts with a seed…
Seeds come in all shapes and sizes. The seeds of some tropical rainforest orchids are very small, about the size of a pinhead. They are so small that they look like dust. The Coco de Merseed is one of the largest seeds in the world. It can be up to 40 centimeters long and weigh as much as 18 kilograms.
Essentially, though, a seed consists of:
- the dormant embryo—a young plant that formed from a fertilized egg cell
- the seed coat—which is a protective layer surrounding the whole package
- the food store—which contains all the nutrients (carbohydrates and protein) an embryonic plant needs to get going. During its early stages of growth, the seedling relies on this store until it’s large enough for its own leaves to begin making food through photosynthesis. Different seeds store food reserved in different ways—some rely on large reserves of endosperm (nutritive tissue around the embryo), while others store food reserves in embryonic leaves.
- the cotyledon/s—which may become the embryonic first leaves of a seedling. In angiosperms (flowering plants), species with one cotyledon are called monocots, while plants with two cotyledons are known as dicots
In flowering plants, seeds develop in a fruit. Fruit not only protects seeds, but also aids in their dispersal to other areas. The fruit is sometimes soft and delicious, like a berry that attracts animals who carry the seed to a new home. At other times, the fruits are hard and woody, like those of a banksia or eucalypt.
THE GLOBAL SEED VAULT—A SEED ‘BACK-UP’
Deep in a mountain on the island of Svalbard off the coast of Norway, tens of thousands of seeds are slumbering. This is the Svalbard Global Seed Vault. It is a seed bank established by the Norwegian government which holds copies of more than 4,000 plant species from seed banks around the world.
The point of these seeds is to have backups in case local seed stocks are destroyed by something like war or a natural disaster. There are over 4,000 plant species in the vault, including essential food crops such as beans, wheat, and rice.
The airport in Svalbard is the northernmost in the world, making it the perfect place to store seeds from around the world. The low temperature on this island will keep the seeds frozen, even if the power fails.
What seeds need to germinate
Seeds may appear to be inactive from the outside, but there is a lot going on inside of them. Some of the tissues inside of seeds remain active, and carry out some basic metabolic processes, such as cellular respiration, according to experiments. This means that seeds use small amounts of stored energy to stay alive until the conditions are right for them to grow.
To germinate, all seeds need:
- the right temperature
You need a suitable place and a little time to be able to do something properly
Most seeds need to absorb water to germinate; this is known as imbibition. Water:
- hydrates enzymes in the seed, activating them. As a result, the seed begins to release energy from its food store for growth.
- causes pressure to build in the embryo’s cells, causing them to enlarge. This often results in the seed coat breaking open.
In order to produce energy for germination and growth, seeds need oxygen.
The embryo gets energy from the food it eats. The process of aerobic respiration, where energy is released from glucose using oxygen, is how all organisms do it. During aerobic respiration:
- glucose and oxygen are used up
- carbon dioxide and water are produced as waste, and energy is released.
Different species of plants require different temperatures in order to germinate successfully. The temperature required also varies depending on the plant’s natural environment. Some need fluctuations in temperature. Some seeds need to be exposed to cold conditions for a period of weeks or months before they will germinate at a higher temperature. This means that cold climate seeds won’t start germinating until after winter.
What about light, you might wonder? Seeds don’t need light to germinate, but seedlings need light to do photosynthesis for energy. The dormancy of a seed is broken when it is exposed to certain types or amounts of light. Seeds that can lie dormant for years are only exposed to light once a tree falls and creates a gap in the forest canopy.
What happens during germination
Now that we know what a seed needs in order to germinate, let’s take a closer look at the germination process.
- The seed takes up water, activating enzymes that begin the growth process.
- The embryo swells and lengthens.
- The embryo breaks through the seed’s covering layers.
- The root meristem is activated and the embryonic root (radicle) pushes through.
- Cotyledons (embryonic leaves) break out.
- The shoot meristem is activated.
- ‘True’ leaves form—the plant is now able to get energy from the sun.
Dormancy: seeds that know when to grow
Dormancy is a state of plants that is typified by suspended animation. This means that the plant’s growth is halted until specific environmental conditions are met. A seed in a state of dormancy will not germinate even when exposed to the proper environmental conditions of water, oxygen, and temperature. The seed requires additional cues, such as light or a change in pH, before it will begin to grow. A seed can be dormant while still on the parent plant, or it can become dormant after it has left the parent plant.
Dormancy is beneficial for plants because it allows them to reduce the risk of their offspring’s death or stunted growth. Delaying germination allows seeds to germinate at ideal temperatures for the seedling to prosper. This also means that seeds can germinate sooner after being separated from the parent plant, which reduces competition with other seeds from the same parent. This allows for the germination of seeds to be spread out, so that if there is bad weather and the first batch of seedlings is destroyed, there will be some reserve seeds.
Dormancy can be caused by a few different mechanisms, some of which happen outside the embryo (exogenous dormancy), and others that happen inside it (physiological dormancy). An exogenous dormancy mechanism is a mechanism that prevents a seed from absorbing water or air. A hard seed coat is an example of an exogenous dormancy mechanism. The tough outer layer of the seed may need to be weakened by exposure to heat, cold, or by exposure to the digestive fluids of an animal. The embryo may also trigger dormancy by experiencing certain internal changes, especially chemical changes, that must occur within the seed before it is able to germinate. Seeds that need a period of light or dark to germinate include, for example, . The Spinifex hirsutus plant, found in the sand dunes of Western Australia, is more likely to germinate and grow when its seeds are buried deep in the dunes where the sand is more stable and there is more moisture and nutrients. The seeds of this plant will only germinate in darkness. A seed will not germinate until the embryo has grown to a certain size in another kind of physiological dormancy.
People in the agricultural industry will often do things that are similar to what happens in nature in order to get seeds to start growing–for example, by putting seeds in a fridge to make them cold like it is in winter, or by using something rough on the outside of the seed to make it easier for the plant to grow.
And Now For Light!
Interesting seed germination fact: most commercial seeds will germinate with or without light because they have been selected to be viable under a variety of conditions. Some things need to be exposed to light. The following text contains more information on some specific seeds that do or don’t need light to germinate.
They have found that light affects not only the rate at which seeds germinate but also the percentage of seeds germinating and the strength of the seedlings. Scientists have conducted research on light requirements for germination over the past century and discovered that light affects not only the rate of seed germination, but also the percentage of seeds that germinate, as well as the seedlings’ overall strength. After testing different combinations of heat and light, Kinzel discovered that some seeds need light to grow, while others need darkness.
Some things that need light don’t need to be covered with dirt. This allows seeds to grow in the right amount of light. To germinate, some plants need darkness, so they need to be planted deeper in the soil to block out the UV rays that could prevent them from growing.
Is Light Even Necessary?
Here’s the truth: all seedlings need light to root. Without a light source, they will get leggy as they strain to reach for it. Some plant seedlings need more light than others.
Those plants that require less light at an adult stage don’t need as much light as those plants that enjoy the full sun throughout their life cycle. Every plant needs the right amount of light and heat to grow, otherwise it will wither and die.
There is a distinction between seeds that need light to germinate and those that do not. It is important to know this when deciding if you should start your seeds indoors or outdoors in your garden. We’re going to take a brief look at photosynthesis and figure out why some seeds need light to germinate while others don’t.
An understanding of which seeds need light to germinate and which need darkness can be gained by studying plant phytochromes. Phytochromes are light receptors in plants that aid in photosynthesis by interacting with red and ultraviolet light.
There are two types of phytochromes that exist in the plant world: type I is activated by far-red UV light, and type II is activated by red UV light.
The trees in the canopy of a rainforest, for example, are able to withstand direct sunlight or red light because of their type II phytochromes. The light that is absorbed by the leaves in canopy trees is reflected back to the source of the light as well as below to the forest floor plants. The plants at the base of the forest contain type I phytochromes. They tend to get burnt if they receive red light, rather than far-red light.
Plants with leaves always have a way to protect themselves from the sun. You need light, but it’s critical to have the right kind. If a plant is not suited for the kind of light it is receiving, it can be damaged. The reverse is true as well. When plants receive the right amount of light, they are able to thrive even when other needs are not perfectly met.
How Much Light Is Enough
If you want your seeds to germinate, you should put them in a tray with grow lights or in a sunny south or north-facing window, depending on which hemisphere you live in.
You can also sow the seeds directly into the soil if the weather is warm and sunny. A greenhouse that is in direct sunlight during early spring can also provide enough light and heat for germination to occur.
You should sow seeds that don’t need light to germinate in trays. The best way to keep them from being exposed to light is to cover them in black plastic. Make sure to monitor the temperature when germination is taking place to ensure that it doesn’t get too hot or cold.
Plastic traps heat and helps keep the soil moist, providing the tray with humidity.
Seeds That Need Light
The majority of vegetables need UV radiation to germinate, so enumerating them all would be superfluous. There are some plants that will not germinate unless they are exposed to light. The seeds of all the plants here have thin seed casings and are very small.
- Lettuce: here we have an example of a seed that doesn’t need to be covered with soil to germinate in your garden. Lettuce seeds must be exposed to light to grow into seedlings. You can sprinkle them on soil or vermiculite and then cover them with a thin layer of soil or vermiculite.
- Carrots: much like lettuce, to get carrot seeds to germinate, expose them to light by sprinkling them on the surface of the soil. Use the same method as lettuce: plant seed in a row on soil or vermiculite and cover with soil or vermiculite.
- Rose: rose seeds germinate best in direct sunlight. Keep soil moisture at the right level and seedlings will emerge in about six weeks.
- Certain salvias: check varieties here because there are some salvia species that prefer darkness. After direct sowing in the ground of your garden, keep the soil moist but don’t wash tiny seeds away.
Plants That Prefer The Dark
Some of these species are animals, while others are plants. Some species prefer to exist in darkness, these include both animals and plants. There are many different types of seeds that germinate well when they are deep in the soil and under a dark cover. Many of these seeds are popular vegetable garden varieties.
- Nasturtium: the benefits of gardening with nasturtium are many. Direct sow nasturtium seeds about three times their diameter, covering them with rich organic soil. Soon after you’ll have happy round-leaved seedlings popping up!
- Calendula: sun inhibits calendula growth. Cover these worm-like seeds in 1 inch of organic soil out of direct sunlight. Keep the soil moist when gardening with calendula.
- Sunflower: mammoth varieties require a planting depth of three times their diameter, and do best under a plastic covering before they germinate. Once the first sign of germination shows, remove the plastic covering and transfer seedlings into direct light.
- Onion: Allium seeds are large with a thick seed casing and will not germinate in direct light. They actually prefer long nights during germination. So you can start them indoors under plastic, remove the plastic and place them under light after they’ve sprouted.