- Just add water? Weird ways plants germinate
- Animals giving seeds a helping hand
- Grass Growth and Regrowth for Improved Management
- What are the factors that determine seed germination?
- Seed Germination
Just add water? Weird ways plants germinate
Animals giving seeds a helping hand
Dispersal: when seeds get carried away
It’s pretty common knowledge that bees help fertilise flowering plants by carrying pollen from one flower to another. But did you know that many plants also rely on insects and animals to transform from a seed to a seedling?
Animals help seeds by carrying them to a place where they can germinate. This may be as simple as a bird knocking seeds to the ground while landing on a branch. Perhaps more significantly, by eating the seeds (often attracted by the ripened fruit surrounding them), birds, bats, insects and other animals may carry them away from the parent plant in their gut, to be deposited somewhere else—in their poo.
Why is this dispersal an advantage for seeds? Well, spreading seeds out means less competition between the seedling and its parent plant, and between the young seedlings. Also, seeds may be moved to places that are more suitable for germination. The cadagai eucalypt, for instance, has a resin that stingless bees like to use in nest building. They collect the resin from inside the eucalypt fruit (capsule) and, inadvertently, the seeds as well. This is beneficial for the seed if it gets carried away to a suitable habitat.
Video: How seeds get a helping hand from animals (Smithsonian Channel / YouTube). View details and transcript.
Plants ❤ ants
With the highest ant biodiversity in the world, Australia has enough ants to keep the scientists who study them happy for a lifetime. And all these ants are essential for keeping plants happy, too.
Like stingless bees help cadagai eucalypts, ants play an important role in the germination of Australian seeds by carrying them away from their source. It’s so important, in fact, that there’s even a name for the dispersal of seeds by ants: myrmecochory. While they usually transport seeds a small distance, scientists have recorded an ant carrying a seed 180 metres—that’s longer than two jumbo jets, a long walk for such a tiny creature! Ants not only move seeds across the ground, but underground to their tunnels. Here, some seeds are free to germinate, safe from predators and away from harsh above-ground weather conditions.
Wattles (acacias) are one type of plant that rely on ants for germination. Ants love the tasty stalk, packed with carbs and protein, that connects wattle seeds to their pods. They take the seed underground to feed the stalk to their larvae, dumping the seed. It’s a perfect environment for a ready-to-germinate seed. Not only have the ants moved it away from predators, but they’ve also loosened and aerated the soil to build their tunnels, introducing oxygen and making space for moisture to enter the soil. Thanks to ants, wattles can thrive in the most arid of areas.
The tasty stalk that attaches a wattle seed to the fruit pod entices ants to carry the seeds away to their nests. Image source: Maurice MacDonald / CSIRO Science Image.
Birds with benefits
The cassowary is a flightless bird of northern Australia, which can stand up to 2 metres tall. It has a spur on its foot and a reputation for being dangerous if you get on its wrong side. It’s also essential for the germination of rainforest plants.
In Australian rainforests, cassowaries are the only animals that consume large native fruits. Hardly what you’d call delicate eaters, they forage for fruits on the forest floor and swallow them whole—seed and all. After carrying the seeds around in their guts for a while, they drop them elsewhere, in their poo. In contrast to other rainforests in the world, Australian rainforests lack large fruit-eating animals such as primates. So, while smaller birds, such as fruit-eating pigeons, and bats do the work of eating (and dispersing) fruit with multiple, small seeds, cassowaries are the only way that large, single-seeded seeded fruits can be dispersed.
What’s more, being deposited with a good dose of cassowary poo may also benefit some seeds when it comes to germination. Not only do droppings provide moisture, but they also protect the seeds from predators such as rodents, who don’t like eating seeds in fresh scats—and, really, who can blame them?
A 2010 study looked into whether the benefits of being eaten by a cassowary go beyond mere dispersal, investigating the effects of gut scarification (weakening of the seed coat in the bird’s digestive tract) and fruit pulp removal. It found that just passing through the gut of a cassowary improved the germination performance of large, single-seeded rainforest plants.
Cassowary poo can contain up to 1 kilogram of seed. Image source: Jeremy Bornstein / Flickr.
Emus are another big bird known to play an essential role in the germination of certain plants. Researchers have discovered that passing through an emu’s gut can help the snottygobble, also known as the nodding geebung (Persoonia nutans), to germinate. An endangered Australian plant, the snottygobble is important as a food source for some animals like possums and emus—and it’s very hard to wake from dormancy. It’s not yet known exactly why being eaten by an emu is good for snottygobble germination, but possible explanations are that the seed coat is scarified by the emu’s digestive tract; that the emu’s gut flora helps the plant grow once it passes out the bird’s other end; or that the seed gets coated with a chemical signal for germination while in the emu’s gut.
Grass Growth and Regrowth for Improved Management
The germination process begins when water is absorbed (imbibed) by the seed. This initiates several biochemical events necessary for seedling development. For example, enzymes secreted from the aleurone layer, break down starch in the endosperm converting it to simple sugars which nourish the embryo.
All structural components of the grass seedling arise from the embryo. The endosperm provides a quick source of energy for the developmental process, whereas the cotyledon (rich in fats and oils) provides energy for later stages of development.
Germination is considered complete when the radical (which becomes the primary root) ruptures the coleorhiza (root sheath) and emerges from the seed.
A “seedling” has developed when the first true leaf appears. Grass seedlings have an important structure called a coleoptile which protects and pushes the plumule through the soil. Once the coleoptile has emerged from the seed, mesocotyl elongation commences, pushing the base of the coleoptile upwards toward the soil surface.
There are actually two types of seedling development among grasses. Most grasses have an elonagated area just below the coleoptile called a mesocotyl. Some grasses, such as crested wheatgrass, have no elongation in the mesocotyl, below the coleoptile, but have a long coleoptile.
When the coleoptile breaks through the soil crust and is exposed to light (particularly red and/or far-red light), the mesocotyl ceases elongation. If the coleoptile is shielded from red light, as with severe shading, the mesocotyl may continue elongation to the extent that is pushes the coleoptile base above the soil surface. This causes the seedling to lodge and later perish due to poor crown development.
The mesocotyl arises from the embryonic axis (cotyledonary or scutellar node) and terminates at the base of the coleoptile. Mesocotyl elongation depends upon energy reserves in the seed. Seedlings often fail with deep seed placement due to inability of the mesocotyl to raise the coleoptile to the soil surface. In this event the leaves may unfurl beneath the soil crust resulting in seedling death.
With exposure to light, seedling leaves begin to supply energy through the process of photosynthesis. At this point the seedling becomes independent of the seed for its food supply.
Seminal and Adventitious Root Development
The primary root, together with the closely associated seminal roots, constitute a root system capable of temporarily supplying water and inorganic nutrients to the seedling. These roots function until adventitious roots, arising from crown tissue, form the permanent root system. The more hairy adventitious roots permeate a large volume of soil and are more efficient than the primary, seminal root system.
Seedling growth and development during germination is totally dependent on energy reserves stored in the seed. Thus, with deep seed placement, the coleoptile may fail to break through the soil surface. In this event, the first true leaf, as yet contained in the coleoptile, will likely unfurl beneath the soil surface. The leaf, without the aid of the coleoptile, is unable to penetrate the soil surface. Failure of the coleoptile to reach the soil surface is due chiefly to insufficient energy reserves to support mesocotyl elongation. The combined forces associated with mesocotyl elongation and coleoptile growth are normally sufficient to break through the soil crust.
Exposure of the coleoptile tip to light stops mesocotyl elongation at the perfect position for leaf emergence. If mesocotyl elongation is insufficient (due to deep planting or poor quality shriveled seed), the first leaf may be unfurled beneath the soil surface and the seedling will perish.
The smaller the seed, the shallower the recommended planting depth. Plant grass seeds no deeper than 5 times their diameter. Thus, small seeded grasses like timothy should be just barely covered by soil, no more than 1/8 to 1/4 inch deep. Large grass seeds like corn may be planted 1 to 1 1/2 inches deep.
Management scenarios that reflect this material are: Shriveled grain implications and Planting depth / soil. These are located in the management segment of this project.
Some of the major factors necessary for seed germination in plants are as follows:
Germination cannot occur unless and until the seed is provided with an external supply of water.
Water is absorbed by a dry seed through the micro Pyle and the seed coat. Water performs a number of functions during the germination of seeds.
(a) It softens seed coat and makes it permeable. Increased permeability allows better gaseous exchange.
(b) Water activates the protoplasm of the seed cells.
(c) Insoluble food materials get solubilised in the presence of water which then diffuses from the storage region to the embryo axis.
(d) Several enzymes which are essential for growth and germination develop only in the presence of water.
Aeration of the soil is absolutely necessary for the germination of the seed because oxygen is necessary for the aerobic respiration by which the seeds get the requisite energy for the growth of the embryo.
Seeds normally germinate within a wide temperature range. However, freshly harvested seeds of several plants germinate only within a narrow temperature range which widens only when after-ripening has taken place.
Plants differ as to the effect of light on their germination. Seeds of many plants are light indifferent or nonphotoblastic, i.e., they are not influenced in the germination by the presence or absence of light. Most of our important crop plants belong to this category. The seeds which are affected by light are described as photoblastic.
Sensitivity to light is a specific character. The photoblastic seeds are of two types, positively photoblastic or light sensitive and negatively photoblastic or light hard. The positively photoblastic seeds require light for germination, e.g., lettuce, tobacco, many grasses and several epiphytes. The negatively photoblastic seeds cannot germinate in the presence of light e.g., Tomato, Onion, Lily, etc.
5. Other factors:
Many orchids and other plants exhibit seed germination only when an appropriate fungus partner is available. Seeds of some parasitic plants will similarly grow only in the vicinity of their host roots because the latter excrete certain growth hormones. Seeds of some aquatic plants germinate only at low or acidic pH.
The ability of a seed to germinate when provided with optimum condition is described as vitality of the seeds. It is dependent upon its stored food, size, health, etc.
2. Longevity or viability:
With the passage of time a seed looses it power to germinate. Thus each seed has longevity or a period within which it can show renewal of growth or germination. Most of the crop plants lose their viability within 2-5 years.
Legumes ordinarily retain their viability for longer periods. A number of seeds have been recorded to remain viable even after 100 years, (e.g., Trifolium, Astragalus, Mimosa species). Many species remain viable only for one season, e.g., Birch, Elm, Tea.
It is due to the internal conditions of the seed. It is, therefore, also described as the inhibition of the germination due to the internal conditions in an otherwise viable seed. These internal restrictions must be offset before germination can occur in dormant seeds.
- Germination Requirements
How do seeds know when spring has arrived? Is it the warm weather? If germination is temperature controlled, why don’t seeds that are dispersed at the end of the flowering season germinate during warm fall days? Or during a January thaw?
Seeds, and the embryos they contain, have evolved the ability to remain in a dormant state until conditions are favorable for growth. Plants vary considerably in how long their seeds will remain viable. Some types of seeds are capable of germinating for only a few weeks after dispersal, while other types will germinate after hundreds of years if the conditions are right.
Germination, the process during which a seed begins to develop, is controlled by both internal and external factors. The external factors include water, temperature, oxygen, and sometimes light.
The presence of water is the most important factor. Without water, cells cannot carry out their necessary activities and the seed will not germinate. When a seed absorbs water, it is called imbibition. Water enters the seed either through a tiny opening in the seed called the micropyle or through the seed coat. The micropyle is also the site at which the pollen tube enters the ovule during fertilization.
Many seeds have thick or waxy seed coats that will not allow water to pass through until their surfaces have been scarified. Scarification is the process of scarring or abrading the seed coat. This can be done chemically or mechanically (using a file, sandpaper, or knife). The scarred seed coat is permeable, allowing water to pass through and germination to begin.
Other seeds require cold temperatures before they can germinate. This is primarily a requirement of seeds in temperate regions, where winter provides this condition naturally. When these seeds are removed from their natural habitats, they must undergo stratification in order to germinate. If we break this word down, we see that the root is stratify, meaning to layer. Historically, seeds were stratified by layering them in damp vermiculite, peat moss, sawdust or sand. Botanists have now discovered that they achieve the same results if the seeds are layered in damp paper towels within a plastic bag and placed in the refrigerator for two to three months.
Temperature is also an important factor for the process of germination itself. For most plants, the optimal germination temperature is 25 to 30oC (77 to 86oF).
Some seeds require light in order to germinate. Since light can reach only those seeds that are positioned at or near the surface of the soil, these seeds-those that require light-will not germinate unless they are located in this portion of the soil. Often these types of seeds do not have a large enough food supply to allow them to grow through a great distance before they reach the surface.
Because exposure to the wrong growing conditions can kill a seed, they also have special internal blocking agents (commonly called germination inhibitors) that prevent them from germinating until the next growing season. These inhibitors do not allow water to enter the seed, thus preventing germination from taking place. A period of drying destroys the blocking agents.
So why is it that seeds don’t germinate inside the fruit? Maybe they do. Your students can explore this idea by planting a fleshy fruit and seeing if the seeds inside germinate. They won’t, because the pulp of most fruits contains blocking agents that prevent germination. Once the pulp has been removed and the seed is allowed a drying period, the blocking agents inside the seed are destroyed, allowing the seed to germinate. This enables seeds that are embedded in fruits to hold off on germinating until they are dispersed (usually by animals).
Why don’t I have to do any of these things to get my store-bought seeds to germinate? Commercial seeds are artificially selected to germinate once exposed to favorable conditions.
Field, laboratory, and greenhouse experiments were conducted to determine the seed production potential and effect of environmental factors on germination, emergence, and survival of texasweed. Texasweed produced an average of 893 seed per plant, and 90% were viable. Seed exhibited dormancy, and prechilling did not release dormancy. Percent germination ranged from 56% for seed subjected to no prechilling to 1% for seed prechilled at 5 C for 140 d. Seed remained viable during extended prechilling conditions, with 80% of seed viable after 140 d of prechilling. Texasweed seed germinated over a range of 20 to 40 C, with optimum germination (54%) occurring with a fluctuating 40/30 C temperature regime. Seed germinated with fluctuating 12-h light/dark and constant dark conditions. Texasweed seed germinated over a broad range of pH, osmotic potential, and salt concentrations. Seed germination was 31 to 62% over a pH range from 4 to 10. Germination of texasweed ranged from 9 to 56% as osmotic potential decreased from − 0.8 MPa to 0 (distilled water). Germination was greater than 52% at less than 40 mM NaCl concentrations and lowest (27%) at 160 mM NaCl. Texasweed seedlings emerged from soil depths as deep as 7.5 cm (7% emergence), but emergence was > 67% for seed placed on the soil surface or at a 1-cm depth. Texasweed seed did not germinate under saturated or flooded conditions, but seed survived flooding and germinated (23 to 25%) after flood removal. Texasweed seedlings 2.5 to 15 cm tall were not affected by emersion in 10-cm-deep flood for up to 14 d. These results suggest that texasweed seed is capable of germinating and surviving in a variety of climatic and edaphic conditions, and that flooding is not a viable management option for emerged plants of texasweed.
Nomenclature: Texasweed, Caperonia palustris (L.) St. Hil. CNPPA.
What are the factors that determine seed germination?
When you buy your packet of squash seeds each spring, you plant them in the ground, and expect plants to sprout! It seems like an easy process, but there’s a lot of plant physiology going on. Besides the current weather conditions, the seeds’ history, and their parents, play a role in how well they germinate. You might even be aware that seed packets are labeled with an expiration date. Let’s talk about what are those top factors that affect seed germination.
Planting a seed, tending to plants, and yielding some home-grown vegetables is quite satisfying. But, what goes into seed germination? Variables like temperature, moisture and oxygen are important. Credit: Justin O’Dea
Seeds are designed to spread throughout the environment and grow into new plants through the process called seed germination. This process causes a seed to sprout. As seeds absorb water, stored food materials become hydrated. Enzymes in the seed become active, producing energy for the growing seed. The root (or radicle) is the first part of the seedling to emerge. It is the first indication of that a seed is viable, meaning it is possible for it to grow into a healthy plant. Roots provide all the necessary nutrients, minerals, and water for the growing shoot. Cotyledons are the parts that form into the first leaves of the seedling. So now, the plant is capable of obtaining energy from sunlight to do photosynthesis in order to make its own food. A few conditions must be present in order to properly germinate a seed.
The root, or radicle, is the first plant part to emerge from a germinating seed. Credit: Kevin Hudson
Environmental conditions must trigger the seeds to grow. Among them, temperature plays the major role. Some plants require moderate to high temperatures, but others may need cold temperatures. For an example, spinach needs cold – if temperatures are 77°F, you’ll only get about half the amount of seeds to germinate than their ideal temperature of 59°F. But, coriander seeds will double their germination rate if the temperature is 77°F (vs the same 59°F). Germination temperature for vegetable crops such as beans, cucumber, okra, tomato, and pepper fall in the range of 60-85°F. Radish, cauliflower, lettuce, cabbage, and carrot need low temperatures, typically in the range of 45-70°F. So the ideal temperature depends on the plant species.
Moisture essentially brings the seed back to life. When the seed fills with water in a process called imbibition, it activates enzymes to initiate the germination process. On the other hand, too much water can cause seeds to rot instead of developing into a seedling. So, suitable moisture is needed to get the best results.
Like humans, seeds also need to breathe. Most seeds will not germinate under waterlogged conditions, because water is taking up all the air space in the soil. So, proper drainage is important to supply enough oxygen to the growing seeds. Aeration by plowing or mixing the soil can increase the available oxygen to grow. This is why getting your soil texture right before planting can really help with your yields. Adding in some compost and making sure your garden is appropriately drained can help in this regard, too.
The soybean plants on the left germinated at more than double the rate than the seeds on the right. Credit: Chathurika Wijewardana
On your package of seeds, there are instructions for how deep to plant your seeds. This is another area where optimal depth will depend on the plant. Small seeds typically need to lay on top of the soil for successful germination. Because of their small size, they only have stored food for a limited period of growth. If we put small seeds in too deep, lack of oxygen will limit seed germination, or the seedling will finish its food reserve prior to reaching the soil surface. On the other hand, large seeds need a deep planting location so that roots can grow deeply for proper anchorage.
In your home garden, when you start planting your seeds (vegetables, flowers, or any kind of herbs), the two most important things that you want to achieve are maximum germination and fast germination rate. However, due to some natural factors and environmental limitations you may have to put an extra effort to achieve the best possible results. These are some tips you can use to achieve the highest germination rate:
- Always plant seeds that are for that particular year. If you feel tempted to use seeds purchased in a previous year, do a germination test first (see bottom of linked page).
- Choose the ideal planting time, as noted on your seed packets. The ideal planting temperature range will be listed.
- Some seeds stay dormant and take a long time to germinate until they have enough moisture to grow. Pre-soaking the seeds before planting (usually overnight, on a wet paper towel), can help.
- If you haven’t had any luck germinating seeds soon after sowing on your outdoor seed bed in the past, you can begin planting seeds indoors. By doing so, you can protect your plant from any damages like wind, frost, or drought and later you can move the seedlings outdoors to continue to develop.
Answered by: Chathurika Wijewardana, Mississippi State University
About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.
Plants come from seeds. This happens when the seeds are planted in the ground and sprout (begin to grow). Before a seed can sprout, it must go through a process called germination. The process of germination happens inside the seed. To learn more about the process of germination, let’s take a look inside a seed…
The parts of a seed
Before we look at the inside of a seed, let’s talk about the outside of the seed. The outside of a seed is called the seed coat. The seed coat is the hard outer layer of the seed. It is the part we see and hold in our hands before we plant them in the ground or a pot of soil.
The inside of a seed has four main parts. The four main parts of the inside of a seed are:
- The Epicotyl
- The Hypocotyl
- The Radicle
- The Cotyledon
Now let’s look at what each of these parts becomes once the seed becomes a plant.
The Epicotyl are the parts of the seed that become the first leaves of a plant.
The Hypocotyl is the stem of the plant.
The Radicle is the first root the plant has.
The Cotyledon is the inner protective layer of the seed that stores food for the seed to use during the process of germination and until the seed comes through the soil and has leaves that can be used for photosynthesis.
The process of germination
If you have ever planted a seed, you know how exciting it is to see the plant that comes from that seed break through the soil. Have you ever thought about how it happens? Let’s find out!
When you plant seeds in some soil, it is important to keep the soil watered (not too much). The reason this is so important is because the seeds you plant need to be able to take in oxygen and minerals from the soil and water through the seed coat’s tiny pores (holes) to give the inside of the seed the food it needs to break open and make its way through the soil so it can grow into a plant.
When the seed is full enough, it pops open. The first parts of the seed to come through the seed coat are the cotyledon and the radicle (root). The root takes hold of the soil and starts to take in food from the soil. But because it is still so small, the cotyledon is still the main source of food for the seed.
The next part of the seed that appears is the hypocotyl. The hypocotyl is sometimes called the understem because it first appears under the cotyledon. The hypocotyl continues to grow upward with the epicotyl. The epicotyl becomes the first leaves of the new plant.
By the time the epicotyl are showing, the plant is now above the ground. When this happens, the cotyledon (which is sometimes called the seed leaves and looks like thin, dried brownish-white skin) has finished its job. Because their job is done, they fall off the plant and become part of the soil.
Once the cotyledon are gone, the plant’s tiny leaves take over the job of supplying food to the new plant. And that is the process called germination.
All seeds are not alike
If you look at different kinds of seeds, you can easily see that they are not all alike. Seeds come in different sizes, shapes and colors And like you’ve already learned, some seeds have softer seed coats than others. All these differences mean that seeds germinate differently.
Seeds with hard seed coats usually germinate slower than seeds with soft seed coats. Why do you think this is?
The reason seeds with hard seed coats take longer to germinate is that it takes longer for the seed to drink enough water to soften the seed coat enough that the inside parts of the seed can break through.
There are also other reasons some seeds take longer to germinate than others. Here are a few of them:
- The amount of sunshine. Seeds don’t see the sun, but the sun heats the soil to make it warm and cozy—which is just what a seed needs to germinate.
- The amount of water in the ground. If the soil is too dry, the seed cannot get the water it needs. If it is too wet, the ground will not have enough oxygen in it to give the seed what it needs to germinate.
- Planting the seed too deep. If you plant a seed too deep, it will use all the energy and food stored in the cotyledon before it can break through the ground so the leaves can come out and take over feeding the plant.
- The seasons. Most seeds will not germinate in the fall or winter. The ground is too cold during these two seasons for a seed to germinate. Instead, the seeds sleep until spring. When a seed sleeps, it is dormant.
Planting seeds and watching them grow is one of the most exciting things ever!
Fill each section of an empty egg carton with moist potting soil. Place a different kind of seed in each section. Make a chart showing what seeds are planted in each section of the egg carton. Keep the seeds in a warm, sunny place and keep the soil moist—but not too wet. Write down how many days it takes for each seed to germinate and pop through the soil.
Read more about Plant Facts for Kids