Have you ever planted a garden? If you have, you will know that plants start as seeds. Seeds are tiny capsules containing everything needed to germinate and grow to a whole new plant! How does this happen? What is inside those mysterious capsules? Let’s find out!
Table of Contents
Why Do Plants Make Seeds?
Why do we look the way we do? Why did we grow five fingers on each hand instead of four? Two ears instead of one? All the answers are in our DNA.
Every cell of every living thing contains DNA. This DNA is a list of instructions for how a living thing should grow and look.
Plants are very different from us, but they still have DNA! Every cell in a plant contains instructions telling the cell how to do its job.
Plants can get very old, but they cannot live forever. They will reproduce so that their species does not disappear. Plants do this in two different ways: through vegetative reproduction and sexual reproduction.
Vegetative reproduction is only possible in plants, not animals. It happens when a piece of the plant, like a stem, is cut off and falls onto the soil. If a plant is very good at reproducing vegetatively, even a tiny piece can grow into a whole new plant! This plant will be an exact clone of the original plant. This means that the DNA will be an exact match!
Sexual reproduction makes offspring that are not exactly the same as either parent. DNA from two different individual plants (parents) mix together and combine to form new DNA. Both parents must usually be from the same species so that their DNA is similar enough to be mixed.
What Does the Inside of a Seed Look Like?
The growing seed has a hard outer shell. This shell protects a tiny plant and some stored food.
The tiny plant is called an embryo. Even when the embryo is only made up of a few cells, the cells are already deciding if they should become part of a leaf, stem, or root. Once they decide, the cells work together to create the tiny plant.
Food stored inside a seed is called the endosperm. This contains enough food to help the tiny plant grow big enough so that it can start making food by itself. Food can also be stored in parts known as cotyledons, which will be discussed shortly.
The embryo and the endosperm are protected by a hard outer shell called the seed coat. This layer does not break open until the seed is in a safe place for the tiny plant to start growing.
Are There Different Kinds of Seeds?
Do all seeds look the same inside? No, there are lots of different kinds of seeds!
The embryo can have two tiny leaves, or just one. These tiny leaves are called cotyledons. Plants with one cotyledon are called monocots. These include grains like corn and wheat; palms like coconuts and bananas; onions, ginger and orchids. Plants with two cotyledons include most other kinds of plants. Tomatoes, potatoes, beans, cucumbers and pumpkins are all dicots!
When seeds are mature and finished growing inside of the flower, they will emerge dormant and ready to travel. The seeds of some species transfer all the food from the endosperm into the cotyledons while the seed is still growing. Have you ever eaten a peanut? Each half of the peanut is a cotyledon full of food!
Some seeds are very small, and some are very big. A small seed can grow a very big plant, and a big seed can grow a small plant. The size of the seed only tells us how long the embryo needs to be fed before it starts making its own food.
The smallest seeds in the world are orchid seeds. The biggest are palm seeds like coconuts.
Why Do Seeds Sprout?
When seeds sprout, the process is called germination. Some seeds germinate right away, while others can stay dormant for a long time. Some plant species have seeds that can stay dormant for centuries! The oldest seed that ever germinated was a 2000-year old-date palm from Masada in the Middle East.
If a seed waits to germinate, it is waiting for the right environmental conditions. When a seed coat breaks, the embryo is in danger. It needs to have the right amount of water, sunlight, nutrients, and heat available right away. If the conditions are not right, the embryo will die.
Many seeds will only germinate after specific environmental events. Some seeds may need a few months of cold. This means that the seed will germinate in spring when it is warm enough to grow. Some seeds may need fire. This means that the seed will have lots of nutrients and sunlight because the forest has just burned. Some seeds need a layer on top of the seed coat to be scraped away by an animal’s digestive process. This means that the seed will germinate in soil with nutrients from the animal’s poop.
What Happens Inside a Seed When It’s Time to Sprout?
When a seed finds the right conditions, it will emerge from dormancy and begin to grow. First the seed will start to absorb water. This process is called imbibition. You may even see the seed increase in size!
Imbibition hydrates the enzymes that digest the food materials in the food source and converts it into energy. The seed coat ruptures from all the pent-up pressure due to imbibition.
Presence of oxygen is important for seed germination. Oxygen and glucose are used up in aerobic respiration to produce energy. Carbon dioxide and water are released as by-products into the environment.
Next, the cells inside the embryo start dividing very fast, and a tiny root emerges. These types of fast-dividing cells constitute the root meristem. Meristems are a class of tissue that has fast growing cells and are found at the tips of plant parts. This first root is called the radicle, and it will grow down into the soil. It is looking for water and minerals to send up to the leaves.
Cotyledons break out next. They can either grow out above the soil which is called epigeal germination, or stay within the soil, known as hypogeal germination. The placement of the cotyledons also determines the length of epicotyl—Part of stem above the cotyledon, and hypocotyl. Part of stem under the cotyledon. In hypogeal germination, the epicotyl extends, restricting the cotyledon below ground and in epigeal germination, the hypocotyl extends pushing the cotyledon out of ground.
Light doesn’t play much significant role; however, the absence of light might keep a seed dormant. These seeds will only germinate when a source of light is available.
After this, the leaves and stem of the embryo break out of the seed coat and start to grow upward. As soon as the first leaves emerge, they start to use energy from the sun to make food! This amazing process is called photosynthesis.
So You Want to Sprout a Seed? Try This Experiment!
Materials
Glass Bottle
Paper Towels
Large Dry Beans
Sunny Windowsill
Step 1
Wet a handful of paper towels
Squeeze them until no water comes out
Step 2
Put the paper towels in a glass jar
Step 3
Wedge the beans in between the jar and the paper towels
Step 4
Cover the jar so that it does not dry out
Step 5
Place the jar on a sunny windowsill
(Add more water to the paper towels if they start to dry out.)
Step 6
Watch the beans sprout!
Can you name the parts of the plant you see emerge?
Here is an amazing video you can check out to witness the whole process of germination in a bean seed.
Keywords
Seed – A tiny capsule with an embryo and stored food inside a hard seed coat.
DNA – A list of instructions inside every cell describing how an organism should look, work, and grow.
Reproduce – When an organism creates more individuals of its same species.
Vegetative Reproduction – When a plant creates an exact copy of itself. The new plant often grows from a small fragment that was separated from the parent plant. This copy has the exact same DNA as the parent.
Sexual Reproduction – When an organism creates another individual by combining its DNA with another individual’s DNA. This creates a new, unique organism.
Pollen – Contains DNA. Produced by one flower and sent to another.
Ovule – Contains DNA. Combines with DNA from pollen. Grows into an embryo.
Embryo – A tiny plant inside a seed.
Endosperm – Food stored inside a seed.
Seed Coat – A hard outer coating that protects the embryo and endosperm.
Cotyledon – An embryo’s leaf.
Monocot – A type of plant that only has one cotyledon.
Dicot – A type of plant with two cotyledons.
Dormant – A state in which the embryo is alive but does not grow. This can last for hundreds of years in some species.
Environmental Conditions – Conditions outside of the seed. Examples are water, temperature, sunlight, and nutrients.
Germination – When a seed breaks dormancy and sprouts
Imbibition – When a seed breaks dormancy and absorbs water.
Radicle – The first root to emerge from the seed.
Photosynthesis – A process during which a plant takes in carbon dioxide and water, and turns it into sugar and oxygen using energy from the sun.
Flesch Kincaid Grade Level: 5.3
Flesch Kincaid Reading Ease: 76.9
How Does A Seed Become A Plant?
Here is a cool video you can watch and know more all about seed germination.
References
Contributors
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