F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and Function

Seeds and Germination

F3.3 F2.4 F3.4 F2.1 A1.6 A1.13About 70 minutes

Learning goals

I can identify the parts of dicot and monocot seeds and state their functions.
I can explain seed dormancy and how it is broken, including cold stratification and scarification.
I can describe the steps of germination, including the role of gibberellin and the conditions seeds need.
I can calculate germination percentage and interpret germination data.

Words to know

seed coat (testa)
The tough outer layer of a seed that protects the embryo.

radicle
The embryonic root; usually the first part to emerge during germination.

plumule / epicotyl / hypocotyl
The plumule is the embryonic shoot with its first leaves; the epicotyl is the stem above the cotyledons; the hypocotyl is the stem below them.

coleoptile
In grasses (monocots), a protective sheath that covers the young shoot as it pushes through the soil.

aleurone layer
A layer of cells around the endosperm of cereal grains that releases enzymes to digest stored starch during germination.

dormancy
A resting state in which a viable seed does not germinate even when given water, until certain conditions are met.

stratification / scarification
Stratification: a period of cold, moist conditions that breaks dormancy. Scarification: scratching, softening or cracking a hard seed coat so water can enter.

imbibition
The rapid uptake of water by a dry seed, which makes it swell and starts germination.

germination percentage
The number of seeds that germinate divided by the number sown, × 100%.

A plant in a lunchbox

A seed is a baby plant packed with a lunch and sealed in a protective box. It can wait, dry and alive, for months or years until the right moment, then burst into growth in a few days. That is a remarkable trick: the seed has to "know" when it is safe to grow. In this lesson you'll open the box, see what's inside, and find out how seeds decide when to germinate.

Remember from Lessons 8 and 9

Double fertilization produces a zygote (which becomes the embryo) and endosperm (food). The ovule becomes the seed and its outer layers become the seed coat. Monocots have one cotyledon and usually large endosperm; dicots such as beans have two cotyledons that store the food.

Inside a seed

Bean (dicot):

Seed coat with a hilum (scar where it was attached in the pod) and a tiny pore, the micropyle, where the pollen tube entered.
Two large cotyledons, full of stored starch and protein.
The embryo axis: the radicle (future root), the hypocotyl (stem below the cotyledons), and the epicotyl with the plumule (the shoot tip and first true leaves).

Corn kernel (monocot):

A thin outer layer made of the fruit wall fused to the seed coat (each kernel is technically a one-seeded fruit).
A large endosperm full of starch, surrounded by an aleurone layer of protein-rich cells.
One cotyledon, the scutellum, which absorbs food from the endosperm.
A coleoptile sheathing the young shoot and a coleorhiza sheathing the radicle, protecting them as they push through soil.

Left: a bean opened to show one of its two cotyledons and the embryo (radicle and plumule). Right: a corn kernel, mostly endosperm, with one cotyledon (scutellum) and a protected shoot and root.

Dormancy: waiting for the right moment

Many seeds won't germinate as soon as they are ripe, even if they get water. This dormancy prevents a seed from sprouting at the wrong time, for example in a warm spell in October, only to be killed by winter. The hormone abscisic acid (ABA) helps keep seeds dormant.

Dormancy is broken in different ways:

Cold stratification: many native Canadian plants, including sugar maple and many wildflowers, need weeks of cold, moist conditions (a winter) before they will germinate. Gardeners imitate this by keeping seeds in damp sand in the refrigerator.
Scarification: seeds with very hard coats need the coat scratched, cracked or softened (by freezing and thawing, by passing through an animal's gut, or by a gardener with sandpaper) so water can enter.
Light: some small seeds need light to germinate, so they only sprout near the soil surface.
Fire and heat: jack pine cones stay sealed with resin until the heat of a fire opens them and releases the seeds onto freshly burned ground (Lesson 17).

Germination step by step

Imbibition: the dry seed absorbs water fast, swells and splits its coat.
Enzymes switch on. The embryo releases gibberellin (a growth hormone). In cereal grains, gibberellin signals the aleurone layer to make amylase, an enzyme that breaks starch in the endosperm into sugars.
Respiration speeds up. Sugars are broken down by cellular respiration to release energy for growth, which needs oxygen.
The radicle emerges first, anchoring the seedling and absorbing water.
The shoot emerges. In beans, the hypocotyl forms a hook that pulls the cotyledons up out of the soil (epigeal germination). In peas and corn, the cotyledon stays underground and the shoot grows up alone (hypogeal germination); in corn the coleoptile protects the shoot tip.
Once the first leaves open, the seedling begins photosynthesis and stops depending on stored food.

Conditions needed: water, oxygen and a suitable temperature (each species has a range; corn, for example, germinates poorly in cold spring soil). Most seeds don't need light to germinate; they need it right after, to grow.

Germination of a bean (not to scale). The radicle emerges first; then the hypocotyl hook pushes up through the soil and straightens, lifting the cotyledons into the light.

Worked example

Let's calculate germination percentage

A student tests 20 seeds at 22 °C and 20 seeds at 12 °C, counting how many have germinated (radicle visible) each day. By day 7, 17 seeds at 22 °C and 9 seeds at 12 °C have germinated. (a) Calculate the germination percentage for each. (b) The graph shows the cumulative results. Describe two differences.

(a) 22 °C: 17 ÷ 20 × 100% = 85%. 12 °C: 9 ÷ 20 × 100% = 45%.
(b) Speed: at 22 °C, germination starts on day 2 and most seeds have germinated by day 4; at 12 °C, it starts later (day 3) and climbs slowly.
Final percentage: 85% at 22 °C versus 45% at 12 °C by day 7. Some 12 °C seeds might still germinate later, so the test could be continued.
Explanation: enzymes and respiration work faster at warmer temperatures (within the species' range), so germination is faster and more complete.

(a) 85% at 22 °C and 45% at 12 °C. (b) Warmer seeds germinated sooner and a higher percentage germinated by day 7.

Cumulative germination for the example: 20 seeds at each temperature. At 22 °C germination is faster and reaches 85%; at 12 °C it is slower and reaches 45% by day 7.

Worked example

Let's design a stratification test

A student wants to know whether white spruce seeds need cold stratification. Outline a fair test.

Question: Does 4 weeks of cold, moist storage increase the germination percentage of white spruce seeds?
Hypothesis: If seeds are stratified, then more will germinate, because many northern conifers' seeds germinate better after a cold period.
Independent variable: stratified (4 weeks at about 4 °C in moist sand in a fridge) or not stratified (dry, room temperature, same 4 weeks).
Dependent variable: germination percentage after 21 days.
Controlled variables: same seed batch, same number of seeds (at least 25 per group, in replicates), same germination temperature, light and moisture.
Analysis: compare mean germination percentages; repeat if possible.

A good test compares two groups of the same seeds that differ only in stratification, with enough seeds and replicates to make the comparison reliable.

Just for fun

In 2005, scientists germinated a date palm seed found at an ancient site in Israel and estimated to be about 2000 years old. The seedling was nicknamed Methuselah. That's one patient lunchbox.

Hands-on activity

Rag-doll germination test

You need: 40 seeds of one kind (beans, peas, radishes or lettuce from a seed packet), 4 paper towels, 2 clear plastic bags, a marker, a spray bottle or cup of water, and a fridge.

Choose a question. Example: Does temperature affect germination? or Does soaking seeds overnight speed up germination?
Dampen two paper towels. Lay 10 seeds on each, roll them up loosely (a "rag doll") and label them. Make two rolls for each condition (so 20 seeds per condition, in two replicates).
Put each condition's rolls in a plastic bag (leave it partly open for air). For a temperature test, keep one bag at room temperature and one in the fridge.
Each day, unroll gently, count seeds with a visible radicle, record, keep the towel moist and roll back up.
After 7 days, calculate germination percentage for each condition and make a line graph like the one in this lesson.

What to look for: at room temperature, beans and radishes usually show radicles in 2 to 4 days; seeds in the fridge are much slower or don't germinate. Soaked seeds often germinate a day sooner. Don't eat seeds from packets (some are treated with fungicides; check the packet) and wash hands afterwards. Plant the strongest seedlings in soil, or compost them.

Your turn

Name three parts of a bean embryo and what each becomes.

What is the job of the coleoptile in a corn seedling?

Why is dormancy useful to a plant growing in Ontario?

Explain the role of gibberellin in the germination of a barley grain.

List three conditions most seeds need to germinate.

Why does the radicle emerge before the shoot?

Of 50 seeds planted, 38 germinate. What is the germination percentage?

More practice: warm-up

What is imbibition?

What is the scar on a bean seed where it was attached to the pod called?

Which hormone helps keep seeds dormant?

What is scarification?

Do beans show epigeal or hypogeal germination?

More practice: on your own

Seeds planted in waterlogged soil often fail to germinate. Explain.

Why do many wildflower seed packets say "store in the fridge in moist sand for 6 weeks before sowing"?

Bean seedlings grown in a dark cupboard become tall, thin and pale. Explain why, and predict what happens if they stay in the dark.

Calculate the germination percentage for each group: Group A 42/60, Group B 51/60. Which group did better, and by how many percentage points?

Why should a germination test use at least 20 seeds per group rather than 2?

Challenge

A barley grain is cut in half. The half with the embryo and the half without it are both placed on starch agar (a jelly containing starch). After two days, starch is digested only around the embryo half. Then gibberellin is added to the non-embryo half, and starch around it is digested too. Explain these results.

Explain why many seeds of plants that grow in northern forests have both dormancy and a cold requirement, using an example of what could go wrong without them.

In the lesson's graph, at 22 °C 85% germinated by day 7 and germination stopped increasing after day 6. Give two possible reasons why the remaining 15% did not germinate, and how you could test one of them.

Check your understanding

Quiz

Which part of a bean embryo becomes the root?

In cereal grains, gibberellin causes the aleurone layer to:

Cold stratification breaks dormancy by:

Why do seeds need oxygen to germinate?

A test of 40 seeds gives 30 germinated. The germination percentage is:

Which structure protects the young shoot of a corn seedling as it pushes through soil?

For parents and teachers

Big idea: F3.3 includes seed germination as part of natural reproduction, and F2.4 includes investigating seed germination as a propagation technique. Students learn seed anatomy, dormancy, the hormonal control of germination (gibberellin, ABA: connecting to F3.4), and how to collect and graph germination data.

How to help: The rag-doll test costs almost nothing and gives real data in a week. Help your teen pick one question and keep everything else the same. Daily counting takes five minutes. Encourage a proper line graph with labelled axes and a title.

Common difficulties:

Thinking seeds need light to germinate. Most need water, oxygen and warmth; light is needed after germination.
Mixing up hypocotyl and epicotyl (hypo = below the cotyledons, epi = above).
Calculating germination percentage with the wrong denominator (it is the number sown, not the number that germinated).