Asexual and Sexual Reproduction in Plants and Microorganisms
Senior 3 · Biology
Asexual and Sexual Reproduction in Plants and Microorganisms
Lesson 5 of 5
25 min
A bag of dry bean seeds can sit in a store for many months and nothing happens. Plant them in dry soil and still nothing happens. But a few days after the first good rain of the season, the soil of a newly planted field cracks and rows of pale, hooked stems push through. Something in the soil switched the seeds on. This lesson finds out what a seed needs to germinate, and how bean and maize seedlings reach the light in two different ways.
Definition
Germination: the process by which the embryo in a seed starts to grow into a seedling, usually beginning when the seed absorbs water and ending when the seedling can make its own food by photosynthesis.
Dormancy: a resting state in which a seed is alive but does not grow, even though it may have water.
Epigeal germination: the cotyledons are carried above the ground, because the stem below them (the hypocotyl) grows longer. Example: common bean.
Hypogeal germination: the cotyledons, or the grain, stay below the ground, because the stem above them (the epicotyl) grows longer. Examples: maize, pea.
Most seeds do not need light to germinate: they germinate underground in the dark. (A few small seeds, such as some lettuce varieties, need light.)
Enzymes in the cotyledons or endosperm digest the stored food: amylase turns starch into maltose, proteases turn proteins into amino acids, and lipases turn fats into fatty acids and glycerol. These soluble products move to the plumule and radicle, where they are used for respiration and to build new cells, until the first leaves can photosynthesise.
Set up four tubes, each with 10 bean seeds, as in Figure 2, and leave them for 5 days. Only the tube that lacks nothing (A) is expected to germinate well.
Warning
Safety in the practical. Boil the water for tube C in a kettle or beaker with your teacher's help, and let it cool before you pour it: hot water and steam scald. Wipe up any spilled oil at once, as it makes the floor slippery. Do not eat the seeds: seeds sold for planting are often treated with fungicide. Wash your hands afterwards.
Figure 1: simplified; not to scale.
| Feature | Epigeal (bean) | Hypogeal (maize) |
|---|---|---|
| Part that grows longer | Hypocotyl (below the cotyledons) | Epicotyl (above the cotyledon) |
| Cotyledons | Carried above the ground; turn green and photosynthesise for a short time | Stay below the ground with the grain |
| How the plumule is protected while pushing through the soil | The hypocotyl is bent into a hook, so the stem, not the delicate plumule, pushes through | The plumule is inside a protective sheath, the coleoptile |
Worked example 1: germination percentage and seed rate
Before the season, a farmer tests her bean seed: she plants 50 seeds and 47 germinate.
Worked example 2: why seedlings first lose mass
Batches of bean seedlings are dried and weighed every 4 days (illustrative data):
| Day | 0 | 4 | 8 | 12 | 16 |
|---|---|---|---|---|---|
| Dry mass of 10 seedlings (g) | 4.0 | 3.4 | 3.0 | 3.3 | 4.6 |
Warning
Common mistakes. (1) Listing light as a condition for germination: most seeds germinate in the dark; light is needed later, for photosynthesis. (2) Mixing up epigeal and hypogeal: "epi-" means above (cotyledons above ground); "hypo-" means below. (3) Forgetting the control in the experiment: tube A shows that the seeds were alive and able to germinate.
To Investigate the Conditions Necessary for Germination of Seeds, Patrick Maye. Watch how each tube removes one condition, and compare it with Figure 2.
Tip
Memory hook: "WOT" a seed needs: Water, Oxygen, Temperature. Light is not on the list.
Practice: germination
Key points
Further reading
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2, the tables, the worked examples (Worked example 2 uses illustrative data) and the practice questions.
