Reproduction in Simple Organisms and Plants: Quick Revision

A brief note on Reproduction in Simple Organisms and Plants. Read it before a test. Try each question at the end before you open its answer.

Copies and variation

• Reproduction makes new individuals of the same kind, so the species survives.
• Its basic event is the copying of DNA. The copying is never perfect, so the young are similar to the parent but not identical.
• These small differences are variations. They help a species to survive when its surroundings change.

Asexual reproduction

• Binary fission: Amoeba splits into two in any plane, Leishmania along a fixed plane.
• Multiple fission: Plasmodium splits into many cells at once.
• Fragmentation: a Spirogyra filament breaks into pieces that grow on.
• Regeneration: a piece of Planaria or Hydra rebuilds the whole body.
• Budding: a bud grows on the side of Hydra and breaks away.
• Spores: Rhizopus makes spores in round sporangia.
• Vegetative propagation: new plants from a root, stem or leaf, as in sugarcane, rose, potato and Bryophyllum. It is quick and keeps a good variety true.

Sexual reproduction in flowers

• Stamen (male): filament and anther, which makes pollen.
• Carpel (female): stigma, style and ovary with ovules.
• Pollination brings pollen to the stigma: self or cross.
• A pollen tube grows down the style. The male gamete joins the egg in the ovule: fertilisation, giving a zygote.
• The zygote becomes the embryo, the ovule becomes the seed, and the ovary becomes the fruit.
• A seed germinates with water, air and warmth. The radicle makes the root and the plumule the shoot.

A worked example

A bean plant has $15$ pollinated flowers. Each ovary holds $6$ ovules, and $4$ of every $6$ are fertilised. How many pods and seeds form?

1. Pods. Each pollinated ovary becomes a pod: $15$ pods.
2. Ovules. $15 \times 6 = 90$ ovules.
3. Seeds. Only the fertilised ones: $\dfrac{4}{6} \times 90 = 60$ seeds.

So the plant forms $15$ pods holding $60$ seeds.

Practice with new numbers

A program to try

The program finds how many pieces of sugarcane a farmer can plant from her stems.

Mistakes to avoid

• Saying asexual young are exact copies. Small variations always appear.
• Mixing up pollination and fertilisation. Pollination comes first. Fertilisation happens later, inside the ovule.
• Saying the ovary becomes the seed. The ovule becomes the seed, and the ovary the fruit.
• Calling spores seeds. Spores are single cells made without sex.

Check yourself

1. Name the kind of fission in Plasmodium, and say how it differs from that in Amoeba.

2. Why is a field of plants grown from cuttings of one parent at risk from a new disease?

3. What does the pollen tube do?

4. After fertilisation, what do the ovule and the ovary become?

5. A seed is kept dry in a warm room for a month. Does it germinate? Why?