Reproduction in Plants

Lessons

Senior 4 · Biology

Reproduction in Plants

5 lessons

Lesson 1 of 5

Asexual and sexual reproduction in plants

25 min

The banana that vanished

Until the 1950s, almost every banana exported to Europe and North America was of one variety, Gros Michel. Then Panama disease, a wilt caused by the soil fungus Fusarium oxysporum, swept through the plantations of Central America. By the 1960s exporters had given up on Gros Michel and replaced it with Cavendish varieties that resisted the fungus. Why could one fungus wipe out a whole variety across a continent? Because every Gros Michel plant was grown from a sucker of another: they were all genetically identical. A disease that could kill one could kill them all.

In S3 you met the main types of asexual reproduction and the parts of a flower. This lesson compares the two ways plants reproduce at A-level depth: how each produces (or prevents) genetic variation, which plant organs carry out asexual reproduction, and why farmers must weigh the benefits of identical plants against their risks.

Definition

Asexual reproduction: reproduction by one parent without the fusion of gametes; the offspring are produced by mitosis and are genetically identical to the parent and to each other.

Clone: a group of genetically identical organisms descended from one parent by asexual reproduction.

Vegetative reproduction (vegetative propagation): asexual reproduction in which a new plant grows from part of a root, stem or leaf, rather than from a seed.

Perennation: surviving an unfavourable season (drought or cold) as a dormant underground organ full of stored food, such as a tuber, bulb, corm or rhizome.

Sexual reproduction: reproduction involving meiosis to form haploid gametes and the fusion of a male and a female gamete (fertilisation) to form a diploid zygote; offspring differ genetically from their parents and from each other.

Apomixis: the formation of seeds without fertilisation, so the seedlings are clones of the mother plant.

Two ways to make a new plant

Feature	Asexual reproduction	Sexual reproduction
Number of parents	One	Usually two (one if the plant self-pollinates)
Gametes and fertilisation	None	Haploid gametes made by meiosis fuse at fertilisation
Cell division involved	Mitosis only	Meiosis to make gametes, then mitosis to grow
Genetic variation in offspring	None (except rare mutations): a clone	Large: crossing over and independent assortment in meiosis, and random fertilisation
Speed and number	Fast; many offspring from one plant	Slower; needs flowering, pollination and seed development
Energy and resources	Little; no flowers, nectar or pollen needed	A lot: flowers, pollen, nectar, fruits and seeds
Dispersal	Offspring grow next to the parent and compete with it	Seeds can be carried far, to new places
Adaptation	Good in a stable environment the parent already suits	Allows adaptation to changing environments and new diseases

Worked example 1: how much variation can sex produce?

The common bean, Phaseolus vulgaris, has $2 n = 22$ chromosomes, that is 11 homologous pairs. (a) Ignoring crossing over, how many genetically different gametes can one bean plant make by independent assortment? (b) How many different chromosome combinations could a zygote have from one cross between two plants? (c) How many genetically different offspring can a plant make by asexual reproduction?

(a) Each pair lines up independently at metaphase I, so each pair gives 2 choices: $2^{n} = 2^{11} = 2048 \text{ different gametes}$
(b) Any of 2048 male gametes can fuse with any of 2048 female gametes: $2048 \times 2048 = 2^{22} = 4\,194\,304$ combinations, and crossing over raises this further still.
(c) One: all offspring made by mitosis carry the parent's genotype (apart from rare mutations).

How plants reproduce asexually

Most vegetative reproduction uses a modified stem, root or leaf that has buds and a food store. A stem can be recognised even underground: it has nodes with scale leaves and buds, which roots never have.

Structure	What it is	Examples
Stem tuber	Swollen tip of an underground stem (stolon) storing starch; its "eyes" are buds	Irish potato
Root tuber	Swollen adventitious root storing food; new shoots grow from its upper end	Sweet potato, dahlia
Bulb	Short, flat stem bearing fleshy leaf bases that store food, with a terminal bud	Onion, garlic
Corm	Short, swollen, upright underground stem with nodes and scale leaves	Taro (cocoyam), gladiolus
Rhizome	Horizontal underground stem with nodes, buds and scale leaves	Ginger, couch grass, canna
Stolon (runner)	Horizontal stem growing along the surface, rooting at the nodes	Strawberry, star grass
Sucker	Shoot growing from an underground stem or root, beside the parent	Banana, pineapple
Leaf plantlets	Tiny plants formed in notches along the leaf edge, which drop and root	Kalanchoe (mother of thousands)

Many of these organs do two jobs: they reproduce the plant and they perennate, letting it survive a dry season underground and then grow quickly using the stored food when rain returns. Fungi, mosses and ferns also reproduce asexually by spores, and many plants regrow from broken fragments (fragmentation), which is why couch grass spreads when a field is hoed.

Worked example 2: how fast does a clone multiply?

A farmer starts with 5 seed potatoes. Each tuber planted produces 8 tubers at harvest, and every tuber is replanted the next season (illustrative figures). (a) Write an expression for the number of tubers $N$ after $t$ seasons. (b) How many seasons are needed to have at least 10 000 tubers?

(a) The number is multiplied by 8 each season: $N = 5 \times 8^{t}$
(b) We need $5 \times 8^{t} \ge 10\,000$ , so $8^{t} \ge 2000$ . Taking logarithms: $t \ge \frac{\log 2000}{\log 8} = \frac{3.301}{0.903} = 3.66$ So 4 seasons are needed (after 3 seasons there are only $5 \times 512 = 2560$ ). Every one of the 10 000 or more tubers is genetically identical to the first five.

Weighing asexual reproduction

Advantages	Disadvantages
Only one parent is needed: no pollinator, no partner	No genetic variation, so a population cannot adapt to new conditions
Fast: many offspring in a short time	One disease or pest can destroy the whole clone (Gros Michel)
Offspring keep all the parent's good characteristics, so a valuable variety stays true	Diseases carried in the parent (viruses in tubers or cuttings) pass to every offspring
Young plants are fed by the parent's food store, so they establish quickly	Offspring crowd round the parent and compete with it for light, water and minerals
Can reproduce where seeds would fail, or in plants that make no viable seed (banana)	Slow spread to new areas, since there is no seed dispersal

Many plants use both methods: grass spreads by rhizomes and stolons when conditions are good, and flowers to make seeds that carry variation to new places. Farmers do the same: they use asexual propagation to keep a good variety true (Lesson 2) and sexual reproduction, through plant breeding, to create new varieties.

Boxlapse: "Potato Growing Underground Time Lapse – 92 Days", filmed through a glass-sided box. Watch the stolons grow out sideways from the stem and swell at their tips into new tubers, each a clone of the seed tuber.

Warning

Common mistakes

Calling an Irish potato a root. It is a stem tuber: it has nodes (eyes) with buds and scale-leaf scars. A sweet potato is a root tuber.
Saying asexual offspring are "similar" to the parent. They are genetically identical, a clone; any differences come from the environment or a rare mutation.
Thinking seeds always come from sexual reproduction. Apomictic seeds form without fertilisation and give clones.
Saying meiosis happens in asexual reproduction. Only mitosis is involved.

Tip

Memory hook. Is it a stem? Look for "N-B-S": Nodes, Buds and Scale leaves. Tubers of potato, bulbs, corms and rhizomes all have them; roots never do.

✦

Practice: asexual and sexual reproduction

Give three differences between asexual and sexual reproduction in plants.
Explain why an Irish potato is classified as a stem but a sweet potato as a root.
Name the type of organ in each: (a) onion, (b) ginger, (c) taro, (d) strawberry runner, (e) banana sucker.
Maize has $2 n = 20$ . Calculate how many genetically different gametes one maize plant can produce by independent assortment alone.
A grass clump spreads by rhizomes, increasing its number of shoots by a factor of 3 each year. Starting from 4 shoots, how many shoots will there be after 5 years?
Explain the term perennation and how a corm allows a plant to survive a long dry season.
A farmer grows only one banana variety, all from suckers. Evaluate the risks and benefits of this practice.

Show answersHide answers

Any three: asexual has one parent, sexual usually two; asexual has no gametes or fertilisation; asexual uses only mitosis, sexual needs meiosis; asexual offspring are genetically identical, sexual offspring vary; asexual is faster and uses less energy; sexual reproduction produces seeds that can be dispersed far.
The Irish potato tuber has nodes ("eyes") with buds in the axils of scale-leaf scars and grows at the tip of a stolon, so it is a stem. The sweet potato is a swollen adventitious root: it has no nodes or scale leaves, and shoots grow only from its upper end.
(a) Bulb. (b) Rhizome. (c) Corm. (d) Stolon (runner). (e) Sucker.
$n = 10$ pairs, so $2^{10} = 1024$ different gametes.
$4 \times 3^{5} = 4 \times 243 = 972$ shoots.
Perennation is surviving an unfavourable season as a dormant organ with stored food. The corm stays underground, protected from drought and heat; its leaves and roots die back, so little water is lost. When the rains return, the bud uses the stored food to grow rapidly before other plants can shade it, and new corms form for the next season.
Benefits: every plant keeps the variety's good yield and taste; suckers are free and quick to establish; bananas make no viable seed, so suckers are the practical method. Risks: the plants are genetically identical, so a disease or pest that attacks one can destroy the whole farm, as Panama disease destroyed Gros Michel; suckers from infected plants carry diseases into new fields. Using clean, disease-free planting material and growing more than one variety reduces the risk.

Quick check

Match each plant to the organ it reproduces by.

Tap one on the left, then its partner on the right.

Key points

Asexual reproduction: one parent, mitosis only, offspring are a clone; sexual reproduction: meiosis, gametes and fertilisation, offspring vary ( $2^{n}$ gamete types from independent assortment alone).
Vegetative organs: stem tubers, root tubers, bulbs, corms, rhizomes, stolons, suckers and leaf plantlets. Stems are recognised by nodes, buds and scale leaves.
Many of these organs also perennate, storing food to survive a dry season.
Asexual reproduction is fast and keeps good varieties true, but clones cannot adapt and can be wiped out by one disease.

Sources and credits

6.3.4: Asexual Reproduction, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for natural methods of asexual reproduction, apomixis and the comparison with sexual reproduction. Adapted by Ganzaa.
Gros Michel and Panama disease: "Gros Michel", Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa. This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Figure 1, the tables, worked examples (the potato figures in Worked example 2 are illustrative), practice and quick check: written by Ganzaa.
Photos: EnrichIndegi, CC BY-SA 4.0; Mk2010, CC BY-SA 4.0; both via Wikimedia Commons.
Video: "Potato Growing Underground Time Lapse – 92 Days", Boxlapse, embedded from YouTube.

1The banana that vanished

2Two ways to make a new plant

3How plants reproduce asexually

4Weighing asexual reproduction

The banana that vanished

Two ways to make a new plant

How plants reproduce asexually

Weighing asexual reproduction