F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and FunctionAsexual Reproduction and Propagation: Cuttings, Layering, Grafting and Tissue Culture
F2.4 F3.3 F2.1 A1.5 A1.10About 70 minutes
Learning goals- I can describe natural vegetative reproduction using runners, tubers, bulbs, rhizomes and root suckers.
- I can explain and carry out artificial propagation by stem, leaf and root cuttings.
- I can explain layering, division, grafting and tissue culture, and why growers use them.
- I can compare the advantages and disadvantages of sexual and asexual reproduction in plants.
Words to know
asexual (vegetative) reproduction
Reproduction from a part of one parent, without seeds from fertilization; the offspring are genetically identical clones.
clone
An organism (or group) that is genetically identical to its parent.
runner (stolon)
A stem that grows along the soil surface and forms new plants at its nodes.
rhizome
A horizontal underground stem that sends up new shoots and roots.
cutting
A piece of stem, leaf or root removed from a plant and grown into a new plant.
rooting hormone
A powder or gel containing a synthetic auxin that helps cuttings form roots.
layering
Encouraging a stem to form roots while it is still attached to the parent plant, then cutting it free.
grafting
Joining a shoot (scion) of one plant to the rooted stem (rootstock) of another so they grow as one.
tissue culture (micropropagation)
Growing many new plants from tiny pieces of tissue on a sterile nutrient jelly containing hormones.
A plant from a piece
Put a sprig of basil from the grocery store in a glass of water, and in a week or two it grows roots. Leave a potato in the cupboard and it sprouts. Cut a leaf from a snake plant, stand it in soil, and a whole new plant appears. Plants can do something most animals can't: a piece of one plant can grow into a whole new plant.
This is asexual or vegetative reproduction. It happens naturally, and people use it on purpose to propagate (multiply) plants.
Remember from Grade 10 and Lessons 9 and 10
In Grade 10 you learned that plants keep meristems (dividing cells) all their lives, and that cells divide by mitosis, making identical copies. In Lessons 9 and 10 you studied sexual reproduction: flowers, pollination, seeds. Asexual reproduction skips flowers and seeds; the new plant is made by mitosis alone, so it is a clone of its parent.
Natural vegetative reproduction
- Runners (stolons): stems that creep over the ground and root at their nodes, making new plants. Strawberries and spider plants do this.
- Rhizomes: underground horizontal stems. Quackgrass, ginger and many ferns spread this way, which is why quackgrass is so hard to pull out of a garden.
- Tubers: swollen underground stems storing food. Each "eye" on a potato is a bud at a node that can grow into a new plant.
- Bulbs: a short stem with thick, fleshy leaves wrapped around a bud (onions, tulips, garlic). Bulbs make smaller side bulbs.
- Root suckers: new shoots from roots. A whole grove of trembling aspen is often one clone, connected underground; in fall, every tree in a clone may change colour on the same day. Raspberries spread the same way.
- Plantlets: some plants, such as the houseplant Kalanchoe, grow tiny plantlets along their leaf edges that drop off and root.
Artificial propagation
Cuttings. A piece of a plant is removed and grown into a new plant.
- Stem cuttings (coleus, pothos, geraniums, willows, basil): cut just below a node, because roots form most easily at nodes. Remove the lower leaves so they don't rot, and keep a few at the top to photosynthesize. Dip the base in rooting hormone (a synthetic auxin) and put it in water or moist soil. Cover with a clear bag to keep humidity high, so the cutting doesn't dry out before it has roots.
- Leaf cuttings (African violet, snake plant, jade plant, begonia): a leaf or piece of leaf forms roots and new shoots at its base or veins.
- Root cuttings (horseradish, raspberries): pieces of root form new shoots.
Layering. A low branch is bent down and covered with soil while still attached to the parent, so it keeps receiving water and sugar while it roots. Raspberry and blackberry canes do this naturally when their tips touch the ground (tip layering).
Division. A clump-forming plant such as a hosta or daylily is dug up and split into pieces, each with roots and shoots.
Grafting. A scion (a twig with buds of the variety you want) is joined to a rootstock (a rooted plant chosen for its hardiness, disease resistance or size). The cambium layers must line up and touch so they grow together and the vascular tissue connects. Almost all fruit trees in orchards are grafted.
Tissue culture (micropropagation). Tiny pieces of tissue, often from a meristem, are placed on a sterile jelly with nutrients and hormones. A high ratio of cytokinin to auxin makes shoots; a high ratio of auxin makes roots. Thousands of identical, disease-free plantlets can be grown from one plant in a small lab space. It is used for potatoes, strawberries, orchids, blueberries and some forestry seedlings.
Why graft an apple tree?
Apple trees don't "come true" from seed. Each seed comes from cross-pollination, so it carries a new mix of genes, and a tree grown from a McIntosh seed will produce a different apple (usually a worse one). The famous McIntosh apple came from a single tree found by John McIntosh on his farm near Dundela, Ontario, in 1811. Every McIntosh tree since then has been grown by grafting, so every McIntosh apple comes from a clone of that one tree.
Sexual vs asexual reproduction
| Sexual (seeds) | Asexual (vegetative) |
|---|
| Parents | Usually two (cross-pollination) | One |
| Offspring | Genetically varied | Genetically identical clones |
| Speed | Slower to reach maturity | Often faster; can skip the seedling stage |
| Advantages | Variation helps a population adapt to disease and change; seeds can be stored and spread far | Keeps desirable traits exactly; uniform crops; no pollinators or seeds needed |
| Disadvantages | Desirable traits may be lost; slower | No variation: one disease can wipe out the whole crop; disease can pass from parent to offspring |
A warning from bananas. Bananas sold in stores are seedless and are propagated asexually, so the plants in a plantation are clones. In the 1900s, the main export variety, Gros Michel, was devastated by a soil fungus (Panama disease) and replaced by the Cavendish variety, which is itself now threatened by a new strain of the same fungus. With no genetic variety, none of the plants could resist it. This is why seed banks and genetic diversity matter (Lesson 16).
Worked exampleLet's analyse a rooting experiment
A greenhouse tests rooting hormone on 40 coleus stem cuttings: 20 dipped in hormone, 20 not. After 14 days, 18 hormone-treated cuttings and 11 untreated cuttings have roots at least 1 cm long. (a) Calculate the rooting success for each group. (b) What conclusion can you draw? (c) Name two variables that must be controlled.
- (a) Treated: 18 ÷ 20 × 100% = 90%. Untreated: 11 ÷ 20 × 100% = 55%.
- (b) The rooting hormone increased rooting success by 35 percentage points. This supports the idea that auxin promotes root formation in cuttings. Repeating the test would make the conclusion stronger.
- (c) Any two: same plant variety and parent plants; same cutting length and number of nodes; cut at the same position (below a node); same temperature, light, humidity and watering; same rooting medium.
(a) 90% with hormone, 55% without. (b) Rooting hormone improved rooting. (c) For example, cutting size and position, and growing conditions.
Worked exampleLet's choose a propagation method
Choose the best propagation method for each grower and explain: (a) an orchard that wants 500 trees of a disease-resistant apple variety; (b) a lab that must produce 50 000 virus-free strawberry plants quickly; (c) a gardener who wants to share a large hosta with neighbours.
- (a) Grafting scions of the variety onto suitable rootstocks: apples don't come true from seed, and grafting keeps the variety exactly while the rootstock controls tree size and hardiness.
- (b) Tissue culture from meristems: many plantlets from little material, quickly, and meristem tissue is often free of viruses.
- (c) Division: dig up the clump and split it into pieces, each with roots and shoots.
(a) Grafting; (b) tissue culture; (c) division.
Just for funEvery McIntosh apple tree in the world is a clone of one tree found in Ontario in 1811. So the McIntosh family tree has exactly one tree in it.
Hands-on activityStart your own clones
You need: scissors or garden snips (a grown-up should do the cutting or supervise), 2 or 3 small glasses or jars, water, a houseplant you have permission to cut (pothos, coleus, spider plant, tradescantia/inch plant) or fresh basil or mint from the grocery store, and optionally a snake plant or jade plant leaf, a potato and a pot of soil.
- Stem cuttings: take 3 stem cuttings about 10 cm long, each cut just below a node. Remove the leaves from the bottom 4 cm. Stand them in water so at least one node is under water. Keep in bright, indirect light and change the water every few days.
- Leaf cutting: push the cut end of a snake plant or jade leaf 2 cm into moist soil. Water lightly; don't let it get soggy.
- Tuber: put a potato with "eyes" in a bright spot (or plant a piece with 2 eyes in soil).
- Record the date when roots first appear and measure the longest root each week. Sketch where the roots come from.
What to look for: stem cuttings in water usually show roots within 1 to 3 weeks, mostly at or near the nodes. Leaf cuttings are much slower (several weeks). Potato eyes sprout shoots in 1 to 3 weeks. All new plants are clones of their parents. Plant rooted cuttings in soil when the roots are 2 to 3 cm long.
Your turn
1What is a clone, and why are plants produced by cuttings clones?
A clone is genetically identical to its parent. Cuttings grow by mitosis from the parent's own cells, with no fertilization, so they have the same DNA.
2Name four structures plants use for natural vegetative reproduction, with an example of each.
Any four: runners (strawberry, spider plant), rhizomes (quackgrass, ginger), tubers (potato), bulbs (onion, tulip), root suckers (trembling aspen, raspberry), plantlets (Kalanchoe).
3Why should a stem cutting be cut just below a node?
Roots form most easily at nodes, so including a node at the base gives the best chance of rooting.
4What does rooting hormone contain, and what does it do?
A synthetic auxin; it promotes root formation at the base of the cutting.
5In grafting, why must the cambium of the scion and rootstock line up?
The cambium is the dividing tissue that makes new xylem and phloem. If the two cambium layers touch, they grow together and connect the vascular tissue, so water and sugar can flow between scion and rootstock.
6Give one advantage and one disadvantage of growing a crop entirely from clones.
Advantage: every plant has the same desirable traits (uniform, predictable crop). Disadvantage: no genetic variation, so a single disease or pest that attacks one plant can destroy them all.
7Why are cuttings often covered with a clear plastic bag?
To keep the humidity high. A cutting has no roots yet, so it can't replace water lost by transpiration; the bag reduces water loss so it doesn't wilt and die before rooting.
More practice: warm-up
1What is the shoot part of a graft called?
2Is a potato tuber a root or a stem?
A stem (its eyes are buds at nodes).
Rooting a stem while it is still attached to the parent plant, then cutting it free.
4Name a plant often propagated by leaf cuttings.
African violet, snake plant, jade plant or begonia.
5In tissue culture, which hormone ratio encourages shoots to form?
A high cytokinin to auxin ratio.
More practice: on your own
1Explain why a gardener might call quackgrass "impossible to get rid of".
Quackgrass spreads by rhizomes. If you pull it, pieces of rhizome break off and stay in the soil, and each piece with a node can grow into a new plant.
2Explain why a grove of trembling aspen might all turn yellow on the same day in fall while a neighbouring grove turns a week later.
Each grove may be a single clone connected by roots, so all its stems share the same genes and respond to shortening days and cool nights in the same way. The neighbouring grove is a different clone with slightly different genes, so its timing differs.
3A rooting test gives 13 rooted cuttings out of 25. Calculate the success rate.
4Why do orchards graft apple varieties onto special rootstocks rather than growing them on their own roots?
Rootstocks are chosen for useful traits: dwarfing (smaller trees that are easier to pick and fruit sooner), cold hardiness, or resistance to soil diseases and pests. The scion provides the desired apple variety.
5Why is tissue culture done on sterile jelly in a clean lab?
The nutrient jelly is full of sugar, which would let bacteria and fungi grow quickly and kill the tiny plant pieces. Sterile conditions keep microbes out.
Challenge
1A farmer plants potatoes from her own tubers each year. After several years, yields fall and many plants show a virus. Explain why and suggest a solution.
Tubers are clones, and viruses in the parent plant pass into the tubers, so infections build up generation after generation. Solution: buy certified seed potatoes produced from virus-free stock (often started by meristem tissue culture, since meristems are frequently virus-free), and remove infected plants.
2Design an experiment to test whether the number of nodes on a stem cutting affects rooting success.
Independent variable: number of nodes (e.g. 1, 2 and 3 nodes, in the buried part). Dependent variable: percentage of cuttings that root within 3 weeks (and/or mean root length). Controlled: same parent plant and variety, same total cutting length, same rooting hormone (or none), same medium, light, temperature and humidity. Use at least 10 cuttings per group and repeat. Compare the percentages.
3Evaluate this statement: "Asexual reproduction is better than sexual reproduction for farmers."
Partly true. Asexual propagation keeps valuable traits exactly (the McIntosh flavour, a seedless banana), gives uniform crops, and can be fast. But a field of clones has no genetic variation, so it is vulnerable to new diseases and pests (as bananas show), and breeders still need sexual reproduction to create new varieties with new combinations of traits. A balanced answer: it depends on the crop and goal; farmers benefit from both, and genetic diversity must be kept somewhere (seed banks, wild relatives).
Quiz
1. Which statement about asexual reproduction in plants is true?
2. A potato tuber is a modified:
3. Rooting hormone usually contains a synthetic form of:
4. In grafting, the part chosen for its roots and hardiness is the:
5. Why are apple trees propagated by grafting rather than seed?
6. The biggest risk of planting a crop of clones is:
Big idea: F2.4 asks students to investigate propagation techniques (leaf, stem and root cuttings, seed germination), and F3.3 asks them to explain natural and artificial reproduction (cuttings, grafting). The big comparison is sexual versus asexual reproduction: variety versus uniformity.
How to help: Cuttings in water are easy and rewarding; grocery-store basil or mint roots reliably. A grown-up should do or supervise the cutting. Encourage your teen to keep a simple log (date, first roots, root length). If you know a gardener or orchard, ask them to show a graft union on a fruit tree.
Common difficulties:
- Thinking potatoes and onions are roots. Tubers and rhizomes are stems; bulbs are mostly leaves.
- Forgetting that a graft only works when the cambium layers meet.
- Saying clones are "weaker". They are identical, which is a risk only when a disease or change comes along that they can't handle.