Control and Coordination · Lesson 7 of 11
Coordination in Plants
“Plants have no nerves, yet a gentle touch can still make them react dramatically.”
- Explain how plants respond despite lacking nerves and muscles. - Distinguish growth-dependent and growth-independent plant movements. - Describe the immediate response of a sensitive plant to touch. - Explain how information travels from the touched region to the moving region. - Relate changes in cellular water content to swelling, shrinking and movement. - Compare sensitive-plant movement with animal muscle movement.
Touch the leaf of a sensitive plant gently and its leaflets fold, although the plant has no brain, nerves or muscles. A germinating seed also responds to its surroundings: the root grows downward and the shoot upward. Plants clearly coordinate responses, but their mechanisms differ from the rapid nerve-and-muscle system of animals.
Plant Responses To Stimuli
Plant movements fall into two broad groups. Some are caused by growth and are therefore relatively slow and directional. Others occur without growth and may happen quickly. The bending of a seedling towards light requires unequal growth. The folding of sensitive-plant leaves after touch uses existing cells and is independent of growth.
| Plant response | Growth involved | Typical mechanism | Example |
|---|---|---|---|
| Immediate movement | No | Rapid changes in cell water content and shape | Sensitive-plant leaf folding |
| Directional growth | Yes | Different growth rates on different sides | Shoot bending towards light |
Immediate Response To Stimulus
In a sensitive plant, the location touched is not always the location where the most visible movement occurs. This shows that information about touch must travel through the plant. Plant cells use electrical-chemical changes to pass information from cell to cell, although plants do not have specialised nervous tissue extending throughout the body.
Movement occurs when cells in a specialised region change their water content. Cells that gain water swell and become firmer; cells that lose water shrink and become less firm. A rapid redistribution of water changes cell shape and support, causing leaflets to fold and the leaf stalk to droop. Because no new cells or permanent elongation are required, the movement can later be reversed when the earlier water balance is restored.
A plant movement produced by rapid changes in existing cells rather than by the formation or permanent enlargement of new tissue.
Plant And Animal Movement
Both plants and animals must communicate a stimulus from the detection site to the response site, and both ultimately depend on cells changing their state. The mechanisms differ. Animal muscles contain specialised contractile proteins and receive signals through neurons. Sensitive plants lack these tissues; information passes between ordinary plant cells and movement results largely from changes in water content.
| Feature | Sensitive plant | Animal leg |
|---|---|---|
| Specialised nervous tissue | Absent | Present |
| Specialised muscle tissue | Absent | Present |
| Information transmission | Electrical-chemical changes from cell to cell | Electrical impulses in neurons and chemicals at junctions |
| Immediate movement | Water-content changes alter cell shape | Contractile proteins shorten muscle fibres |
Problem
Does folding of a sensitive-plant leaf require growth?
- 1.The response occurs rapidly after touch.
- 2.Existing cells change water content and shape.
- 3.No permanent addition of new tissue is needed.
- 4.Therefore the movement is independent of growth.
Problem
The leaf is touched at one point but the stalk droops elsewhere. What can be inferred?
- 1.The stimulus is detected at the point of contact.
- 2.Movement at a different point requires information transfer.
- 3.Plant cells must pass an electrical-chemical signal between regions.
- 4.The response is coordinated even without specialised nerves.
Problem
Why is sensitive-plant movement reversible but a shoot's growth towards light is not simply reversed?
- 1.Sensitive-plant movement depends on temporary redistribution of water in existing cells.
- 2.Restoring water balance can return cells and leaflets towards their earlier state.
- 3.Phototropic bending involves differential growth and permanent elongation.
- 4.Changing the light direction affects new growth but does not remove tissue already produced.
Plants do respond to stimuli even though they lack a nervous system. Rapid sensitive-plant movement is not growth, and it is not caused by animal-like muscles; it depends mainly on cell-to-cell signals and changes in water content.
Quiz
What causes the rapid folding of sensitive-plant leaves?
Why must information travel within a sensitive plant?
Which response is growth-dependent?
What specialised tissue for information conduction is absent in plants?
Which feature is shared by sensitive-plant and animal movement?
Practice Problems
- Classify the folding of a sensitive-plant leaf. Model answer: It is a rapid growth-independent movement produced by changes in water content and shape of existing cells.
- Why does movement away from the touched point demonstrate coordination? Model answer: Information about touch must be transmitted to the separate region that changes shape and produces movement.
- Describe how loss of water from movement cells affects a sensitive leaf. Model answer: Cells become less swollen and less firm, changing support at the leaf base so leaflets fold or droop.
- Compare information transmission in plants and animals. Model answer: Plants pass electrical-chemical information from cell to cell without nerves; animals rapidly conduct impulses through specialised neurons.
- Explain why stopping growth would prevent seedling bending but not necessarily immediate leaf folding. Model answer: Seedling bending requires unequal cell growth; leaf folding uses reversible water changes in existing cells and does not require growth.
Key Takeaways
• Plants detect and respond to stimuli without nerves or muscles. • Plant movement may be dependent on growth or independent of growth. • Sensitive-plant leaf folding is a rapid growth-independent response. • Information about touch passes from cell to cell by electrical-chemical changes. • Redistribution of water changes cell firmness and shape, producing movement. • Plant and animal responses both require communication but use different effector mechanisms.