Directed path: taxis. Changed speed or turning: kinesis. Shoot bending toward light: auxin moved to the shaded side.
Organisms detect changes around them and respond. Selection keeps the responses that help them survive and reproduce. Animals often move or signal. Plants can't walk away, so they change how they grow and use hormones as messages.
Key terms
- Taxis — movement steered toward or away from a stimulus, like a flatworm crawling straight away from a lamp.
- Kinesis — a change in speed or turning rate, with no set direction. If conditions vary, animals pile up where they slow down.
- Phototropism — growth of a plant toward or away from light. Shoots bend toward it because their shaded side elongates more.
- Auxin — a plant hormone that makes shoot cells elongate. Light on one side drives it to the other, shaded side.
- Abscisic acid (ABA) — a plant stress hormone. Roots in drying soil send it up the xylem, and it closes stomata.
- Honest signal — a signal that is hard to fake because a real trait sets it, like call pitch set by body size.
How it works
- Most responses follow one chain: stimulus → receptor → internal signal → response. Light, drying soil, and a rival's call are all stimuli.
- Phototropism: light hits one side of a shoot → auxin moves to the shaded side → shaded cells elongate more → the shoot curves toward the light.
- The bend is growth, not force. Both sides keep the same number of cells; the shaded cells get longer.
- Drought: soil dries → roots make abscisic acid → it rides the xylem to the leaves → guard cells lose K+ and water → stomata close.
- Animals send visual, sound, touch, chemical, or electrical signals. Selection favors whichever channel reaches receivers in that habitat, and receivers keep heeding signals that reliably predict a benefit.
Why it matters
Expect a scenario: worms in a dish, a leaning seedling, a frog chorus, a shrub in dry soil. Name the response, then trace its cause. Answers often turn on one detail: directed or not, lit side or shaded side, chemical message or physical one. Photoperiodism works the same way: night length tells many plants when to flower.
Exam traps
| Trap | Actually |
|---|
| Kinesis is a taxis at a lower stimulus intensity. | Taxis has a heading toward or away from the source. Kinesis changes only speed or turning, with no heading. |
| Auxin builds up on the lit side, or light shrinks the lit cells. | Auxin moves to the shaded side. Those cells elongate more, so the shoot curves toward light. |
| Stomata close only once the leaves start to wilt. | Roots in drying soil send abscisic acid up the xylem. Stomata close while the leaves are still fully hydrated. |
| Chemical signals beat visual ones because they carry more information. | The habitat picks the channel. Dim, still forest floors favor chemical trails; open, sunlit ground favors visual displays. |
Worked example
Pill bugs are placed in a chamber with a dry half and a damp half. After 20 minutes, 85% are on the damp side. Tracking shows they walk at 1.4 cm/s on the dry side and 0.3 cm/s on the damp side. Pill bugs released one at a time at the border set off in random directions. Is this response a taxis or a kinesis?
- Check for a heading. Released at the border, the pill bugs start off in random directions. Humidity is not steering them.
- Find what does change: speed. They move fast where it is dry and slowly where it is damp.
- Follow the consequence. Fast walkers leave dry ground quickly; slow walkers linger on damp ground. Over time, most end up damp.
Kinesis. Humidity changes how fast the pill bugs move, not which way they go.
A pile-up on one side looks like a taxis, and that is the near-miss. A taxis needs paths aimed toward or away from the stimulus. Here the paths start out random, and slowing down in damp air is enough to collect the animals there.