Biology · Plant Reproduction, Signals & Responses

Tropisms, biological clocks and seasonal responses

A shoot bends while its base remains in place. Another plant changes its flowering behaviour as days lengthen. Both respond to light, but direction and duration ask different biological questions.

Plant Structure, Transport & Nutrition supplies meristems, tissue boundaries, water movement and source–sink transport. Development & Differentiation introduces changing cell identities; Meiosis & Patterns of Inheritance supplies haploid and diploid chromosome sets. The first lesson explains the plant life-cycle transition again before using n. Cell Regulation & Signalling is a useful return path for receptors and feedback, and Evolutionary Processes for the final lesson’s selection argument.

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By the end, you can

Distinguish directional growth, rhythmic timing and seasonal cue interpretation, and read a bounded coincidence model without treating it as a universal flowering rule.

Tropisms depend on the direction of a cue

A tropism is a directional growth response to a stimulus. Positive phototropism of a shoot directs growth toward light. Growth can reorient an organ without locomotion of the whole plant. Photoreceptors, signalling and differences in growth across the organ contribute to the response.

In a selected shoot example, the initially equal sides are each 10 mm long. Over the assigned interval the lit side becomes 16 mm and the shaded side 18 mm. Their extensions are 6 and 8 mm, respectively. The longer shaded side belongs on the outside of the bending shoot; the selected shoot bends toward the light.

The straight side-length bars display the measured extensions, while a separate curved outline shows the qualitative orientation. The 2 mm extension difference does not determine a bending angle: width, the distribution of growth and geometry are not measured here. Root responses and other contexts should not be inferred from this one selected shoot case.

Unequal extension reorients a shoot

The bar axis measures extension; the curved outline is not a reconstruction from measured geometry.

Read the diagram in words

Both sides begin at 10 mm. The lit side ends at 16 mm, extension 6 mm; the shaded side at 18 mm, extension 8 mm. The difference is 2 mm. The qualitative outline bends toward light on the right with the longer shaded side on the outside.

Unequal extension reorients a shoot
Shoot side	Initial length (mm)	Ending length (mm)	Extension (mm)
Lit side	10	16	6
Shaded side	10	18	8

Unequal extension reorients a shoot
Displayed axis	Axis minimum	Axis maximum	Displayed ticks	Unit
Extension during the assigned interval	0	10	0, 2, 4, 6, 8, 10	mm

Selected shoot bends toward light
Qualitative curve point	Schematic x coordinate	Schematic y coordinate
Base	0	0
First control point	0	2
Second control point	0.5	3
Tip	2	4

Schematic width: 0.6
Light arrives from the right: Right

Unequal extension reorients a shoot
Series	Displayed marker
Lit side extension	Circle
Shaded side extension	Diamond

Light arrives from the right
Shaded side is the longer outer side
Selected shoot bends toward light
No angle inferred from the length difference

Constructed two side-length measurements; curved outline is qualitative and has no angle scale.

A daily cue can synchronise an internal rhythm

A circadian rhythm is an internally generated roughly daily pattern. Environmental cycles can entrain it: align its timing with outside conditions. A pattern that continues under a constant condition can support an internal timing interpretation; a daily response observed only during a daily light cycle cannot establish that by itself.

The schematic clock trace repeats every 24 assigned hours after an earlier light–dark cycle. Its phase markers recur at 0, 24 and 48 hours during the depicted constant-light interval. This constructed trace represents a persisting internal pattern; it is not an experimental measurement of an oscillator or evidence that all plants have exactly the same free-running period.

Photoperiod is the duration of daylight within a cycle. Clock-regulated components can interact with light-dependent processes, making a cue’s timing matter as well as its duration. Different plants interpret seasonal cues differently. Temperature, developmental stage and other conditions can also affect flowering.

A rhythm and a separate coincidence rule

Persistence, phase and coincidence are distinct ideas. The assigned timing rule is not a species flowering schedule.

Read the diagram in words

The constructed constant-light trace repeats peaks at 0, 24 and 48 hours. In the separate 0–24 hour panel, the active window is 14–18. Light from 0–12 overlaps it for zero hours; light from 0–16 overlaps it for 2 hours, from 14–16. The assigned output threshold is at least 1 hour. Shifting the window to 18–22 removes overlap with the same 0–16 light interval.

Schematic persisting rhythm
Displayed axis	Axis minimum	Axis maximum	Displayed ticks	Unit
Time under constant light	0	48	0, 6, 12, 18, 24, 30, 36, 42, 48	h
Constructed internal pattern	0	1	0, 0.5, 1	Relative readout

Schematic persisting rhythm
Time (h)	Relative readout
0	1
6	0.5
12	0
18	0.5
24	1
30	0.5
36	0
42	0.5
48	1

Assigned period (h): 24
Constant light in this panel: Yes

Schematic persisting rhythm
Phase marker time (h)
0
24
48

A rhythm and a separate coincidence rule
Displayed axis	Axis minimum	Axis maximum	Displayed ticks	Unit
Hours after dawn	0	24	0, 4, 8, 12, 16, 20, 24	h

Assigned overlap threshold (h): 1

Assigned clock-associated active window
Assigned timing case	Light starts (h)	Light ends (h)	Active window starts (h)	Active window ends (h)	Overlap starts (h)	Overlap ends (h)	Overlap duration (h)	Output under the teaching rule
12-hour light interval	0	12	14	18	No shared interval	No shared interval	0	Output absent under the teaching rule
16-hour light interval	0	16	14	18	14	16	2	Output present under the teaching rule

Alternative window with the same light interval
Assigned timing case	Light starts (h)	Light ends (h)	Active window starts (h)	Active window ends (h)	Overlap starts (h)	Overlap ends (h)	Overlap duration (h)	Output under the teaching rule
Alternative window with the same light interval	0	16	18	22	No shared interval	No shared interval	0	Output absent under the teaching rule

Schematic persisting rhythm
Light
Assigned clock-associated active window
Shared interval
Output present under the teaching rule
Output absent under the teaching rule

Two distinct constructed panels; the 24-hour period, window and 1-hour output threshold are invented teaching inputs.

Read one selected timing rule

The second panel is a separate constructed coincidence model. A clock-associated active window is assigned to hours 14 through 18 after the cycle’s dawn at hour 0. One light interval runs from 0 to 12; another from 0 to 16. Both panels use the same 0–24 hour axis.

The 12-hour light interval has no overlap with the active window. The 16-hour interval overlaps it from hour 14 to hour 16, for 2 hours. The teaching rule labels an output present when the overlap is at least 1 hour. Under this assigned rule, the first case has no output and the second has output. These particular hours and threshold are invented, not a flowering schedule for a named species.

The model isolates temporal coincidence, a useful idea for photoperiodic pathways. In Arabidopsis, FT protein provides a well-supported example of a signal associated with communication from leaves to the shoot apex during floral induction. That evidence does not imply that every plant uses the simplified threshold or that FT is a sugar reserve flowing to a sink.

A rhythm and a separate coincidence rule

Persistence, phase and coincidence are distinct ideas. The assigned timing rule is not a species flowering schedule.

Read the diagram in words

Schematic persisting rhythm
Displayed axis	Axis minimum	Axis maximum	Displayed ticks	Unit
Time under constant light	0	48	0, 6, 12, 18, 24, 30, 36, 42, 48	h
Constructed internal pattern	0	1	0, 0.5, 1	Relative readout

Schematic persisting rhythm
Time (h)	Relative readout
0	1
6	0.5
12	0
18	0.5
24	1
30	0.5
36	0
42	0.5
48	1

Assigned period (h): 24
Constant light in this panel: Yes

Schematic persisting rhythm
Phase marker time (h)
0
24
48

A rhythm and a separate coincidence rule
Displayed axis	Axis minimum	Axis maximum	Displayed ticks	Unit
Hours after dawn	0	24	0, 4, 8, 12, 16, 20, 24	h

Assigned overlap threshold (h): 1

Assigned clock-associated active window
Assigned timing case	Light starts (h)	Light ends (h)	Active window starts (h)	Active window ends (h)	Overlap starts (h)	Overlap ends (h)	Overlap duration (h)	Output under the teaching rule
12-hour light interval	0	12	14	18	No shared interval	No shared interval	0	Output absent under the teaching rule
16-hour light interval	0	16	14	18	14	16	2	Output present under the teaching rule

Alternative window with the same light interval
Assigned timing case	Light starts (h)	Light ends (h)	Active window starts (h)	Active window ends (h)	Overlap starts (h)	Overlap ends (h)	Overlap duration (h)	Output under the teaching rule
Alternative window with the same light interval	0	16	18	22	No shared interval	No shared interval	0	Output absent under the teaching rule

Schematic persisting rhythm
Light
Assigned clock-associated active window
Shared interval
Output present under the teaching rule
Output absent under the teaching rule

Two distinct constructed panels; the 24-hour period, window and 1-hour output threshold are invented teaching inputs.

Worked example

Find the actual shared interval

To calculate overlap, choose the later start and the earlier end. With 16 hours of light the later start is 14, and the earlier end is 16; overlap = 16 − 14 = 2 hours. With 12 hours of light the earlier end, 12, comes before the active window begins; overlap is zero, rather than a negative duration.

The result depends on timing, not just total light. A separate hypothetical light interval from 0 to 16 with a clock window from 18 to 22 would have no overlap. It has the same 16-hour duration but a different relation to the assigned window. The example describes the constructed logic and supplies no cultivation advice.

Check your understanding

The shaded side extends by 8 mm and the lit side by 6 mm. Which side is longer, and can an angle be calculated?
Show reasoning
The shaded side is longer by 2 mm and lies on the outer side of the selected bend toward light. No exact angle follows because width and the distribution and geometry of growth are unassigned.
Why does a 24-hour pattern seen only during alternating daily light and darkness fail to prove an internal clock?
Show reasoning
The outside cycle could directly drive the pattern. Persistence under appropriate constant conditions supplies different evidence for internal timing; entrainment concerns alignment with external cues.
Under the assigned window 14–18 and light interval 0–16, how many hours coincide?
Show reasoning
Hours 14–16 coincide, a duration of 2 hours. This passes the model’s invented 1-hour threshold; it is not a universal flowering condition.

What to carry forward

Direction, phase and duration carry different information; keep the response and timing rule specific to the case.

References

Resources supporting this lesson’s explanations and examples.