Tropic Responses

A tropism = directional growth in response to a stimulus.

Tropism	Stimulus	Shoots	Roots
Phototropism	Light	Positive (toward)	Negative (away)
Geotropism / gravitropism	Gravity	Negative (away from)	Positive (toward)
Hydrotropism	Water	—	Positive (toward)
Thigmotropism	Touch	Climbing plants wrap around	—

Why these directions?

• Shoots toward light → leaves can photosynthesise.
• Shoots away from gravity → grow upward, find sun.
• Roots toward gravity → reach soil for water + minerals.
• Roots away from light → stay underground (in dark soil).
• Roots toward water → maximise water uptake.

Worked qualitative. Why do houseplants 'lean' toward windows?

• Light is uneven (more from window).
• Phototropic shoot growth bends them toward the light.
• Rotating the plant occasionally helps it grow evenly.

Cambridge tip. Memorise the chart. Cambridge often gives a scenario ('a root grew downward') and asks students to identify the tropism (geotropism, positive).

Step-by-step explanation:

1. Auxin produced at shoot tip.

• Made constantly at the very tip (apical meristem).
• Diffuses down the shoot.

2. Light from one side.

• Auxin redistributes — moves AWAY from the light, TOWARD the shaded side.
• (Mechanism: light affects auxin transport proteins.)

3. Auxin causes cell elongation.

• Higher auxin on shaded side → cells take up more water + grow longer there.

4. Differential growth.

• Shaded side: cells longer.
• Lit side: cells shorter.
• Result: shoot CURVES toward the light.

Diagram you should be able to draw:

      LIGHT
        |
        v
   [shoot tip]    ← auxin made here
   /        \
  /          \
 [lit]      [shaded]
  short      long  ← cells elongate more here

The shoot leans toward the light source.

Worked qualitative. Why are shoots that grow in pure darkness EXTRA TALL but skinny and yellow?

• Without light, plants can't tell where to bend → grow straight up.
• Auxin still works; just no directional cue.
• Without light, no chlorophyll made → leaves yellow.
• Cells elongate furiously trying to find light.
• Called ETIOLATION — common in plants in dark cellars.

Cambridge tip. Always state: 'auxin moves to the SHADED side'. The misconception 'moves to lit side' is so common Cambridge specifically tests it.

Auxin works DIFFERENTLY in roots:

• In SHOOTS: auxin PROMOTES elongation.
• In ROOTS: auxin INHIBITS elongation.

This opposite effect is why the SAME hormone can produce opposite responses in shoots and roots.

A horizontal root in a Petri dish:

1. Gravity makes auxin SETTLE to the LOWER side of the root.
2. The lower side has MORE auxin.
3. More auxin → cells INHIBITED → grow LESS.
4. Upper side has LESS auxin → cells grow MORE.
5. Differential growth: upper side longer than lower side.
6. Root BENDS DOWNWARD (positive geotropism).

A vertical root growing straight down.

• Auxin distributed evenly → cells elongate equally → straight growth.
• This is why a normally-positioned seedling grows straight down with its root.

Worked qualitative. A scientist places a young bean shoot HORIZONTALLY in a dark room. After 2 days, what does the shoot do? What does the root do?

• The SHOOT bends UPWARD: auxin gathers on lower side → promotes growth there → upper grows less → bend up (negative geotropism).
• The ROOT bends DOWNWARD: auxin gathers on lower side → inhibits growth there → upper grows more → bend down (positive geotropism).
• Both due to gravity (no light involvement) — pure geotropism experiment.

Cambridge tip. Cambridge often asks 'why does auxin have OPPOSITE effects?'. Answer: roots and shoots have different sensitivities to auxin — same hormone, different cellular response.