Responding to the Environment — Plants

Tropisms — directional growth responses

A tropism is a growth response of a plant towards or away from a directional stimulus. Growth towards the stimulus is called a positive tropism; growth away from it is a negative tropism.

Phototropism — growth response to light. Shoots are positively phototropic (grow towards light, to maximise photosynthesis); roots are usually negatively phototropic.
Geotropism (gravitropism) — growth response to gravity. Roots are positively geotropic (grow downwards, anchoring the plant and reaching water/minerals); shoots are negatively geotropic (grow upwards).
Hydrotropism — growth response to water. Roots are positively hydrotropic, growing towards moist soil in search of water.

The role of auxin

Auxin is a plant hormone produced in the growing tips (apical meristems) of shoots and roots. It causes cell elongation, but is unevenly distributed when a directional stimulus (like light) is present.

When light shines from one side of a shoot, auxin migrates towards the shaded side. Since auxin promotes cell elongation, cells on the shaded side elongate more than cells on the illuminated side. This causes the shoot to bend towards the light — positive phototropism.

Key mechanism: unequal auxin distribution → unequal cell elongation → curvature towards (or away from) the stimulus. The same basic mechanism explains phototropism and geotropism, just with auxin responding to light or gravity respectively.

In roots, the situation is reversed: root cells are far more sensitive to auxin, so a build-up of auxin on the lower side of a horizontal root actually inhibits elongation there, causing the root to bend downward — positive geotropism.

Nastic movements — a contrast to tropisms

Unlike tropisms, nastic movements are non-directional responses — the direction of the plant's response does not depend on the direction of the stimulus. A classic example is Mimosa pudica ("sensitive plant"), whose leaflets fold up rapidly when touched, regardless of which direction the touch came from. Nastic movements are usually caused by rapid changes in water pressure (turgor) in specialised cells, rather than by differential growth.

	Tropisms	Nastic movements
Direction of response	Depends on direction of stimulus	Independent of stimulus direction
Mechanism	Differential growth (auxin-driven)	Rapid turgor (water pressure) changes
Speed	Slow (hours to days)	Fast (seconds)
Example	Shoot bending towards light	Mimosa pudica leaf-folding when touched

Answer in your exercise book.

Question 1 · (2 marks)

Define "tropism", and distinguish between a positive and a negative tropism.

Question 2 · (3 marks)

Name and describe THREE types of tropism found in plants.

Question 3 · (4 marks)

Explain, in terms of auxin distribution and cell elongation, how a shoot bends towards a light source.

Question 4 · (3 marks)

Explain why roots grow downwards (positive geotropism) in terms of auxin sensitivity.

Question 5 · (3 marks)

Distinguish between a tropism and a nastic movement, using Mimosa pudica as an example of the latter.

Question 6 · (6 marks)

In an investigation, a shoot was illuminated from one side only. The length of cells on the illuminated side and on the shaded side was measured every 6 hours:

Time (hours)	0	6	12	18	24
Illuminated side — cell length (mm)	2.0	2.3	2.6	2.8	3.0
Shaded side — cell length (mm)	2.0	2.6	3.4	4.3	5.2

(a) Calculate the total increase in cell length on the illuminated side, and on the shaded side, over the full 24 hours.
(b) Calculate the average rate of elongation (in mm per hour) for the shaded side over the 24-hour period.
(c) Using this data, explain how the difference in elongation rate between the two sides causes the shoot to bend, and state which direction (towards or away from the light) it will bend. Name the hormone responsible.

Responding to the Environment — Plants

Tropisms — directional growth responses

The role of auxin

Nastic movements — a contrast to tropisms

Phototropism Simulator

Seedling growth response