Directional growth responses
Plants can improve their chances of survival by altering their growth when environmental conditions change. In flowering plants, some growth responses are orientated according to the direction of a stimulus.
A tropism is a growth response whose direction is determined by a directional stimulus.
| Tropism | Stimulus |
|---|---|
| Phototropism | Light |
| Gravitropism | Gravity |
Positive tropism
The plant organ grows towards the stimulus.
Negative tropism
The plant organ grows away from the stimulus.
Growth factors
Plant cells produce chemical growth factors that regulate growth responses. Their action is similar in principle to the action of hormones in animals.
Growth factors are specific, so different growth factors can produce different responses. They are often made in actively growing regions and then transported to other tissues where they influence growth.
Exam Tip: Use the A-level term growth factor. When describing a tropism, state both the stimulus and whether growth is towards or away from it.