4.3 Tropism Flashcards
Tropism.
Definition
Growth response to an environmental stimulus in which the stimulus determines the direction of growth.
Growth responses.
Definition
Permanent directed movements of plant parts.
Brought about by asymmetric growth.
Occurs in some constant relationship to an external stimulus which occurs in some constant relationship.
Nastic movements
Movements of plant parts not related to an external stimulus
Difference between positive and negative tropisms.
Positive - moving towards stimulus.
Negative - moving away from source of stimulus.
Phototropism
Relates to the external stimulus of light.
Most commonly seen when plants grow towards to light:
Growth is stimulated on dark side.
No growth on lighted signs.
Plant bends towards light.
Auxin stimulates cell expansion on dark side.
In asymmetric light, there is more auxin in the dark side of the plant and this leases to more growth in this side. Plant grows bending towards the light.
Most effective light colour for phototropism and how phototropism works.
Blue light.
How phototropism works is uncertain.
Possibilities include:
Light destroys auxin?
Light causes auxin to move to dark side?
This allows the plant to gather the maximum amount of sunlight in order to photosynthesise most efficiently.
Geotropism.
Directional growth of a plant as a result of earth’s gravitational forces.
Difference between positively geotropic and negatively geotropic responses.
Positively geotropic response - eg. Primary roots, they grow downwards.
Negatively geotropic - eg. Stem and stalks, they grow away from earth’s centre of gravity.
Diageotropism
More horizontal growth.
Found in secondary roots, branches,petioles, rhizomes and stolons.
Effect of auxin in asymmetrical orientation in a gravitational field
There is more auxin on the ‘down’ side.
Promotes cell expansion in stems ; results in more growth in lower side.
Result - stem bends upwards.
Effect if high auxin levels in root system.
High auxin levels inhibit cell expansion in roots.
Moor growth on upper levels, where there is less auxin.
Result - root bends downwards.
Example of effect of geotropism.
Antirrhinum flowers exhibit geotropism.
Cannot be stored flat , in boxes, as they will bend upwards.
Flowers must be kept vertical throughout harvesting and shipping.
Thigmotropism (seismotropism) - definition.
Growth of plant in response to the stimulus of touch - contact with solid object.
Thigmotropism/seismotropism. - example of type of growth.
Demonstrated by curling of tendrils around objects such as canes and trellis.
Tendrils
Tendril grips support.
Coils very tightly.
Rigidity is conferred on plant.
Offers stability to plant prior to secondary thickening.
