Page 19 - Biology_Form_3

Biology for Secondary Schools

Another example of chemotropism is
the growth of plant roots towards the
location of fertilizer placed at one side
of the plant.

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Thermotropism or thermotropic
movement
This is a movement of plant parts in
response to temperature. Some flowers
close when the temperature drops, and

Figure 1.9: Thigmotropism in plant some leaves for example maize leaves
curl when the diurnal temperature is rises
Importance of thigmotropism above normal. For example, petals of
Thigmotropism enables twining plants to climb tulip (Tulipa sp.) close at low temperature
up higher plants and expose their leaves to and re-open at high temperature.
sunlight for optimum photosynthesis. Nastic responses

Chemotropism or chemotropic movement These are non-directional movements of
This is a growth movement of plants towards part of a stationary plant in response to
chemicals. This type of movement can be external stimuli such as folding of leaves
seen in the growth of the pollen tube from in warm weather, opening and closing of
the stigma through style toward the ovary and flowers in response to intensity of light,
finally the ovule . The growth of pollen tube is and closing of leaves when touched. Such
induced by a sugary substance in the stigma. movements occur as a result of changes
See Figure 1.10. in turgor pressure in a certain cell from

Pollen any direction of the stimulus. Nastic
movements are reversible movements
Generative which occur repeatedly in response to
nucleus stimuli. Based on the nature of stimuli,
nastic movements can be divided into

Pollen tube photonasty, chemonasty, thigmonasty,
nyctinasty and thermonasty.
Tube nucleus
Photonasty
Extending tip This is a movement of plant parts
of pollen tube in response to light intensity but not
Ovule
toward light. For instance, some
Pollen tube flowers or leaves fold in response to
Micropyle reaches embryo
Sperm nuclei (n) sunshine, or the opening and closing
Figure 1.10: Chemotropism in plant of flowers during the day and night.

12 Student’s Book - Form Three

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