Page 257 - Biology_F5

Biology for Advanced Level Secondary Schools

taxis. Positive taxis is when an organism Nyctinasty
or cell moves towards the source of This is a nastic movement which
stimulation, while negative taxis is when occurs in response to diurnal changes
an organism or cell moves away from of temperature (thermonasty) or light
the source of stimulation. For example, intensity (photonasty). An example of
Chlamydomonas usually moves towards nyctinasty is the opening and closing
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light of low intensity and away from light of petals of some flowers that occur in
of high intensity. This type of response response to alternation in the duration of
is known as phototactic response or day and night. Another example is sleep
phototaxis. In this example, the movement movements of leaves of some leguminous
towards light of low intensity is a positive plants in response to the onset of darkness.
phototaxis, whereas, that away from light
of high intensity is a negative phototaxis. Thermonasty
This is a movement of plant parts in
Types of tactic movements (taxes) response to temperature. For example,
Based on the nature or kind of a stimulus some flowers close when the temperature
involved, different types of tactic drops, or some leaves fold when the
movements (taxes) can be identified.
Examples of taxes include phototaxis temperature is too high.
(response to light), which can be seen
in Euglena and blue green algae and Chemonasty
chemotaxis (response to chemicals) as seen This is the movement of plant parts
in lower plants such as Funaria, where by in response to chemicals stimuli. For
male gamete moves towards the chemical example, sundew plants close glandular
substance released by egg cells. Others hairs due to chemicals produced when
include geotaxis (gravity), aerotaxis insects land on those plants. This prevents
(oxygen), magneotaxis (magnetic field) escape of the insects.
and rheotaxis (water current).
Photonasty
4.4.2 Nastic movement This is the movement of plant parts in
Nastic movement is a non-directional response to light. For instance, some
movement of a part of stationary plant in flowers or leaves fold following the sun
response to an external stimulus. Based on shine, or the opening and closing of
the nature of stimuli, nastic movements
can be divided into the following groups: flowers depending on whether it is day or
night (Figure 4.38).

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