Page 11 - Biology_Form_3

Biology for Secondary Schools

the plants to survive, grow, develop and
Exercise 1.1 reproduce. There are two types of growth
1. Citing examples, explain why Auxin movements in plants, namely tropic
hormone is sometimes referred to as movement and nastic movement.
“master regulator” in the context of
FOR ONLINE READING ONLY
plant growth and development. Task 1.3
Search the library and internet sources
2. Using relevant examples explain the
concept of coordination in plants. animations on tropic and nastic responses
in plants. Watch the videos and then write
3. Providing specific examples to short notes about the information learned.
support your points, analyze how
plant hormones can affect plant’s Tropic responses
productivity. These are growth movements which
4. From the statement “every plant occur in the direction of the stimulus.
function is hormonally based” Tropic movement is the unidirectional
analyse the role of plant hormones growth movement shown by a fixed part
in different plant processes. of a stationary plant towards or away
from stimulus. Tropic movements are

Concept of paratonic movement also known as tropism movements. If
Plant movements stimulated by external the response is toward the stimulus, the
movement is referred to as (+) positive
factors such as light and chemicals are tropic movement. But if the response is
called paratonic movement. They include away from the stimulus it is referred to as
tropic movements and nastic movements. (-) negative tropic movement.
Tropic movements are controlled by plant
hormones and are directional meaning that Tropic movements take place at very slow
the movement is towards or away from rates and are irreversible. The growth
the direction of stimuli. The changes in movement is caused by an increased or
plant growth resulting from tropism are decreased rate of growth on the side of
irreversible and they occur at a slower rate. the organ which is under the influence of

Nastic movements are controlled by plant the stimulus relative to the opposite side.
hormones but they have non-directional This unequal growth results in growth
responses because the movement is not of curvature. There are various types of
related to direction of stimulus. The changes tropic movements, these are phototropism,
are reversible and occur at much faster rates geotropism, hydrotropism, thigmotropism
compared to tropic movements. and chemotropism.
Plants detect and respond to a variety of Phototropism or phototropic movement
stimuli that are important for their survival This is the growth movement of a plant
in the environment. These responses allow part or organ in response to unidirectional

4 Student’s Book - Form Three

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