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18 Growth responses of green plants (Extension)
Class Practice
18.1 Do plants respond to external stimuli?
18.2 What are tropisms?
A. 1. F
2. F
3. T
4. T
B. 1. A
2. C
18.3 The growth response of shoots to light
18.4 The growth responses of roots and shoots to gravity
18.5 The growth response of roots to water
A. (a) phototropism
(b) gravity
(c) hydrotropism
B. 1.
F
2.
T
3.
T
C.
4.
1.
2.
F
(a)
(b)
(c)
(d)
(e)
(a)
(b)
(c)
(d)
Clinostat
Geotropism
It grows and bends upwards.
It grows without a change in curvature.
It cancels out the effect of gravity.
Hydrotropism
It avoids the effect of light.
It absorbs water vapour so as to provide a dry environment.
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(e)
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The effect of hydrotropism is stronger than negative geotropism in roots.
18.6 What are the substances which control growth movements in plants?
18.7 More about auxins
A. 1. F
2. T
3. F
4. F
5. T
B.
1.
2.
3.
B
B
B
18.8 The role of auxins in phototropic and geotropic responses
1. (a) shaded
(b) illuminated
(c) towards
2. (d) lower
(e) gravity
(f )
(g)
(h)
inhibits
faster
downwards
Exam Practice
A.
Multiple Choice Questions
1. C
2. B
3. A
4. D
5.
6.
7.
8.
9.
10.
B.
D
D
A
A
B
C
Structured Questions
1. (a) The shoot tip of the young shoot is sensitive to light. It is because in B when the tip
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was covered it did not show any response while in C the tip grew and bent towards
the light.
2.
(b) When the shoot is exposed to unilateral light, it causes auxins to move away from it.
More auxins accumulate on the shaded side.
A higher concentration of auxins stimulates shoot growth.
The shaded side grows faster. As a result the shoot grows and bends towards light.
A – first from above, later from right hand side;
B – in complete darkness;
C – right, left and then right;
D – evenly from all sides
3. (a)
4.
(b) The auxins are mainly distributed in the outside of both the root and shoot due to
centrifugal force. A higher auxin concentration in the root retards growth while in
the shoot this stimulates growth.
(c) same apparatus but with the motor switched off
(d) to avoid the influence of light
(i) The shoot grows and bends to the light.
5.
(ii)
(iii)
(iv)
(v)
(vi)
(a)
The shoot does not show growth and curvature.
The shoot grows without bending.
The shoot grows without bending.
The shoot grows and bends away from the side under the shoot tip.
The shoot grows and bends away from the side under the agar.
(b) The auxin accumulates at the lower side due to gravity.
The higher concentration of auxin at the lower side of the radicle inhibits growth.
The higher concentration of auxin at the lower side of the plumule stimulates
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growth.
The upper side of the radicle grows faster so it bends downloads.
The lower side of the plumule grows faster so it bends upwards.
(c) Upward bending of the plumule ensures absorption of a larger amount of light.
Downward bending of the radicle ensures absorption of more water and minerals.
C.
STS Connections
1. (a) The plant can receive maximum amount of sunlight for photosynthesis.
(b) No. Phototropism is an irreversible growth movement while 'suntracking' is a
reversible movement.
(c)
speed
reversibility
2.
Suntracking
Phototropism
faster
slower
reversible
irreversible
(a) Reaction: closure of leaf lobes to trap the insects
Stimulus: two trigger hairs are brushed in close succession, or one hair is touched
twice
Receptor: trigger hairs
Effector: leaves
(b) Secretion of digestive enzymes
(c) It is not a kind of tropism because it is not a kind of directional growth movement.
(d) Human irritability involves the central nervous system or hormonal system while
Venus Flytrap’s irritability does not.
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