Page 15 - Biology_Form_3

Biology for Secondary Schools

Importance of phototropism On the other hand, the growth rate of the
(a) Phototropism drives the plant shoot root will be slower in the lower half of the
toward light for photosynthesis to take root since higher concentration of auxins
place. inhibits the growth of roots. This will result

(b) The light absorbed by plants through into the downward growth of the roots as
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phototropic response, influences the indicated in Figure 1.5(b) and (c).
opening of the stomata pores. This is
important for gaseous exchange.

Geotropism or geotropic movement Equal distribution of auxin hormone
Geotropism is the movement of a part of immediately after germination
the plant towards the force of gravity. In (a)
plants, roots grow downward in response
to the force of gravity while shoots grow Gravity Gravity
upward against gravity. Growth of plant
roots towards gravity is said to be a positive
geotropic response whereas plant shoots
growth away from gravity is said to be
negative geotropic response. When a seed Auxin hormone concentration is more on the lower
half of shoot and root due to gravitational force
is planted it germinates into seedling. As (b)
the seed germinates immediately before
any environmental stimulus affects it, the Low auxin Low auxin
auxin hormone is equally distributed in the concentration concentration
shoot and root of the seedling, as shown in
Figure 1.5 (a).
High auxin High auxin
When the seedling is placed horizontally concentration in concentration
stimulates it to
shoot stimulates
more auxins will be concentrated at the it to curve upward curve downward
lower half of the shoot and root due to the (c)
force of gravity as shown in Figure 1.5 (b). Figure 1.5 (a), (b) and (c): Geotropism

Higher concentration of auxins in the shoots In the shoot, the area with high concentration
facilitates growth of shoots while lower of auxin hormone grow faster than the area
concentration in the roots facilitates growth with low concentration of auxin hormone.
of roots. The lower half of the shoot with In the root, the area with high concentration
higher auxin concentration will therefore of auxin hormone grows slowly compared
grow faster than the upper half causing the to the area with low concentration of auxin
shoot to grow vertically upward as shown hormone. The effect of gravity on auxin
in Figure 1.5(c). hormone distribution also affect mature
plants as shown in Figure 1.6.

8 Student’s Book - Form Three

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