Page 259 - Biology_F5

Biology for Advanced Level Secondary Schools

4.4.3 Tropic movement positive geotropism. If a potted plant is kept
This is the movement of part of a plant in a horizontal position, the shoot will bend
associated with growth of plant tissue and start growing in an upright direction
caused by a differential concentration of while the roots change their ground direction
plant hormones, usually auxins, under a of growth downwards (Figure 4.41).
specific stimulus. The stimuli involved
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include light, gravity, water/humidity, and Negative
pressure or touch, among others. Some geotropic
common examples of tropic movement in
plants include phototropism, geotropism,
and hydrotropism.

Phototropism: Plant growth movement in
response to light stimulus. Shoots tend to Positive
grow towards light (positive phototropic geotropic
movement), and if the light is coming Figure 4.41: Geotropism in plant
from a single direction the plant will bend
towards that source of light (Figure 4.40). Hydrotropism: Hydrotropism takes
The roots show the opposite tendency place in response to moisture. Roots are
(negative phototropism). generally positively hydrotropic (Figure
4.42 a and b).

(a)

Root grows
straight

Moist soil

(b) Water Clay pot

Figure 4.40: Shoot grow towards light
Root grows
towards water
Geotropism: This is the plant growth
movement in response to the earth’s
gravitational pull. Shoots show negative Dry soil Moist soil
geotropic movement, that is, they grow away
from the force of gravity, while roots show Figure 4.42: Movement of roots towards (a)
moisture (b) water

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