Page 112 - NCERT Science Class 10 English Medium

Activity 6.2
Activity 6.26.2
Activity 6.2
Activity
Activity 6.2
n Fill a conical flask with water.
n Cover the neck of the flask with a wire mesh.
n Keep two or three freshly germinated bean
seeds on the wire mesh.
n Take a cardboard box which is open from one
side.
n Keep the flask in the box in such a manner
that the open side of the box faces light coming
from a window (Fig. 6.5).
n After two or three days, you will notice that
the shoots bend towards light and roots away
from light.
n Now turn the flask so that the shoots are away Figure 6.56.5
Figure
6.5
Figure 6.5
Figure
Figure 6.5
from light and the roots towards light. Leave it
Response of the plant to the direction of light
undisturbed in this condition for a few days.
n Have the old parts of the shoot and root
changed direction?
n Are there differences in the direction of the new
growth?
n What can we conclude from this activity?
Environmental triggers such as light, or gravity
will change the directions that plant parts grow in.
These directional, or tropic, movements can be either
towards the stimulus, or away from it. So, in two
different kinds of phototropic movement, shoots
respond by bending towards light while roots
Figure
Figure 6.66.6
6.6
Figure 6.6 Plant showing geotropism
Figure
respond by bending away from it. How does this help Figure 6.6
the plant?
Plants show tropism in response to other stimuli as well. The roots
of a plant always grow downwards while the shoots usually grow
upwards and away from the earth. This upward and downward growth
of shoots and roots, respectively, in response to the pull of earth or gravity
is, obviously, geotropism (Fig. 6.6). If ‘hydro’ means water and ‘chemo’
refers to chemicals, what would ‘hydrotropism’ and ‘chemotropism’
mean? Can we think of examples of these kinds of directional growth
movements? One example of chemotropism is the growth of pollen tubes
towards ovules, about which we will learn more when we examine the
reproductive processes of living organisms.
Let us now once again think about how information is communicated
in the bodies of multicellular organisms. The movement of the
sensitive plant in response to touch is very quick. The movement of
sunflowers in response to day or night, on the other hand, is quite slow.
Growth-related movement of plants will be even slower.
Even in animal bodies, there are carefully controlled directions to
growth. Our arms and fingers grow in certain directions, not haphazardly.
So controlled movements can be either slow or fast. If fast responses to
stimuli are to be made, information transfer must happen very quickly.
For this, the medium of transmission must be able to move rapidly.

Control and Coordination 107

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