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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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