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 respond by bending away from it. How does this help 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. 7.6). 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.
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Answer:
Plants have a variety of developmental, physiological, and growth responses to light—sometimes only to particular wavelengths of light.
In phototropism a plant bends or grows directionally in response to light. Shoots usually move towards the light; roots usually move away from it.
In photoperiodism flowering and other developmental processes are regulated in response to the photoperiod, or day length.
Short-day plants flower when day length is below a certain threshold, while long-day plants flower when day length is above a certain threshold.
In many plants, photoperiodism is controlled by the overlap between the day length cue and the plant's internal circadian rhythms.
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