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More to Movement than Growth
The flower of Taraxacum officinale (common Dandelion) blooms in the morning and evening it closes (Photonasty). Ipomoea alba (Moon ower) does exactly the opposite. They open in the night and hence the name moon ower and closes in the day time. Just a casual touch is enough to make the Mimosa pudica (Touch-me-not) leaves fold up and droop (Seismonasty or thigmonasty).
The root and shoot move towards the direction of the stimuli, whereas the movement of the opening and closing of the flowers are not directed towards stimuli. Such movements in plants are called as Nastic movements. Unlike tropic movement, nastic movements are independent of the stimuli direction and may or may not be growth movement.
1. Thigmonasty or Seismonasty – touch - Example: Brunnichia ovata and Mimosa pudica
2. Nictinasty – darkness - Example Leucaena leucocephala
3. Thermonasty – temperature Example: Tulipa sp
4. Photonasty – changes in light intensity - Samanea saman
Plant parts move as they have clear adaptive advantages. Roots going down are more likely to find water and minerals they need. Stem growing up and towards light is more likely to get sunlight for photosynthesis and display its flowers prominently for pollinators to arrive. It also has better chance of spreading its seeds (Table 1).
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